U.S. patent application number 10/166197 was filed with the patent office on 2002-10-17 for diagnostic and therapeutic detector system for imaging with low and high energy x-rays and electrons.
Invention is credited to Brahme, Anders, Danielsson, Mats.
Application Number | 20020149305 10/166197 |
Document ID | / |
Family ID | 26654971 |
Filed Date | 2002-10-17 |
United States Patent
Application |
20020149305 |
Kind Code |
A1 |
Danielsson, Mats ; et
al. |
October 17, 2002 |
Diagnostic and therapeutic detector system for imaging with low and
high energy X-rays and electrons
Abstract
A detector unit for detecting photons in the energy range 1 keV
to 100 MeV, includes at least two converter layers adapted to
interact with incident X-ray photons and to cause electrons to be
emitted therefrom, at least one amplifier adapted to interact with
the electrons emitted from the converters and adapted to produce a
multiplicity of secondary electrons and photons representing a
signal proportional to the incident fluence of X-ray photons, a
connector connecting the detector to an electric field generator
providing an electric drift field for secondary electrons in the
detector, and a sensor device arranged to receive the signal and
provide an input to electronic signal processor.
Inventors: |
Danielsson, Mats;
(Stocksund, SE) ; Brahme, Anders; (Dandeyrd,
SE) |
Correspondence
Address: |
YOUNG & THOMPSON
745 SOUTH 23RD STREET 2ND FLOOR
ARLINGTON
VA
22202
|
Family ID: |
26654971 |
Appl. No.: |
10/166197 |
Filed: |
June 11, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10166197 |
Jun 11, 2002 |
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09236596 |
Jan 26, 1999 |
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6429578 |
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Current U.S.
Class: |
313/105CM |
Current CPC
Class: |
A61N 5/1048 20130101;
A61N 2005/1054 20130101; H01J 47/02 20130101; G01T 1/185
20130101 |
Class at
Publication: |
313/105.0CM |
International
Class: |
H01J 043/00 |
Claims
What is claimed is:
1. A system for X-ray therapy and diagnosis, comprising at least
one source of X-rays; collimators for said X-rays for providing a
defined amount of radiation to a patient; and a detector for
photons in the energy range 1 keV-100 MeV, said detector comprising
a stack of amplifiers and converters.
2. A system for X-ray control of transport devices such as bags,
containers, trucks trailers, comprising at least one source of
X-rays; collimators for said X-rays for providing a defined amount
of radiation to an object to be examined; a detector for photons in
the energy range 1 keV-100 MeV, said detector comprising a stack of
amplifiers and converters; and a place of location for said
transport device to position said device between said source of
X-rays and said detector.
3. The system as claimed in claim 2, further comprising a scanning
unit enabling scanning of said X-rays over an area on said device;
and an X-Y movement generating device coupled to said detector for
enabling movement of said detector in correspondence with said
scanning.
4. A method for performing X-ray therapy, comprising the following
steps: selectively providing an X-ray beam in the energy range 1
keV-100 MeV; providing a detector responsive to said X-ray beam
over the entire energy range; directing an X-ray beam of diagnostic
or therapeutic energy towards a patient; selectively detecting
diagnostic or therapeutic radiation as needed using the same
detector in order to get optimal information from each
modality.
5. A method as claimed in claim 4, comprising making a first
detection of diagnostic energy X-rays, for generating an image
representative of the position of the patient; in response to
position information obtained form said image, immediately
adjusting the position of the patient if needed; and exposing the
patient to therapeutic radiation.
6. A charge collection device, comprising a multilayer printed
circuit board having a multiplicity of chargeable pads distributed
over the surface of the board; and separate electrical connections
connected to each pad and drawn between the layers of said
multilayer board to the edges thereof.
7. The charge collection device as claimed in claim 6, wherein said
pads are of metal.
8. The charge collection device as claimed in claim 6, wherein said
board comprises 2-20 layers.
9. The charge collection device as claimed in claim 6, further
comprising electronic read-out devices coupled to said electrical
connections, said read-out devices being mounted at the edges of
said printed circuit board.
10. The charge collection device as claimed in claim 6, wherein the
pads are of a metal selected from the group good charge collectors
consisting of Cu, Au and Al.
11. A detector unit for detecting photons in the energy range 1 keV
to 100 MeV, comprising a housing having walls, a top cover and a
bottom and enclosing a gas; a plurality of integrated converter and
amplifier structures arranged in a stack in said housing; a charge
collection device arranged at the bottom of the housing; connection
means comprising a variable resistor chain coupled in series across
the entire detector, connectable to an external voltage source;
electronic read out devices coupled to said charge collection
device for providing an input to a control unit; wherein said
integrated converter and amplifier structures each comprise a
central metal layer on both sides of which there is provided an
insulating layer, and wherein the respective insulating layers each
are provided with a metal film, wherein the central metal layer is
thicker than each insulating layer, and wherein each insulating
layer is thicker than each metal film, and wherein said composite
dipole layered structure is perforated with holes; said holes
having an entrance opening for electrons and an exit opening
wherein said exit opening is wider than said entrance opening.
Description
[0001] The present invention relates to a general X-ray and
electron imaging device, particularly useful for verification,
control and optimization of radiation treatment of cancer as well
as for applications like diagnostic X-rays, non-destructive testing
and screening of containers and vehicles in airports and in
customs. More particularly it relates to a detector system with
high efficiency over a wide range of photon and electron energies,
from diagnostic X-rays starting from the low energies of a few keV
all the way up to a hundred MeV, i.e. energies that are of interest
and used in radiation therapy or for imaging of large and/or dense
objects.
BACKGROUND OF THE INVENTION
[0002] Real time electronic detectors have during the last 30 years
revolutionized many areas of X-ray imaging. This includes
diagnostic modalities like computed tomography for detailed imaging
of the human head and body as well as image intensifiers and video
techniques for imaging of the cardiovascular system and for airport
security. There are several advantages with real time electronic
detectors including improved detection efficiency, wider dynamic
range and instantaneous response. Digital images also allow
immediate display, electronic storage, diagnosis through
telecommunication and computer-aided detection, on-one image
enhancement and diagnosis. In spite of the obvious advantages with
digital imaging it has turned out to be very hard to replace
current film-screen combinations in applications demanding high
spatial resolution over large areas, in particular when constraints
like high tolerance to radiation damage and reasonable cost are
added. Despite its advantages film has a number of disadvantages
such as low efficiency, limited dynamic range, noise and the need
for chemical development.
[0003] The working principle of the present range of electronic
detectors is that photons transmitted through the irradiated object
are converted to electrons through electromagnetic interactions.
Those electrons are in some devices collected directly by dedicated
sensors or they are guided through some fluorescent material where
secondary light is created and this light is in turn detected by a
sensor like e.g. a CCD. In imaging devices for higher X-ray
energies, a special converter is added in front of the detector to
increase the probability for electromagnetic interaction of the
X-rays. This is needed to increase the efficiency of the devices
since higher energy X-rays are much more penetrating and would
otherwise pass the detector undetected. The converter is usually
made as a thin plate of some heavy metal like copper or iron, but
molybdenum, chromiun or tungsten are equally suitable. In principle
any material could be used, but the efficiency of the device will
increase with increasing atomic number. Thus, an atomic number
greater than 20 is preferable.
[0004] For the purpose of this application the term
"electromagnetic interactions" should be taken to encompass all
physical interactions between photons and matter that causes
generation of at least an electron, i.e. via Compton effect,
pair-production or photo electric effect.
[0005] The term "conversion" is meant to encompass any process
involving a photon, wherein some or all of the energy of that
photon is transferred to some other corpuscle and wherein a free
electron is produced as a result of said energy transfer. Thus, a
"converter" is any device capable of producing this effect. It
could simply be a gas enclosed in a volume, wherein incident
photons interact with the gas in the photo-electric effect to
produce electrons. It can also be a sheet or other type of
structure of a solid material, in which electrons are generated via
the Compton effect or by pair production (electron--positron
generation).
[0006] "Amplification" is to be construed as a process where one
electron interacts with atoms or molecules of a gas thereby causing
ionization thereof to produce a plurality of electrons and "holes"
(positive gas ions). Thus, "amplification" is meant to encompass
both primary ionization regardless of whether there is an electric
field present or not, as well as the well known avalanche fenomenon
that occurs in electric fields of the order of 10.sup.4 V/m or
more.
[0007] Thus, an "amplifier" will encompass any structure that
causes such "amplification"; it could e.g. simply be a gas enclosed
in a volume where incident electrons will interact with the gas, or
a more complex device where an electric field is generated.
[0008] Radiation therapy and surgery remain the main modalities for
cancer cure in the industrialized world. Radiation therapy is used
for more than half of the new cancers with permanent eradication of
the tumor without severe complications in more than half of the
cases. The radiation dose is delivered to the patient in different
fractions, one fraction a day over a period of a couple of weeks.
Alignment of the radiation field relative to the tumor is of
paramount importance. The alignment has to be particularly accurate
when intensity modulation is used and the tumor is close to
sensitive organs like the spinal cord. Positioning errors should by
no means exceed 2 to 5 mm depending on treatment site. Monitoring
and controlling the treatment with a detector behind the patient is
usually referred to as portal imaging. More recently, it has been
shown that a correction of the patient set-up using the information
from an Electronic Portal Imaging Device (EPID) increases the
probability of a complication free tumor cure in the order of 10%.
However, as already indicated, film still remains the most common
tool for verification and quality control of the treatment and is
used in more than 90% of the cases. The EPID's has proven valuable
since digital images allow electronic storage and processing of the
data. They also in principle enable an on-line control and
verification of the treatment even if this is difficult because of
the low efficiency of the present EPID's and the corresponding
relatively long times for data acquisition. They also facilitate an
adaptive real time control during the course of delivery of the
different fractions of the treatment. In portal imaging, it is
obvious that the detectors need to be highly radiation tolerant and
this is a severe constraint one has to take into account when
designing the detector.
[0009] There are two main types of EPID systems available
commercially today: One is a mirror-based video system and the
second is an electronically scanned liquid-ionization chamber
system. In both cases, the incident photons are converted to
electrons with an efficiency of about 5%-8% through interactions in
a metal plate, typically 1.5 mm of copper. If the metal is made
thicker, scattering of the electrons in secondary reactions is
becoming a problem and electrons will stop in the metal. The
typical range for 1 MeV electrons in Cu is less than 0.7 mm. This
range is approximately proportional to the energy of the electrons.
This puts a fundamental limit on the obtainable efficiency for
these devices. Both approaches have proven valuable in localizing
the patient in the radiation field and verification of the
radiation therapy. A major drawback is that the contrast and
quality of the resulting images only makes the bone structure
visible and not internal organs and the tumor itself, the exact
position of these organs remains unknown. The only way of being
sure about these positions would be diagnostic X-ray images taken
with the patient in the actual treatment position, without movement
of the patient, and right before the actual treatment starts since
any movement would cause change in position of the internal organs.
Unfortunately existing EPID's are almost insensitive to X-rays of
diagnostic energies.
[0010] The main specific drawback with the video system is its low
efficiency due to loss of photons in the process of de-magnifying
the fluorescent screen through a mirror, lens or fiber optic taper
to the camera. This efficiency is in fact less than 0.01%. Another
problem is the inherent bulkiness of the system that may hamper
patient set-up and make them difficult to use in machines with beam
stoppers to stop the radiation beam after passing the patient.
[0011] In the liquid-ionization chamber the pixels are scanned by a
switched high voltage one row at a time and the currents from the
pixels are read out by a row of 256 electrometers, the whole
detector consist of an array of 256.times.256 pixels with a spacing
of 1.27 mm. This generates a current of typically 50 pA and the
noise is around 0.5 pA. The liquid is integrating the created
charge for around 0.5 s and it takes around 5 s to get an image.
The drop in efficiency due to the scanning is thus a factor 10.
Limitations are long-time stability of the ultra-clean liquid and
pick-up due to the high-voltage switching.
[0012] The most promising emerging EPID seems to be amorphous
silicon arrays. They have been developed since around 1990 but are
not yet a commercial product. Advantages compared to the video
system are much better optical coupling (around 50%) between the
fluorescent screen and the detector since the array is positioned
in close proximity to the screen and there is no demagnification.
Each pixel is controlled by an a-Si transistor, one row of pixels
is gated at a time, and the accumulated charge is amplified by a
row of preamplifiers and digitized by a 12 bit ADC. Amorphous
silicon has the advantage that it can be deposited over large areas
but is not ideal for fabrication of transistors; the ON resistance
usually exceeds mega-ohms and this slows down the readout of the
charge. In spite of enormous investments from the flat-panel
display industry it is not trivial to manufacture large arrays
without defects and the cost for a large instrumented a-Si array
for X-ray imaging (.about.25.times.25 cm.sup.2 size) is very high.
The efficiency is also for this device limited by the fact that
only 6%-8% of the incident photons interacts at all in the
detector.
[0013] The trend in radiation therapy is towards conformal
intensity modulated treatments and hyperfractionation that reduces
the dose per treatment field. This increases the demands on the
EPID in terms of efficiency, high quality image for aligrunent
checks should be obtained at dose levels of 0.01 Gy corresponding
to an image acquisition time of 0.25 s at a dose rate of 2 Gy per
minute. For a total dose for the field of 1 Gy this means the
treatment maybe aborted at radiation levels of less than 1% of the
single field dose in case of misalignment. The intended set-up may
be documented through either a simulator or a digitally
reconstructed radiograph (DRR), which has been reconstructed for a
certain beam set-up using computed tomography. Potentially this
will enable computer-aided on-line detection of misalignments of
the radiation field.
[0014] If one compares the EPID to for example an upgrade in
accelerator equipment for the treatment unit the cost for an EPID
would be less than 0.15M$ while a new accelerator would cost about
2M$. Since a portal imager would have very significant impact on
estimated benefits for the patient in terms of increased
probability of eradicating the tumor, it is in reality a very
cost-effective device compared to other investments. If the effect
on the outcome of the treatment is 10%, this corresponds to about
1,5 million more patients saved in the US per year.
SUMMARY OF THE INVENTION
[0015] Thus, there is still a strong need in this field for a
detection means that allows an adaptive real time control during
the course of delivery of radiation during treatment. In addition,
it would be advantageous if the same detection system could be used
for both low and high-energy photons, such that for quality control
purposes in medical care, a high quality image could be obtained
before therapeutic irradiation begins. Furthermore, it would be
advantageous if there need be no physical shift or replacement of
the detection unit between high and low energy detection, i.e. the
detector units should not need to be moved or damaged due to
exposure to high energy radiation.
[0016] These objects are achieved with a device, method and system
as defined in the appended claims.
[0017] In particular the present invention in a preferred
embodiment concerns detectors comprising a plurality of amplifier
and converter stages.
[0018] The spatial resolution is determined by the pixel pitch,
which will be around 1 mm in the prototype detector, but could be
taylored to suit the application in question. This is not a very
competitive resolution for diagnostic medical imaging but is
sufficiently high for portal imaging. The portal imager according
to the invention will also be used as a detector for diagnostic
X-rays. It may not be the optimum detector for this task but it
will provide valuable additional high-contrast images to correct
for internal displacements of sensitive organs as well as the
target with the patient in the actual treatment position. To use
separate X-ray detectors for all these tasks is impractical. With
some modifications the system can also be used for precision
dosimetry and current mapping of therapeutic radiation fields. It
can thus be used to optimize the dose delivery with different
radiation treatment units and techniques.
[0019] A particular advantage with a preferred embodiment of the
detection system according to the present invention is that it
allows the contrast of images produced to be optimized to a high
degree and also makes it possible to determine the elemental
composition of different parts of the object. This is achieved by
the provision of gain control for each individual amplifier in the
stack, whereby detection of photons can be discriminated between
high and low energies.
[0020] Further advantages with the invention are:
[0021] 1) Possibility of an order of magnitude higher efficiency
compared to present detectors for high energy X-rays due to use of
multiple conversion layers in combination with efficient collection
of the signals from each conversion layer. The integral signals
from all the individual layers are detected by one single matrix of
sensors.
[0022] 2) A high signal to noise ratio due to the amplification of
the signal in the gas.
[0023] 3) High radiation resistant since no active electronics need
to be directly exposed to the beam, if desired.
[0024] 4) Very fast parallel read-out enabling acquisition of the
whole image matrix in less than 3 ms, if desired.
[0025] 5) The energy response of the detector can be changed simply
by altering the potential on the different electrodes.
[0026] 6) Rugged design where the amplification is geometrically
stable but adjustable.
[0027] 7) Highly efficient over a wide range of energies. This
enables the combination of an detector for diagnostic and
therapeutic X-rays in one single device by using a thin entrance
window and gas volume on top of the first converter layer. If a
diagnostic X-ray tube is inserted above the patient a high contrast
diagnostic X-ray image could be obtained right before the treatment
starts and thus the exact position of any organs and the tumor
itself could be determined.
[0028] 8) Energy sensitive if desired. This makes it possible to
optimize the contrast for any given imaging task and also opens the
possibility to determine the elemental composition of the object.
This energy sensitivity also enables dual-energy imaging in the
sense that it is possible to determine not only the X-ray
attenuation in the object but also the different elements the
object consist of by comparing images with different weighting of
low and high energy X-rays.
[0029] 9) The invention also offers the possibility to weight the
information from X-rays of different energies in such a way that
the contrast in the resulting image is optimized for the object of
interest.
BRIEF DESCRIPTION OF THE DRAWINGS
[0030] FIG. 1a is a perpective view of a basic embodiment of a
detecor according to the invention;
[0031] FIG. 1b is a schematic top view of a charge collector
according to the invention;
[0032] FIG. 1c is a schematic side view of a stack of converters on
top of a charge collector;
[0033] FIG. 2a is a schematic view of a GEM structure suitable for
use with the invention;
[0034] FIG. 2b shows equipotential lines for a biased structure as
shown in FIG. 2a;
[0035] FIG. 3 is a schematic overview of a preferred embodiment of
a detector unit according to the invention;
[0036] FIG. 4 is a perspective view of an embodiment of the
detector comprising a mesh structure;
[0037] FIG. 5 is a perspective view of a charge collector according
to the invention;
[0038] FIG. 6 schematically shows a set-up for radiation
therapy;
[0039] FIG. 7a illustrates an alternative structure for a further
embodiment of the detector according to the invention;
[0040] FIG. 7b shows schematically equipotential lines for biased
structure of FIG. 7a;
[0041] FIG. 8 illustrates schematically a set-up for cargo
screening; and
[0042] FIG. 9 is a schematic overview of still another preferred
embodiment of a detector unit according to the invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0043] The basic principle behind the invention will now be
described with reference to FIGS. 1a and b showing the most general
embodiment of a detector according to the invention.
[0044] This figure shows an X-ray to e.sup.- converter 100 in the
form of a sheet 102 of a heavy metal, e.g. Cu, about 1.5 mm thick.
Below this converter there is an air gap 104 of about 1 mm, and at
the bottom there is a multi-layer PCB 106 (Printed Circuit Board)
(described more in detail with reference to FIG. 5 below). Briefly
it comprises pads 107b, 1 mm.sup.2, distributed over the surface of
a polymer board, and connected to ASIC inputs 108 positioned at the
edges of the PCB (see FIG. 1b).
[0045] X-rays on the converter sheet 102 will generate electrons in
the metal, exiting into the air gap 104. The negatively charged
electrons will ionize the gas, and the electron signal will be
amplified through an avalanche in the gas if the field in the gap
104 is 10.sup.4 V/m or higher. The secondary electrons will be
collected on the electrode pads at ground potential. If there is a
gas volume above the converter sheet 102 and the top cover (not
shown) is made essentially transparent to diagnostic X-ray
energies, the device can be used as a detector for diagnostic
purposes too. This requires that the converter 102 be perforated
such that the electrons can be drifted through the holes 110.
[0046] If several converters are stacked the efficiency will be
increased. This is shown schematically in FIG. 1c. By applying an
appropriate voltage across the stack and by selecting the thickness
and hole diameter appropriately for the converter sheets, it is
possible to obtain avalanche amplification in the holes 110 in the
perforated converter sheets 102.
[0047] Another way of achieving amplification is to provide a
separate amplifier device. Several such devices are known, and one
type is designated Gas Electron Multipliers (GEM).
[0048] Referring now to FIG. 2a there is shown the main component
of such a GEM, generally designated 200. It is a thin composite
mesh 202 acting as proportional avalanche amplifier in gas counters
(U.S. patent pending, Sauli et al).
[0049] The mesh consists of a thin insulating foil 204, e.g. of
Kapton.RTM., which is metal-clad 206, 208 on both sides, a suitable
metal being Cu, and perforated by a regular matrix of holes 210.
The holes may be 100 .mu.m wide. The insulating foil 204 may be 50
.mu.m thick; the thickness of the metal cladding 206, 208 being 5
.mu.m. This structure is located in a confinement containing a gas.
This gas, when exposed to ionizing radiation of some kind, will
dissociate into electrons with negative charge and corresponding
ions with positive charge. If a potential difference (typically 500
V) is applied across the insulator/between the two metal clad sides
of the composite mesh 202 structure, a dipole field F will develop
in the holes 210, see FIG. 2b (the lines shown are equipotential
lines, and thus the filed lines are perpendicular to these lines).
Electrons released by the ionization in the gas will drift towards
the high field through the holes/channels 210, and will be focussed
therein. The focussed electrons will then be amplified through
avalanche multiplication of the electrons in the high electric
field region. The amplified signal of electrons could be detected
by e.g. a Multi-Wire Proportional Chamber (MWPC), a Micro Strip Gas
Chamber (MSGC) or a Printed Circuit Board (PCB). With a device like
this amplification factors above 10000 have been reached. It is
also known to combine two GEM's by arranging them in a cascade at
some distance, or in electrical contact.
[0050] Devices of this kind were originally developed for the
detection of ionizing radiation in high-energy physics experiments.
However, as indicated previously, these known devices are also
suitable for detecting X-rays of diagnostic energies up to around
100 keV through conversions in the gas. At higher energies, the
probability for the X-rays to interact in the gas will decrease and
the efficiency will drop towards zero.
[0051] Turning now to FIG. 3 there is shown schematically a
preferred embodiment of the detector according to the invention
generally designated 300.
[0052] It comprises a stack of alternating converters 302 and
amplifiers 304. The entire stack is located inside a housing (not
shown) containing a suitable gas, e.g. Xe, although there is a
large spectrum of possible gases to choose from like other noble
gases such as e.g. Ar, Ne. Also mixtures of gases are conceivable,
in particular it is a standard technique to mix in a so called
quencher that will make the avalanches more controllable and make
the detector less prone to sparks and discharges. Examples would be
CO.sub.2 or dimethylalcohol (DME). The gas or gas mixture may or
may not be pressurized, or it could be provided at sub-atmospheric
pressure. The higher the gas pressure, the more ionizations in the
gas will take place per unit path length of a charged particle such
as e.g. an electron.
[0053] The top cover 306 of the housing is preferably thin and
light to maximize the number of low energy diagnostic X-rays
reaching the gas volume at the top, i.e. as many of the X-ray
photons as possible should be transmitted therethrough. A suitable
material would be thin metal foil, e.g. Al, or the like. Other
possible materials are polymers of various kinds, e.g. MYLAR.RTM..
Generally speaking materials with low atomic numbers are suitable.
An at present preferred embodiment of the device according to the
invention comprises as an amplifier device the above mentioned GEM
(Gas Electron Multiplier) (for simplicity the perforations of
converters and amplifiers have been left out). At present it is
believed that the use of GEM's is the best mode of operating the
invention.
[0054] The other component, the converter 302, comprises a sheet of
material having the ability to convert the incident photons into
electrons through electromagnetic interactions. Preferably, a
material with high cross-section for this reaction will be used.
The converters have been positioned below each said GEM type
structure. However the uppermost layer is an amplifier, for
amplifing signals generated in the top gas volume 308, just below
the top cover 306. In this volume low energy photons (diagnostic
X-rays) will react and generate electrons.
[0055] The converter sheet 302 made of heavy metal is perforated
(not shown in this figure), which are aligned with the holes (not
shown) in said GEM structure. The converter sheet is preferably 0.1
mm to 1 mm thick depending on the field of application of the
invention. The sheets may also be progressively thicker towards the
bottom of the stack compared to the sheets in the top in order to
match the higher occurrence of lower energy X-rays in the top
layers relative to the bottom layers.
[0056] These sheets are referred to as converter layers since the
photons impinging in such a sheet will create charged particles
(electrons and positrons) and the forward momentum of the photons
will be transferred to the electrons, such that electrons will exit
from the sheet into the gas volume 308 beneath the top cover 306.
There the electrons will cause ionization of the gas and give rise
to the order of 10 electron-ion pairs on average. Those electrons
will be collected and multiplied in the first amplification stage
304a. As indicated above, the amplifier in the structure may
consist of said GEM type structure. The metal layers of the GEM's
are biased with a voltage of typically 500 V each. However, the
voltage is variable in certain ranges such as from negative voltage
(if you want to block the signal from above) up to 1000 V. By
controlling the voltage certain beneficial effects are obtainable,
which will be described below.
[0057] Each amplifier layer can be individually biased at a desired
voltage/potential difference across the insulator with a suitable
voltage source 310. This could most easily be provided by arranging
a series of variable resistors 312, such that the voltage across
each amplifier layer 304 simply is adjusted by adjusting the series
resistance associated with that particular layer. Thus, each
amplifier stage is coupled in a way such that it can be set at the
desired potential by a common external voltage source. In the shown
embodiment there is a resistor chain 312a, 312b, . . . 312o,
coupled in series across the entire stack. The means for
controlling the voltage across each amplifier is not critical and
any other means that achieves the same result is usable within the
scope of the invention. For example, each layer could of course be
connected to an individual voltage source. In the shown example the
external voltage across the entire stack is about 6000 V, and the
voltage across each amplifier layer may be adjusted to between 0
and 1000 V.
[0058] For the top amplifier layer 304a it is contemplated that the
voltage bias for special purposes may be set to be reversed, i.e.
in some cases the potential difference is set such that electrons
scattered from the patient or other object and containing no
relevant information, will not contribute to the signal.
[0059] GEM's would be only one possibility to achieve the desired
charge collection and amplification that is needed in combination
with the converter layers.
[0060] Another means of obtaining the desired charge collection and
amplification would be to combine the converter layers (perforated
with holes) with wire meshes 402 (see FIG. 4). The potential
difference between the wire mesh 402 and the converter should be
high enough to start avalanche multiplication.
[0061] This double layer structure of "converter and amplifier" can
be repeated a number of times, where a practical number could be
5-8, but could also be higher, or lower, for specific
applications.
[0062] At the bottom of the stack of converter and amplifier layers
there is a Printed Circuit Board (PCB) 314 for collecting charge
and which is coupled to read-out electronics 316 that provide data
to a control unit 318, e.g. a computer. The signals can be routed
to the edge of the board through state of the art multi-layer PCB's
(see FIG. 5 which is a schematic illustration of a charge collector
according to the invention comprising a PCB), and the electronics
sensitive to the radiation (such as diodes, transistors or other
semiconductor devices) can thus, in accordance with the invention,
be positioned at the edge outside the radiation field and can even
be shielded to be protected against scattered radiation. As
indicated such PCB's per se are state of the art and in FIG. 5 a
PCB based charge collector 500 is shown in a perspective view.
[0063] It may comprise 20 layers of an insulator such as FR4.
Between each layer there are metal conductors provided, each of
which are connected to one charge collection pad 502 each through
holes, metal plated on the inner circumference, said conductors
extending towards the edges of the board. The pads are preferably
made of Cu, Au or Al, although other metals are conceivable, and
are about 0.8 mm wide and about 5 .mu.m thick. They are made using
conventional photolithografic techniques well known in the art. The
pads are distributed so as to correspond to the geometry of the
holes in the converter and amplifier matrices, i.e a center to
center distance of about 1 mm.
[0064] The most important feature of the PCB is that there is a
large number of pads 502 distributed over the board. Each pad must
have its own connection to an input of e.g. an ASIC 504. If, as
preferred, the ASIC's are mounted at the edges of the board, the
PCB is made in a layered structure, wherein the leads connecting
the pads with the electronics are drawn in respective layers. The
optimum design of such a PCB is made by so called auto-routing, a
standard technique well known to the skilled man in the field of
printed circuit board design. Suitable software for auto-routing
can be obtained from Cadence, under the trade name SPECTRA.
[0065] Other types of charge collecting means are conceivable in
less radiation intense environments, e.g. an ordinary CCD. Most
likely different tools to achieve the charge collection and
avalanche amplification will be used for different applications,
the essential idea according to the invention being the mixing of
layers of converters with layers for amplification and charge
collection means to achieve an efficient detector for high energy
X-rays.
[0066] Instead of collecting the electrons with pads on a printed
circuit board and measure the charge for each pads as the signal
proportional to the number of x-ray photons (or fluence) in one
pixel there are other possibilities to read out the signal for each
pixel. One way is to convert the electrons to visible photons at
the bottom of the detector and have the bottom transparent to those
photons.
[0067] In an ordinary avalanche in the gas there is at least as
many photons produced as electrons and thus the photons may be
produced in an amplifier at the bottom of the detector. Another way
of producing the photons is to have the electrons incident on a
fluorescent screen at the bottom of the detector where the
electrons will induce emission of photons in this screen The
photons created in any of these ways will in turn be detected by a
sensor sensitive to photons such as for example a CCD. The photons
may be guided to the CCD sensor through suitable optics such as
mirrors and lenses.
[0068] In FIG. 6, there is shown a schematic of a set-up 600 for
radiation therapy with external X-ray photons or electrons. An
accelerator (not shown) is situated in a neighboring room, and a
beam of electrons B is directed through collimators towards the
patient's body. If X-ray photons are desirable, a suitable target
602 is placed in the electron beam at appropriate location. If
electrons are desired of course, no target is used. The beam
(X-rays or electrons) is swept with a modulated intensity. A portal
imager 604 according to the invention may be positioned either
directly below the patient or below the treatment coach, as
indicated in the figure. Also, there is provided a complementary
diagnostic X-ray tube 606, which is positioned above the patient.
When the patient is exposed to diagnostic X-rays, the portal
imaging device will function as a detector for diagnostic X-rays in
the 50 keV range.
[0069] The function of a portal imaging device according to the
embodiment shown in FIG. 3 will now be described with reference to
the set-up of FIG. 6 and to FIG. 3. We assume operation with
photons, i.e. X-rays. Thus, for the production of therapeutic
radiation, a linear accelerator is provided. It produces electrons
in the energy range 1-50 MeV. These electrons are directed via
suitable optics into the space where a patient to be treated is
placed. A target (e.g. Be) is positioned such that the electrons
impinging thereon produces X-ray photons, which are collimated and
directed to the area on the patient's body where therapy and/or
diagnosis is to be performed.
[0070] For diagnostic purposes, a standard X-ray tube positioned
above the patient is used. The energy of such photons is around 50
keV. The photons exiting beneath the patient will impinge on the
gas volume 308 at the top of the detector unit, comprising the
stack of amplifiers 304 and converters 302. A significant fraction
of the diagnostic X-rays will interact in this gas volume 308,
mainly through the photoelectric effect. The ionization in form of
electrons created by the photoelectron will be collected and
amplified by the uppermost amplification structure 304a. Almost all
diagnostic X-rays remaining after passing the gas volume 308 will
be stopped in the first converter 302 (intended for high energy
X-rays) and will create a negligible amount of detectable
ionization. This first diagnostic image can be used for aligning
the patient appropriately, e.g. if there should any risk of
sensitive tissues being exposed to the highly energetic therapeutic
X-rays that are to follow.
[0071] In the following moment the radiation therapy beam will be
turned on, if there is no need to correct the position of the
patient based on the information from the diagnostic X-ray image.
The gas volume 308 will be more or less transparent to those
high-energy X-rays. Since the gas volume 308 may still get hit by
scattered electrons from the patient that does not contain much
image information, it may be preferable to e.g. put the drift field
to zero in the gas volume 308 to get rid of this background noise.
The majority of these electrons will stop in the first converter
layer 302a. The high-energy photons will penetrate into the stack
and the photons with lower energy will predominantly convert in the
top layers 302a, b, c while the photons with relatively higher
energy will dominate in the bottom layers 302n, n-1, n-2. Secondary
Compton photons will also penetrate down the stack and depending on
the angle through which they are deflected, this will smear the
position resolution as you go down in the stack. Since the contrast
is higher for low energy photons the information content will
presumably be higher in the top layers. By tuning the voltage
determining the amplification for each layer, it is possible to
weight the contribution from the different layers to obtain a
maximum contrast. It is also possible by way of comparing two
images where higher and lower energy photons are weighted
differently to roughly estimate the energy of the incident X-rays.
From this estimate it is possible to deduce an elemental
composition of the object. This may in particular be of value for
non-medical applications, e.g. when looking for explosives in
screening for air port security.
[0072] The attenuation of the x-rays in the object will mainly
depend on the density (rho) and the atomic number (Z) of the
object. Those two quantities are usually not possible to
distinguish from each other, but since the dependence for each
quantity is different as a function of x-ray energy this is
actually possible with a detector that gives an estimate of the
energy of the incident x-rays. As an example one could imagine
first obtaining an image with the photons converting in the first
half of the stack of converters/amplifiers weighted much higher
comparing to photons converting in the second half. This will be an
image predominantly made up of lower energy photons. Secondly one
could register an image with the photons converting in the second
half of the stack of converters/amplifiers. This will be an image
consisting predominantly of higher energy photons. The number of
x-ray photons detected in the two images will depend on the
elemental composition of the object. This difference can be
measured for different test objects of known composition. It is
thus possible to calibrate the expected response for different
materials. One can also compare the measurements to computer
simulations of the spectrum and record images with one multiple
weightings of the x-ray spectrum and compare the results to get a
more detailed estimate of the elemental composition.
[0073] In FIGS. 7a-b a preferred embodiment of an
amplifier/converter structure according to the present invention is
disclosed, generally designated 700.
[0074] FIG. 7a is a cross section of one perforated sheet 700,
forming a composite layered dipole structure, comprising holes 702
through which electrons may move. The sheet is comprised of a first
(or top) metal layer 704, about 5 .mu.m thick. This metal layer 704
has been deposited on an insulating material forming a first
insulating layer 706, similar to the known GEM structure described
above with reference to FIG. 2. Underneath the insulating layer 706
there is a thick metal layer 708, which is at least one order of
magnitude thicker than the top metal layer, in the shown embodiment
it is 150 .mu.m thick. Below the thick metal layer 708 there is a
second insulating layer 710, on which there is deposited a second
(bottom) metal layer 712. The layers 710 and 712 preferably have
the same compositions and thicknesses as the layers 704 and 706.
The entire sheet is made by suitable known depostion methods.
[0075] When the composite sheet has been made, the holes 702 are
made by etching. Being an unisotropic process, the etching will
have the effect of creating "funnel" like holes, as is clearly
shown in FIG. 7a. Thus, the "entrance" opening 714 for electons has
a smaller diameter that the "exit" opening 716. The actual slopes
of the inner walls of the holes 702 is not necessarily as shown in
the figure, but will vary depending on the materials selected for
the layers and on the particular etching process employed. The
known GEM structure shown in FIG. 2, having a much thinner overall
thickness does not exhibit such outspoken funnel-like holes. This
particular structure with such "funnel"-like holes 702 has certain
benefits for the purpose of the invention, which will be described
below.
[0076] The structure of FIG. 7a will function as a composite
amplifier/converter. Thus, as shown schematically in FIG. 7b, which
is an image obtained by simulation, a voltage is applied across the
entire structure such that the first (top) metal layer 704 is at
approximately +300 V, the thick converter layer 708 is at
approximately 100 V, and the second (bottom) metal layer 712 is at
-100 V. Of course these values can vary within relatively wide
limits depending on where in the stack the actual structure is
situated, and what one wants to achieve in the structure in
question. The lines are equipotential lines, and thus the field has
a direction perpendicular to the potential curves. As can be seen
in FIG. 7b there will be an electric field inside the holes 702,
the density of which is highest in the upper part of the holes 702,
i.e. in the region of the first insulating layer 706. By virtue of
the holes "flaring" out downfield, electrons passing through the
holes 702 will have less probability of diffusing into the thick
metal layer 708, and thereby the efficiency of the structure
becomes higher. The bottom part of the structure comprising
insulating layer 710 and bottom metal layer 712, does not function
as an amplifier in the sense of the corresponding structure of
layers 704 and 706. Rather the function is to provide enhanced
guiding of the electrons out from the holes 702 and to further
prevent the potential diffusion of electrons into the converter
metal layer 708. Of course by suitable selection of voltage applied
also the lower part of the structure could be used for
amplification purposes.
[0077] These composite dipole layered structures 700 may be
arranged in the same way as shown in FIG. 3, just substituting the
alternating converters 302n and amplifiers 304n for such a
composite structure 700. A set-up for use of the embodiment of the
structure shown in FIG. 7 is disclosed in FIG. 9 and generally
designated 900.
[0078] Thus, a plurality of composite layered structures 902a-d,
904a-d is stacked above each other enclosed in a housing (not
shown). A top cover 906 is provided and a gas volume 908 is formed
between the top cover and the first composite structure 902a, 904a.
At the bottom of the stack there is provided a PCB 914 having
read-out electrinocs 916 connected thereto, which in turn are
coupled to a control unit.
[0079] The composite structures can be individually biased at a
desired voltage/potential difference across the insulator with a
suitable voltage source 910, similar to the embodiment in FIG. 3.
This could most easily be provided by arranging a series of
variable resistors 912a-o, such that the voltage across each
amplifier 904 in the composite structure simply is adjusted by
adjusting the series resistance associated with that particular
layer. Thus, each amplifier stage is coupled in a way such that it
can be set at the desired potential by a common external voltage
source. In the shown embodiment there is a resistor chain 912a,
912b. . . . 912o, coupled in series across the entire stack. The
means for controlling the voltage across each amplifier is not
critical and any other means that achieves the same result is
usable within the scope of the invention. For example, each layer
could of course be connected to an individual voltage source. In
the shown example the external voltage across the entire stack is
about 6000 V, and the voltage across each amplifier layer may be
adjusted to between 0 and 1000 V.
[0080] In FIG. 8 an application of the invention for cargo
screening is shown schematically in a top view. The set-up
comprises a source of X-rays 800, in the energy range up to 50 MeV,
e.g. a linear accelerator. Collimators 802 are provided for
collimating the X-rays. There is provided an arrangement for
scanning/sweeping the beam over a relatively large area, indicated
schematically by arrows S. Furthermore there is provided a detector
804 according to the invention, arranged such there will be a space
806 between the radiation source 800, 802 and the detector 804,
large enough for a large object, such as a lorry 808 to be
positioned therebetween. Because the detector can be made only in a
limited size, there is provided means for moving the detector in an
X-Y plane such that when the radiation beam has scanned one area
corresponding to the size of the detector, it can be moved so as to
cover a previously unscanned area (the movement of the detector is
indicated by arrows M). In operation of the system a large cargo
carrying object, such as a container, truck, trailer etc, will be
positioned in the space 806 between an X-ray source and a detector
according to the invention. The radiation will be turned on and the
interior of the object can be checked for its content, in
relatively short time. As an alternative to move the detector, the
entire object could be moved. A truck or other vehicle could e.g.
be moved by its own engine.
[0081] The invention having thus been described, it should be
understood that various modifications can be made without departing
from the inventive concept, which is defined by the appended
claims.
[0082] For example, the detector can be used to study dose
distributions. This can be done in air or in a so called water
phantom, which is a simulation of body tissue. For such
applications the material in the detectors are selected to mimic
body tissue, i.e. polymers having carbon and nitrogen contents
similar to that of living tissue.
[0083] Also, it is conceivable to use the device for imaging during
electron therapy. In such a case of course the converter would not
be operable since the therapeutic electrons themselves are
detected.
[0084] It is also to be understood that the detector may be
optimized by a skilled man for other types of particles, such as
neutrons, protons, atomic nuclei of various kinds etc.
* * * * *