U.S. patent application number 15/501193 was filed with the patent office on 2018-04-19 for direct conversion radiation detector.
The applicant listed for this patent is KONINKLIJKE PHILIPS N.V.. Invention is credited to Cornelis Reinder RONDA, Frank VERBAKEL, Herfried Karl WIECZOREK.
Application Number | 20180106910 15/501193 |
Document ID | / |
Family ID | 53525135 |
Filed Date | 2018-04-19 |
United States Patent
Application |
20180106910 |
Kind Code |
A1 |
VERBAKEL; Frank ; et
al. |
April 19, 2018 |
DIRECT CONVERSION RADIATION DETECTOR
Abstract
The present invention relates to a direct conversion radiation
detector for wherein the direct conversion material comprises a
garnet with a composition of Z.sub.3(Al.sub.xGa.sub.y)O.sub.12:Ce,
wherein Z is Lu, Gd, Y, Tb or combinations thereof and wherein y is
equal to or greater than x; and preferably Z comprises Gd. Suitable
garnets directly convert radiation, such as x-rays or gamma-rays,
into electronic signals. Preferably photoluminescence of the garnet
is low or absent. The detector is particularly suitable for use in
x-ray imaging devices, such as computed tomography. In some
embodiments photoluminescence of garnets might be used to construct
a hybrid direct-indirect conversion detector, which may be
particularly suitable for use with Time-of-Flight PET.
Inventors: |
VERBAKEL; Frank; (Helmond,
NL) ; RONDA; Cornelis Reinder; (Aachen, DE) ;
WIECZOREK; Herfried Karl; (Aachen, DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
KONINKLIJKE PHILIPS N.V. |
Eindhoven |
|
NL |
|
|
Family ID: |
53525135 |
Appl. No.: |
15/501193 |
Filed: |
June 28, 2016 |
PCT Filed: |
June 28, 2016 |
PCT NO: |
PCT/EP2016/064930 |
371 Date: |
February 2, 2017 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G01T 1/24 20130101; G01T
1/241 20130101 |
International
Class: |
G01T 1/24 20060101
G01T001/24 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 9, 2015 |
EP |
15176076.6 |
Claims
1. A direct conversion radiation detector comprising: a direct
conversion layer comprising a direct conversion material for
directly converting incoming radiation from a radiation source into
electron and hole pairs; a first electrode mounted on the direct
conversion layer facing the radiation source; a second electrode
mounted on an opposite side of the direct conversion layer compared
to the first electrode; and means for applying an electrical
potential between the first electrode and the second electrode,
characterized in that the direct conversion material comprises a
garnet with a composition of
Z.sub.3(Al.sub.xGa.sub.y).sub.5O.sub.12:Ce, wherein Z is Lu, Gd, Y,
Tb or combinations thereof and wherein y is equal to or greater
than x; and preferably Z comprises Gd.
2. The direct conversion radiation detector according to claim 1,
wherein the second electrode is pixelated.
3. The direct conversion radiation detector according to claim 1,
further comprising a photosensor mounted behind the second
electrode with respect to the direct conversion layer for
converting visible light formed in the direct conversion layer to
an electronic signal, wherein the second electrode is transparent
to visible light and wherein y is preferably between 0.4 and 0.6,
more preferably y is about 0.5, and/or wherein the garnet is
preferably a garnet wherein Z comprises Gd and Lu with a Gd:Lu
ratio of about 2:1.
4. The direct conversion radiation detector according to claim 1,
further comprising an integrated circuit for processing electronic
signals generated in the direct conversion radiation detector.
5. The direct conversion radiation detector according to claim 4,
further comprising a transparent re-routing layer for re-routing
each pixel of the pixelated electrode to the integrating
circuit.
6. A radiation imaging method, comprising the steps of: emitting a
radiation beam from a radiation source; detecting the emitted
radiation beam with a direct conversion radiation detector
according to claim 1; generating a first electronic signal
indicative of a number of detected charge carriers generated in the
direct conversion layer.
7. The radiation imaging method according to claim 6, wherein the
direct conversion radiation detector is a direct conversion
radiation detector and a first electronic signal is generated for
each detector pixel of the pixelated second electrode.
8. The radiation imaging method according to claim 6, wherein the
direct conversion radiation detector is a direct conversion
radiation detector (, further comprising the step of: generating a
second electronic signal indicative of a number of detected
electrons generated in the photosensor.
9. The radiation imaging method according to claim 7, wherein the
radiation source is a decaying radioactive material, further
comprising the following steps: detecting two simultaneously formed
photons with at least the radiation detector; determining a
difference in detection time between the two simultaneously formed
photons; generating a timestamp based on the determined difference
in detection time, wherein the step of generating a first
electronic signal includes using the generated timestamp as
input.
10. The radiation imaging method according to claim 6, further
comprising the step of: generating image data based on the first
electronic signal.
11. The radiation imaging method according to claim 8, further
comprising the steps of: generating image data based on the first
electronic signal and on the second electronic signal.
12. The radiation imaging method according to claim 10, further
comprising the step of: displaying the image data.
13. An imaging system comprising a direct conversion radiation
detector according to claim 1.
14. The imaging system according to claim 13, selected from a group
comprising X-ray imaging device, computed tomography imaging
device, preferably a spectral computed tomography imaging device,
position emission tomography imaging device, preferably a
time-of-flight positron emission tomography imaging device,
single-photon emission computed tomography device, or combinations
thereof.
Description
FIELD OF THE INVENTION
[0001] The present invention generally relates to a direct
conversion radiation detector, a radiation imaging method and an
imaging system.
BACKGROUND OF THE INVENTION
[0002] Scintillators are widely used as detectors for spectroscopy
of X-rays and gamma-rays. Incoming ionizing radiation is absorbed
by the scintillator, which re-emits a photon of a different (e.g.
visible) wavelength, which then, in a photosensor, such as a
photodiode, an avalanche photodiode or a silicon photomultiplier,
may be used to generate an electronic signal, which may
subsequently be processed to imaging data. Radiation detectors
based on scintillators are commonly used in e.g. medical imaging,
security scanning or astrophysics. Important properties for the
scintillation crystals used in these applications include high
light output, high gamma-ray stopping efficiency, fast response,
low cost, good proportionality, and minimal afterglow. There is
continued interest in new scintillator materials that have these
properties. In particular garnets are a group of materials that
have shown to be of interest for use as scintillator material.
[0003] A garnet is an inorganic crystalline material, in many cases
comprising a mixed oxide composition containing Gd, Lu, Al and/or
Ga. Often dopants like Cerium, which forms an emission center, is
included to increase light output upon X-ray irradiation, as is for
instance known from US patent application US2012/0223236A1.
[0004] X-ray or gamma-ray detection with scintillators is an
indirect detection method, since it requires the photosensor to
detect the light emitted by the scintillator. A drawback of such an
indirect detection method is (high) loss of energy due to the two
steps: there is a loss in converting the radiation to light and
afterwards in the photodiode to electrons. Due to the resulting
(relatively) low number of electrons in the photosensors, the
energy resolution of the detector is limited.
[0005] An alternative method to detect radiation is direct
detection. This uses a semiconductor to directly convert the energy
of absorbed X-ray or gamma-ray photons into electron-hole pairs.
The electrons may be processed into an electrical signal without
the use of, and therefore without the above-mentioned losses
associated with a further functional layer. Cadmium Telluride
(CdTe) or Cadmium Zinc Telluride (CZT) are the most commonly used
direct conversion materials in direct conversion radiation
detectors. If performed in a so-called photon counting mode, this
enables measuring the energy of each of the radiation quanta
absorbed with much higher energy resolution (spectral response).
This spectral information is very important to improve image
resolution and quality, e.g. for diagnostics. WO2014/032874A1
discloses a hybrid photodiode with an organic direct conversion
layer with scintillating garnet fillers dispersed therein. However,
these materials are typically single crystals, which are very
difficult to make and therefore expensive. Also, it is quite
difficult to modify these materials to optimize or tune their
properties for different detector systems.
SUMMARY OF THE INVENTION
[0006] Embodiments according to the present invention are directed
to a direct conversion radiation detector comprising a direct
conversion layer comprising a direct conversion material for
directly converting incoming radiation from a radiation source into
electron and hole pairs; and a first electrode mounted on the
direct conversion layer facing the radiation source; and a second
electrode mounted on an opposite side of the direct conversion
layer compared to the first electrode; and means for applying an
electrical potential between the first electrode and the second
electrode. The direct conversion material comprises a garnet. The
garnet has a composition of
Z.sub.3(Al.sub.xGa.sub.y).sub.5O.sub.12:Ce, wherein Z is Lu, Gd, Y
or Tb (or combinations thereof) and wherein y is equal to or
greater than x; and preferably Z comprises Gd.
[0007] In another preferred embodiment the second electrode is
pixelated.
[0008] In another preferred embodiment the detector comprises a
photosensor mounted behind the second electrode with respect to the
direct conversion layer for converting visible light formed in the
direct conversion layer to an electronic signal, wherein the second
electrode is transparent to visible light and wherein the garnet
has a composition of Z.sub.3(Al.sub.xGa.sub.y).sub.5O.sub.12:Ce,
wherein Z is Lu, Gd, Y, Tb or combinations thereof and wherein y is
equal to or greater than x; and preferably Z comprises Gd.
[0009] In another preferred embodiment the detector comprises an
integrated circuit for processing electronic signals generated in
the direct conversion radiation detector.
[0010] In another preferred embodiment the detector comprises an
integrated circuit for processing electronic signals generated in
the direct conversion radiation detector.
[0011] In another preferred embodiment the detector comprises a
transparent re-routing layer for re-routing each pixel of the
pixelated electrode to the integrating circuit.
[0012] Further embodiments according to the present invention are
directed to a radiation imaging method using the direct conversion
detector according to the present invention.
[0013] A particularly interesting embodiment of the radiation
imaging method wherein the radiation source is a decaying
radioactive material, comprises detecting two simultaneously formed
gamma-ray photons with at least the radiation detector; determining
a difference in detection time between the two simultaneously
formed photons; generating a timestamp based on the determined
difference in detection time, wherein the step of generating a
first electronic signal includes using the generated timestamp as
input.
[0014] Further embodiments according to the present invention are
directed to an imaging system comprising the direct conversion
detector according to the present invention.
[0015] Still further aspects and embodiments of the present
invention will be appreciated by those of ordinary skill in the art
upon reading and understanding the following detailed description.
Numerous additional advantages and benefits will become apparent to
those of ordinary skill in the art upon reading the following
detailed description of preferred embodiments.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] The present invention is illustrated by drawings of
which
[0017] FIG. 1 shows a schematic representation to explain the
principle of direct conversion detection with a pixelated direct
conversion radiation detector.
[0018] FIG. 2 shows a schematic representation of a first
embodiment of a direct conversion radiation detector according to
the present invention.
[0019] FIG. 3 shows a schematic representation of a second
embodiment of a direct conversion radiation detector according to
the present invention.
[0020] FIG. 4 shows a schematic representation of a pixelated
electrode of a direct conversion radiation detector according to
the present invention.
[0021] FIG. 5 shows a flowchart for a method for a radiation
imaging method according to the present invention.
[0022] FIG. 6 shows a flowchart for a hybrid radiation imaging
method according to the present invention based on FIG. 5 with
additional steps.
[0023] FIG. 7 shows a flowchart for a radiation imaging method
wherein the radiation source is a decaying radioactive material
according to the present invention.
[0024] The invention may take form in various components and
arrangements of components, and in various process operations and
arrangements of process operations. The drawings are only for the
purpose of illustrating preferred embodiments and are not to be
construed as limiting the invention. To better visualize certain
features may be omitted or dimensions may be not be according to
scale.
DETAILED DESRIPTION OF EMBODIMENTS
[0025] FIG. 1 shows a highly schematic depiction of the known
principle of direct conversion radiation detection. This principle
and the invention are illustrated using x-ray (x) and gamma-ray
radiation (Y), but the concept would be valid for any other type of
radiation that may be directly converted into electronic signals by
a direct conversion layer.
[0026] In FIG. 1 several essential layers of a direct conversion
radiation detector 1 are shown. A bulk of the detector 1 is formed
by direct conversion layer 11 comprising a direct conversion
material. The direct conversion material 11 may be composed of a
single-crystal semiconductor material, which is an intrinsic
material or has a fully depleted p-i-n structure (due to electrical
contacts). Cd.sub.xZn.sub.1-xTe (Cadmium Zinc Telluride, commonly
abbreviated to CZT) is a suitable known semiconductor material.
Also Cadmium Telluride (CdTe) is often used as a direct conversion
material. The direct conversion layer 11 is placed between a first
electrode (cathode) 12 and a second electrode (anode) 13, wherein
the first electrode faces a direction from which radiation x,
.gamma. may be emitted towards the radiation detector 1. The first
electrode 11 and second electrode 12 are connected to an electrical
power source 14 (or more than one source). The first electrode 11
is held at a negative bias potential, while the second electrode 12
is held at a less repelling (usually an attracting positive)
potential. The first electrode 12 forms a continuous layer on the
direct conversion material layer 51 and is generally transparent to
photons x, .gamma. with an energy level to be detected by the
radiation detector 1. The second electrode 13 is on the opposite
side of the direct conversion layer 11 and is normally subdivided
into a row or grid of detector pixels 131.
[0027] When a photon x, .gamma. passes the first electrode 12 and
penetrates into the direct conversion material layer 11, the photon
x, .gamma. interacts with direct conversion material to generate
numerous electron-hole pairs. The positively charged holes drift
towards the strongly negatively charged first electrode 12, while
the negatively charged electrons drift towards the more positively
charged second electrode 13. When the electrons approach second
electrode 13, a signal is induced from each detector pixel 131,
which, after collection, is indicative of a count of electrons that
approached that particular electrode pixel 131. Which may then be
further processed by processing units and eventually displayed on a
display unit to a user as written information or as a reconstructed
image of (part of) an examined object.
[0028] FIG. 2 shows a schematic depiction of an embodiment of a
radiation detector 1 according to the present invention. This
embodiment comprises the same elements as those shown in FIG. 1,
but in this case the direct conversion layer 11 comprises a garnet
as a direct conversion material. The direct conversion material may
be made up completely, or at least mostly, made from a garnet. The
used garnet acts in the same way as direct conversion material as
described previously: when a photon x, .gamma. is absorbed in the
garnet electron-hole pairs are formed, which are transported to the
second electrode 13 and first electrode 12 respectively. An
electronic signal indicative of the electron count for each
detector pixel 131 is sent to an integrated circuit 15, which is in
electrical connection with the second electrode 13 to process the
electronic signals into image data. In this embodiment the
integrated circuit 15 is directly mounted to the second electrode
13, but it may also be placed away from the second electrode
13.
[0029] Most types of garnets that do not show very low (or
preferably no) photoluminescence emission under ionizing radiation
are suitable for use in context of this invention. It is an insight
of the present invention that garnets, particularly non-or
low-luminescing garnets, actually may be used as direct conversion
materials. Normally garnets are used in the field of radiation
detection for their good photoluminescence properties, which could
be detrimental to the efficiency as direct conversion materials and
a skilled person would therefore not contemplate to use a garnet he
is familiar with as a direct conversion material. Examples of
particularly suitable garnet materials are Cerium (Ce) doped
Aluminium (Al)-Gallium (Ga) based garnets with a composition of
Z.sub.3(Al.sub.xGa.sub.y).sub.5O.sub.12:Ce, wherein Z is chosen
from Lutetium (Lu), Gadolinium (Gd), Yttrium (Y) or Terbium
(Tb).
[0030] An advantage of using garnets instead of known direct
conversion materials such as CdTe or CZT is that garnets are
cheaper to produce, there is more variety in types and they can be
tuned to optimize properties for a certain system, which is another
insight of the present invention. The fabrication process of
garnets enables tuning garnet material properties to a desired
specification. For example, Cerium content is tuned for maximum
light output and speed for garnets that are used as indirect
scintillator materials. For the present invention the garnet
material properties need to be tuned in such a manner that the
garnet can be used for direct conversion. As such, it is necessary
to limit recombination of electron-hole pairs and to enable charge
separation to occur (which is actually opposite of what is needed
for scintillator materials). Garnets used for the present invention
should preferably not show luminescence, since this is a loss
process for direct conversion. As in general undoped materials show
lattice related emission (e.g. due to self-trapped excitons),
preferably Ce.sup.3+ doped garnets are used in which the excited
Ce.sup.3+ ion ionizes to Ce.sup.4+, rendering an electron in the
conduction band, which is counted. In the garnets used in the
context of the present invention, holes are trapped on the
Ce.sup.3+ ions. Such ionization of Ce.sup.3+ ions is especially
seen in garnets that have a low energetic distance between the
excited d-level states of Ce.sup.3+ ions and the conduction band.
From literature it is known that Gd, Lu or Y-based Al--Ga garnets
with a Gallium content higher than the Aluminum content are
especially prone to ionization of Ce.sup.3+ ions. As such,
compositions described by (Lu, Gd, Y,
Tb).sub.3(Al.sub.xGa.sub.y).sub.5O.sub.12:Ce, with Ga content
greater or equal than the Al content (y>=x), are particularly
interesting garnets as direct converters in the context of the
present invention. The composition may be tuned such that both
luminescence and direct conversion can be detected. When done
properly, the total received signal increases and better quality
images may be obtained.
[0031] Furthermore, garnets are sintered in their fabrication
process and said sintering step will need to be performed such that
grain boundaries are limited to prevent conduction pathways and
defect centers. This requirement is similar for the current garnet
based systems for indirect scintillation as defect minimization
limits recombination and increases light output. Sintering is
typically carried out at temperatures above 1600.degree. C.,
preferably in a temperature range between 1650.degree. C. and
1780.degree. C., most preferably in a temperature range between
1675.degree. C. and 1750.degree. C. in vacuum. As garnets can be
produced using sintering processes in ceramic form, the garnets
used in the present invention will be significantly cheaper than
the common direct converters based on CZT/CdTe, which are applied
as single crystal. Moreover, a plurality of ceramic garnet
compositions is already available and new types are still developed
and produced, which offers the possibility of fine tuning a number
of significant parameters, like the ratio of direct- and indirect
conversion, the stopping power and the Ce.sup.3+ emission spectrum.
This is much more difficult, if not impossible, with single
crystals.
[0032] FIG. 3 shows a further embodiment of a radiation detector
according to the present invention which combines direct conversion
and indirect detection. As with the previously described
embodiment, the direct conversion layer 11 comprises a garnet.
However in this case the garnet is one chosen from scintillator
garnets that are currently used for indirect detection. For
instance a garnet wherein y is preferably between 0.4 and 0.6, more
preferably y is about 0.5 (e.g.
Gd.sub.3Al.sub.2.5Ga.sub.2.5O.sub.12:Ce). The term `about` means
that in the context of the present application a property value may
in practice vary somewhat, e.g. 10% in either direction. These are
highly efficient group of garnets used in known scintillators.
Another good option (on its own or in combination with the
previously mentioned types) would be a garnet wherein Z comprises
Gd and Lu with a Gd:Lu ratio of about 2:1. These garnets are
particularly suitable for use in PET imaging. As with common
indirect detectors a photosensor 16, such as a photodiode, an
avalanche photodiode or a silicon photomultiplier, is placed
between the direct conversion layer 11 and the integrated circuit
15. The second electrode 13 is sandwiched between the direct
conversion layer 11 and the photosensor 16. This arrangement is a
hybrid between an indirect conversion detector (which does not have
the first electrode 12 and second electrode 13) and a direct
conversion detector (which does not have the photosensor 16). This
arrangement allows the electrical power source 14 to apply an
electrical field on the direct conversion layer to separate part of
the electron-hole pairs (direct conversion), while others recombine
to generate visible light (scintillation).
[0033] The visible light generated within the direct conversion
layer 11 is transmitted to the photodiode, where it is converted
into a second electronic signal. Because of this the second
electrode 13 needs to be transparent to the appropriate visible
light spectrum in order not to block the visible light for the
indirect conversion detector. The transparent second electrode 13
may comprise known materials for transparent electrodes, such as
for instance Indium Tin Oxide (ITO) or Aluminium-doped Zinc Oxide
(ZnO:Al).
[0034] The directly converted separated electron-hole pairs are
collected by the pixelated second electrode 13 and converted by the
integrated circuit 15 into spectral information of the absorbed
X-ray quanta. FIG. 4 shows a top view of the second electrode 13
with a grid of transparent pixels 131. In this embodiment the
second electrode 13 is electrically connected to the integrated
circuit 15 by flexible electrical connection means 132, but other
known connection means may be contemplated by the skilled person as
well. For a connection of the transparent pixelated second
electrode 13, it is necessary to apply re-routing layers (not
shown) to re-route every single pixel to the electronics. Also the
re-routing materials need to be transparent, for the conductive
wires, similar materials as for the electrodes can be used, and for
the isolation layer materials such as for instance Silicondioxide
(SiO.sub.2) or Silicon Nitride (Si.sub.3N.sub.4) may be used.
[0035] Hybrid embodiments such as the radiation detector shown in
FIG. 3 produce at least two separate electronic signals indicative
of the detected radiation, which may be used separately or combined
to obtain additional and/or improved image data, which will better
assist a user with analyzing a scanned object, e.g. allowing a
physician to provide a more detailed and/or improved diagnosis of a
scanned patient. By both counting the number of photons and the
number of electrons, a larger signal is obtained. This effect
relies on the fact that prior to reaching an emitting Ce.sup.3+
ion, the charges always travel a finite distance through the
converting material. The ratio between direct and indirection
conversion can e.g. be tuned via the Ce.sup.3+ concentration and
also via the Ce.sup.3+ ionization energy, which in turn can be
tuned by varying the host lattice composition, requiring ceramics
rather than single crystals.
[0036] A higher indirect conversion signal is obtained when
choosing a high Ce.sup.3+ concentration (while not inducing
concentration quenching) and a high ionization energy of Ce.sup.3+
in the excited state.
[0037] The radiation detector according to the present invention is
particularly suitable for x-ray imaging and computed tomography
(CT) imaging, particularly spectral CT imaging, in which x-ray
radiation is emitted from a radiation source to the radiation
detector. The present invention is also suitable for use in any
other imaging system to image an object which uses radiation that
may be directly converted into an electronic system by a direct
conversion layer, such as single-photon emission computed
tomography device (SPECT) or position emission tomography (PET)
imaging device or combinations of different types of imaging.
[0038] It is particularly interesting to use a hybrid
direct-indirect radiation detector according to the present
invention, similar to the embodiment depicted in FIG. 3, in
Time-of-Flight PET imaging. In PET imaging a radiation detector
detects gamma photons emitted consequent to the radioactive decay
of a radioactive tracer material which was previously introduced
into an object to be scanned (e.g. a patient's organ) to obtain
three-dimensional image data of said object. In Time-of-Flight PET
imaging a difference in time between the detection, by a pair of
detectors, of two simultaneously formed gamma photons may be
determined. This information may then be used to more precisely
localize a point of origin of the annihilation event that caused
the photon emission. Time-of-Flight PET needs high temporal
accuracy time stamp to accurately measure the position of the
photon emission center on the line-of-response. With the hybrid
direct-indirect radiation detector according to the present
invention, the indirect conversion process is used to provide the
time stamp in PET. Photon emission from garnets is a very fast
process, which makes it suitable for Time-of-Flight PET.
Additionally, an electrical field separates part of the generated
electron-hole pairs for direct detection. As a time stamp is now
already available from the indirect conversion process, the speed
of the direct conversion process becomes less important, thereby
relaxing material requirements and the direct conversion system.
However as the number of electrons collected from the direct
conversion process is related to the energy of the absorbed
radiation, spectral information may also be obtained from the
direct conversion process with high energy resolution. This allows
obtaining an imaging system with time resolution of Time-of-Flight
PET combined with spectral information of X-ray or CT imaging,
allowing for even more accurate analysis a scanned object,
resulting in e.g. an even further improved diagnosis for a
patient.
[0039] The present invention also provides another advantage,
particularly for PET imaging. Luminescence quenching decreases
light yield, but also the decay time, but basically the ratio
between the two even remains the same. So the figure of merit for
coincidence resolving time (CRT) in PET remains the same. As
coincidence resolving time and counting (for energy resolution) are
decoupled in case of the present invention, there is now more time
to count in case of PET. Therefore the tasks of CRT and energy
resolution between scintillation and counting may be distributed.
As the decay time of the emission in garnets is rather long (due to
trapping of charges), it is a viable option to measure
photoconductivity, because a current may be already measured before
trapping of the charges occurs, whereas emission of trapped charges
and charges that will be trapped only occurs after the charges have
been released. The emission is needs to be quenched by at least
50%, but more preferably by at least 90%.
[0040] FIG. 5, which solely includes everything left of the dotted
line, shows a schematic depiction of a radiation imaging method
according to the present invention. In step 501 radiation is
emitted from a source, for instance x-ray radiation or gamma
radiation towards a direct conversion radiation detector according
to the present invention, so comprising a garnet in the direct
conversion layer. In step 502 incoming photons are converted into
charge carriers (in this embodiment electrons and holes) in the
direct conversion layer. In step 504 a first electronic signal
indicative of a number of detected charge carriers is generated
(photon counting). In step 505 image data is generated based on the
first electronic signal. In step 506 the image data is displayed to
a user, e.g. as two or three dimensional images.
[0041] FIG. 6 depicts a schematic overview of a hybrid radiation
imaging method according to the present invention and includes all
steps of FIG. 5, as well as all steps right of the dotted line.
Direct conversion occurs similar as described for steps 502, 504
and 505 of FIG. 5. In parallel, in step 602, part of the incoming
photons cause photoluminescence in the garnet, which then emits
photons at a different wavelength, usually in the visible spectrum.
In step 603 the photons generated in the garnet are then converted
to electrons in a photosensor. In step 604 a second electronic
signal indicative of a number of detected charge carriers is
generated. In step 505 image data is now generated based on both
the first and second electronic signal. Single image data may be
generated based on both signals and/or two different sets of image
data may be generated based on each of the signals. As with FIG. 5,
the image data is displayed to a user in step 506.
[0042] FIG. 7 depicts a schematic overview of a specific embodiment
of the hybrid radiation imaging method according to the present
invention wherein the radiation source is a decaying radioactive
material, for instance a radioactive tracer in an object. In step
701 the radioactive tracer is detected with a radiation detector
according to the present invention that also has an indirect
detection option. In step 702 a timestamp is determined from
indirectly converted photons. In step 703 spectral information is
determined from directly converted photons. In step 704 image data
is generated from the spectral information and using the timestamp.
In step 705 the image data is displayed to a user.
[0043] While the invention has been illustrated and described in
detail in the drawings and foregoing description, such illustration
and description are to be considered illustrative or exemplary and
not restrictive; the invention is not limited to the disclosed
embodiments.
[0044] Other variations to the disclosed embodiments can be
understood and effected by those skilled in the art in practicing
the claimed invention, from a study of the drawings, the
disclosure, and the appended claims. In the claims, the word
"comprising" does not exclude other elements or steps, and the
indefinite article "a" or "an" does not exclude a plurality. A
single processor or other unit may fulfill the functions of several
items recited in the claims. The mere fact that certain measures
are recited in mutually different dependent claims does not
indicate that a combination of these measured cannot be used to
advantage.
* * * * *