Method and system for reading a data signal emitted by an active pixel in a sensor

A method for reading a data signal emitted by an active pixel in a sensor having a plurality of addressable pixels, comprising the steps of (a) grouping the plurality of pixels into at least two groups each having a fraction of the plurality of addressable pixels, (b) identifying an active group of addressable pixels in which the active pixel is located, (c) providing a reading circuit for the active group of addressable pixels, and (d) reading a magnitude of the data signal in respect of each pixel in the active group of addressable pixels so as to identify the active pixel. A system uses such a method for reading a data signal emitted by an active pixel in a sensor module having a plurality of addressable pixels arranged into at least two groups, and comprises an identifying circuit commonly coupled to each group of pixels and responsive to the data signal for identifying an active group containing the active pixel without identifying the active pixel itself. A reading circuit is responsively coupled to the identifying circuit for reading a magnitude of the data signal in respect of each pixel in the active group, so as to identify the active pixel.

FIELD OF THE INVENTION 
This invention relates to a method for reading a data signal associated 
with an active pixel in a sensor having a plurality of addressable pixels 
in such a manner to optimize the reading efficiency. 
BACKGROUND OF THE INVENTION 
A known diagnostic technique used in tomography for locating tumors 
involves injecting into a patient's bloodstream a radioactive isotope 
which targets the tumor, so that the location of the tumor can be derived 
by detecting the location of the radioactive isotope. Typically, the 
radioactive isotope emits high energy .gamma.-rays which are dispersed 
from the tumor site. In order to achieve the desired detection so as to 
determine the precise location of the tumor, it is necessary to image the 
patient's body in such a manner as to detect only those .gamma.-rays which 
are emitted normally from the body and to ignore those .gamma.-rays which 
are dispersed in other directions. 
Known prior art approaches to achieving this requirement, include the use 
of a mechanical collimator made out of lead having a plurality of spaced 
apart holes which are sufficiently narrow in diameter to allow only those 
.gamma.-rays to pass which are emitted parallel to the collimator holes. 
The collimator is moved until a signal is detected whereupon the location 
of the collimator allows the location of the radioisotope to be inferred. 
However, since most of the radioactive energy is dispersed and therefore 
not detected, such an approach is highly inefficient and the detector 
requires lengthy exposure time which is expensive in terms of the time 
required to perform a reliable measurement as well as being uncomfortable 
for the patient. The resolution of such a system depends on the diameter 
of the holes in the collimator and is typically 8 mm. It can be improved 
be reducing the diameter of the holes in the collimator at the expense of 
decreasing even further the efficiency which is in any case typically no 
better than 10.sup.-5. 
It is obviously desirable to reduce the measurement time as far as possible 
without compromising on the detection accuracy. This requirement has been 
partially addressed by the use of a Compton camera using ring geometry so 
that scattered photons are detected by the ring rather than being lost as 
is the case with mechanical collimators. This obviates the need for a 
collimator and allows the angle of emanation of the .gamma.-rays to be 
calculated. For the purpose of the present invention which is not 
concerned with the physics of the Compton effect, the Compton camera may 
be regarded as just another type of 2-dimensional image sensor having a 
plurality of addressable pixels, one of which emits a signal when 
stimulated by a .gamma.-ray. Specifically, each pixel is a diode which 
generates a charge signal when hit by a .gamma.-ray. A .gamma.-ray emitted 
by the radioisotope will be detected only if it creates a Compton effect 
by creating a charge signal thereby giving up some of its energy. In 
practice, it is usual to employ a composite sensor having several 
spaced-apart sensor layers each containing at least one sensor module so 
as to increase the probability that an incident .gamma.-ray will produce a 
Compton effect in at least one of the layers. The multilayer sensor module 
constitutes a first detector of the Compton camera. Having thus produced a 
Compton effect, the .gamma.-ray then emerges from the first detector. 
However, in order to calculate the angle of the incident .gamma.-ray, the 
emergent .gamma.-ray is directed to a second detector in which it is 
absorbed completely, thereby giving up all of its residual energy. 
As a result of such a geometry, it is necessary to read out data in the 
first detector from a large number of addressable pixels along respective 
channels in order to detect which pixel is "active". This is done by first 
integrating the charge associated with each pixel using an integrator in 
the form of an operational amplifier (OP AMP) having a feedback capacitor. 
The integrated charge pulse is then amplified and shaped and the resulting 
analog signal is sampled and held, allowing its magnitude to be measured. 
In order to measure the peak magnitude of the shaped signal, the shaped 
signal must be very accurately sampled at the peak value. This requires an 
accurate determination of the peak time which occurs a fixed time 
difference t.sub.P after the emission of charge by the excited pixel. The 
fixed time difference t.sub.P is a function of the RC time constant of the 
shaper circuit and is therefore known. 
Thus, in order to know when to sample the integrated charge signal, the 
time of occurrence t.sub.o of each charge emission must itself be 
accurately determined. This having been done, all that is then necessary 
is to sample the held integrated charge sample at a time t.sub.P. A 
reading system for reading out the charge signals must therefore generate 
an accurate trigger coincident with the occurrence of each charge 
emission. Self-triggering systems are known in which the channel in which 
the charge emission occurs generates the trigger by means of a 
level-sensitive discriminator. The pulse height is also latched so that it 
can be read out. However, such a system provides information regarding the 
pulse height only in the specific channel in which the charge emission 
occurred and not in other channels, except sometimes in the nearest 
neighboring channels. Moreover, no data is provided relating to the time 
of occurrence of the charge emission. 
It is also known to generate the trigger by means of a separate electronic 
device on the common "back plane" of the image sensor. However, such an 
arrangement constrains the image sensor to being a "single-sided" detector 
rendering it impossible to determine where, in the sensor, the charge 
emission occurred, as well as being impractical to implement. 
Obviously, if during every scan of the composite image sensor, each pixel 
is read sequentially only one at a time, then the current scan can be 
terminated when an "active" pixel is detected. However, it is impractical 
to read each pixel in such a manner because of the time overhead involved 
in addressing each pixel separately and downloading the pixel data along a 
dedicated channel for further processing. Furthermore, it will be 
appreciated that in addition to the one pixel with which the .gamma.-ray 
stimulation is associated, the other pixels too emit noise. Such noise may 
occur, for example, owing to the common mode drift of the OP AMPs 
associated with the reading circuit. When pixels are read only one at a 
time, it is difficult to quantify accurately the common mode noise 
component in the "active" pixel. Such considerations militate against 
addressing each pixel separately and favor batch addressing of a plurality 
of pixels in a single read operation using multiple channels each in 
respect of a corresponding pixel. This adds to the expense of the reading 
circuit, since various components thereof must be repeated for each 
channel. Having thus read a large number of data signals on separate 
channels each in respect of one pixel in the image sensor, it is then 
necessary to process the data in order to determine which pixel is 
"active", whereupon the location of the radioisotope may be inferred. 
Moreover, as explained above, a non-zero common mode noise signal is 
associated with all of the pixels, including the "active" pixel. In order 
to measure the "active" pixel data accurately, the average common mode 
noise must be determined and subtracted from the "active" pixel data 
itself. This adds to the processing time and, obviously, the more pixels 
are processed simultaneously, the more time-consuming is the required 
processing. 
Consequently, there is trade-off between reading the data sequentially 
pixel by pixel, with the consequent high addressing time and inability to 
compensate for common mode noise; and reading too many pixels 
simultaneously, with the consequent high processing time and added 
expense. 
Yet a further consideration relates to establishing time coincidence of 
.gamma.-ray stimulated emissions in the two parallel detectors of a 
Compton camera. As has been explained above, in order to calculate the 
angle of the incident .gamma.-ray, the emergent .gamma.-ray from the first 
detector is directed to a second detector in which it is absorbed 
completely, thereby giving up all of its residual energy. It is obviously 
necessary to correlate events in the two detectors in order to establish 
that they derive from the same .gamma.-ray. This is done by establishing 
that the two events are substantially simultaneous. However, accurate time 
coincidence of the two events can be determined accurately only if the 
.gamma.-ray emission is measured fast. Prior art detectors employ a filter 
having a slow time constant for shaping the data signal resulting from the 
.gamma.-ray emission. A slow time constant is necessary to improve signal 
to noise ratio and to improve locking on to the peak value of the shaped 
signal. However, using a slow time constant detracts from the accuracy 
with which the peak time can be measured and this, in turn, reduces the 
accuracy with which time coincidence of corresponding events in two 
detectors can be established. 
There is therefore clearly a need to optimize the reading of an array of 
pixels in a 2-dimensional image sensor so as to reduce the time taken to 
detect a single "active" pixel. Associated with this need is the 
requirement to provide an accurate trigger when a charge emission occurs 
so as to determine the time of emission (and thus the peak time) 
accurately thereby permitting time coincidence of events in more than one 
detector to be properly established, and to eliminate the effect of common 
mode noise from "active" pixel data so that only the actual data is read. 
SUMMARY OF THE INVENTION 
It is therefore an object of the invention to provide a method and system 
for reading an "active" pixel in a 2-dimensional image sensor or a stack 
of spaced apart image sensors each having a plurality of pixels. 
According to a broad aspect of the invention there is provided a method for 
reading a data signal emitted by an active pixel in a sensor having a 
plurality of addressable pixels, the method comprising the steps of: 
(a) grouping said plurality of pixels into at least two groups each having 
a fraction of said plurality of addressable pixels, 
(b) identifying an active group of addressable pixels in which the active 
pixel is located, 
(c) providing a reading circuit for said active group of addressable 
pixels, and 
(d) reading a magnitude of the data signal in respect of each pixel in said 
active group of addressable pixels so as to identify the active pixel. 
According to another aspect of the invention, there is provided a system 
for reading a data signal emitted by an active pixel in a sensor having a 
plurality of addressable pixels arranged into at least two groups, the 
system comprising: 
identifying means commonly coupled to each of said groups of pixels and 
responsive to said data signal for identifying an active group containing 
the active pixel without identifying the active pixel itself, 
a reading circuit responsively coupled to the identifying means for reading 
a magnitude of the data signal in respect of each pixel in the active 
group, so as to identifying the active pixel. 
Thus, the invention provides a compromise between reading each pixel 
sequentially one at a time with the consequent overhead in pixel 
addressing and lack of common mode noise correction; and reading all 
pixels simultaneously with the consequent overhead in processing 
Specifically, in the invention, all the addressable pixels are divided 
into groups and in a first step only the active group containing the 
"active" pixel is identified. This can be done very quickly by using a 
threshold comparator for each pixel in a selected group and wire OR-ing 
the outputs of all the comparators to discriminate whether the output of 
the group of pixels exceeds a predetermined threshold. Having thus 
identified the active group, only the pixels in this group are now read 
sequentially, one at a time, so as to identify the "active" pixel, 
whereupon the location of the radioisotope may be inferred. Specifically, 
such an approach obviates the need to read the pixels in the non-active 
groups, thereby saving considerable reading time. 
In the active group only the "active" pixel will have a signal level 
exceeding the discriminating threshold. However, the remaining pixels give 
rise to common mode noise which also affects the "active" pixel and must 
therefore be compensated for, in order that the common mode noise 
component may be eliminated from the signal level of the "active" pixel. 
By reading the signal levels of the remaining pixels in the active group, 
the average common mode noise level may be determined for the non-active 
pixels and subtracted from the data signal in respect of the "active" 
pixel. 
Preferably, the data signal is a fast rising current pulse derived from an 
emission of electric charge consequent to the pixel being struck by a 
.gamma.-ray. The current pulse is integrated by a preamplifier so as to 
produce an analog voltage step having a sharp change in level upon 
emission of the data signal. The voltage step constitutes an initiation 
signal indicative of the time of emission to and whose magnitude is 
proportional to the accumulated charge produced by the current pulse and 
which is collected by a feedback capacitor in the preamplifier. The 
reading circuit further includes at least one shaper in respect of each 
pixel in the active group which is responsive to the voltage step for 
amplifying and shaping the integrated charge in order to generate a slowly 
rising analog voltage signal having a high signal to noise ratio. An 
important feature of such an embodiment resides in the precision with 
which the shaped analog voltage signal is sampled at its peak. 
Specifically, the reading circuit includes in respect of each pixel in the 
active group: 
a fast shaper having a fast time constant and being responsive to said data 
signal for shaping the data signal so as to generate a fast response curve 
which quickly rises above a predetermined threshold, 
a slow shaper having a slow time constant and being responsive to said data 
signal for shaping the data signal so as to generate a slow response curve 
having high signal to noise ratio, 
delay means coupled to the fast shaper for determining a time delay 
.DELTA.t for the fast response curve to exceed said predetermined 
threshold, and sampling means coupled to the delay means and to the slow 
shaper 
for sampling the slow response curve at a further time interval t.sub.P 
-.DELTA.t where t.sub.P is the time at which the slow response curve 
reaches its peak value so as to sample the slow response curve 
substantially at its peak value.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
FIGS 1A and 1B are schematic diagrams showing a system depicted generally 
as 10 comprising a sensor module 11 having an array of 512 pixels 12 
constituted by silicon diodes which are responsive to an incident 
.gamma.-ray for producing a charge signal. By way of example, the sensor 
module 11 may be part of a Compton camera for use in tomographic imaging 
of a patient's body. Such a Compton camera is provided with two separate 
detectors: one of which is a multilayer sensor whose multiple layers serve 
to increase the Compton effect and each of which is provided with its own 
independent readout circuit for enabling the data signal to be read out. 
The second detector, which is not itself a feature of the invention, may 
be a similar pixel array or any other suitable sensor for absorbing the 
.gamma.-ray which emerges from the first detector. As a result, the 
.gamma.-ray gives up its residual energy in the second detector thereby 
allowing the angle of the .gamma.-ray to be calculated in known manner. 
The present invention is thus applicable primarily to the first detector in 
which the pixel arrays 12 are arranged into 16 groups of pixels each 
having 32 pixels. The sensor module 11 includes on-chip discrimination 
circuitry in respect of each of the 16 groups of pixels in the pixel array 
12, in the form of a pair of application specific integrated circuits 
(ASICs) 13a and 13b which may, if desired, be combined in a single ASIC. 
Details of the ASICs 13a and 13b are given below with reference to FIG. 5 
of the drawings. Respective output channels of the discrimination 
circuitry are fed to a readout circuit 14 for sequentially selecting a 
different group of pixels and coupling the pixels in the selected group to 
respective first and second data buses 15 and 16. Specifically, the pixels 
in each group are fed to a multi-input discriminator so as to derive a 
composite signal which is fed to a corresponding data line in the first 
data bus 15. Thus, the composite signal level is derived from all of the 
pixels in the respective group and is HIGH if any of the pixels in the 
group is "active" and is otherwise LOW. Consequently, only one data line 
will be HIGH indicating which of the 16 groups of pixels is "active" 
whilst the other 15 data lines corresponding to the remaining 15 groups 
containing, in total, some 480 pixels are inactive are LOW, showing that 
none of the 480 pixels is "active". Thus, by grouping the 512 pixels as 
described into 16 groups and discriminating between the one active group 
and the remaining 15 inactive groups, the location of the one "active" 
pixel is very significantly narrowed down; although of course, at this 
stage, it is not yet known which of the 32 pixels in this group is the 
"active" pixel. 
All 16 data lines in the first data bus 15 are connected to respective 
inputs of a 16 input OR-gate 20 (constituting a first logic means) whose 
output is consequently HIGH (constituting a first logic level) if any one 
or more of the 16 data lines is HIGH indicating that the corresponding 
group is "active". If none of the 512 pixels in the sensor module 11 is 
"active", then the output of the OR-gate 20 is LOW (constituting a second 
logic level). As will be explained below, with particular reference to 
FIG. 2 of the drawings, several sensor modules may be assembled so as to 
form a multi-layer image sensor each having a plurality of sensor modules. 
Each sensor module has an associated OR-gate 20 and the output of the 
OR-gate in each module is fed to one input of a 2-input AND-gate 21 to 
whose second input is fed an output from a second detector. Consequently, 
the output of the AND-gate 21 in each sensor module is HIGH only if the 
outputs of at least one of the OR-gates 20 in the respective sensor module 
is HIGH, and at the same time the output of the second detector is HIGH. 
The AND-gate 21 thus allows time coincidence of charge emissions emanating 
from both the first and second detectors to be established in real time. A 
.gamma.-ray may strike the image sensor obliquely and if it is partially 
absorbed in one layer then its angle will change thus permitting its 
subsequent detection. 
A decoding means 22 is coupled to the first data bus 15 and is responsive 
to the composite signal on each of the data lines thereof for determining 
which of the data line is HIGH thereby establishing an identity of the 
active group. A timing means 23 is coupled to the output of the OR-gate 20 
via the AND-gate 21 and is responsive to the output of the OR-gate 20 
being HIGH (i.e. the first logic level) for generating a time stamp 
corresponding to a time of creation of the first logic level which is 
substantially coincident with the pixel response. A delay gate 24 
(constituting a delay means) coupled to the timing means 23 is responsive 
to the output of the OR-gate 20 being HIGH for generating a time delay 
t.sub.D which is fed via a delay line 25 to a latch in a sample and hold 
circuit (not shown) within the ASIC 13 so that the data signal generated 
by the "active" pixel may be sampled at a time delay t.sub.P 
=.DELTA.t+t.sub.D after its generation, At being the time difference 
between the actual charge emission and its discrimination. 
The second data bus 16 is constituted by an analog data line 26 which is 
coupled via a sequencer 27 (constituting a selection means) to each of the 
pixels in a selected one of the groups for receiving thereon respective 
signal levels of each of the pixels in the selected group. The sequencer 
27 is responsive to a start signal which is fed to a start input thereof 
via the delay line 25 for cycling through the pixel addresses in the 
active group so as to output serially on the analog data line 26 an analog 
signal corresponding to the signal level of each pixel in turn in the 
active group. The analog data line 26 is coupled to an analog-to-digital 
converter (ADC) 30, the output of which is a digital signal representative 
of the signal level of the corresponding pixel in the active group. 
The digital signal output by the ADC 30 is fed to a digital signal 
processor (DSP) 31 (constituting a discriminating means) which is 
programmed to compare the signal level of each pixel in the selected group 
with a discriminating threshold so as to identify the active pixel as that 
pixel whose signal level exceeds the discriminating threshold. 
The DSP 31 also functions as a common mode noise level determination means 
for determining an average common mode noise level associated with the 31 
pixels in the selected group whose signal levels are not commensurate with 
an "active" pixel. The common mode noise levels of the 31 inactive pixels 
is averaged and then subtracted from the signal level of the "active" 
pixel so as to correct for common mode noise. The DSP 31 produces a 
digital data stream containing data representative of the module number, 
the active group identity, the active pixel identity, the time stamp of 
the active pixel and the common mode noise-corrected signal level thereof. 
The resulting digital data stream is then transmitted to a computer 32 
where it is processed as required: this not being a feature of the present 
invention. The module number is a pre-programmed code which is downloaded 
into the corresponding sensor module 11 and which thus identifies from 
which sensor module a data signal was emitted. The time stamp specifies 
the time based on a common real time clock (not shown) at which a charge 
emission occurred by the identified pixel. These data are relevant when 
coincident data from the first detector is combined with the corresponding 
data from the second detector so as to calculate the location of the 
incident .gamma.-ray and its angle of incidence. It should, however, be 
noted that the time stamps are not used to establish time coincidence of a 
charge emission from the first and second detectors: this being 
established in real time using logic gates as explained above with 
reference to FIGS. 1A and 1B of the drawings. 
Separate power supplies (not shown) are provided for the analog and digital 
sections of the reading circuit 14, so that the analog and digital power 
is distinct as shown by the chain-dotted line. In order to maintain the 
desired differentiation between analog and digital components whilst 
nevertheless allowing for unimpeded data transfer between the two, 
opto-couplers are employed. 
FIG. 2 shows pictorially a multi-layer sensor 40 being part of a first 
detector for a Compton camera and having five parallel identical sensor 
planes each designated 41 and comprising an array of nine sensor modules 
11 as described above with reference to FIGS. 1A and 1B of the drawings. 
Each of the sensor modules 11 in each sensor plane 41 is coupled to a 
respective reading circuit 42, so that nine reading circuits are required 
for each sensor plane. Respective outputs 43 of each of the reading 
circuits 42 are coupled via a data bus 44 to a computer 45. A .gamma.-ray 
impinging on the sensor 40 has sufficient energy to penetrate through all 
the layers 41 thereof, but produces a charge data signal only if it is 
partially absorbed by a pixel in at least one sensor plane. As noted 
above, the provision of multiple layers increases the probability that a 
Compton effect will occur in at least one pixel of the sensor. It should 
be noted that the same objective can also be realized by increasing the 
surface area of each sensor plane. 
In order that the charge signal emitted by an "active" pixel can be read by 
the reading circuit 42, the charge signal, after pre-amplification, must 
first be shaped whereupon its peak magnitude may be sampled and measured. 
FIG. 3 shows graphically three curves 50, 51 and 52 representing 
respective charge signals emanating at the same time t.sub.0 and each 
having a different peak value V.sub.P. 
Referring back to the description of the reading circuit with reference to 
FIGS. 1A and 1B of the drawings, it will be recalled that a time delay 
t.sub.D is fed via a delay line 25 to a latch in a sample and hold circuit 
(not shown) within the ASIC 13a so that the data signal generated by the 
"active" pixel may be sampled at a time delay t.sub.P after its 
generation. The time delay t.sub.D may be predetermined based on the value 
of .DELTA.t and the peak time t.sub.P which is known from the RC 
time-constant of the shaper, so that if the start of the signal at time 
t.sub.0 is known then the curve may be sampled exactly at the peak time 
t.sub.P so as to obtain the peak value V.sub.P. In fact, this is not 
feasible because the time t.sub.0 of the charge signal emanating from the 
"active" pixel can never be determined precisely since it is first 
necessary to discriminate between actual pixel data resulting from an 
incident .gamma.-ray and the signal baseline level. Such discrimination is 
performed by comparing the signal with a predetermined threshold 53 using 
a conventional comparator. The time taken for each of the signals to pass 
the threshold 53 depends on the peak value V.sub.P of the signal and thus 
varies from one signal to another. This effect is known as "time walk" and 
must be compensated for in order to sample each of the three signals at 
the correct time so as to obtain the respective peak value. Without such 
compensation, there is no constant delay between the time at which each 
curve passes through the threshold and the time at which the curve reaches 
its peak value. 
FIGS. 4a and 4b show graphically a solution to the problem of time walk by 
means of which the desired compensation can be effected. Thus, FIG. 4a 
shows a typical integrated charge curve 55 having a known peak time 
t.sub.P. As explained above, this in itself is not sufficient to measure 
accurately the peak value V.sub.P because it is first necessary to 
determine a reliable time origin to using a threshold discriminator as 
explained above. 
In order to achieve this objective, the invention provides a second shaper 
having a very much faster time constant so as to produce a sharp curve 56 
(shown in FIG. 4b) which crosses the threshold after a time .DELTA.t very 
much less than the peak time t.sub.P of the slow shaper. Having thus 
determined from the curve 56 that the signal corresponds to pixel data and 
not the signal baseline level, the first curve 55 may be sampled after a 
time delay t.sub.D equal to t.sub.P -.DELTA.t. It is, of course, true that 
.DELTA.t is not known precisely because the fast rise time curve 56 is 
also subject to time walk and therefore .DELTA.t depends on the peak value 
thereof. However, since the value of .DELTA.t is very small compared to 
the value of t.sub.P, any error in .DELTA..sub.t has negligible effect on 
the delay t.sub.P -.DELTA.t after which the first curve 55 is sampled in 
order to read the value of t.sub.P. 
FIG. 5 shows a detail of the reading circuit 42 illustrating the 
application of duplicated shapers having different time constants in order 
to compensate for time walk. To the extent that the reading circuit 42 
contains components which are shown in other figures also, identical 
reference numerals will be employed. Thus, each pixel 12 in a selected 
group is fed to an integrator 60 formed by a preamplifier 61 having a 
feedback capacitor 62. The integrated output of the preamplifier 61 is 
filtered by a first CR-RC shaper 63 (constituting a slow shaper) and then 
passed to a sample and hold unit 64. The analog output of the sample and 
hold unit 64 is multiplexed by a multiplexer 65 so that the signal 
corresponding to each of the 32 pixels in each group can be sampled and 
processed. 
The output of the preamplifier 61 is also fed to a second CR-RC shaper 66 
(constituting a fast shaper) having an integration time (i.e. peak time) 
which is in the order of ten times shorter than that of the first shaper 
63. The output of the second shaper 66 is fed to a level discriminator 67 
whose threshold is of sufficient magnitude to discriminate between a 
genuine signal and the signal baseline level. The output of the level 
discriminator 67 is fed to a monostable 68 whose output is coupled to the 
gate of a MOSFET 69. The monostable 68 is thus responsive to an "active" 
pixel within the respective group for producing a short trigger pulse for 
switching on the MOSFET 69. The MOSFETs 69 of each group are connected in 
wired OR configuration so that if any of the pixels in the selected group 
is "active" the combined output of the MOSFETs 69 will be HIGH. 
Thus, the second shaper 66 permits very fast discrimination in respect of 
an "active" pixel whereafter the slow integrated signal generated by the 
first shaper 63 may be accurately sampled after a constant delay time 
t.sub.D in order to establish its peak value V.sub.P. 
FIGS. 6a to 6f summarize the various signal levels associated with an 
"active" pixel all drawn according to a common time base. Thus, FIG. 6a 
shows the actual charge signal emitted by a pixel consequent to being 
struck by a .gamma.-ray. As explained above, this signal is a sharp 
current pulse starting at time t.sub.0, almost instantaneously rising to a 
peak value and then trailing off to zero. 
FIG. 6b represents the corresponding waveform after pre-amplification. As 
noted above, the preamplifier integrates the charge data signal so as to 
produce an analog voltage signal having a sharp change in level upon 
emission of the data signal. The sharp change in level defines the start 
time to of the data signal. 
FIGS. 6c and 6d show respectively the slow and fast shaped data signals. In 
FIG. 6c, the shaped signal rises to a peak value V.sub.P at a time t.sub.P 
after the start time t.sub.0 and then trails off to zero as shown by the 
dotted line. The waveform must be sampled and held at the time t.sub.P in 
order to capture the peak value V.sub.P. It is clearly shown in FIG. 6d 
that the fast shaped signal rises through the threshold after a time 
interval .DELTA.t following the start time t.sub.0. 
FIG. 6e shows the output of the monostable 68 (shown in FIG. 5) which is a 
sharp square wave pulse generated at time t.sub.0 +.DELTA.t and which is 
fed via the MOSFET 69 to the delay circuit 24 (shown in FIG. 1A) so as to 
trigger the delay circuit 24 whereby after a further delay time t.sub.D 
equal to t.sub.P -.DELTA.t the slow integrated signal shown in FIG. 6c is 
sampled at its peak value V.sub.P. 
It will be appreciated that whilst the use of parallel discrimination using 
slow and fast shapers has particular benefit to the sensor according to 
the invention, the principle of parallel discrimination may find more 
general application. More particularly, it is to be noted that, where high 
speed is not essential, such discrimination may advantageously be employed 
with known image sensors all of whose pixels are read out, so as to allow 
the pixel data to be read with greater accuracy. Likewise, it will be 
apparent that other modifications may be effected to the particular 
embodiments as described without departing from the spirit of the 
invention. 
Thus, for example, whilst the invention has been described with particular 
regard to the detection of .gamma.-ray emissions, it is to be understood 
that the same principles are equally well applicable for the detection of 
other high energy particles. As will further be appreciated, such high 
energy particles may be photons or charged particles. 
Likewise, although the use of the multi-pixel sensors within a Compton 
camera has been described, it is to understood that the same principles 
are equally well suited for use with a hybrid photon detector and for 
readout of photomultiplier tubes. 
It should also be pointed out that when the sensor modules are based on 
silicon, each pixel is effectively a diode. However, other semiconductor 
sensors may also be employed in which case the pixels are high resistive 
elements.