U.S. patent application number 10/934829 was filed with the patent office on 2005-02-17 for method and apparatus for determining depth of interactions in a detector for three-dimensional complete body screening.
Invention is credited to Crosetto, Dario B..
Application Number | 20050035297 10/934829 |
Document ID | / |
Family ID | 32869072 |
Filed Date | 2005-02-17 |
United States Patent
Application |
20050035297 |
Kind Code |
A1 |
Crosetto, Dario B. |
February 17, 2005 |
Method and apparatus for determining depth of interactions in a
detector for three-dimensional complete body screening
Abstract
The present invention is directed to a system and method for
efficiently and cost effectively determining an accurate depth of
interaction for a crystal that may be used for correcting parallax
error and repositioning LORs for more clear and accurate imaging.
The present invention is directed to a detector assembly having a
thin sensor (e.g., APD) deployed in front of the detector (the side
where the radioactive source is located and the photon is arriving
to hit the detector) and a second sensor (APD or photomultiplier)
on the opposite side of the detector. The light captured by the two
interior and exterior sensors which is proportional to the energy
of the incident photon and to the distance where the photon was
absorbed by the detector with respect to the location of the two
sensors, is converted into an electrical signal and interpolated
for finding the distance from the two sensors which is proportional
to the location where the photon hit the detector.
Inventors: |
Crosetto, Dario B.; (DeSoto,
TX) |
Correspondence
Address: |
JONES DAY
222 EAST 41ST ST
NEW YORK
NY
10017
US
|
Family ID: |
32869072 |
Appl. No.: |
10/934829 |
Filed: |
September 2, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10934829 |
Sep 2, 2004 |
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10721345 |
Nov 25, 2003 |
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10721345 |
Nov 25, 2003 |
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10376024 |
Feb 26, 2003 |
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Current U.S.
Class: |
250/367 |
Current CPC
Class: |
G01T 1/2018 20130101;
G01T 1/1642 20130101; G01T 1/1644 20130101; A61B 6/037
20130101 |
Class at
Publication: |
250/367 |
International
Class: |
G01T 001/20 |
Claims
1. A detector assembly for improved depth of interaction
determinations comprising: a scintillator crystal for interacting
with a photon and creating a plurality of optical signals, said
scintillator crystal having a first end and a second end; a first
transducer for receiving one the plurality optical signals from
said scintillator crystal and converting the one the plurality
optical signals to a first electrical signal, said first transducer
having a first active area for receiving optical signals, and said
first active area of said first transducer being optically coupled
to the first end of said scintillator crystal; a second transducer
for receiving another of the plurality optical signals from said
scintillator crystal and converting the one the plurality optical
signals to a second electrical signal, said second transducer
having a second active area for receiving optical signals and said
second active area of said second transducer being optically
aligned for receiving optical signals from second end of said
scintillator crystal; and an optical guide, said optical guide
optically coupled between said second end of said scintillator
crystal and said active area of said second transducer, said
optical guide being conducive to direct optical signals to said
active area of said second transducer.
2. The detector assembly recited in claim 1 above, wherein said
first active area is larger than said second active area.
3. The detector assembly recited in claim 1 above, wherein said
scintillator crystal further comprising: a plurality of slits, each
of said plurality of slits being approximately equal in length.
4. The detector assembly recited in claim 1 above, wherein said
second transducer is one of a photodiode and further comprises: a
semiconducting material, said semiconducting material having a low
photon absorption rate and a low photon scattering rate.
5. The detector assembly recited in claim 4 above, wherein said
second transducer is one of a photodiode and an avalanche
photodiode (APD).
6. The detector assembly recited in claim 4 above, wherein said
second transducer is an avalanche photodiode (APD).
7. The detector assembly recited in claim 5 above, wherein said
first transducer is a photomultiplier (PMT).
8. The detector assembly recited in claim 1 above further
comprises: a third transducer for receiving one of the plurality of
optical signals from said scintillator crystal and converting the
one of the plurality of optical signals to a third electrical
signal, said third transducer having a third active area for
receiving optical signals, and said third active area of said first
transducer being optically coupled to the first end of said
scintillator crystal.
9. The detector assembly recited in claim 5 above, wherein said
first and third transducers are photomultipliers (PMT) and said
second transducer is an avalanche photodiode (APD).
10. The detector assembly recited in claim 9 above, wherein said
scintillator crystal being optically coupled between a plurality of
optical guides and a plurality of photomultipliers (PMT).
11. The detector assembly above in claim 1 recited, wherein said
first electrical signal is related to a first distance from the
first active area of the first transducer and an interaction point
where said photon interacted with said scintillator crystal.
12. The detector assembly recited in claim 11 above, wherein said
second electrical signal is related to a second distance from the
second active area of the second transducer and the interaction
point.
13. The detector assembly recited in claims 12 above, wherein a
depth of interaction (DOI) for the photon in said scintillator
crystal is determined from said first electrical signal, said
second electrical signal and a distance between said first and
second ends.
14. The detector assembly recited in claim 1 above, wherein said
scintillator crystal further comprises: a bismuth germanate (BGO)
crystal.
15. The detector assembly recited in claim 1 above, wherein said
scintillator crystal further comprises: a plurality of bismuth
germanate (BGO) crystals.
16. The detector assembly recited in claim 1 above, wherein said
scintillator crystal further comprises: a sodium iodate (NaI)
crystal.
17. The detector assembly above in claim 1 recited, wherein said
scintillator crystal further comprises: a plurality of sodium
iodate (NaI) crystals.
18. The detector assembly above in claim 1 recited, wherein a
distance between said first and second ends exposes an oblique
angle to a photon.
Description
CROSS REFERENCES TO RELATED APPLICATIONS
[0001] The present application is a divisional of, and claims the
benefit of priority from co-pending U.S. Non-Provisional Patent
Application entitled, "Method and Apparatus for Three-Dimensional
Complete Body Screening," having application Ser. No. 10/376,024,
and filed on Feb. 26, 2003, for inventions not disclosed in U.S.
Provisional No. 60/360,301. Therefore, the present patent
application does not seek benefit from U.S. Provisional No.
60/360,301 for the subject matters not disclosed therein. The
present application is also related to, and claims the benefit of
priority from co-pending U.S. Non-Provisional Patent Application
entitled, "Method And Apparatus for Improving Pet Detectors,"
having application Ser. No. ______, and filed on Nov. 11, 2003,
attorney docket no. 510974-600005, which claims priority to U.S.
Provisional Patent Application No. 60/424,933, of the same title
and filed on Nov. 9, 2002 and each of which is incorporated by
reference herein in its entirety.
[0002] The present application is also related to the following
patent applications:
[0003] U.S. Pat. No. 5,937,202 filed Feb. 15, 1996 entitled
"High-Speed, Parallel, Processor Architecture for Front-End
Electronics, Based on a Single Type of ASIC, and Method Use
Thereof."
[0004] U.S. patent application Ser. No. 09/506,207 filed Feb. 15,
2000 entitled "Method and Apparatus for Extending Processing Time
in One Pipeline Stage," which claims priority from: U.S.
Provisional Patent Application No. 60/120,194 filed Feb. 16, 1999;
U.S. Provisional Patent Application No. 60/112,130 filed Mar. 12,
1999; U.S. Provisional Patent Application No. 60/129,393 filed Apr.
15, 1999; U.S. Provisional Patent Application No. 60/132,294 filed
May 3, 1999; U.S. Provisional Patent Application No. 60/142,645
filed Jul. 6, 1999; U.S. Provisional Patent Application No.
60/143,805 filed Jul. 14, 1999; U.S. Provisional Patent Application
No. 60/154,153, Sep. 15, 1999; U.S. Provisional Patent Application
No. 60/161,458 filed Oct. 25, 1999; U.S. Provisional Patent
Application No. 60/164,694 filed Nov. 10, 1999; and U.S.
Provisional Patent Application No. 60/170,565 filed Dec. 14,
1999.
[0005] U.S. patent application Ser. No. 10/185,904 filed Jun. 27,
2002 entitled "Method and Apparatus for Whole-Body,
Three-Dimensional Dynamic PET/CT Examination," claiming priority
from U.S. Provisional Patent Application No. 60/301,545 filed Jun.
27, 2001; and U.S. Provisional Patent Application No. 60/309,018
filed Jul. 31, 2001.
[0006] U.S. patent application Ser. No. 10/296,532 filed Nov. 25,
2002 entitled "Method and Apparatus for Anatomical and Functional
Medical Imaging," which claims priority from: PCT/US01/15671 filed
May, 15, 2001; U.S. Provisional Patent Application No. 60/204,900
filed May 16, 2000; U.S. Provisional Patent Application No.
60/215,667 filed Jun. 30, 2000; U.S. Provisional Patent Application
No. 239, 543 filed Oct. 10, 2000; U.S. Provisional Patent
Application No. 60/250,615 filed Nov. 30, 2000; U.S. Provisional
Patent Application No. 60/258,204 filed Dec. 22, 2000; and U.S.
Provisional Patent Application No. 60/261,387 filed Jan. 15,
2001.
[0007] U.S. patent application Ser. No. 10/376,024 filed Feb. 26,
2003 entitled "Method And Apparatus For Determining Depth of
Interactions in a Detector for Three-Dimensional Complete Body
Screening," claiming priority from U.S. Provisional Patent
Application No. 60/360,301 filed Feb. 26, 2002.
[0008] U.S. patent application Ser. No. 10/453,255 filed Jun. 2,
2003 entitled "Gantry for Geometrically Configurable and
Non-Configurable Positron Emission Tomography Detector Arrays,"
claiming priority from U.S. Provisional Patent Application
60/385,140 filed Jun. 2, 2002.
[0009] The above-identified patent applications are incorporated by
reference herein in their entireties.
BACKGROUND OF THE INVENTION
[0010] 1. Field of the Invention
[0011] The present invention relates to radiation detectors. More
particularly, the present invention relates to a system and method
for correcting parallax error in a detector resulting from
inaccurately assessing where the photon interacted with the
detector, and thereby increasing efficiencies of traditional
Positron Emission Tomography (PET) devices on a photons per unit of
radiation basis.
[0012] 2. Description of Related Art
[0013] These devices (detectors) are about 200 times smaller than
the large detectors for high-energy physics and require
identification of only one particle, the photon. The task to be
solved of capturing and identifying the particles is relatively
easier than before: one particle instead of five on a detector 200
times smaller.
[0014] The use of positron emissions for medical imaging has been
well document from the early 1950's, see "A History of Positron
Imagining," Brownell, Gordon, presented on Oct. 15, 1999,
Massachusetts General Hospital and available at
http://neurosurgery.mgh.harvard.edu/docs/PEThis- tory.pdf, which is
incorporated herein by reference in its entity. PET imaging have
advantages over other types of imaging procedures. Generally, PET
scanning provides a procedure for imaging the chemical
functionality of bodily organs rather than imaging only their
physical structure, as is commonly available with other types of
imaging procedures such as X-ray, Computerized Tomography (CT), or
Magnetic resonance imaging (MRI). PET scanned images allow a
physician to examine the functionality heart, brain, and other
organs as well as diagnosing disease groups which cause changes in
the cells of a body organ or in the manner they grow, change,
and/or multiply out of control, such as cancers.
[0015] Positron Emission Tomography (PET) is a medical imaging
technique that involves injecting a natural compound, such as sugar
or water, labeled with a radioactive isotope into a patient's body
to reveal internal biological processes. As the isotope (positron)
circulates within the patient's body. The positron annihilates with
and electron and emits pairs of photons in diametrically opposed
directions (back-to-back). A PET device is made of a set of
detectors coupled to thousands of sensors that surround the human
body. These detectors (crystals) capture the photons emitted by the
isotope from within the patient's body at a total rate of up to
hundreds of millions per second, while the sensors (transducers
such as PMTS) convert them to electrical signals, and send the
signals to the electronics.
[0016] Other applications for detecting particles (photons,
electrons, hadron, muon and jets) are well known, such as with
regard to experiments in high energy physics. While particle
detection in high energy physics and medical imaging have some
common ground, differences between the disciplines are sticking. On
distinction between the usages is that the detectors used in
medical imaging are approximately 200 times smaller than the larger
detectors employed in high-energy physics applications, and what is
more, medical imaging PET applications require the identification
of only a single type of particle, the photon.
[0017] Typically, prior art Positron Emission Tomography (PET)
devices require the injection into the patient's body of a
radiation dose that is 10 to 20 times the maximum radiation dose
recommended by the International Commission on Radiological
Protection (ICRP). This amount is necessary because, at best, prior
art PET devices only detect 2 photons out of 10,000 emitted in the
patients' body. Currently the largest manufacturers of PET (General
Electric Company and Siemens AG (ADR)) which command in excess of
90% of the world market, are manufacturing two different PET
(PET/CT) systems with very similar performance and are selling them
at very similar prices. However, although the price and performance
of the systems from the different manufactures are comparable, one
manufacturer's system (Siemens) uses nearly ideal crystal
detectors, while contrastingly, the other manufacturer's system
(General Electric) uses cheaper, lower quality crystal detectors
with slower decay time. Consequently, the manufacturer using the
cheaper, lower cost detectors, expend on the order of only 10% the
price of the ideal crystals used in their competitor's systems.
Thus, the question arises: how it could be that even though one
manufacturer uses crystals detectors that are ten times more
expensive that the other manufacturer, the price and performance of
the two PET systems from the different manufacturers are very
comparable.
[0018] Anecdotally, the present inventor has analyzed the progress
of the most significant PET improvements made in the most recent 17
years, see "400+times improved PET efficiency for lower-dose
radiation, lower-cost cancer screening," 3D-Computing, Jun. 30,
20010, ISBN: 0970289707, which is incorporated herein by reference
in its entity. During that time period the efficiency of PET
improved at a rate of between two and three times every five years.
The analysis included technical literature, patents (including
those assigned to GE and Siemens) and also PETs that were built as
prototypes at several universities but were never commercialized.
At the current improvement rate of PET advancement, conservatively
it would take several decades of improvements for the radiation
dose necessary for a PET procedure to come within the maximum
radiation dose recommended by the ICRP.
[0019] What is needed is a means for increasing the accuracy and
efficiencies of PET devices enabling caregivers to more accurately
diagnose aliment related to the functionality of body organs and
not just inferences from the structure of the organs. Additionally,
what is needed is a quantum advance forward in PET devices and
procedures wherein patients can receive the benefits of PET imaging
without the associative risks from the radioactive doses necessary
for the procedures. Finally, what is needed is a means for reducing
the associated risks and increasing detection efficiencies
associated with PET imaging procedures to such an extent that the
benefits of PET imaging can be applied in well body care and
preventative medicine strategies for apparently healthy
individuals; as a standard health assessment and diagnostic tool
for regular, periodic checkups.
SUMMARY OF THE INVENTION
[0020] The present invention is directed to a system and method for
efficiently and cost effectively determining an accurate depth of
interaction for a crystal that may be used for correcting parallax
error and repositioning LORs for more clear and accurate imaging.
The present invention is direct to a detector assembly having. A
thin sensor (e.g. Avalanche Photodiode (APD)) is deployed in front
of the detector (side where the radioactive source is located and
the photon is arriving to hit the detector) and a second sensor
(APD or photomultiplier) on the opposite side of the detector. The
light captured by the two sensors interior sensor and exterior
sensor, which is proportional to the energy of the incident photon
and to the distance where the photon was absorbed by the detector
with respect to the location of the two sensors, is converted into
electrical signal and interpolated for finding the distance from
the two sensors, which is proportional to the location where the
photon hit the detector.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] The novel features believed characteristic of the present
invention are set forth in the appended claims. The invention
itself, however, as well as a preferred mode of use, further
objectives and advantages thereof, will be best understood by
reference to the following detailed description of an illustrative
embodiment when read in conjunction with the accompanying drawings
wherein:
[0022] FIG. 1 is a diagram depicting the arrival of information
about the particles from several electronic channels at one time in
accordance with an exemplary embodiment of the present
invention;
[0023] FIGS. 2A-2C are diagrams of the sequence of operations in a
3D-Flow sequentially implemented parallel-processing architecture
in accordance with exemplary embodiments of the present
invention;
[0024] FIG. 3 is a diagrammatic roadmap of the construction of the
3D-CBS with different technologies in accordance with exemplary
embodiments of the present invention;
[0025] FIGS. 4A-4D are diagrammatical comparisons of the
relationship between the increasing FOV in LOR in accordance with
exemplary embodiments of the present invention;
[0026] FIGS. 5A-5D depict a scintillation detector assembly as is
well known in the prior art;
[0027] FIG. 6 is a diagram of a detector assembly having two
sensors for measuring the depth of interaction to correct the
parallax error in accordance with an exemplary embodiment of the
present invention;
[0028] FIGS. 7A-7B depict a scintillation detector assembly having
a sensor on either end of the detector is depicted in accordance
with an exemplary embodiment of the present invention; and
[0029] FIG. 8 a is a flowchart of the process performed by the
3D-CBS system for determining DOI from the interior and exterior
sensors on a crystal detector in accordance win an exemplary
embodiment of the present invention.
[0030] Other features of the present invention will be apparent
from the accompanying drawings and from the following detailed
description.
DETAILED DESCRIPTION OF THE INVENTION
[0031] The present invention relates to improvements in processing
data acquired from sensors coupled to detectors, enabling the
alteration of altering detector placement, detector array spacing
and detector field of view for increasing the capture rates of
photons, and thereby increasing efficiencies of traditional
Positron Emission Tomography (PET) devices on a photons per unit of
radiation basis. It is a method and apparatus consisting of:
[0032] a) A detector of photons covering a large surface of a human
body (field of view --FOV);
[0033] b) A particular detector assembly that best couples and
transfers to the transducer and electronics the information
generated by the interaction of a photon with the detector.
[0034] c) An electronics with the capability to process most
information arriving from the detector without the limitation of
saturation or processing dead-time for any given radiation to the
patient.
[0035] The electronics can acquire data faster than the decay time
of any specific detector (e.g. crystals), it can process the data
captured by all detector/transducer elements at a specific time
(synchronous or asynchronous) in parallel form, it can execute
different programmable real-time algorithms on the acquired data,
each algorithm suitable to a different detector for best extraction
of all parameters of the interaction of the photon with the
detector. When the processing time is longer than the time interval
between two consecutive sets of input data, the electronics route
the received information to a different set of processors via
bypass switches. A particular arrangement of processor arrays and
bypass switches are implemented in a hardware system made of boards
and chassis (VME or IBM PC). This electronics compared to the prior
art electronics used by other inventors/manufacturers, although has
intrinsic features and advantages that improve sensitivity of
current PET, however, when:
[0036] a) coupled to a detector which is assembled differently from
the assembly used in current PET, and when
[0037] b) coupled to a transducer array which is in a different
relation and array segmentation compared to the one used in current
PET,
[0038] has the capability to extract more accurately, more
information from the interaction of the photons with the detector,
allowing:
[0039] 1. to capture more accurately more photons emitted by the
patient's body (which allow to improve the sensitivity of current
PET and which allows to increase the length of the detector in a
cost-effective manner, thus allowing a further great increase of
the sensitivity of the instrument and thus it allows to reduce the
radiation to the patient),
[0040] 2. to use more economical crystals (which reduces detector
cost),
[0041] 3. to accurately measure the depth of interaction by using
two sensors on both sides of the detector (which improves spatial
resolution),
[0042] 4. to measure more accurately the energy of the incident
photon (which allows to reject more accurately scatter events and
thus reduce "false positives" and "false negatives", and
[0043] 5. to measure more accurately the location of the incident
photon (which improves spatial resolution).
[0044] This inventions enables:
[0045] a) to use a PET device for preventive health care screening.
(Without this invention, current PET cannot be used in preventive
health care screening because they require to deliver to the
patient 20 to 30 times the radiation accepted by the International
Commission for Radiation Protection)
[0046] b) to have a revolutionary change in the way images will be
displayed (The image resulting from an examination with the 3-D
Complete Body Screening tool (3D-CBS) is three dimensional,
visualizes the whole body at one time, because data are recorded at
the same time over the entire body, and has greatly increased
definition. It provides dynamic imaging, allowing for motion
studies of real-time metabolic activity. The 3D-CBS has the unique
capability of recording data continuously and simultaneously over
the entire body. This makes it possible to view images of
biological processes, blood flow, and organ movements as a running
film instead of a static picture. Current PET cannot provide this
because the information is acquired at different time in different
section of the body. No more slices of the body, but real 3-D
images of any organ of the body. No more need to take several
cancer screening examinations, but only a single, more efficient
examination that will detect not only cancer, but also other
diseases).
[0047] c) it will lower the cost of health care because more
economical crystals can be used with this innovative technology.
This will lower the examination cost and will combine in a single
exam the examination of many organs, elimination the need of
several, different, expensive (and sometimes invasive) of screening
for cancer such as mammogram, colonoscopy, etc.
[0048] d) it will be an essential tool to develop and study the
effect of new, experimental pharmacopeia.
[0049] The cumulative effect of the combination of improvements
disclosed herein yields increased detection efficiencies to the
extent necessary to reduce the reducing the associated risks and
associated with PET imaging procedures to such an extent that the
benefits of PET imaging can be realized with radiation dosages far
less than that recommended by the International Commission on
Radiological Protection (ICRP). Thus, PET imaging can be realized
as a standard health assessment and diagnostic tool for regular,
periodic checkups in conjunction with well body care and
preventative medicine plans for apparently healthy individuals.
Moreover, because the present invention realizes a substantial
increase in detection efficiencies, more comprehensive and higher
quality PET images are obtainable in substantially less time than
is required for prior art PET imagining procedures, consuming far
less radioactive materials. Consequentially, the present invention
allows for substantially more procedures to be performed in the
time period as a single prior art PET procedure. Thus, even
factoring the in additional support and diagnostic personnel
necessary for the additional procedures, the cost per scan
procedure is far less than prior art imaging techniques.
[0050] In the year 2001 the two major companies in the field
introduced four new machines which were the result of their
planning before innovation appeared in "400+Times Improved PET
Efficiency For Lower-Dose Radiation, Lower-Cost Cancer Screening,"
3D-Computing, Jun. 30, 20010, ISBN: 0970289707 (hereinafter
400+Times). These machines are a PET/CT called "Biograph" by
Siemens, one called "Discovery LS" from GE, a new PET called "ECAT
ACCEL" from Siemens, and a new PET from GE called "Advance Nxi."
All the above new machines had a field of view (FOV, length of the
detector) of about 16 cm. This followed the previous trend of
limiting the capturing of photons and requiring high radiation to
the patient. (The new PET from Siemens even showed a step backwards
in FOV from their previous model "EXACT3D" which had 25 cm FOV). An
indication of the revolution caused by the advent of the new
discovery described herein and in other U.S. and international
patents that the direction shown by the large companies in the year
2001 will reverse in the year 2002 and in the coming years. It is
expected that the above identified companies (and new companies)
following the trend introduced with this discovery: they will
increase the FOV instead of decreasing it as taught by the present
inventor in co-pending U.S. Non-Provisional patent application Ser.
No. 10/296,532, entitled "Method And Apparatus For Anatomical And
Functional Medical Imaging," relating to and claiming priority from
PCT/US01/1567160/204,900, filed May 15, 2001 which relates to and
claims priority from U.S. Provisional Patent Application No.
60/204,900 filed May 16, 2000, U.S. Provisional Patent Application
No. 60/215,667 filed Jun. 30, 2000, U.S. Provisional Patent
Application No. 60/239,543 filed Oct. 10, 2000, U.S. Provisional
Patent Application No. 60/250,615 filed Nov. 30, 2000, U.S.
Provisional Patent Application No. 60/258,204 filed Dec. 22, 2000
and U.S. Provisional Patent Application No. 60/261,387 filed Jan.
15, 2000 which are each incorporated herein by reference in their
entirety (see also 4000+ Times by the present inventor). In fact,
the new PET model called Discovery VI just introduced by GE has a
longer FOV of 50 cm and is using crystal detectors even cheaper
than the one used before. This improvement is still far from
reaching the level of efficiency achieved by the inventor's design
and described in U.S. patent application Ser. No. 10/296,532 and
its prodigy, because the new GE PET has still a resolution of about
6 mm, which is lower than the previous PET, and a coincidence
window of about 12 nanosecond, which is longer than previous
PET.
[0051] It will take time and involve technological change,
replacing the display of information in slices by real-time 3-D
over the entire body, and the obsolescence of gamma camera, SPECT
and all such equipment that captures only 1 out of 300,000 photons.
These, and other parameters listed below are achieved by the
present invention:
[0052] radiation lowered from 1100 mrem per exam to less than 100
mrem acceptable to the ICRP and which will allow screening without
hazard to the patient);
[0053] the quality of the picture is improved from the capture of
about 60 million pairs of photons per examination in more than one
hour (with Fluorodeoxyglucose--FDG) to about 240 million in 4
minutes. (The identification of the photons also improves,
improving the image quality, with the reconstruction of the total
energy of photons in the 3D-CBS; this is not performed in prior art
PET)
[0054] the cost of one examination can be realized from the current
cost of $2,000-$4,000 per exam to about $400 per exam
[0055] The new GE PET Discovery VI with 50 cm FOV, about 6 mm
resolution, about 12 nanosecond coincidence window, and with no
significant advantage in lower radiation and lower examination
price is still far from reaching those goals (some parameters are
even worse than previous PET).
[0056] The ICRP and the U.S. National Council on Radiation
Protection and Measurements (NCRP) recommends a limit of 100 mrem
per year (average over five years) of exposure to ionizing
radiation for the general population (ICRP Publication 60, Annuals
of the ICRP 21, pp. 25; 1991 and Ordonnance sur la radioprotection
(OraP) Le conseil federal suisse. 19 decembre 2000). A single PET
(or CT) examination using devices currently available in hospitals
gives the patient 10 to 20 times this dose.
[0057] Prior art PETs require a high radiation dose to the patients
because they can capture only a few of the photons emitted from the
patient's body: at most they can capture about two out of 10,000
photons emitted. The sensitive area of the prior art detector that
can capture photons (the axial field of view (FOV), or the length
of the detector) covering the patient's body is very small and the
electronics inefficient. Until now, the greatest impediment to
extending the FOV has been the electronics of prior art PET, which
could not efficiently capture the photons and was saturating with
even the short FOV.
[0058] The unique architecture of the presently described invention
in the embodiment of the 3D-CBS electronics permits the extension
of the FOV to over one meter in length and captures about 1,000 out
of 10,000 photons in time coincidence. The innovations that reduce
the required radiation to the patient to {fraction (1/30)} of
current requirement lie partly in the way existing components
(available off the shelf) are assembled together with the
innovative section of the electronics (the 3D-Flow system). Such
technology allows:
[0059] 1. an increase in the input bandwidth of the electronics
from the 10 million events per second of prior art PET to over 36
billion in the 3D-CBS PET (a high bandwidth of the electronics is
required because the photons arrive at the detector randomly) as
described in co-pending U.S. Non-Provisional patent application
Ser. No. 10/185,904 entitled "Method And Apparatus For Whole-Body,
Three-Dimensional, Dynamic PET/CT Examination," filed on Jun. 27,
2002 by the present inventor, relating to and claiming priority
from U.S. Provisional Patent Application No. 60/301,545, filed Jun.
27th, 2001, Ser. No. 60/301,545, entitled Method And Apparatus For
Whole-Body Annual PET/CT Examination, and U.S. Provisional Patent
Application No. 60/309,018 entitled "Method And Apparatus For
Whole-Body, Three-Dimensional, Dynamic PET/CT Examination," which
are each incorporated herein by reference in their entirety. (See
also Section 4 of Crosetto, D. "Saving lives through early cancer
detection: Breaking the prior art PET efficiency barrier with the
3D-CBS" (referred to as "Saving Lives" hereinafter).
www.3d-computing.com/pb/3d-c- bs.pdf);
[0060] 2. an increase in the field of view to over one meter,
providing good efficiency in photon detection (e.g. a 3D-CBS with
only three times the FOV of prior art PET could detect nine times
the number of photons compared to prior art PET when they are used
in 3-D and 27 times when prior art PET are used in 2-D mode);
and
[0061] 3. the accurate identification of most photons, using
digital signal processing with neighboring data exchange performed
by a set of DSPs on each electronic channel. Each DSP executes a
complex real-time photon detection algorithm, as taught in patent
application Ser. No. 10/185,904 (see also FIG. 34 in Saving
Lives);
[0062] This 3D-Flow architecture has been designed in intellectual
property (IP) form, suitable to be targeted to several
technologies, and has been built into field programmable gate array
(FPGA) (2.6 million gates 0.18 micron CMOS technology, 8 aluminum
layers) with four 3D-Flow processors in a single chip as described
in co-pending U.S. Non-Provisional patent application Ser. No.
09/506,207 entitled "Method And Apparatus For Extending Processing
Time In One Pipeline Stage," filed Feb. 16, 2000 by the present
inventor, relating to and claiming priority from claiming U.S.
Provisional Patent Application No. 60/120,194, entitled
"Implementation of Fast Data Processing With Mixed-Signal And
Purely Digital 3D-Flow Processing Boards," filed Feb. 16, 1999,
U.S. Provisional Patent Application No. 60/112,130, entitled
"Design Real-Time," filed Mar. 12, 1999, U.S. Provisional Patent
Application No. 60/129,393, entitled "Novel Instrumentation For Pet
With Multiple Detector Types," filed Apr. 15, 1999, U.S.
Provisional Patent Application No. 60/132,294, entitled "System
Design And Verification Process For Electronics," filed May 3,
1999, U.S. Provisional Patent Application No. 60/142,645, entitled
"Real-Time System Design Environment For Multi-Channel High-Speed
Data Acquisition System And Pattern-Recognition," filed Jul. 6,
1999, U.S. Provisional Patent Application No. 60/143,805, entitled
"Design And Verification Process For Breaking Speed Barriers In
Real-Time Systems," filed Jul. 14, 1999, U.S. Provisional Patent
Application No. 60/154,153, entitled "Novel Idea That Can Bring
Benefits In Proven HEP Applications," filed Sep. 15, 1999, U.S.
Provisional Patent Application No. 60/161,458, entitled "System
Design And Verification Process For LHC Trigger Electronics," filed
Oct. 25, 1999, U.S. Provisional Patent Application No. 60/164,694,
entitled "Advantages Of The 3D-Flow System Compared To Current
Systems," filed Nov. 10, 1999, and U.S. Provisional Patent
Application No. 60/170,565, entitled "Novel Instrumentation For
PET/SPECT Suitable For Multiple Detector Types," filed Dec. 14,
1999 each filed by the present inventor and are each incorporated
herein by reference in their entirety by the present inventor which
is incorporated herein by reference in its entirety (see also "LHCb
Base-Line Level-0 Trigger 3D-Flow Implementation," Nuclear
Instruments and Methods in Physics Research, Section A, vol. 436
(November 1999) pp. 341-385 (referred to as "LHCb Base-Line"
hereinafter). Full simulations of the system with fault-tolerant
capabilities have been performed for the entire system. A hardware
prototype implementing these functions in real time, using Altera's
FPGA described in patent application Ser. No. 09/506,207 (also
described in "Proven Technology for the New 3D Complete-Body-Scan
(3D-CBS) Medical Imaging Device," 3D-CBS Progress report (November
2001) pp. 1-4 presented at the seminar at the Industrial Program of
the IEEE Nuclear Science Symposium and Medical Imaging Conference
at San Diego, Calif., U.S.A., on Nov. 6, 2001 (referred to as
"Proven Technology" hereinafter,
www.3d-computing.com/pb/reportl.pdf). These publications
demonstrate its hardware feasibility. This hardware construction is
the basic element of the project and is described in, for example,
The Changing of Positron Imaging System. Clinical Positron
Imaging," by Phelps, M. E. et al. in "vol. 1(1): 31045, 1998.
Several of these basic elements replicated hundreds of times makes
the electronic system of the 3D-CBS. The staging of the
construction of the 3D-CBS is also presented herein, and taught by
Crosetto in U.S. Pat. No. 5,937,202 entitled "High-Speed, Parallel,
Processor Architecture for Front-End Electronics, Based on a Single
Type of ASIC, and Method Use Thereof," and also in patent
application Ser. No. 09/506,207 (see also Saving Lives).
[0063] The present invention is predicated on advancements in PET
systems, circuitry, detectors, processors and processing
architectures described in patent application Ser. No. 09/506,207
(see also LHCb Base-Line). Full simulations of the system with
fault-tolerant capabilities have been performed for the entire
system.
[0064] The new discovery was first described logically and compared
to the existing technology. Then was analyzed and proven by
computer simulation, the results of which were published in Nuclear
Instrument and Methods in Physics Research (see vol. 436 (1999) pp.
341-385). Recently the electronics, which is directed to
efficiency, has been implemented in hardware in field-programmable
gate array (FPGA), Altera 20K1000, (see also Proven Technology").
The other elements of the new 3-D Complete Body Screening (3D-CBS)
design (detector, software reconstruction, etc.) are available as
off-the-shelf components and therefore an operable embodiment is
possible, while the improvements to the off-the-shelf components to
take advantage of the additional information provided by the
3D-CBS, which acquires more accurate data at the same time over the
entire body, are formulated in the future. This will allow to
reconstruct a real three dimensional object of all organs and of
all body.
[0065] The more noteworthy improvements the 3D-CBS offers over the
prior art PET are: (a) capturing more data from the emitting source
and (b) processing the acquired data with a real-time algorithm
which best extracts the information from the interaction between
the photons and the crystal detector.
[0066] If more data from a radioactive source at the level of
radiation currently used (or from a source with lower radiation
activity) is captured by the detector, sent to the PET electronics,
and processed correctly, then the examination time, radiation
dosage, and consequently also the cost per examination can be
significantly reduced. In order to obtain more data, the axial
field of view (FOV, the total length of the rings of crystals in
the PET detector) must be lengthened to cover most of the body. In
order to process these data, the electronics must be designed to
handle a high data input rate from multiple detector channels. The
3D-CBS can handle up to 35 billion events per second with zero dead
time in the electronics versus the 10 million events per second
with dead time that the prior art PET can hand (using a system with
1,792 channels as described by Crossetto in patent application Ser.
No. 10/296,532 (Method And Apparatus For Anatomical And Functional
Medical Imaging) (see also "A Modular VME or IBM PC Based Data
Acquisition System For Multi-Modality PET/CT Scanners Of Different
Sizes And Detector Types," presented at the IEEE Nuclear Science
Symposium and Medical Imaging Conference, Lyon, France, 2000,
IEEE-2000-563, (hereinafter referred to as "Modular VME,"
http://www.3d-computing.com/pb/ieee2000-563- .pdf)). High input
bandwidth of the system is necessary because the photons arrive at
random time intervals.
[0067] Crossetto further describes both: (a) a novel architectural
arrangement of connecting processors on a chip, on a Printed
Circuit Board (PCB), and on a system; and (b) a new method of
thoroughly processing data arriving at a high rate from a PET
detector using the 3D-Flow sequentially-implemented parallel
architecture, in patent application Ser. No. 10/296,532 (Method And
Apparatus For Anatomical And Functional Medical Imaging) and in
patent application Ser. No. 09/506,207 (see also Modular VME and
LHCb Base-Line).
[0068] Put simply, the processing of the electronics on the data
arriving from the detector can be compared to the task of reuniting
families that have been separated by a catastrophic natural event,
i.e., the family reunion paradigm. The following analogy in human
terms is made: the two groups of signals generated by the sensors,
that are coupled to the detectors hit by the two back-to-back
photons of a single event are similar to the two halves of a
families split apart, the mother with some of her children being
separated far from her husband with the other children. The task of
the detector is to find the back-to-back photons that came from the
same annihilation event, or to reunite the two half families. The
sequence of events in the family reunion example is one billion
times slower than the sequence of annihilation events in the
PET:
[0069] A catastrophic event separates on average 17 families every
50 seconds. During the attempt to reunite the families,
unfortunately, only about 12% of the husbands and wives can arrive
at a reunion center. The reduction of families is analogous to the
reduction of photons that are absorbed by the patient's body, or
not captured by the detector because of the limited field of view
(FOV) and solid angle of the detector.
[0070] When a family was split, the husband and wife went in
opposite directions, each with some of their children. In the
analogy, the children in neighboring paths and the parent represent
signals on neighboring sensors (or electronic channels) that have
been generated by a photon striking the detector. The analogy
illustrates the fact that the total energy of the incident photon
that was split among several neighboring electronic channels must
be reconstituted, just as the children must be first reunited with
the parent.
[0071] The family reunion takes place in two phases. During the
first phase, the father and the children who went with him but
followed a neighboring path are reunited. The same process is
followed independently, in a separate venue, by the mother with
their other children; however, that takes place far from where the
father is. During the second phase the two half-families are
reunited.
[0072] FIG. 1 shows an example of information split over several
channels (or wires). FIG. 1. depicts the "Family reunion" paradigm
used herein. A solution, that identifies family members and checks
in detail for their characteristics, is needed for the reunion of
the families. The figure shows an example of the arrival of
information about the particles from several electronic channels at
one time. As an analogy, several members of a family arriving at
the same time on different channels (e.g., see four members of a
family in the second row from top) are compared to a photon that
has its energy split among several electronic channels. (The size
of a family member is proportional to the area of the signal in a
given example).
[0073] A photon striking in such a way that its information is
divided among several electronic channels is analogous to one
parent with some children going down several paths. (See on the
second row of the FIG. 1 in the dotted lines, the split of a family
among four paths, or wires, and on the third row the split of a
family between two wires).
[0074] Because there are on average about four groups of fathers
with their children (or mothers with their children) arriving 26 at
random time intervals every 50 seconds at any place in the 1,792
channels at the reunion center, it is necessary to clearly identify
family members and reunite the half-family (or to rebuild the
energy of the incident photon) at their arrival site, before the
children are mixed with millions of unrelated people.
[0075] The first problem involves reuniting the half-family
(rebuild the energy of each incident photon, determine its exact
arrival time, measure the exact position of its center of gravity,
measure the Depth of Interaction (DOI), and resolve pile-up). The
solution to the problem, which is illustrated in the image of the
"family reunion" of FIG. 2A, is mainly provided by the "bypass
switch" (or multiplexer) of the 3D-Flow architecture (see FIG. 2B
and FIG. 2C). Information concerning the father and children, that
is, the signals generated by the photon, arrives at the top of the
channel (wire) and moves down one step each time new data arrive at
the input. The numbers in FIG. 2A correspond to the positions of
the objects (data set or smiling face) at time 13t of FIG. 2B.
Objects outlined in dotted lines correspond to the status one
instant before time "13t." With more specific reference to FIG. 2,
the illustration is subdivided into three discrete parts. FIG. 2A
(left side) depicts an illustration of the "family reunion"
paradigm for time 13t of FIGS. 2B and 2C. Each photon remains in
the measuring station (processor) for a duration five times longer
than the time interval between two consecutive input data. The
result from any measuring station will not be an input to the next
station (as it is in a typical pipeline system) but will be passed
on with no further processing in the 3D-Flow sequentially
implemented, parallel-architecture until it exits (see description
below).
[0076] FIG. 2C (lower right side) depicts an illustration of the
stages 1d-5d of the input data and output results in the registers
of the 3D-Flow pipelined system at a time 13t. The depicted example
demonstrates how the 3D-Flow system extends the execution time in a
pipeline stage beyond the time interval between two consecutive
input data (sequentially-implemented, parallel architecture). An
identical circuit (a 3D-Flow processor) is copied 5 times at stage
d (the number of times the circuit is copied corresponds to the
ratio between the algorithm execution time and the time interval
between two consecutive input data). A bypass switch (or
multiplexer) coupled to each processor in each 3D-Flow stage 1d,
2d, 3d, 4d, and 5d sends one data packet to its processor and
passes four data packets along to the next stage ("bypass switch").
Thus, the execution time at each substation d will be
t.sub.P=4(t1+t2+t3)+t1. The numbers in the rectangles below the
switches identify the input data packets to the CPU of the 3D-Flow
processor. (See also FIG. 2B for the sequence of operations during
the previous clock cycles). A 3D-Flow processor is shown in the
figure with the three functions of (a) a bypass switch (dotted
right arrow in the rectangle), (b) an output register (rectangle to
the right), and (c) a CPU (rectangle below).
[0077] FIG. 2B (upper right side) is a tabular listing depicting a
sequence of the data packet at different times in the pipeline
stage (see FIG. 2C). One data packet in this application contains
64-bit information from one channel of the PET detector. The clock
time at each row in the first column of the table is equal to
t=(t.sub.1+t.sub.2+t.sub.3) of FIG. 2C. The number in the lower
position in a cell of the table is the number of the input data
packet that is processed by the 3D-Flow processor at a given stage.
The values in the raised position, indicated as ix and rx, are the
input data and the result data, respectively, which flow from
register to register in the pipeline to the exit point of the
system. Note that input data 1 remains in the processor at stage 1d
for five cycles, while the next four data packets arriving (i2, i3,
i4, and i5) are passed along (bypass switch) to the next stage. It
should be understood that, although not shown in FIG. 2B, at last
clock (14t), while stage 4d is fetching 9, it is at the same time,
outputting r4. This r4 value is then transferred to the exit of the
3D-Flow system without being processed by any other d stages. Note,
however, that clock 13t shows the status represented in FIG. 2C and
that input data and output results are intercalculated in the
registers of the 3D-Flow pipelined system.
[0078] The 3D-Flow architecture allows a high throughput at the
input because (a) each data packet relative to the information
about the photon (or about the family member) has to move only a
short distance at each step, from one station to the next, and (b)
complex operations of identification and measurement can be
performed at each station for a time longer than the time interval
between two consecutive input data.
[0079] Every time a new data packet arrives at the top of the
channel, all other data packets along the vertical wire move down
one step, but the wire is broken (equivalent to a bypass switch in
input/output mode) in one position where the station is free to
accept a new input data packet and is ready to provide at the same
time the results of the calculations of the previous data
packet.
[0080] At any time, four switches in "bypass mode" and one switch
in "input/output mode" (or the wire broken at a different place)
are always set on the vertical wire. This synchronous mechanism
will prevent losing any data at input and will fully process all of
them.
[0081] When a data packet relative to a photon enters a measuring
station (that is, a 3D-Flow processor, or the station represented
on the right side of FIG. 2A), it remains in that station for its
complete identification, measurements, and correlation with its
neighbors. Several operations are performed at each station:
[0082] 1. A "picture" is taken and sent along with the time of
arrival to the neighbors, while "pictures" from the neighbors,
along with their time of arrival are also received and checks are
performed to see if there were any family members in the
neighboring channels. Similarly, the energy and arrival time of
photons are exchanged between neighboring elements to check if the
energy of the incident photon was fragmented between several
channels.
[0083] 2. Local maxima (checking to see if the signal is greater
than the neighbors) are calculated to determine if the parent
arrived at that channel; this is equivalent to comparing the
photon's energy and arrival time to similar information in the
neighboring channels. If the parent did not arrive at that channel,
the process at that channel is aborted to avoid duplication. The
neighboring channel that finds the father will carry on the
process.
[0084] 3. Center of gravity is calculated (that is the point at
which the weight, (or photon's energy in this specific case) of an
object is equally distributed). This calculation will provide an
accurate location where the half-family was found; this is
equivalent to the spatial resolution of the incident photon.
[0085] 4. Pile-ups, which occur when two half-families belonging to
two different families arrive within a very short time interval, or
when two events occur in a nearby detector area within a time
interval shorter than the decay time of the crystal. When this
happens, the apparent integral of the second signal will show it
riding on the tail of the previous signal. Digital Signal
Processing (DSP) techniques of the 3D-Flow processor can detect the
change of slope of the tail of the signal and separate the two
signals.
[0086] 5. The accurate arrival time of the half-family group is
calculated and assigned to be carried for the rest of the trip.
Similarly, the accurate arrival time of the photon is
calculated.
[0087] 6. Other measurements are performed on the input data
(half-family or photon), such as the depth-of-interaction (DOI) on
the incident photon.
[0088] DOI measurements solve the problem of identifying the
affected crystal when the incident photon arrives at an oblique
angle instead of perpendicularly to the face of the crystal. The
3D-Flow processor can utilize several DOI measurement techniques
known well to those of ordinary skill in the art, such as may be
found in "A Novel APD-based detector module for multi-modality
PET/SPECT/CT scanners," by Saoudi, A., and Lecomte, R., IEEE Conf.
Rec. Nucl. Sci. Symp. and Med. Imag., pp. 1089-1093, 1998; "Effect
of Detector Scatter on Decoding Accuracy of a DOI Detector," by
Miyaoka, R. S., et al., IEEE Conf. rec. of the Nucl. Sci. Symp. and
Med. Imag. M3-34, Seattle, Oct. 24-30, 1999; and "Development of a
64-channel PET detector module with depth of interaction
measurement," by Huber, J., et al., IEEE presentation at the Nucl.
Sci. Symp. and Med. Imag., M4-6, Seattle, Oct. 24-30, 1999, for
correcting the effect commonly referred to as "parallax error."
[0089] 7. Finally, the half-family is reunited (the total energy of
the photon is calculated), all measurements are performed, and
results are sent to the channel for its trip to the exit (See in
FIG. 2A the object r4 in the fourth station from the top, which is
the result of the input data No. 4).
[0090] Only some of the above processing is carried on in the prior
art PET devices. The most important task of rebuilding the energy
of the incident photon (equivalent to reuniting a half-family) is
not performed. On the contrary, prior art PET techniques add analog
signals before checking whether the signals belong to the same
incident photon; this is equivalent to grouping father and children
before checking if they belong to the same half-family.
[0091] Adding several analog signals before checking whether the
signals belong to the same incident photon, as is done in prior art
PET, turns out to be very counterproductive at the next electronic
stage because the analog signal (which is the sum of several
signals) cannot be separated into its original components and the
information on the single photons that is needed for several
subsequent calculations is lost forever.
[0092] In the most advanced prior art PET devices, the electronics
cannot complete the processing before the arrival of another data
set; therefore, consequently, dead time is introduced and photons
are lost.
[0093] The conclusion is that the limitation of the electronics of
the prior art PET (front-end and coincidence detection, described
later) prevents it from detecting many photons, and the overall
performance of the best prior art PET device detects about two
photons in time coincidence out of 10,000 emitted by the
radioactive source. This should be compared to 1,000 photons out of
10,000 captured by the 3D-CBS, with its improved electronics and
extended axial FOV. In addition, of the two out of 10,000 photons
in coincidence captured by prior art PET devices, many will be
discarded by subsequent processing, or they will not carry accurate
information.
[0094] Conversely, the advantage of the 3D-Flow architecture of the
3D-CBS is a result of using of several layers of stations
(processors) with the data flow controlled by "bypass switches" (or
multiplexer) allowing more than, for example, 50 nanoseconds to
weigh the subject, to take the picture, to exchange them with the
neighbors, to calculate the local maxima, the center of gravity,
etc. The use of 50 ns herein is not intended as a limitation of the
scope of the invention; it is merely an exemplary value. The
3D-Flow system can be designed to sustain a sampling rate higher
than the faster crystal detector currently employed in the
industry. Five layers of stations (or processors at the same level)
allow 250 nanoseconds in each station to perform all of the above
calculations. In the event this processing time is not sufficient,
more layers are added.
[0095] The bypass switches will provide good synchronization of
input data and output results at each station (or processor) by
simply taking one data packet for its station and passing four of
them along.
[0096] Using the scheme depicted in FIG. 2A, it is possible to
follow the path of a data packet of photon (i3) through the entire
system. At time 5t shown in FIG. 2B, the data packet of photon i3
enters the channel at the top of FIG. 2A. If it finds a busy
station (processor) on the right, it rests for one cycle on the
platform (or register, shown in FIG. 2C as a rectangle next to the
bypass switch).
[0097] During the next cycle, 6t of the table in FIG. 2B, this data
packet of photon (i3) advances to the next station. If this station
is also busy, then it will rest on the next platform, and so on
until it finds a free station.
[0098] When the data packet of photon (i3) finds a free station (at
time 7t in FIG. 2B), it enters the station and stays there for five
cycles for processing. After the data packet of photon r3 (which
contains the results of the processing performed on i3) leaves the
station and goes to the platform on the left, adjacent to the
station (at time 12t), another data packet of photon (i8) enters
the station from the upper left platform. The result from photon
(i3) cannot go straight to the exit but can advance only one
platform at a time until it reaches the exit.
[0099] In summary, the 3D-Flow sequentially implemented
parallel-processing system is synchronous; it has a fixed number of
steps and a fixed sampling rate, the data flows in an orderly
fashion from input to output according to the time clock, and there
is no congestion in the flow. The sequence is as follows:
[0100] synchronously receive a data packet from the input of the
system
[0101] synchronously send out a data packet from the output of the
system with a fixed time latency from when it was received by the
system and with a tag identifying the result as either a non-data,
a good CT photon, a good PET photon, or a Compton scatter photon,
etc.
[0102] process each data packet fully, with information exchange
with neighbors, by a 3D-Flow processor in one layer of the system,
regardless of whether or not it contains relevant data; no data
packet is skipped or lost. The 3D-Flow system is dimensioned with
the correct number of layers needed to fulfill the requirements of
executing the real-time algorithm in full (a fixed maximum number
of steps) on each data packet and of sustaining the maximum input
data rate. There is always a free processor waiting to receive a
data packet. If a processor finds no meaningful results and
terminates its process in fewer steps, it waits its turn (because
it is a synchronous system) before it sends out the result and
fetches a new data packet at the input. If either the input data
rate or the complexity of the algorithm increases, one or more
layers are added to satisfy the requirement of zero dead-time. (See
FIG. 2A).
[0103] The next phase is to reunite husbands and wives (the two
half-families reunited in description above) from distant
locations, or find the back-to-back photons in time coincidence.
The measurements performed during phase I have reunited the
half-families (each parent with some children), creating good
candidates for the final entire family reunion. The result of the
previous process is that, at most, four new fathers (or mothers)
are found every 50 seconds over the 1,792 channels. It is initially
assumed at the beginning of this analogy of the need to reunite at
the reunion center only 12% of 17 families (17 fathers+17 mothers)
separated every 50 seconds 3 which is equivalent to about four
photons (two photons back-to-back per event) arriving at the
coincidence circuit on average, for example, every 50 nanoseconds
(which corresponds to a radiation activity of about 9 mCi
administered to the patient). Six comparisons every 50 nanoseconds,
for example, are necessary in order to find all possible matches
among the four photons. A coincidence circuit with the capabilities
of performing six comparisons every 50 ns (or 120 million
comparisons per second) can handle a radioactivity of about 9 mCi
of FDG which is far more than the expected 0.3 mCi of FDG estimated
to be required by the 3D-CBS for cancer screening. The
implementation of a coincidence circuit that will perform more
comparisons per second will not be a challenge even if higher doses
of radioisotopes with shorter half-life, such as .sup.15O-water or
.sup.82R rubidium are used. The calculation of the rate of the
photons that hit the detector is as follows: 9
mCi.times.3.7.times.10 7=333.times.10 6 disintegrations per second
(or about 17 families separated every 50 seconds in the family
reunion paradigm, which, recall, has an event rate one billion
times slower).
[0104] The approach used in prior art PET devices in the final
reunion is that the fathers and mothers do not move from the
location where they are and each location interrogates about half
of all the other locations explaining that it is not necessary to
test Lines of Response--LOR--which do not pass through the
patient's body in order to find out whether there is a companion in
that location.
[0105] Because, as mentioned elsewhere herein, there are about
2,000 locations (electronic channels) in the system, the total
number of comparisons that must be performed in order to find the
companion will be enormous. For instance, for a PET with 1,792
channels, the number of comparisons 28 necessary would be:
(1,792*1,791)/4=802,368 comparisons every 50 ns; that is equivalent
to sixteen trillion comparisons/second (The division by 4 in the
formula is required because approximately half the LORs do not pass
through the patient's body). Although in our human analogy, family
events are one billion times slower, it would still require sixteen
thousand checks of matching families per second.
[0106] In order to avoid making that many comparisons per second,
manufacturers of prior art PET have reduced the number of locations
(electronic channels). This has several drawbacks, such as
increasing dead-time, reducing resolution, etc. For example, with a
reduction to 56 channels, the number of comparisons in prior art
PETs is still (56*55)/4=770 comparisons every 250 ns, or equivalent
to about 3 billion comparisons/second, which are performed in seven
ASICs in the current GE PET as taught by Mertens et al. in U.S.
Pat. No. 5,241,181 entitled "Coincidence detector for a PET
scanner" incorporated by reference herein in its entirety.
[0107] The approach used in the proposed 3D-CBS greatly simplifies
the circuit and requires only 120 million comparisons per second as
discussed in the present invention in co-pending patent application
Ser. No. 10/296,532 (and as described in more detail in Section
13.4.14 and shown in detail in FIG. 13-22 of Modular VME. This
efficiency is equivalent to that of the PET with 1,792 channels,
which, as noted above, would require instead sixteen trillion
comparisons per second.
[0108] Again, using the family paradigm, the approach can be
explained as follows: the husbands and wives should move from their
location to the reunion center. At that location an average of four
groups of parents with their children arrive every 50 seconds (when
an original family separation rate of 17 every 50 seconds is
assumed); thus, in order to make all possible combinations among
four elements and avoid accumulation in the room, six comparisons
every 50 seconds are necessary. This would still be manageable in
the world of the family reunion, only 7.2 comparisons per minute
being required instead of sixteen thousand comparisons per second
with the prior art PET approach, and with the 3D-CBS it would also
be manageable in the world of photons requiring only 120 million
comparisons per second.
[0109] As mentioned above, the advantages of the presently
described invention are partly a result of how existing technology
is coupled with the newly described electronics of the 3D-Flow
system and partly due to the 3D-Flow system itself. These include
increased input bandwidth of the electronics with less radioactive
isotope in the patient, greater field of view (FOV) and more
accurate identification of most photons.
[0110] In accordance with one exemplary embodiment of the present
invention, the 3D-CBS can be built using off-the-shelf detector
components and the 3D-Flow processor implemented in FPGA. Such
combination provides a system input bandwidth of the electronics of
10 billion events per second (instead of the current maximum 10
million events per second). Thus, the goal of reducing the
radiation dose to a level lower than 100 mrem/yr is achieved. In
accordance with one exemplary embodiment of the present invention,
the 3D-Flow processor used in the 3D-CBS device is implemented in
ASIC, thereby further reducing the radiation dose with the use of
the 3D-Flow (which can provide an input bandwidth higher than 36
billion events per second) will further lower the radiation to the
patient and will provide better images.
[0111] FIG. 3 is a chart of the correspondence between radiation
dosage and PET technology. Essentially the chart depicts a
diagrammatic roadmap of the construction of the 3D-CBS with
different technologies in accordance with exemplary embodiments of
the present invention. The "Current PET" column describes salient
attributes of the prior art PET device. Accordingly, an 1100 mrem
dosage of radioactive isotope yields an input bandwidth of
approximately 10 million events/sec. for a 16 cm detector field of
view (FOV), according to "The Changing of Positron Imaging System.
Clinical Positron Imaging," vol. 1(1):31045, 1998 Phelps, M. E., et
al. Thus, the prior art PET devices operate well above the
recommended limit of 100 mrem/yr (average over five years) for
exposure to ionizing radiation for the general population set by
the ICRP and the NCRP (ICRP Publication 60, Annuals of the ICRP 21,
pp.25; 1991 and Ordonnance sur la radioprotection (OraP) Le conseil
federal suisse. 19 decembre 2000). Thus, one PET (or CT) procedure
using prior art technology exposes a patient 10 to 20 times the
allowable dosage.
[0112] The "3D-CBS" column is subdivided into two subheadings,
3D-CBS implemented in Field Programmable Gate Array (FPGA)
(programmable logic chip embodiments) and 3D-CBS implemented in
Application Specific Integrated Circuit (ASIC)
(application/design-specific chip embodiments). In comparison with
the prior art PET, the 3D-CBS with FPGA realizes an input bandwidth
of 10 billion events/sec through a detector that has a FOV about
137 cm long (calculated as 6 MHz input bandwidth of each 3D-Flow
processor times 1792 electronic channels). The capability to
execute an 18-step real-time algorithm (during each step, the
3D-Flow processor can execute up to 26 operations simultaneously)
allows for accurately identifying the characteristics of the
interaction between the incident photon and the detector. The
3D-CBS with FPGA embodiment is configured six IBM PC chassis, five
IBM PC chassis contains 19 Data Acquisition (DAQ) boards each, and
one chassis with 17 DAQ boards. Each DAQ board is equipped with 16
channels with five sequentially implemented parallel-processing
stages, implemented on 25 FPGAs. Each FPGA contains the
functionality of 4.times.3D-Flow processors. Notice that
radioactive dosage requirement for this embodiment is reduced to
approximately 100 mrem from the 1100 mrem required by the prior art
PET.
[0113] Finally, with regard to the 3D-CBS with ASIC embodiment,
notice that while the FOV remains constant at about 137 cm from the
FPGA embodiment, the input bandwidth increased to 36 billion
events/sec from 10 billion of the previous embodiment. The
bandwidth is calculated as 20 MHz input bandwidth of each 3D-Flow
processor, times 1792 electronic channels. The ASIC embodiment
enables the execution of a more sophisticated (longer than 18
steps) real-time algorithm which allows capturing more photons and
improving the quality of the images. The more powerful algorithms
are possible using application specific integrated circuit even
though only two IBM PC chassis are employed, each containing 14 DAQ
boards equipped with 64 channels on 25 ASICs, each with
16.times.3D-Flow processors. Consequently, the radioactive dosage
body burden to a patient for this embodiment is reduced by half
from the previous embodiment and far below the 100 mrem upper
dosage threshold set by the ICRP.
[0114] FIGS. 4A-4D are diagrammatical comparisons of the
relationship between the increasing FOV in Line of Response (LOR)
in accordance with exemplary embodiments of the present invention.
A PET with an axial FOV that is twice as long as the short FOV of
the prior art PET can detect four times the number of photons in
time coincidence from an organ emitting photons from the center of
FOV. FIG. 4A and FIG. 4B and assume the detector has only three
rings of detector elements. Only the LOR connecting opposite sets
of detectors within the three rings are considered instead of all
possible LORs passing through the patient's body. The top detector
elements are elements A, B, C, and the bottom detector elements are
depicted in the figure as elements D, E, F. For a linear source at
the center of the FOV emitting pairs of photons in time coincidence
in opposite directions, one could only capture three possible
combinations AD, BE, and CF (See FIG. 4A) when SEPTA are used
(septa are lead rings between the ring-detectors that prevent
photons arriving with an angle from hitting the detector). Thus,
FIG. 4A depicts the prior art PET devices with short FOV and
further LOR limiting septa.
[0115] For the purpose of understanding how the capturing of
photons is greater than doubled when the FOV is doubled, assume
that the representation of the detector is simplified as shown in
FIG. 4B, depicting a prior art PET device with the same short FOV
as in FIG. 4A, but the number of photons captured increases from 3
to 9 when the SEPTA are removed. In the absence of SEPTA lead
rings, there are nine possible combinations of pairs of photons:
AD, AE, AF, BD, BE, BF, CD, CE, CF which can be captured.
[0116] FIG. 4C depicts the effect of doubling the axial FOV has on
LOR. Doubling the FOV, thereby doubling the number of detector
element rings, increases the Lines of Response four times over
prior art PET devices with half the number of rings (or 12 times if
compared to 2-D mode, shown in FIG. 4A). If the FOV is doubled and
with new top detector elements G, H, L, and the new bottom detector
elements M, N, P, then 36 combinations of pairs of photons emitted
in opposite directions from a linear source in the center of the
FOV are captured. The possible pairs for which a LOR could be drawn
are: AD, AE, AF, BD, BE, BF, CD, CE, CF, plus the new GM, GN, GP,
HM, HN, HP, LM, LN, LP, plus the combination of old top and new
bottom AM, AN, AP, BM, BN, BP, CM, CN, CP, plus the combination of
the new top and the old bottom GD, GE, GF, HD, HE, HF, LD, LE,
LF.
[0117] Finally the LOR algorithm described above is infinitely
extendable, for instance if the FOV is increased three times from
that depicted in FIG. 4B, the number of pairs of photons that can
be captured increases nine times (or 27 times if compared to the
current use of the PET in 2-D shown in FIG. 4A). If the FOV is
increased four times from that depicted in FIG. 4B, the number of
pairs of photons that can be captured increases sixteen times (or
48 times if compared to the current use of the PET in 2-D shown in
FIG. 4A).
[0118] Considering that most of the PET (even the most advanced)
available currently in hospitals use a 2-D mode for the torso,
where only the combinations AD, BF, and CF are detected, the
difference between the prior art PET and the 3D-CBS when the FOV is
doubled, is from 3 to 36 (or 12 times). If the FOV of the prior art
PET is tripled from 16 cm to 48 cm, then the difference in captured
pairs of photons will increase 27 times when using the 3D-CBS
approach.
[0119] Cost of the 3D-CBS device is reduced, or kept at least to a
minimum, through the use of low cost detector crystals. One type of
scintillator crystal known for its cost effectiveness is the
bismuth germanate (BGO) crystal. An even lower cost crystal is the
sodium iodate (NaI) crystal; however, the disadvantages associated
with NaI crystals have discouraged a large segment of the PET
industry from using other more expensive crystal detectors, as
mentioned elsewhere above. NaI crystals are less dense, and have
less "stopping power" of the 511 keV photons than BGO crystals. BGO
is more rugged, and allows for higher detection efficiency.
Additionally, BGO is not count-rate limited, thus practitioners are
encouraged to inject even larger dosages of isotopes in their
patients because it has been surmised that the BGO can detect more
counts and more counts result in clearer scans and sharper images.
In fact, some estimates place BGO crystal usage at almost ten time
that of NaI. Although the NaI crystal may have lower stopping power
than the BGO, it provides a stronger signal.
[0120] Therefore, in accordance with another exemplary embodiment
of the present invention, an improvement in the PET spatial
resolution may be achieved by means of a more accurate measurement
of the depth of interaction (DOI) using either low cost crystals
such as BGO, or the NaI crystal which has even a lower cost. The
photon's stopping power of the NaI crystal is increased by
fabricating a thicker NaI detector in proportion to a comparable
BGO detector, with a stronger signal. With a renewed interest in
NaI detectors, there is a likelihood that NaI crystals will be
grown ever larger, in fact, it is technologically possible to build
a single barrel to cover the entire surface of the patient's body.
Although, cost-efficiency criteria will most probably dictate an
optimal segmentation and separation of the crystal that will cover
most, but not all, of the patient's body.
[0121] Measuring the DOI is important for correcting the parallax
error. Parallax is the error that results from assuming that
photons strike the detector at 90 degrees to its face. A better
understanding of the parallax problem may be realized through a
discussion of the prior art PET photon detector assemblies. With
regard to FIGS. 5A-5D, a scintillation detector assembly is
depicted as is well known in the prior art. FIG. 5A is an oblique
view of a typical photomultiplier (PMT) module employed by, for
example, prior art PET devices. Prior art assembly 500 utilizes a
block detector design concept in which single crystal 502 is
optically coupled to a 2.times.2 block (or module) of transducers
504A-504D.
[0122] Crystal 502 might be any type which interacts with a photon
to produce a scintillation, or rapid flash of light, in the
interior lattice structure of the crystal. However, recently prior
art PET manufacturers have moved away from less efficient and
cheaper crystal, focusing instead on more expensive crystals in an
effort to increase the detector's efficiency. Notice that crystal
502 has interior face 501 which faces the patient on the interior
of the barrel, exterior face 503 which is optically coupled to
interior face 505 of the transducer. Notice also that prior art
crystal 502 has been cut or slit into smaller crystals. The purpose
of the cut (slits) between small crystals (pixels) is to reduce the
number of photomultipliers affected by the light generated by an
event (or interaction between the incident photon and the crystal).
The length of these cuts which separates two crystals has to be
determined experimentally and is different from crystal to crystal.
Crystal 502 (coupled to prior art PMT module 504) is typically
subdivided into an 8.times.8 block of variable length slits. The
8.times.8 block does not share light well with adjacent 8.times.8
crystal blocks associated with neighboring detector module
assemblies. In general, the variable length slits allow only the
PMTs in the module assembly that are coupled to a crystal receive
light from that crystal. Moreover, edge and corner subdivisions of
each prior art 8.times.8 crystal block contribute only a small
signal compared to the contribution of the inner subdivisions of
the crystal making the identification of photon events more
difficult, and lowering the overall efficiency for the PET.
Furthermore, if a photon strikes the boundary edge between adjacent
2.times.2 PMT modules (between the edge and/or corner subdivisions
of two 8.times.8 crystal blocks), neither PMT may receive
sufficient energy to recognize the strike as a photon and the
photon is lost, further reducing the efficiency of capturing
photons for the prior art PET device.
[0123] Transducers 504A-504D may be Photomultipliers (PMTs),
Avalanche Photodiodes (APDs) or some other type of light emitting
diode; however, each transducer-detector combination will have a
signal output (a channel) for outputting the amplified signal to
the processing electronics. Those of ordinary skill in the art will
readily understand that a PMT is typically described as having an
amplification section for amplifying the photon's energy and a
sensor for receiving the amplified energy and converting it to an
electrical signal.
[0124] With regard to the parallax effect, notice from FIG. 5B that
incident photon y 550 is approaching crystal 502 at an oblique
penetration (instead of being perpendicular) to the face of the
crystal looking toward the emitting source. Only transducers 504A
and 504B are depicted for simplicity. When a photon enters the
crystal at 90 degrees, its X-Y position can be easily calculated
from the detectors which perceive the scintillation effect in the
crystal, the XY position through a centroid calculation. An
exemplary centroid calculation for 2.times.2 detector array
(detectors A, B, C and D) is: 1 X m = ( A + B ) - ( C + D ) A + B +
C + D Y m = ( B + D ) - ( A + C ) A + B + C + D
[0125] (A better calculation for determining .DELTA..sub.x is the
ratio of the sum of the energies of all sensors at the west of the
central element, divided by the sum of all sensors at the east of
the central element (.DELTA..sub.x=.SIGMA.E.sub.W/.SIGMA.E.sub.E).
Similarly, for the calculation of .DELTA..sub.y, the ratio of the
sum of the energies of all sensors at the north of the central
element, divided by the sum of all sensors at the south of the
central element (.DELTA..sub.y=.SIGMA.E.sub.N-
/.SIGMA.E.sub.S.)).
[0126] The depth at which the photon interacts with the crystal is
unimportant in this case where the photon penetrates the crystal
perpendicular to the face because it will interact somewhere along
a line oriented in the Z direction formed by the intersection of an
X plane and a Y plane, i.e. the LOR is found perpendicular to the
X-Y planes. This presumes that all lines of response between
coincidental pairs of detectors intersect the center point of the
barrel which is very imprecise. In practice, once the detector
elements 506A and 506B receive an optical signal, an analog signal
is produced at output 508 and sent to the PET electronics (the
coincidence board(s)). Generally, the PET electronics which
compares all of the possible LOR for coincidences, even those
connecting two detectors that did not receive a hit. When a
coincidence is determined, the resulting LOR is used for generating
the image. However, the parallax effect shifts the placement of the
endpoints of the LOR along the Z axis to some default depth, such
as the mid point or face of the crystal. The error is apparent on
FIG. 5D, where both LOR 520 and LOR 522 are correctly spatially
positioned on the X-Y plane of detector 502, but only LOR 520 is at
the proper depth. Often, if a DOI calculation is not performed, the
LOR is found by correspondence using a default depth, e.g. midway
down the detector, on its face, etc. The results of not calculating
a DOI are graphically illustrated in FIG. 5D by the separation
between LOR 520 and LOR 522.
[0127] Therefore, the parallax error resulting from incident
photons with angles different from a 90-degree measurement is
corrected by determining an accurate interaction depth, and using
the depth to properly place the LOR. DOI is determined by comparing
the photon's energy, as captured by two different detectors, and
relating the difference to the interaction depth of the photon in
the crystal. Best results are obtained when the two detectors are
positioned to make maximized variations in energy based on the
depth of interaction. One detector should offset depth with respect
to the Z axis. In accordance with an exemplary embodiment of the
present invention, the measurement of the depth of interaction to
correct the parallax error of incident photons with angles
different from 90 degrees can be performed by using two sensors,
for instance, Photomultipliers (PMT) and/or Avalanche Photodiodes
(APD) on either side of the detector crystal, one being positioned
internal to the barrel and the other being positioned external to
the barrel. For instance, by using an array of photomultipliers
internally and externally and then interpolating the signals
received by the two sensors.
[0128] FIG. 6 is a diagram of a detector assembly having two
sensors (transducers) for accurately measuring the depth of
interaction of a photon in the crystal in order to correct the
parallax error in accordance with an exemplary embodiment of the
present invention. Detector assembly 600 generally comprises
crystal 602 having an interior face 601 and an exterior face 603,
which is optically coupled directly to exterior face 605 (or
window) of external transducer 604 which is a sensor (e.g., APD or
photomultiplier) on the opposite side of the detector from where
the radioactive source is located. Interior face 601 of crystal 602
is connected to light guide 616. The interior opening of light
guide 616 is coupled to interior face 615 (or window) of internal
transducer 614 which is a thin sensor (e.g., an APD) in front of
the detector (the side where the radioactive source is located and
the photon is arriving to hit the detector). In accordance with an
exemplary embodiment of the present invention, detector assembly
600 employs Photodiodes or APD as internal transducer 614, rather
than a PMT, to improve efficiency. The semiconductor material
comprising a photodiode or APD will not absorb or scatter many
photons that penetrate the face of the crystal because it is
comprised of an extremely thin material of only a few hundred of
microns.
[0129] Here, it should be also noted that in contrast with prior
art detector assemblies configured for DOI calculations, the 3D-CBS
processor stack uses the signal outputs from the exterior PMTs for
the vast majority of the data to be used for image generation. As
mentioned above, the present system is hundreds, if not thousands,
of time more efficient than the prior art PET device using only the
photomultipliers. Therefore, while the 3D-CBS architecture could
easily accommodate a complex interior sensor arrangement, such as
an array of interior sensors, there is simply no need to expend the
resources on developing interior sensors and signal channels that
will be used for only one purpose, that is to be compared to the
exterior signals for an interaction depth. To that end, the present
interior sensors are chosen and configured with cost effectiveness
as a primary intent. The results of the choices on the detector
configuration are strikingly different than any interior sensor
arrangement hereto. For instance, one means for achieving cost
effectiveness is by reducing the coverage area of the APD.
[0130] Notice from FIG. 6 that, although the detector 602 has
approximately the same area as the face of PMT 604, the coverage
area of APD is much smaller than the face of crystal 604. For the
purposes of the present invention, this makes absolute perfect
logic. The faces of crystal detector 602 and PMT 604 should be
comparable for better optical coupling and lowering the risk of
missing an event. The requirements for coupling APD 614 are much
less stringent. In fact, since what is sought from APD is a
reasonably accurate signal, the diode utilizes optical guide 616 to
collect and channel the scintillation from crystal 602. In stark
contrast with prior art DOI schemes, it is simply not necessary to
use the interior sensor for anything other than collecting an
optical signal to be compared with the exterior channel
signals.
[0131] In accordance with an exemplary embodiment of the present
invention, the area of window 605 for external transducer 604
(D.sub.PMT) is greater than that of window 615 for internal
transducer 614. As depicted in the figure, the diameter of window
605 (D.sub.PMT) is greater than the diameter of window 615
(D.sub.APD), D.sub.PMT>>D.sub.APD, and therefore, the surface
area (D.sub.PMT).sup.2 of window 605 of external transducer 604 is
proportionally larger than the surface area (D.sub.APD).sup.2 of
window 615 of internal transducer 614. Keeping the surface area of
internal transducer 614 smaller than that of external transducer
604 has two advantages. First, both APDs and photodiodes typically
cost much more than PMTs. Therefore, reducing the size of the APD
reduces the cost of employing APDs. It should also be noted that
presently, in addition to being more costly, Photodiodes and APDs
also have a lower gain; however, it is expected that those
deficiencies will probably abate somewhat as the convenience of
using Photodiodes or APD internally and externally becomes more
apparent. Second, because the detector obscures only a portion of
the face of crystal 602, not every photon penetrating from the
crystal's face will pass through detector 614.
[0132] The operation of detector assembly 600 shown in FIG. 6 will
now be described with regard accurately determining DOI for
reducing the parallax error. In the following discussion,
transducer 604 is optionally referred to as a PMT, while transducer
614 is optionally referred to as an APD; however, these references
are not intended to limit the scope of present invention.
[0133] The light captured by the two transducers is proportional to
the energy of the incident photon and to the distance where the
photon was absorbed by the crystal with respect to the location of
the two transducers. Light captured by the two transducers is
converted into electrical signals 608 and 618. The two signals are
then converted into digital form and sent to the 3D-Flow processor,
which computes the interpolation of the distance from the two
sensors, which is proportional to the location where the photon hit
the detector. This measurement more accurately determines the
location where the photon hit the crystal (the depth of
interaction), rather than assuming the photon strikes that crystal
face at 90.degree. and traverses the crystal along its optical
axis. Parallax error due to poor DOI assumptions is therefore
eliminated and spatial resolution is correspondingly improved.
[0134] Here, it should be understood that the present invention for
improving DOI determinations is extremely adaptable. The detector
assembly can be arranged in several configurations for use in a
PET. For example, and as generally depicted in FIG. 6, detector
assembly 600 may have a 1:1 correspondence between the number
interior transducers 614 and exterior transducers 615. As mentioned
above, exterior transducer 615 may have a smaller surface area than
that of interior transducer 614. In accordance with another
exemplary embodiment of the present invention, the correspondence
between the number of interior transducers 614 and exterior
transducers 615 may instead be 1:M (one to many), as depicted in
FIGS. 7A-7C below. M is defined as an NxN grouping of PMTs having
their interior windows coupled to the exterior opening of the light
guide (2.times.2, 3.times.3, 4.times.4, 5.times.5 and so on). DOI
determinations are made in exactly the same manner as described
above, between the interior transducer and local maxima. The local
maxima is defined as the head of a cluster of PMTs for an impact
(the cluster of PMTs is NOT the same N.times.N grouping of PMTs
coupled to the light guide but may instead extend beyond that group
and be coupled to other light guides). Finding a local maxima and
an XY position of an interaction in a boundary-free cluster of PMTs
is disclosed in U.S. patent application Ser. No. ______, attorney
docket no. 510974-600005; U.S. patent application Ser. No.
09/506,207; U.S. patent application Ser. No. 10/185,904; and U.S.
patent application Ser. No. 10/296,532 identified above and are
each incorporated by reference herein in their entireties.
[0135] Regardless of whether the detector assembly is arranged in
1:1 interior transducers 614 to exterior transducers 615
correspondence or a 1:M, crystal 602 may be configured in one of
several shapes. The first being similar to that known in the prior
art wherein the crystal is coupled to a single PMT, alternatively
to a PMT module having four individual PMTs. Although FIG. 6, and
others, depict the detector as having been segmented into small
rectangular shapes, that depiction is not intended to limit the
scope of the present invention. Despite the fact that the crystal
detectors may be cut in small pieces, alternatively, and as stated
above, the entire barrel can be fabricated from several sectors
(two, four or eight arc segments). Still further, the barrel may be
constructed as a single piece surrounding the entire body of the
patient. In those cases, and with regard to a 1:1 transducer
arrangement, the external opening of light guide 616 covers an area
of crystal 602 proximate to, and equivalent in size to window 605.
Alternatively, the external opening of light guide 616 covers an
area of crystal 602 proximate to, and equivalent in size to
multiple windows for multiple PMTs (see FIGS. 7A-7C below).
Moreover, because the processors in each signal channel of the
3D-CBS processor stack share information with each of their
neighbors, photons interacting with an edge or corner of crystal
602 are properly identified, thereby allowing the DOI determination
to proceed as described.
[0136] Turning now to FIGS. 7A-7C, a scintillation detector
assembly having a sensor on either end of the detector is depicted
absorbing a photon in accordance with an exemplary embodiment of
the present invention. Assembly 700 comprises crystal 702,
amplifiers 704A and 704B and corresponding sensor/transducer 706A
and 706B (generally referred to cumulatively as transducers). Here
again, crystal 702 may be any known or heretofore unknown type of
detector which interacts with a photon so as to produce a
scintillation, or rapid flash of light, in the interior lattice
structure of the crystal. Crystal 702 may be coupled to one or more
optical amplifier/sensors which have a detector integrated therein.
Also, as discussed with regard to FIG. 6, transducer 704 is
depicted as a PMT, while transducer 714 is illustrated as an APD.
Notice from FIG. 7B, however, that transducer 714 was the first to
receive an optical signal from crystal 702, resulting in output
electrical signal 718, while at a later time transducer 704
receives the optical signal from crystal 702, resulting in output
electrical signal 708. It should be cautioned, however, that the
order in which the optical signals are received and the timing are
relatively unimportant. The present invention utilizes the energy
levels, not the arrival times, at the respective sensors to
determine the DOI of the photon in crystal detector 702. The depth
of interaction, not the arrival times, is proportional to the
respective signal strengths. In any case, once electrical signals
708 and 718 have been generated, they are passed to the 3D-CBS DOI
electronics for integration and depth determination. To that end,
optical guide 716 collect sand redirects the optical signal toward
the active portion of APD 714 in an extremely cost effective
manner.
[0137] Manufacturers of prior art PET devices often rely on highly
efficient scintillator crystals for increasing PET efficiency which
substantially increases the cost of the PET device. Therefore, the
particular crystals are chosen to be relatively short (10 mm) to
limit the cost associated with the crystal. Shorter crystals have
the added benefit of minimizing parallax in the prior art PET
because less of the crystal is exposed for a photon to penetrate at
an oblique angle. Because the present invention enables a highly
accurate DOI determination, the crystals selected can be longer to
compensate for lower efficiency, and therefore cost substantially
less than prior art PET devices. The longer crystal results in more
photon stopping power and better overall efficiency for less cost
than is typically achieved in the prior art. Moreover, because the
DOI can be accurately determined, higher resolution images are
possible even when using a lower cost, less efficient scintillator
crystal, such as a bismuth germanate (BGO) crystal or a sodium
iodate (NaI) crystal.
[0138] At present, the exterior sensors are PMTs for the reasons
discussed above. However, correction of parallax errors from
incident photons with angles different from 90 degrees can be
performed by using two sensors (Photomultipliers or Avalanche
Photodiodes APD) on both sides of the detector, one internal to the
barrel and the other external to the barrel. For instance, by using
an array of photomultipliers internally and externally and then
interpolating the signals received by the two sensors. In
accordance with one aspect of the present invention, using a
Photodiodes or APD internally that will not absorb or scatter many
photons will significantly improve efficiency of the system because
of its small thickness of material of a few hundred of microns, and
PMT externally. Photodiodes or APD will cost more than PMTs and
have a lower gain; however, future technology advances will show
that it will be convenience to use Photodiodes or APD internally
and externally. Although many figures on this non-provisional
patent show the symbol of the detector cut in a small rectangular
shape, the idea described in this non-provisional patent is not
limited to crystal detectors cut in small pieces, but, as stated
above, the entire barrel can be made of several sectors, four
sectors, two sectors or at the limit a barrel in a single piece
surrounding the entire body of the patient. This detector can have
sensors (PMT, APD, or photodiodes) internally or externally to the
barrel.
[0139] FIG. 8 is a flowchart of the process performed by the 3D-CBS
system for determining DOI from the interior and exterior sensors
on a crystal detector in accordance with an exemplary embodiment of
the present invention. The DOI process begins with any sensor
receiving an optical signal generating the crystal 702 by a photon
being decimated (step 802). Typically, the signal is in analog form
from the sensors and should be converted to a digital signal prior
to inputting to the DOI hardware. From the perspective of the DOI
algorithm, it is unimportant whether interior sensor 714, receives
the input prior to exterior sensor 504/506, or vice versa. However,
because the orientation of the Z direction of the crystal is
important, the sensor must be identified as being one of an
interior channel or exterior channel sensor, and then the specific
detector should be identified in some manner enabling the DOI
hardware to look for signals on opposite and corresponding channels
which correspond to the received channel signal for the detector
(step 804).
[0140] What is intended is that the Z (perpendicular) depth of the
interaction in the crystal be determined. Therefore, corresponding
signal channels to be used by the DOI algorithm are taken, if
possible, from interior and exterior sensors lying on the
detector's Z axis. In other words, logic dictates that the optimal
choices for a single corresponding pair of interior and exterior
corresponding sensor pair are those having identical X-Y positions
and vary only in the Z direction, i.e., the axial direction of the
detector. However, as has been discussed repeatedly, the 3D-CBS
processing architecture is extremely powerful and programmable to
accommodate a variety of algorithms. Moreover, the majority of
important signal information is derived from the exterior channels.
Therefore, the interior channel signals are far less important,
being used only as a basis for comparison with the exterior
channels for calculating DOI and correcting parallax errors. Noise
and transients can be filtered using the 3D-CBS and the DOI
determined by interpolating signals from a multitude channels.
[0141] Once one channel signal has been received and digitized, the
DOI processing hardware "watches" the channels associated with the
corresponding sensors for an input. Normally, the signal arrives
almost instantaneously with another, but the possibility exists
that no signal will be forthcoming, and time out.
[0142] Upon receiving the channel signal from the corresponding
sensor, it is also A/Ded and passed to the DOI hardware (step 810).
Here again, the 3D-CBS architecture is a very powerful system and
can easily process multiple channels from a sensor, such as an
array of Photodiodes, APDs or PMTs, but given the single purpose of
the interior channels, it is doubtful that the added expense could
result in any better DOI values. Thus, the interior and exterior
channel signals are interpolated toward a value that is indicative
of the depth in the detector barrel where a photon was observed
causing the scintillation. Using the depth measurement, the
parallax error can be corrected using simple trigonometric
functions and a more accurate placement of the LOR is
determined.
[0143] The present invention will change the way health care is
practiced. Preventive health care will receive a boost because,
with this discovery, a safe, low-radiation preventive screening
examination will be available. Prior art PET examinations require
1100 mrem of radiation, more than 10 times the exposure deemed
acceptable in one year by the International Commission for
Radiation Protection, and cannot be approved for preventive
screening. The 3D-CBS, on the other hand, requires less than 100
mrem of radiation, well within the guidelines of the ICRP.
Moreover, there will be a revolution in the way the images will be
displayed. The image resulting from an examination with the 3D-CBS
is three dimensional, visualizes the whole body at one time because
data are recorded at the same time over the entire body, and has
greatly increased definition. No more slices of the body, but real
3-D images of any organ of the body can be seen. There will be no
more need to take several cancer screening examinations, but only a
single, more efficient examination that will detect not only
cancer, but also other diseases. Several current diagnostic
devices, such as Single-Photon Computed Tomography (SPECT), gamma
camera, etc. which capture only one out of 300,000 photons emitted,
will become obsolute
[0144] Just to mention a few consequences of this revolutionary
discovery among those that will change the way medicine is
practiced: There will be an impulse toward preventive medicine
because this discovery opens up the possibility to perform
non-invasive annual examinations at radiation doses accepted by the
ICRP on asymptomatic people. This discovery will eliminate the
current display of the body in slices and will making it possible
to obtain real 3-D pictures of any organ in the body at the fused
anatomical and molecular level because the data are acquired at the
same time over the entire body. It will be an essential tool to
develop and study the effects of new, experimental pharmacopeia. It
will lower the cost of health care by combining in a single exam
the examination of many organs, thus eliminating the need for
several different, expensive (and sometimes invasive) procedures of
screening for cancer such as mammogram, colonoscopy, etc.
[0145] The corresponding structures, materials, acts, and
equivalents of all means or step plus function elements in the
claims below are intended to include any structure, material, or
act for performing the function in combination with other claimed
elements as specifically claimed. The description of the present
invention has been presented for purposes of illustration and
description, but is not intended to be exhaustive or limited to the
invention in the form disclosed. Many modifications and variations
will be apparent to those of ordinary skill in the art. The
embodiment was chosen and described in order to best explain the
principles of the invention and the practical application, and to
enable others of ordinary skill in the art to understand the
invention for various embodiments with various modifications as are
suited to the particular use contemplated.
* * * * *
References