U.S. patent application number 14/795915 was filed with the patent office on 2016-01-14 for photodetector and computed tomography apparatus.
The applicant listed for this patent is Kabushiki Kaisha Toshiba. Invention is credited to Shunsuke Kimura, Keita Sasaki.
Application Number | 20160011323 14/795915 |
Document ID | / |
Family ID | 50841613 |
Filed Date | 2016-01-14 |
United States Patent
Application |
20160011323 |
Kind Code |
A1 |
Sasaki; Keita ; et
al. |
January 14, 2016 |
PHOTODETECTOR AND COMPUTED TOMOGRAPHY APPARATUS
Abstract
A photodetector according to an embodiment includes: a
photodetector element unit including a first cell array including a
plurality of first cells arranged in an array and a second sell
array including a plurality of second cells arranged in an array,
each of the first and second cells including a photoelectric
conversion element, the second cell array being arranged to be
adjacent to the first cell array; a first pulse height analyzer
unit analyzing a pulse height of an electrical signal outputted
from the first cell array; a second pulse height analyzer unit
analyzing a pulse height of an electrical signal outputted from the
second cell array; and a signal processing unit determining
non-uniformity of a distribution of photons entering the first and
second cell arrays using an output signal of the first pulse height
analyzer unit and an output signal of the second pulse height
analyzer unit.
Inventors: |
Sasaki; Keita;
(Yokohama-shi, JP) ; Kimura; Shunsuke;
(Kawasaki-shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Kabushiki Kaisha Toshiba |
Tokyo |
|
JP |
|
|
Family ID: |
50841613 |
Appl. No.: |
14/795915 |
Filed: |
July 10, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
14282048 |
May 20, 2014 |
9109953 |
|
|
14795915 |
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Current U.S.
Class: |
250/370.08 ;
250/208.2 |
Current CPC
Class: |
G01J 1/44 20130101; G01T
1/2018 20130101; G01T 1/248 20130101; G01T 1/1647 20130101 |
International
Class: |
G01T 1/24 20060101
G01T001/24; G01T 1/20 20060101 G01T001/20 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 12, 2013 |
JP |
2013-123821 |
Claims
1.-13. (canceled)
14. A photodetector comprising: a first array in which a plurality
of first photoelectric conversion elements are arranged in an
array; a second array in which a plurality of second photoelectric
conversion elements are arranged in an array, the second array
being arranged to be adjacent to the first array; a first electrode
electrically connected to the first array and a first analyzer
analyzing a pulse height of an electrical signal outputted from the
first array; and a second electrode electrically connected to the
second array and a second analyzer analyzing a pulse height of an
electrical signal outputted from the second array.
15. The photodetector according to claim 14, further comprising a
signal processing unit that determines a distribution of photons
entering the first array and the second array using an output
signal of the first analyzer and an output signal of the second
analyzer.
16. The photodetector according to claim 14, further comprising a
signal processing unit that receives an output signal of the first
analyzer and an output signal of the second analyzer and determines
a distribution of photons entering the first array and the second
array.
17. The photodetector according to claim 14, wherein the second
array is arranged to surround the first array.
18. The photodetector according to claim 14, wherein the second
array is divided into a plurality of arrays.
19. The photodetector according to claim 14, wherein the first and
second photoelectric conversion elements are avalanche
photodiodes.
20. The photodetector according to claim 19, wherein the avalanche
photodiodes are connected in parallel with each other.
21. A computed tomography apparatus comprising: a radiation
generating emitting radiation; a display displaying an image; a
photodetector comprising; a first array in which a plurality of
first photoelectric conversion elements are arranged in an array; a
second array in which a plurality of second photoelectric
conversion elements are arranged in an array, the second array
being arranged to be adjacent to the first array; a first electrode
electrically connected to the first array and a first analyzer
analyzing a pulse height of an electrical signal outputted from the
first array; and a second electrode electrically connected to the
second array and a second analyzer analyzing a pulse height of an
electrical signal outputted from the second array.
22. The apparatus according to claim 21, further comprising a
signal processing unit that determines a distribution of photons
entering the first array and the second array using an output
signal of the first analyzer and an output signal of the second
analyzer.
23. The apparatus according to claim 21, further comprising a
signal processing unit that receives an output signal of the first
analyzer and an output signal of the second analyzer and determines
a distribution of photons entering the first array and the second
array.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This is a continuation of application Ser. No. 14/282,048,
filed May 20, 2014, which is incorporated herein by reference.
[0002] This application is based upon and claims the benefit of
priority from prior Japanese Patent Application No. 2013-123821
filed on Jun. 12, 2013 in Japan, the entire contents of which are
incorporated herein by reference.
FIELD
[0003] Embodiments described herein relate generally to
photodetectors and computed tomography apparatuses.
BACKGROUND
[0004] A silicon photomultiplier (SiPM) is a photodetector element
including two-dimensionally arranged avalanche photodiodes
(hereafter referred to as "APDs"), which operate in a mode called
"Geiger mode" when a reverse-bias voltage higher than a breakdown
voltage of the APDs is applied thereto.
[0005] The gain of an APD operating in Geiger mode is very high,
1.times.10.sup.5 to 1.times.10.sup.6. Therefore, a very weak light
emission of a single photon can be measured using the APD.
[0006] A resistor having a high resistance value called "quenching
resistor" is connected in series to each APD of a SiPM. When a
single photon enters the APD to cause a Geiger discharge, the
quenching resistor causes a voltage drop to terminate the
amplification. As a result, a pulsed output signal can be obtained.
Each APD of the SiPM acts in this manner. Accordingly, if the
Geiger discharge occurs in a plurality of APDs, an output signal
can be obtained, the output signal indicating a charge value or
pulse height value obtained by multiplying an output signal of a
single APD by the number of APDs in which Geiger discharge occurs.
Therefore, the number of APDs in which the Geiger discharge occurs,
i.e., the number of photons entering the SiPM, can be determined
from such an output signal. This enables the counting of the number
of photons.
[0007] As described above, if a plurality of photons enters the
SiPM, the number of photons can be correctly counted as long as a
single photon enters each APD of an APD array, since the Geiger
discharge occurs in each APD. However, it takes some time for an
APD in which a Geiger discharge occurs to recover to the original
reverse-bias potential state. If a photon enters thereto during
such a time, a sufficient reverse-bias is not applied to the APD.
As a result, the photon is not counted. Therefore, the recovery
time is called "dead time." If a large number of photons reach the
APD array during the dead time, there would be a loss in the
counting of photons. Accordingly, the output signal shows nonlinear
values relative to the number of photons. As a result, the photon
counting accuracy is considerably degraded.
[0008] The spatial distribution and temporal distribution of the
photons entering the SiPM greatly relate to the cause of such a
degradation. For example, cases where light rays having the same
energy enter the SiPM uniformly and non-uniformly are considered.
If the light rays enter non-uniformly, a frequency with which a
single APD receives a light ray during a short period increases.
Accordingly, the output signal in such a case becomes lower than
that in the case where the light rays enter uniformly. However, the
APDs of the SiPM are connected in parallel, and there is no
information on which APDs are in the Geiger mode at which timing.
Therefore, such an output signal, which is an erroneous signal
having information that a lower number than the actual number of
photons enter, the energy resolution of the SiPM is degraded.
[0009] In order to improve the characteristics of SiPMs, the number
of APD arrays for receiving photons is increased, or the dead time
is shortened in some SiPMs. However, if the number of APD arrays is
increased, the area of each APD may be reduced. This would degrade
the photon detection efficiency and the gain. The shortening of
dead time is in a trade-off with an increase of noise or a decrease
of gain. Accordingly, this cannot solve the problem
fundamentally.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] FIG. 1 is a block diagram showing a photodetector according
to the first embodiment.
[0011] FIG. 2 is a plan view showing a first example of a SiPM
included in the first embodiment.
[0012] FIG. 3 is a plan view showing an APD cell array including
3.times.3 APD cells, which is an enlarged view of a region A of the
SiPM shown in FIG. 2.
[0013] FIG. 4 is a cross-sectional view of the SiPM shown in FIG.
3, taken along line B-B.
[0014] FIG. 5 is a cross-sectional view of a Geiger mode APD cell
for explaining the operational principle of the SiPM according to
the first embodiment.
[0015] FIG. 6 is an equivalent circuit diagram of the SiPM
according to the first embodiment, for explaining the operational
principle of the SiPM.
[0016] FIG. 7 is a plan view of a SiPM according to a comparative
example.
[0017] FIG. 8 is a histogram showing outputs from the SiPM
according to the comparative example.
[0018] FIG. 9 is a plan view of the SiPM for explaining the
photodetector according to the first embodiment.
[0019] FIG. 10 is a diagram for explaining the configuration of the
photodetector according to the first embodiment.
[0020] FIGS. 11A to 11C are histograms showing outputs from the
pulse height analyzer units and the signal processing unit of the
photodetector according to the first embodiment.
[0021] FIG. 12 is a plan view showing a second example of the SiPM
included in the first embodiment.
[0022] FIG. 13 is a plan view of a third example of the SiPM
included in the first embodiment.
[0023] FIG. 14 is a plan view of a fourth example of the SiPM
included in the first embodiment.
[0024] FIG. 15 is a plan view of a fifth example of the SiPM
included in the first embodiment.
[0025] FIG. 16 is a block diagram showing a photodetector according
to a modification of the first embodiment.
[0026] FIG. 17 is a timing chart of the signal processing circuit
according to the first embodiment.
[0027] FIG. 18 is a block diagram showing a photodetector according
to the second embodiment.
[0028] FIG. 19 is a cross-sectional view showing a photodetector
element according to the second embodiment.
[0029] FIGS. 20A and 20B are diagrams each showing a characteristic
relationship between the scintillation depth and the frequency of
occurrence obtained by a simulation.
[0030] FIG. 21 is a perspective view of the photodetector element
according to the second embodiment.
[0031] FIGS. 22A to 22D are diagrams each showing the distribution
of scintillation photons reaching a SiPM, at a scintillation
position.
[0032] FIG. 23 is a block diagram showing a photodetector according
to the third embodiment.
[0033] FIG. 24 is a cross-sectional view showing a photodetector
element according to the third embodiment.
[0034] FIG. 25 is a schematic external view in a case where the
photodetector according to the third embodiment is applied to a
computed tomography apparatus.
DETAILED DESCRIPTION
[0035] A photodetector according to an embodiment includes: a
photodetector element unit including a first cell array in which a
plurality of first cells are arranged in an array, each of the
first cells including a photoelectric conversion element that
detects a photon incident thereto and converts the photon to an
electrical signal, and a second cell array in which a plurality of
second cells are arranged in an array, each of the second cells
including a photoelectric conversion element that detects a photon
incident thereto and converts the photon to an electrical signal,
the second cell array being arranged to be adjacent to the first
cell array; a first pulse height analyzer unit that analyzes a
pulse height of an electrical signal outputted from the first cell
array; a second pulse height analyzer unit that analyzes a pulse
height of an electrical signal outputted from the second cell
array; and a signal processing unit that determines non-uniformity
of a distribution of photons entering the first cell array and the
second cell array using an output signal of the first pulse height
analyzer unit and an output signal of the second pulse height
analyzer unit.
[0036] Embodiments will now be explained with reference to the
accompanying drawings.
First Embodiment
[0037] FIG. 1 is a block diagram showing the structure of a
photodetector 3 according to the first embodiment. The
photodetector 3 includes a photodetector element 1 for detecting
photons to be counted and converted to electrical signals, and a
signal processing circuit 2 for processing the electrical signals
photoelectrically converted by the photodetector element 1. The
photodetector element 1 includes a silicon photomultiplier (SiPM)
10 serving as a photoelectrically converting device. The signal
processing circuit 2 includes pulse height analyzer units 20 and 21
for analog-to-digital converting analog electrical signals
outputted from the SiPM 10, and a signal processing unit 22 for
processing the digital signals from the pulse height analyzer units
20 and 21. The signal processing circuit 2 also includes circuits
relating to the driving and the characteristics of the
photodetector such as a voltage power supply circuit and a
temperature compensation and control circuit, which are not shown
for the simplicity of the descriptions of the first embodiment.
Although the pulse height analyzer units 20 and 21 are described to
be included in the signal processing circuit 2, they can be formed
as an on-chip circuit on a common chip together with the SiPM 10
formed on a semiconductor substrate. Output signals 4, which are
subjected to the analog-to-digital signal processing performed by
the signal processing unit 22, are transferred to an information
terminal such as a personal computer via a USB cable, for
example.
[0038] Next, the specific structure and operation of the
photodetector 3 according to the first embodiment will be
described.
[0039] FIG. 2 is a plan view of the SiPM 10 according to the first
embodiment. The SiPM 10 includes two arrays, a first APD cell array
101 and a second APD cell array 102, in which APD cells 5
performing photoelectric conversion are two-dimensionally arranged.
The first APD cell array 101 and the second APD cell array 102 are
electrically isolated from each other by an element isolation
region 59.
[0040] In each APD cell array 101, 102, the APD cells 5 are
connected in parallel with each other. The first APD cell array 101
is connected to an electrode 101a and the second APD cell array 102
is connected to an electrode 102a.
[0041] FIG. 3 is a plan view of an APD cell array including
3.times.3 APD cells, which is an enlarged view of the region A
shown in FIG. 2. FIG. 4 is a cross-sectional view showing the SiPM
10 cut by a line B-B shown in FIG. 3. Each APD cell 5 includes a
p-type epitaxial layer 50 epitaxially grown on an n.sup.+ type
semiconductor substrate 52. The p-type epitaxial layer 50 is
isolated by an element isolation region 57 formed of an insulating
film. The p-type epitaxial layer 50 is a p.sup.- layer with a low
impurity concentration. By implanting an acceptor impurity into the
p-type epitaxial layer 50, p.sup.+ layers 50a, 50b are formed. The
p-type epitaxial layer 50 and the p.sup.+ layers 50a, 50b make an
avalanche layer 51.
[0042] In order to obtain electric charges generated by the
avalanche layer 51, a contact 55, a quenching resistor 53, and a
signal wiring line 54 connecting to the p.sup.+ layer 50a are
formed. The contact 55, the quenching resistor 53, and the signal
wiring line 54 are covered by an interlayer insulating film 58.
[0043] A metal electrode 56 is formed at the back side of the
semiconductor substrate 52 by sputtering or plating. The planar
layout shown in FIG. 3 is only an example for briefly explaining
the SiPM, and is not limited to FIG. 3. Furthermore, although FIG.
4 shows a vertical APD cell structure in which the p-type epitaxial
layer 50 is formed on the n.sup.+-type semiconductor substrate 52
and an acceptor impurity is implanted thereto, another type of
vertical structure in which the internal impurity concentration
distribution differs, or a lateral APD cell structure may also be
employed.
[0044] Next, the operational principle of the SiPM 10 will be
described with reference to FIGS. 5 and 6. FIG. 5 is a
cross-sectional view of a Geiger mode APD cell 5 for explaining the
operational principle of the SiPM 10. The cross-sectional structure
of the APD cell 5 shown in FIG. 5 is the same as that in FIG. 4.
This APD cell 5 has a vertical structure in which a p-type
epitaxial layer 50 is formed on the n.sup.+-type semiconductor
substrate 52 and an acceptor impurity is implanted thereto.
Therefore, the metal electrode 56 serves as a cathode electrode,
and when a reverse-bias voltage is applied to the metal electrode
56, an electric field is generated in the avalanche layer 51. If
the reverse-bias voltage is increased further, the electric field
intensity is increased to cause avalanche breakdown at a certain
reverse-bias voltage to allow a large current to flow. Such an APD
that is activated by applying thereto a voltage more than the
breakdown voltage for causing avalanche breakdown is called a
Geiger mode APD.
[0045] In order for a Geiger mode APD to cause avalanche breakdown,
electrons or holes serving as seeds are required in a depletion
layer region to which a high electric field is applied. If photons
are absorbed in this region to cause photoelectric conversion to
generate electron-hole pairs, avalanche breakdown is caused to
allow a large current to flow continuously. In order to prevent
this, the quenching resistor 53 is connected in series on the anode
side, from which the charges are obtained. Since the quenching
resistor 53 is connected in series, a voltage drop occurs at the
same time as the avalanche breakdown occurs to allow a large
current to flow. Accordingly, the potential between the anode and
the cathode falls to the breakdown voltage to terminate the
multiplication function of the avalanche layer 51. As a result, the
output signal becomes a pulsed signal. Since a single photon makes
such a pulsed signal, a photon counting can be performed.
[0046] FIG. 6 is an equivalent circuit diagram of the SiPM 10. In
the equivalent circuit, a plurality of APD cells 5, each connected
to a quenching resistor 53 located on the anode side, is connected
in parallel. As has been described with reference to FIG. 5, each
APD cell 5 is capable of detecting a single photon. Accordingly,
each of the APD cells 5 connected in parallel detects a photon, and
thus the limit value of the number of photons that can be detected
is determined by the number of APD cells 5 in the SiPM 10.
[0047] As described above, photon counting can be performed by
using the APD cells 5 operating in Geiger mode. However, it
requires a recovery time to recover the potential, which has
decreased to the breakdown voltage, to the Geiger mode operating
potential to enable the photon detection again, the recovery time
being in accordance with an RC time constant determined by the
capacity of the APD cell 5 and the quenching resistor 53. If a
photon enters the APD cell 5 during the recovery time, the
multiplication function cannot be satisfactorily obtained to have
an output signal. For this reason, the recovery time is also called
"dead time." Similarly, the output signal does not change if a
photon enters during the avalanche breakdown time. Thus, depending
on the state of the SiPM 10, omission in counting occurs, which
reduces the photon counting accuracy. A specific example of such a
case will be described as a comparative example.
[0048] FIG. 7 is a plan view of a SiPM 10A according to a
comparative example. The SiPM 10A of the comparative example
includes two-dimensionally-arranged APD cells 5 that are connected
in parallel with each other and further connected to an electrode
101a of an APD cell array. Cases of three photon receiving regions
6a, 6b, 6c of the SiPM 10A are considered, which are each in a
different size and located at a different portion, and to each of
which the same number of photons enter. The photon receiving region
6a covers the entire area of the SiPM 10A, the photon receiving
region 6b covers a fourth of the area of the SiPM 10A, and the
photon receiving region 6c is located at a central portion of the
SiPM 10A. FIG. 8 shows a histogram of output signals from the SiPM
10A. The lateral axis of the histogram shown in FIG. 8 indicates
the number of APD cells 5 in which Geiger discharge occurs (which
are fired), i.e., the number of photons detected, and the
longitudinal axis indicates the frequency of occurrence. As can be
understood from FIG. 8, although the condition on the number of
photons entering each region of the SiPM 10A is the same, the
histogram of the output signals shows as if the number of photons
differs in each region. This is caused from such reasons that the
number of APD cells 5 in each photon receiving region changes, and
the proportion of photons received during the dead time increases
due to the increased photon density.
[0049] FIG. 9 is a plan view of the SiPM 10 according to the first
embodiment, for explaining the photodetector 3. The photon
receiving regions 6a, 6b, 6c of the photodetector 3 according to
the first embodiment shown in FIG. 9 are located at the same
positions of those in the comparative example shown in FIG. 7. As
shown in FIG. 10, analog electrical signals outputted from the
first APD cell array 101 and the second APD cell array 102 are
inputted to the pulse height analyzer unit 20 and the pulse height
analyzer unit 21 of the signal processing circuit 2, respectively,
converted to digital signals, and then inputted to the signal
processing unit 22. The pulse height analyzer units 20, 21 each
have a waveform shaping function for shaping the waveforms of input
pulses, an AD conversion function for analog-to-digital converting
the pulse height values of the shaped pulses, a memory function for
storing the number of signals in each group classified depending on
the converted values, and a pulse height analyzing function for
analyzing the frequency distribution of pulse height. FIGS. 11A,
11B, and 11C show outputs of the pulse height analyzer unit 20, the
pulse height analyzer unit 21, and the signal processing unit 22,
respectively. The lateral axis of each of the histograms shown in
FIGS. 11A, 11B, and 11C indicates the number of APD cells 5 in
which Geiger discharge occurs (which are fired), i.e., the number
of photons detected, and the longitudinal axis indicates the
frequency of occurrence. The outputs of the pulse height analyzer
units 20, 21 are values corresponding to the numbers of APD cells 5
to which photons enter. Therefore, as in the case of the SiPM 10A
of the comparative example shown in FIG. 7, the number of photons
detected varies in each of the photon receiving regions 6a, 6b, and
6c. Furthermore, since the number of corresponding APD cells 5
differs between the first APD cell array 101 and the second APD
cell array 102, the number of detected photons differs between
photon receiving regions. The signal processing unit 22 calculates
the ratio between the signals of the pulse height analyzer unit 20
and the pulse height analyzer unit 21, and if the ratio is in a
predetermined range (for example, .+-.a few percent of the ratio of
the number of the APD cells 5 in the first APD cell array 101 and
the number of the APD cells 5 in the second APD cell array 102 of
the SiPM 10), the signal analyzed by the pulse height analyzer unit
101 is recorded, and if the ratio is beyond the predetermined
range, the signal analyzed by the pulse height analyzer unit 101 is
not recorded. Therefore, as shown in FIG. 11C, the signal
processing unit 22 only outputs the signal of the pulse height
analyzer unit 101 in the case of the photon receiving region 6a of
the SiPM 10, to which photons are uniformly incident, but does not
output the signals in the cases of the photon receiving regions 6b,
6c, to which the photon are not incident uniformly.
[0050] In the comparative example, even if the number of photons to
be counted is the same, each region of the SiPM 10 shows a
different count value. In contrast, the photodetector 3 according
to the first embodiment is capable of determining non-uniformity in
area to which photons are incident, and not outputting the counted
value if photons are incident in a non-uniform manner to suppress
variations, thereby improving the photon counting accuracy.
[0051] The structure of the photodetector 3 of the first embodiment
is not limited to that shown in FIG. 10. For example, the second
APD cell array 102 of the SiPM 10 may be electrically divided into
a plurality of second APD cell arrays 102A, 102B as shown in FIG.
12.
[0052] Furthermore, as shown in FIG. 13, the SiPM 10 may be formed
of a first APD cell array 101 including APD cells 5 and second APD
cell arrays 102A, 102B including second APD cells 5a with a cell
pitch and an aperture ratio different from those of the APD cells
5. In FIGS. 2 and 12, the element isolation region 59 is present
between the first APD cell array 101 and the second APD cell
array(s) 102, which is considerably wider than the element
isolation layer 57 between the APD cells 5.
[0053] The APD cells 5 may be arranged in a matrix form in an array
as shown in FIGS. 14 and 15, as in the SiPM 10A of the comparative
example. In this case, the first APD cell array 101 and the second
APD cell array 102 can be located at arbitrary positions by
appropriately patterning the signal wiring line 54.
(Modification)
[0054] FIG. 16 shows a photodetector 3A according to a modification
of the first embodiment. The photodetector 3A according to the
modification includes the SiPM 10 shown in FIG. 12 and a signal
processing circuit 2A. The signal processing circuit 2A may have a
configuration depending on the number of signals to be dealt with.
The signal processing circuit 2A includes a non-uniformity
detecting unit 24 for detecting non-uniformity in incident photons,
a pulse height analyzer unit 20, and a signal processing unit 22.
The non-uniformity detecting unit 24 includes a difference output
circuit 24a for outputting an absolute value of a difference
between pulses outputted from the two second APD cell arrays 102,
and a disabling signal output circuit 24b for outputting a
disabling signal when an output signal of the difference output
circuit 24a exceeds a predetermined threshold value. The pulse
height analyzer unit 20 includes a waveform shaping unit 20a for
shaping waveforms of pulses outputted from the first APD cell array
101, a disabling signal detecting unit 20b for not passing the
output of the waveform shaping unit 20a only when a disabling
signal generated by the non-uniformity detecting unit 24 is
detected, a peak detecting and holding unit 20c for detecting and
holding a peak of an output pulse of the waveform shaping unit 20a
passing through the disabling signal detecting unit 20b, and an AD
conversion unit 20d for analog-to-digital converting the output of
the peak detecting and holding unit 20c.
[0055] The elements included in the signal processing circuit 2A,
the non-uniformity detecting unit 24, and the pulse height analyzer
unit 20 are not limited to those described above. If photons are
uniformly enter the SiPM 10, the output signals of the two second
APD cell arrays 102 are at the same level as shown in FIG. 17, and
the output of the difference output circuit 24a becomes low.
[0056] On the other hand, if photons enter non-uniformly, the
output signals of the two second APD cell arrays 102 differ from
each other as shown in FIG. 17, and the output of the difference
output circuit 24a becomes high. If the output of the difference
output circuit 24b exceeds the threshold value set by the disabling
signal output circuit 24b, it is decided that photons enter
non-uniformly, and a trigger pulse for generating a disabling
signal is generated and outputted from the disabling signal output
circuit 24b. The pulse height analyzer unit 20 does not output a
signal if the disabling signal is inputted to the disabling signal
detecting unit 20b. Accordingly, only when photons enter uniformly,
a signal is inputted to the signal processing unit 22. Thus, the
output of the signal processing unit 22, i.e., the photon counting
accuracy, is improved. In this circuit configuration, the load of
the signal processing unit 22 is reduced.
[0057] The process by the signal processing unit 22 can be adjusted
so as not to decrease the photon counting ratio. For example, in
the photodetector shown in FIG. 10, the ratio between the number of
photons detected by the first APD cell array 101 and the number of
photons detected by the second APD cell array 102 would change
depending on where photons enter. In such a case, the digital value
of the first APD cell array 101 is corrected based on digital
signal values of the first APD cell array 101, the ratio between
digital values of the first APD cell array 101 and digital values
of the second APD cell array 102, by using a correction table
including correction coefficients stored in the signal processing
unit 22 in advance. In this manner, all the signals outputted from
the first APD cell array 101 can be counted, and the photon
counting ratio is not reduced.
[0058] As described above, the photodetector of the first
embodiment is not affected by the arrangement of the APD cells 5 in
the SiPM 10 and the number of output signals from the second APD
cell array, and not limited by the configuration or signal
processing method of the signal processing circuit. In any case,
the photodetector of the first embodiment detects the distribution
of photons detected by the SiPM 10, decides not to output signals
that reduce the photon counting accuracy, or corrects the signals,
for example, thereby improving the photon counting accuracy
obtained from signals outputted finally.
Second Embodiment
[0059] FIG. 18 is a block diagram showing a photodetector 3
according to the second embodiment. The photodetector 3 according
to the second embodiment includes a photodetector element 1A for
detecting photons to be counted and converting them to electrical
signals, and a signal processing circuit 2 for processing the
electrical signals photoelectrically converted by the
photo-detecting element 1A. The photodetector element 1A includes a
scintillator 11 for emitting fluorescent light when it receives
radiation, and a SiPM 10 for detecting the fluorescent light
emitted from the scintillator.
[0060] The signal processing circuit 2 includes pulse height
analyzer units 20, 21 for analog-to-digital converting analog
electrical signals outputted from the SiPM 10, and a signal
processing unit 22 for processing digital signals outputted from
the pulse height analyzer units 20, 21. The output signals 4 that
are analog-to-digital processed by the signal processing unit 22
are transferred to an information terminal such as a personal
computer via a USB cable, for example.
[0061] FIG. 19 is a cross-sectional view showing a photodetector
element 1A according to the second embodiment. As in the case of
the first embodiment, the photodetector element 1A according to the
second embodiment includes a SiPM 10 which is formed in a
semiconductor substrate 52 and in which APD cells each including an
avalanche layer 51 are arranged in an array form. The SiPM 10 has
the same configuration as the SiPM 10 of the first embodiment shown
in FIG. 4. The photodetector element 1A of the second embodiment
further includes a scintillator 11 located above the SiPM 10. The
scintillator 11 and the SiPM 10 are bonded to each other by an
adhesion layer 13. Five surfaces of the scintillator 11 other than
the surface to be bonded to the SiPM 10 are covered by a reflector
12. Furthermore, as in the first embodiment, a metal electrode 56
is disposed on the backside of the semiconductor substrate 52.
[0062] FIGS. 20A and 20B each show characteristics of the
scintillation depth and the frequency of occurrence obtained from a
simulation. The simulation is performed on a scintillator material,
LGSO (Lu.sub.2-xGd.sub.xSiO.sub.5:Ce), having thicknesses of 10 mm
and 2 mm, to which a radiation energy of 120 keV is incident.
[0063] FIG. 21 is a perspective view of the photodetector element
according to the second embodiment. FIGS. 22A, 22B, 22C, and 22D
show characteristics of photon distribution at the SiPM 10 when
scintillations occurs at the points B, C, D, and E in the
scintillator 11, obtained by a simulation. A correlation can be
found between the positions B, C, D, and E in the scintillator 11
and the photon distributions. If scintillation occurs near the
incident surface of the scintillator, photons are distributed
uniformly and spread over the entire region of the SiPM 10 (FIGS.
22A and 22B). In contrast, if scintillation occurs near the surface
of the scintillator from which photons are emitted, photons are
distributed non-uniformly (in a concentrated manner) (FIGS. 22C and
22D). As can be understood from FIGS. 20A and 20B, the
scintillation frequency is exponentially decayed from the surface
to which the radiation enters. As a result, as the thickness of the
scintillator 11 increases, the probability of the occurrence of
scintillation near the photon-emitting surface decreases. Thus, the
probability of the occurrence of non-uniform photon distribution
can be reduced by increasing the thickness of the scintillator 11,
but this would cause a new problem of increasing the costs and
making the scintillator processing difficult. Furthermore,
depending on the frequency and the timing of the counting, photons
that are distributed non-uniformly may be counted at a ratio higher
than an expected ratio. Thus, increasing the thickness could not
essentially improve the counting.
[0064] On the other hand, in the photodetector 3A according to the
second embodiment, the analog electrical signals outputted from the
first APD cell array 101 and the second APD cell array 102 are
inputted to the pulse height analyzer unit 20 and the pulse height
analyzer unit 21 of the signal processing circuit 2, respectively,
converted to digital signals, and inputted to the signal processing
unit 22. The signal processing unit 22 obtains a ratio between
signals outputted from the pulse height analyzer unit 20 and the
pulse height analyzer unit 21, and if the ratio is within a
predetermined range (for example, .+-.a few percent of the ratio
between the number of APD cells 5 in the first APD cell array 101
and the number of APD cells 5 in the second APD cell array 102 in
the SiPM 10), the signals analyzed by the pulse height analyzer
unit 101 are recorded, and if the ratio is beyond the range, the
signals analyzed by the pulse height analyzer unit 101 are not
recorded. Thus, signals are outputted from the signal processing
unit 22 only in the case where photons are distributed uniformly
over the SiPM 10. Therefore, the photons distributed non-uniformly
due to the scintillation position of the scintillator 11 do not
affect the output signals.
[0065] Conventional devices provide different photon counting
values depending on distributions of photons entering the SiPM 10,
which is affected by where scintillation occurs in a scintillator,
even if the number of photons to be counted is the same.
[0066] If the photodetector of the second embodiment is used,
non-uniformity in region to which photons reach is detected, and
the photon counting value in such a case is not outputted to
prevent variations the counted values. In such a manner, the photon
counting accuracy can be improved.
[0067] The configuration of the photodetector according to the
second embodiment is not limited to that shown in FIG. 18. For
example, the arrangement of the APD cells 5 in the SiPM 10, the
number of output signals from the second APD cell array, the
configuration of the signal processing circuit 2, and the method of
processing signals may follow those shown in FIG. 16. For example,
the distribution of photons detected by the SiPM 10 is obtained,
and the output signals that degrade the photon counting accuracy
would not be outputted. Furthermore, a correction of signals may be
performed to improve the photon counting accuracy obtained from
final output signals.
Third Embodiment
[0068] FIG. 23 is a block diagram showing a photodetector according
3B according to the third embodiment. The photodetector 3B includes
a light generating unit 61 for generating photons to be counted, a
photodetector element 1B for detecting photons and converting them
to electrical signals, a signal processing circuit 2 for processing
the electrical signals photoelectrically converted by the
photodetector element 1B, and a control unit 7 for analyzing the
output signals from the signal processing circuit 2, and
controlling the light generating unit 61 and the photodetector.
[0069] If the wavelength of light emitted from the light generating
unit 61 is in a radiation range, the photodetector element 1B
includes an array with scintillators 11 for emitting fluorescent
light beams in response to radiation, and SiPMs 10 for detecting
the fluorescent light beams emitted from the scintillators, as
shown in FIG. 24. As in the case of the second embodiment, the
SiPMs 10 are arranged in an array on a semiconductor substrate 52.
Each SiPM 10 has the same configuration as the SiPM 10 according to
the first embodiment shown in FIG. 4. In the third embodiment, the
scintillators 11 arranged in an array are located above the SiPMs
10 arranged in an array. The scintillators 11 and the SiPMs 10 are
bonded by an adhesion layer 13. Five surfaces of each scintillator
11 other than a surface to be bonded to the corresponding SiPM 10
are covered by a reflector 12. As in the case of the first
embodiment, a metal electrode 56 is disposed on the backside of the
semiconductor substrate 52. If light emitted from the light
generating unit 61 is any of ultraviolet light, visible light and
infrared light having a wavelength of 300 nm or more, the
photodetector element 1B may be formed only of the SiPMs 10.
[0070] In the photodetector 3B according to the third embodiment,
the control unit 7 controls light energy and emission timing of the
light generating unit 61 by means of a controller 71, and also
controls the signal processing circuit 2 to be in sync with the
output of the photodetector element 1B. The analog electrical
signals outputted from the first APD cell array 101 and the second
APD cell array 102 of each SiPM 10 are inputted to the pulse height
analyzer unit 20 and the pulse height analyzer unit 21 of the
signal processing circuit 2, respectively, converted to digital
signals, and inputted to the signal processing unit 22. The signal
processing unit 22 obtains a ratio between the signals sent from
the pulse height analyzer unit 20 and the signals sent from the
pulse height analyzer unit 21, and if the ratio is within a
predetermined range (for example, .+-.a few percent of the ratio of
the number of APD cells 5 of the first APD cell array 101 and the
number of APD cells 5 of the second APD cell array 102 of the SiPM
10), the signals analyzed by the pulse height analyzer unit 20 are
recorded, and if the ratio is beyond the predetermined range, the
signals analyzed by the pulse height analyzer unit 20 are not
recorded.
[0071] Therefore, signals are outputted from the signal processing
unit 22 only when photons are uniformly distributed to the SiPM 10.
As a result, non-uniform distribution of photons caused by the
position of scintillation in the scintillator 11 does not affect
the output signals. The output signals are recorded and stored in a
data storage unit 72 of the control unit 7, converted to arbitrary
image data by an image reconstruction unit 73, and displayed by a
display unit 74.
[0072] The photodetector 3B according to the third embodiment can
be applied to a computed tomography (CT) apparatus for medical
imaging diagnosis. FIG. 25 shows a schematic external view of a
case where the photodetector 3B according to the third embodiment
is applied to a computed tomography apparatus. The light generating
unit 61 and the photodetector 3B are fixed to a gantry 81 so as to
be opposed to each other. Radiation emitted from the light
generating unit 61 passes through the body of a person 8 and
detected by the photodetector 3B. Photons of the radiation pass the
body or are absorbed by substances in the body. Accordingly, an
output signal histogram would show that the frequency is reduced by
the amount of radiation energy absorbed by the substances in the
body. By reconstructing the image, the substances in the body can
be discriminated, and the positional relationship among them is
clarified. If the photon counting accuracy is low, the counted
value of the energy of radiation passing through the substances of
the body is reduced, and the counted value of the energy of
radiation absorbed by the substances of the body is increased. This
would considerably affect the reconstructed image to be obtained,
and there is a possibility that an existing disease may be
overlooked.
[0073] In contrast, the computed tomography apparatus including the
photodetector 3B according to the third embodiment does not
increase the counted value of energy of radiation.
[0074] As described above, the photodetector according to the third
embodiment is capable of judging non-uniformity of photons emitted
from the light generating unit and reaching the SiPM, and
determining not to output a count value if the non-uniformity
occurs, thereby preventing variations in data, and improving the
photon counting accuracy.
[0075] The configuration and the features of the photodetector
according to the third embodiment are not limited to those shown in
FIGS. 23 to 25.
[0076] While certain embodiments have been described, these
embodiments have been presented by way of example only, and are not
intended to limit the scope of the inventions. Indeed, the novel
methods and systems described herein may be embodied in a variety
of other forms; furthermore, various omissions, substitutions and
changes in the form of the methods and systems described herein may
be made without departing from the spirit of the inventions. The
accompanying claims and their equivalents are intended to cover
such forms or modifications as would fall within the scope and
spirit of the inventions.
* * * * *