U.S. patent application number 09/815433 was filed with the patent office on 2001-10-25 for flat panel x-ray imager with gain layer.
This patent application is currently assigned to Hologic, Inc.. Invention is credited to Cheung, Lawrence K.F., Lee, Denny L.Y., Smith, Andrew P..
Application Number | 20010032934 09/815433 |
Document ID | / |
Family ID | 26887561 |
Filed Date | 2001-10-25 |
United States Patent
Application |
20010032934 |
Kind Code |
A1 |
Lee, Denny L.Y. ; et
al. |
October 25, 2001 |
Flat panel x-ray imager with gain layer
Abstract
A flat panel x-ray imager includes a gain layer (charge
multiplication layer) that facilitates imaging at low x-ray
exposure levels. The gain layer can be a gas chamber or a solid
state material operating in an avalanche mode.
Inventors: |
Lee, Denny L.Y.; (West
Chester, PA) ; Cheung, Lawrence K.F.; (Wilmington,
DE) ; Smith, Andrew P.; (Medford, MA) |
Correspondence
Address: |
Ivan S. Kavrukov
Cooper & Dunham LLP
1185 Avenue of the Americas
New York
NY
10036
US
|
Assignee: |
Hologic, Inc.
|
Family ID: |
26887561 |
Appl. No.: |
09/815433 |
Filed: |
March 22, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60191943 |
Mar 24, 2000 |
|
|
|
Current U.S.
Class: |
250/370.09 ;
257/E27.146 |
Current CPC
Class: |
G01T 1/2935 20130101;
H01L 27/14676 20130101; G01T 1/248 20130101 |
Class at
Publication: |
250/370.09 |
International
Class: |
G01T 001/24; H01L
025/00; H01L 027/00 |
Claims
1. A flat panel x-ray imager comprising: a charge generator layer
locally generating electrical charges in response to x-ray
exposure; a gain layer receiving electrical charges generated in
the charge generator layer and locally multiplying received
charges; and a charge collector locally collecting electrical
charges multiplied by the gain layer to produce electrical signals
related to local x-ray exposure of the charge generator layer.
2. A flat panel x-ray imager as in claim 1 including a charge
emission layer interposed between the charge generator layer and
the gain layer.
3. A flat panel x-ray imager as in claim 1 in which the gain layer
comprises a gas-filled chamber.
4. A flat panel x-ray imager as in claim 3 in which the gas filled
chamber is filled with a mixture of approximately 90% argon and 10%
methane.
5. A flat panel x-ray imager as in claim 1 including a biasing
arrangement producing an electrical field in the charge generator
layer and in the gain layer, and wherein the field is higher in the
gain layer than in the charge generator layer.
6. A flat panel x-ray imager as in claim 1 in which the gain layer
comprises a solid state material.
7. A flat panel x-ray imager as in claim 6 in which the gain layer
comprises a solid state material operating in an avalanche
mode.
8. A flat panel x-ray imager as in claim 1 in which the charge
generator layer comprises a scintillator layer and an electron
emission layer.
9. A flat panel x-ray imager as in claim 1 in which the charge
generating layer comprises one or more of amorphous selenium,
PbI.sub.2, CdTe, CdZnTe, TIBr, HgI.sub.2, silicon, germanium,
PbO.sub.2, and doping materials.
10. A flat panel x-ray imager as in claim 1 in which the gain layer
comprises a microchannel plate.
11. A flat panel x-ray imager as in claim 1 in which the gain layer
comprises a chamber filled with a substance comprising xenon.
12. A flat panel x-ray imager as in claim 11 in which the xenon is
in liquid form.
13. A method of imaging x-rays comprising: receiving x-rays at a
charge generator layer locally generating electrical charges in
response to x-ray exposure; locally receiving at a gain layer
electrical charges generated by the charge generating layer and
locally multiplying charges received at the gain layer to thereby
produce locally multiplied electrical charges; and locally
collecting electrical charges multiplied by the gain layer to
produce electrical signals related to local x-ray exposure at the
charge generator layer.
14. A method of imaging low exposure level x-rays comprising:
receiving x-rays at a charge generator layer locally generating
electrical charges in response to x-ray exposure; locally receiving
at a gain layer electrical charges generated by the charge
generating layer and locally multiplying charges received at the
gain layer to thereby produce locally multiplied electrical charges
suitable for low x-ray exposure level imaging; and locally
collecting electrical charges multiplied by the gain layer to
produce electrical signals related to local x-ray exposure at the
charge generator layer.
15. A method of imaging low exposure level x-rays suitable for
real-time fluoroscopy comprising: receiving x-rays at a charge
generator layer locally generating electrical charges in response
to x-ray exposure; locally receiving at a gain layer electrical
charges generated by the charge generating layer and locally
multiplying charges received at the gain layer to thereby produce
locally multiplied electrical charges suitable for low x-ray
exposure level imaging in real-time fluoroscopy; and locally
collecting electrical charges multiplied by the gain layer to
produce electrical signals related to local x-ray exposure at the
charge generator layer.
Description
REFERENCE TO RELATED APPLICATION
[0001] This present application claims the benefit of provisional
Application Ser. No. 60/191,943, filed on Mar. 24, 2000, which is
hereby incorporated herein by reference.
FIELD
[0002] This patent specification is in the field of radiography and
pertains more specifically to x-ray imaging using a digital flat
panel detector.
BACKGROUND
[0003] Flat panel x-ray imaging devices that generate electrical
signals related to local x-ray exposure have been developed in
recent years. An example is discussed in U.S. Pat. No. 5,319,206,
the contents of which are hereby incorporated by reference, and a
current version is commercially available from the assignee of this
patent specification. These panels offer good spatial resolution
and dynamic range properties and can replace x-ray film in a
variety of radiographic procedures, such as, without limitation,
chest x-ray imaging. However, when the exposure level is much lower
than used for procedures such as chest x-rays, for example at the
low exposures used for procedures such as real-time fluoroscopy,
electronic noise and other adverse characteristics can detract from
optimal performance of these flat panel devices. In addition, in
certain applications other that real-time fluoroscopy, such as in
procedures where it is particularly important to reduce patient
x-ray dosage, it may also be desirable to image at lower than
conventional x-ray exposure levels.
[0004] Real-time fluoroscopic radiation exposures can be 10 to 1000
times lower than typical radiographic exposures such as chest
x-rays. Image intensifiers are commonly used in these low-exposure
applications, and because of their high gain they can generate an
acceptable signal-to-noise performance even at very low x-ray
exposure levels. Image intensifiers unfortunately have undesirable
properties, such as spatial distortions, poor contrast, and are
heavy and large.
[0005] Because of other advantages of flat-panel detectors used as
imagers, there exists a need for a flat-panel device that can
operate with high performance at the very lowest x-ray exposure
levels. There also exists a need for a flat panel device to operate
at these low x-ray exposure (dose) levels that is also capable of
generating many images per second for real-time imaging. There also
exists a need for a flat panel device that operates at these low
dose levels that does not have spatial distortions and other
imaging non-uniformities and that is small and lightweight as
compared with an image intensifier of a comparable image plane
size.
SUMMARY
[0006] The flat panel x-ray imaging device such as disclosed in
U.S. Pat. No. 5,319,206 comprises an image charge generation layer
such as selenium, and a flat panel thin film transistor layer with
a multiplicity of pixels and signal control and output lines. A
pixel includes a charge collector, image charge storage capacitor,
and field effect transistor. (The term pixel is used here to denote
a portion of the flat panel detector that can generate an
electrical signal for a positionally corresponding display pixel on
a screen displaying an x-ray image.) The x-ray detection lower
limit of such a device depends to a large extent on the charge
signal generated from the incoming x-rays and the level of
electronic noise in the readout circuit.
[0007] Such a flat panel device can be enhanced to improve its
ability to image at low x-ray exposure through enhancing the charge
signal produced from the incoming x-ray and therefore increasing
the sensitivity of image formation and producing a radiation image
from a very low level of radiation. This can be done through the
use of a gain or charge multiplication layer in addition to known
layers of a flat panel detector, as described in greater detail
below.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] FIG. 1 illustrates a flat-panel x-ray detector with an
x-ray-absorbing photoconductor layer and a gain layer using a gas
chamber proportional chamber for charge gain.
[0009] FIGS. 2a and 2b illustrate two examples of support
structures used to separate at a fixed distance an electron
emission layer from a collector electrode in a flat panel
imager.
[0010] FIG. 3 illustrates a flat-panel x-ray imager with an
x-ray-absorbing photoconductor layer and a gain layer using a
material such as a photoconductor biased in the avalanche mode.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0011] FIG. 1 illustrates one preferred embodiment in which a basic
structure of a flat panel imager 10 comprises the following
layers:
[0012] 1. An x-ray transparent bias electrode 100;
[0013] 2. An x-ray transparent electron blocking layer 200;
[0014] 3. A charge generation layer 300, comprising a
photoconducting material such as selenium or other suitable
photoconducting material. Layer 300 can comprise, for example,
amorphous selenium, PbI.sub.2, CdTe, CdZnTe, TIBr, HgI.sub.2,
silicon, germanium, PbO.sub.2, with or without doping materials,
and combinations or subcombinations thereof. It is also possible to
use, in place of layer 300, the combination of a scintillator layer
and a photoelectron emission layer (e.g., SbCs.sub.3) to achieve
local conversion of x-rays to an electrical signal;
[0015] 4. A thin charge (electron) emission layer 400 comprising a
material with a low work function such as barium oxide or another
suitable material having a low work function. Preferably, this
material should be electrically insulating, or a poor conductor of
electricity. If not, it can be divided into patches that correspond
to pixel positions and are substantially electrically insulated
from each other. In certain embodiments, depending on the materials
chosen for the layers above and below, layer 400 may not be needed,
or may be replaced or augmented by a separation layer that prevents
undesirable interactions between layers;
[0016] 5. A gain or charge multiplication layer 500, which in this
embodiment comprises a thin gas chamber filled with a mixture of
approximately 90% argon gas and 10% methane gas, although another
suitable gas or gas mixture can be used that has comparable charge
multiplication properties.
[0017] For example, liquid or pressurized xenon can be used, or a
solid state charge multiplier can be used (as described in
connection with another preferred embodiment below), or a
microchannel plate charge multiplier can be used, or another device
carrying out a similar function can be used;
[0018] 6. A thin film transistor (TFT) array comprising a
multiplicity of pixel charge collectors 600, image forming
transistor/capacitor circuits 700;
[0019] 7. Readout electronics 800 similar to those in prior art
flat panel imagers; and
[0020] 8. A source of bias voltage 900.
[0021] The preferred embodiment illustrated in FIG. 1 is understood
to operate as follows. A negative electrical bias potential 900 is
applied across the structure. In the case of using selenium as the
photoconductor layer 300, an electric field of approximately 5 to
15 volts per micron(V/.mu.m) is typically used. According to
Maxwell's Equations (see, e.g., J D Jackson, Classical
Electrodynamics, 2.sup.nd Ed. Wiley 1975), the electric field in
the gas layer 500 will be K1/K2 times higher than the field in the
photoconductor selenium layer 300, where K1 is the dielectric
constant of selenium (.about.6.6) and K2 is the dielectric constant
of the gas mixture (.about.1).
[0022] In the presence of the electric field in the photoconductor
300, x-ray energy 1000 absorbed in the selenium 300 will generate
electron-hole pairs 1010 & 1020, with the electrons 1010
drifting across the selenium layer 300 to reach the charge emission
layer 400. The charge emission layer 400 is a layer of material
such as barium oxide with a low work function such as from 0.3 eV
to 1.5 eV and, as earlier indicated, preferably is electrically
non-conductive or a poor conductor, or is divided into patches
matching pixel positions and electrically insulated from each
other. Those electrons that have reached this charge (electron)
emission layer 400 and have acquired sufficient energy from the
bias electric field to exceed the work function energy will
traverse the emission layer and escape its lower surface to enter
the gas chamber 500.
[0023] Upon entering the gas chamber 500, the electrons are
accelerated by the stronger electric field in the chamber to
collide with atoms of the inert gas such as argon gas. The
collision with the inert gas will produce more free electrons that
are accelerated in the chamber 500, thus producing a charge
multiplication effect. The physics of such a gas chamber is known
to scientists in the field of nuclear instruments as a proportional
counter. See, e.g., P W Nicholson, Nuclear Electronics, Wiley 1974,
hereby incorporated by reference. The magnitude of charge
multiplication can be controlled by varying the gas mixture, the
gas pressure, and the applied field across the gas chamber as well
as across the photoconductor layer, as in known in the art of
proportional counters. A multiplication factor such as 10 to 100
can be desirable for single x-ray photon detection and
quantum-limited imaging.
[0024] The image charge after such a multiplication through gas
chamber 500 is collected by the collector electrode 600 and the
thin film transistor (TFT) structure 700, and subsequently readout
as an image by the readout electronics 800 and processed into an
image by an associated computer 3000 that can include a suitable
display for a resulting x-ray image, as known in the art of flat
panel x-ray imagers.
[0025] The electron blocking layer 200 is a material that allows
the passage of the holes 1020 into the bias electrode 100 but
impedes the injection of electrons from the bias electrode 100 into
the photoconductor 300. This structure reduces the magnitude of the
dark current that flows in the device in the absence of x-ray
illumination. The flow of hole current generated from the x-ray
absorption through this layer helps prevent charge buildup and
facilitates the construction of a device allowing continuous
imaging.
[0026] The separation of the gain stage 500 from the charge
generation stage 300 makes for a signal gain that is independent of
the position of absorption of the x-ray in photoconductor 300. The
charge gain that occurs in the gas chamber layer 500 allows the
imaging of very low levels of radiation.
[0027] As a practical example of how to construct a device such as
illustrated in FIG. 1, layer 300 can be amorphous selenium, biased
at 10 V/.mu.m electric field. A thin (.about.1 .mu.m) electron
emission layer 400 of barium oxide provides a barrier preventing
the escape of the gas in the chamber 500. A thickness of the gas
chamber of 20 .mu.m would provide a gain of approximately 10 in the
presence of the 66 V/.mu.m electric field, when the gas (90% argon
gas and 10% methane) is filled to a pressure of about 1.4
atmospheres. A pressure of 1.4 atmospheres will approximately
balance the electrostatic attraction force between the collector
electrode layer and the electron emission layer. Different
pressures can be used if desired.
[0028] Construction of the device and device reliability can be
facilitated by manufacturing small support posts 1600 that extend
from the plane of the collector electrodes 600 to the electron
emission layer 400. See FIG. 2a. These posts 1600 can comprise an
insulating material deposited using photolithography, with the base
of each post 1600 attached in a 10 .mu.m insulating spacing 1700
typically used between collector electrodes 600. The tops of the
posts 1600 support the electron emission layer 400. An alternative
to posts can be walls 1610, as illustrate in FIG. 2b, or some other
support structure that provides the desired mechanical support
function but does not adversely affect imaging performance.
[0029] An alternative preferred embodiment of a flat panel imager
10, illustrated in FIG. 3, is otherwise similar to the embodiment
illustrated in FIG. 1 but uses a solid semiconductor 2500 in place
of the electron emission layer 400 and the gas chamber 500. The
photoconductor layer is labeled 2300 in this embodiment, and
carries out a function similar to that of layer 300 in the
embodiment illustrated in FIG. 1. In the embodiment of FIG. 3, the
material for layer 2500 is chosen so that the electric field
strength in the material is sufficient to bias the material in the
avalanche mode. This is known in nuclear electronics as a
solid-state proportional counter. See, e.g., P W Nicholson, Nuclear
Electronics, Wiley 1974. Electrons accelerating in this material
2500 generate additional charges and thus gain similarly to the
operation in the gas chamber 500. The electric field in the
material 2500 will be given by K1/K2 of the field strength in the
photoconductor 2300, where K1 is the dielectric constant of the
photoconductor layer 2300 and K2 is the dielectric constant of the
gain layer 2500.
[0030] For example, amorphous selenium can be used as the
photoconductor 2300 and another material such as HgI.sub.2 can be
used as the gain layer 2500. The amorphous selenium layer can be
biased with a typical field strength of, for example, .about.10
V/.mu.m. With suitable deposition and sample preparation HgI.sub.2
can be made with similar dielectric constant as the selenium layer
2300 and will have a similar electric field strength of, for
example, 10 V/.mu.m. This can be sufficient to operate the gain
layer 2500 in the avalanche mode. Other materials can be chosen for
layers 2300 and 2500, as long as the field strength in the gain
layer is sufficiently high to create a charge gain.
[0031] One advantage of a device where the gain multiplication
occurs separately from the original charge generation is that the
signal gain is substantially independent of the position of
absorption of the x-ray, although it is true that the energy with
which charges arrive at the gain region 2500 (or 500 in the earlier
embodiment can differ). X-rays absorbing anywhere within the
thickness of the photoconductor 2300 create charges that drift to
the amplifier region 2500. They are then accelerated across a
constant thickness of layer 2500, resulting in a constant charge
multiplication. X-rays absorbed in the second layer 2500 and
generating charges create a depth-dependent gain; absorption near
the entrance surface of layer 2500 can create a larger gain than
absorption near the collector electrodes 600. If the absorption
cross-section of the gain layer 2500 is much smaller than that of
the photoconductor (charge generation) layer 2300, there will be
negligible absorption in the second layer and a negligible
depth-dependent gain. This can be controlled, for example, by
making layer 2500 much thinner than layer 2300, or making layer
2500 of a material of much lower atomic number so its x-ray cross
section is lower. The system in FIG. 1 has a similarly negligible
x-ray absorption cross-section in layer 500 because the density of
the gas chamber layer 500 is much lower than the density of the
photoconductor layer 300, and thus does not have a significant
depth-dependent gain.
[0032] While specific preferred embodiments have been described
above as examples of the improvements disclosed in this patent
specification, it should be understood that many variations are
possible. As some non-limited examples, additional layers can be
used in the flat panel imager, or different materials can be used
for the layers, layers can be omitted so long as a gain or charge
multiplication is still achieved, and the bias voltage can be the
opposite of that used in the preferred embodiments described above.
The flat panel image can be of any desired size and shape, and need
not even be absolutely planar, so long as it is relatively thin as
compared with the combination of a scintillator and an image
intensifier known in the art as an x-ray imager. While the term
x-rays has been used above, it should be clear that the term is
intended to include other ionizing radiation as well such as, for
example, gamma rays.
[0033] The electric field in the examples above is higher in the
gain layer than in the charge generator layer, but in case of other
materials for these layers, the electric fields may be the same, or
the field in the charge generator layer may be higher.
* * * * *