Scintillation camera compensation for shifting the center channel of the energy spectrum due to photomultiplier gain change

A scintillation camera apparatus includes a scintillation camera which generates X and Y position signals representing a detection position of gamma rays radiated from an object to be examined and a Z signal representing the energy level of the gamma rays, and an image memory for storing data indicating the detection frequency of the Z signal having an energy exceeding a predetermined level at a location accessed by the X and Y position signals. A first compensation table for compensating for the nonlinearity of the X and Y position signals and a second compensation table for the Z signal are provided for compensating for the X, Y, and Z signals to be supplied to the image memory. For compensating a change with time in gains of the photomultipliers, a data memory is provided for storing an energy spectrum of gamma rays in a real time manner. The peak valve position of the energy spectrum is calculated based on the data stored in the data memory. If the peak valve position changes, the content of the second compensation table is rewritten in accordance with the shifting of the peak valve position of the energy spectrum.

BACKGROUND OF THE INVENTION 
The present invention relates to a scintillation camera apparatus for 
obtaining a scintigram of a living body into which a gamma-emitting radio 
isotope (RI) is injected. 
Medical equipment which utilizes a scintillation camera is popular. In this 
type of equipment, the distribution of gamma rays radiated from an object 
to be examined, into which a radio isotope is applied, is detected for 
diagnosis of the object. A scintillation camera comprises a collimator, a 
scintillator, and photomultipliers arranged in a matrix form. Gamma rays 
radiated from an object to be examined bombard the scintillator through 
the collimator to generate fluorescent light. The fluorescent light is 
incident on the respective photomultipliers which produce electrical 
signals proportional to the intensity of the incident gamma rays. The 
electrical signals are supplied to an electronic circuit comprising a 
position calculation circuit, an addition circuit, and the like, thus 
generating position signals X and Y representing the detection position of 
the gamma rays and an energy signal Z representing the intensity of the 
gamma rays. The detection frequency of the energy signal Z whose magnitude 
is within a certain range (window) is stored in an image memory for each 
detection position, and data in the image memory is read out to a display 
device having a cathode ray tube (CRT) display to display the scintigram 
of the object. Due to its structural limitations, the scintillation camera 
is inevitably subject to errors caused by the nonlinearity of radiation 
detection positions and energy detection errors caused by variations in 
gains of the photomultipliers. For this reason, at the manufacturing stage 
of the scintillation camera apparatus, it is essential to compensate for 
the nonlinearity of the detection positions (i.e., position signals X and 
Y) and variations in energy detection response (i.e., energy signal Z), in 
order to obtain accurate scintigrams. 
For example, the characteristics of the scintillation camera are measured 
using a uniform radiation source to prepare a nonlinearity compensation 
table (i.e., an X-Y compensation table) which stores compensation vectors 
for compensating for the nonlinearity of the detection positions and an 
energy detection sensitivity compensation table (i.e., a Z compensation 
table) for compensating for variations in energy detection response at 
pixel positions of the camera. In actual diagnosis of the object, position 
signals X and Y and energy signal Z obtained from the scintillation camera 
are corrected by compensation data read out from the correction tables 
comprised of random access memories (RAMs) accessed by these signals. 
However, a change in gain characteristics of the photomultipliers with 
passage of time (i.e., a change in peak values of Z signal) is inevitable. 
When such a change with time in gain occurs, the initially prepared Z 
compensation table becomes useless. For this reason, an operator is 
required to update the Z compensation table at intervals of a constant 
time in accordance with the current gain characteristics of the 
photomultipliers. This operation is complicated and time consuming. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide an improved 
scintillation camera apparatus. 
It is another object of the present invention to provide a scintillation 
camera apparatus which is free from the necessity of rewriting a 
correction table even if a change with time in gain characteristics of 
photomultipliers in a scintillation camera occurs. 
According to the present invention, there is provided a scintillation 
camera apparatus comprising scintillation camera means, having a 
two-dimensional radiation detector, for detecting gamma rays radiated from 
the interior of an object to be examined, into which a radio isotope is 
injected, to produce X and Y position signals representing a detection 
position of gamma rays and a Z signal representing the intensity of gamma 
rays for each detection position, image memory means, coupled to said 
scintillation camera means, for storing data indicating a detection 
frequency of gamma rays exceeding a predetermined energy level at a memory 
location addressed by the X and Y position signals when the Z signal 
exceeds the predetermined energy level and providing image data 
representing a distribution of the radio isotope in the object, first 
compensation table means for storing compensation data for compensating 
for nonlinearity of the X and Y position signals, said first compensation 
table means being accessed by the X and Y position signals from said 
scintillation camera means to read out corresponding X and Y compensation 
data therefrom, second compensation table means for storing energy 
compensation data corresponding to an energy detection response for gamma 
rays of said scintillation camera means, said second compensation table 
means being accessed by the Z signal from said scintillation camera means 
to read out corresponding Z compensation data therefrom, compensation 
means coupled between said scintillation camera means and said image 
memory means, and responsive to the X, Y, Z compensation data from said 
first and second compensation tables for compensating the X and Y position 
signals and the Z signal to be supplied from said scintillation camera 
means to said image memory means, energy spectrum data memory means, 
coupled to said scintillation camera means, for storing an energy spectrum 
of gamma rays, energy spectrum center-of-gravity detecting means, coupled 
to said energy spectrum data memory means, for detecting the center of 
gravity of energy distribution, and rewriting means, coupled to said 
energy spectrum data memory means and said second compensation table 
means, for, when a current center-of-gravity position detected by said 
energy spectrum center-of-gravity detecting means is shifted from a 
previously detected center-of-gravity position, forming energy 
compensation data in accordance a shifting amount between the 
center-of-gravity positions, and rewriting said second compensation table 
means.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, position signals X and Y representing detection 
positions of gamma rays and an energy signal Z representing the intensity 
of the gamma rays are obtained in an analog form from a scintillation 
camera 11 having a known configuration. Signals X, Y, and Z are applied to 
corresponding analog-to-digital (A/D) converters 12 to be converted to 
12-bit digital signals. Digital signals X, Y, and Z are applied to a 
compensation circuit 13 for the nonlinearity and sensitivity to produce 
compensated signals X', Y', and Z', as will be described later. 
Signals X' and Y' are applied to a memory controller 14 and signal Z' is 
applied to a window circuit 15. Window circuit 15 issues a write command 
to memory controller 14 only when input signal Z' represents an energy 
level within a predetermined range. Memory controller 14 applies position 
signals X' and Y' to an image data memory 16 as address signals in 
response to application of the write command thereto, and increments, by 
unity, data in a memory location in the memory designated by address 
signals X' and Y'. This means that data indicating the detection frequency 
of gamma rays exceeding a predetermined level (i.e., scintigram data) is 
stored at memory locations corresponding to the respective pixel positions 
on the detection surface of scintillation camera 11. The scintigram data 
is read out from image data memory 16 to a display device (not shown) for 
observing the scintigram of the object. Memory controller 14, window 
circuit 15, and image data memory 16 are controlled by a central 
processing unit (CPU) 17 in the same manner as in existing systems. 
Position signals X and Y from A/D converters 12 are applied to a linearity 
compensation table (RAM) 18 as address signals so that compensation data 
.DELTA.X and .DELTA.Y stored in advance for linearity compensation are 
read out therefrom. Energy signal Z from A/D converter 12 is applied as an 
address signal to a Z compensation table (RAM) 19 to read out energy 
detection response compensation data a(x,y) obtained from a measurement 
and previously stored therein. Compensation data .DELTA.X, .DELTA.Y, and 
a(x,y) read out from RAMs 18 and 19 are applied to compensation circuit 13 
to correct signals X, Y, and Z in accordance with predetermined 
compensation calculations (to be described later) and to form compensated 
signals X', Y', and Z'. 
The compensation calculations are made as follows: 
EQU X'=X+.DELTA.X 
EQU Y'=Y+.DELTA.Y (1) 
EQU Z'=a(x,y).times.Z 
.DELTA.X and .DELTA.Y are expressed by 
EQU .DELTA.X=a0X+b0Y+c0X.multidot.Y+d0 
EQU .DELTA.Y=a1X+b1Y+c1X.multidot.Y+d1 
Where a0, a1, b0, b1, c0, c1, d0, and d1 are constants determined by X and 
Y. 
The above-mentioned compensation method of position signals X and Y and 
energy signal Z is well known to those skilled in the art. 
The main feature of the present invention resides in monitoring the energy 
spectrum of energy signal Z at all times, and rewriting compensation 
coefficients a(x,y) of RAM 19 when a center position (channel) of the 
energy spectrum is shifted. 
To this end, according to this invention, a memory 20 which is accessed by 
position signals X and Y and stores energy spectrum data, and an energy 
analyzer 21 for analyzing the energy of signal Z are provided. 
FIG. 2 shows energy spectrum Z(x,y,m) of signal Z. In FIG. 2, the abscissa 
represents value m of signal Z, which ranges from 0 to 4096 since signal Z 
is a 12-bit signal. In this embodiment, the energy analysis is made with 
respect to the .+-.10% range of a central value (peak value) of the 
spectrum. Energy analyzer 21 samples the energy spectrum of the .+-.10% 
range into, for example, 0 to 31 channels, as shown in FIG. 3, and k 
represents the channels. Energy analyzer 21 generates a 5-bit output 
signal k representing one of 32 channels in response to 12-bit input 
signal Z. Output signal k of analyzer 21 is applied to energy spectrum 
memory 20 as an address signal together with position signals X and Y. 
Memory 20 has 64.times.64 matrix storage areas of a two-dimensional plane 
defined by position signals X and Y, as shown in FIG. 4. The 64.times.64 
storage areas defined by signals X and Y each include storage subareas 
corresponding to the 32 channels of signal Z. Therefore, memory 20 has 
60.times.60.times.32 memory locations, in each of which is stored data 
Z(i,j,k) with i=x and j=y. Z(i,j,k) represents the energy detection 
frequency of gamma rays, corresponding to k, stored in the kth channel of 
a memory area defined by signals X and Y. Energy analyzer 21 issues 
address data k as well as address data X and Y to energy spectrum memory 
20 to read out data Z(x,y,k) therefrom, and returns data Z(x,y,k)+1 in the 
same memory location accessed by address data X, Y, and k. As a result, 
data representing energy spectrum as indicated by a solid curve 51 in FIG. 
5 is stored in 32 channels of a memory area defined by X=i and Y=j. A 
center channel indicating the peak value of the energy spectrum is 
indicated by .alpha.. 
An energy spectrum peak detector 22 is provided which is coupled to memory 
20, and detects a peak value (the center) of the energy spectrum 
distribution, on the two-dimensional plane defined by the X and Y 
coordinates, stored in memory 20. As described previously, when a change 
in gains of the photomultipliers with passage of time occurs, the center 
channel of the energy spectrum distribution may change from .alpha. to 
.beta., as shown in FIG. 5. This would cause a change in the center (i.e., 
peak value) of the energy spectrum. Such a change requires a correction of 
the Z compensation table. 
Detector 22 detects the center the of energy spectrum distribution and 
supplies peak value data to CPU 17. CPU 17 calculates compensation 
coefficient a'(x,y) from the shifting of the center of the energy spectrum 
distribution resulting from the shifting of the center channel as shown in 
FIG. 5, and corrects Z compensation table 19. For this reason, the 
necessity of a working of correcting RAM 19 is obviated regardless of a 
change with time in gains of the photomultipliers. 
FIG. 6 shows energy analyzer 21 and energy-spectrum peak detector 22 in 
detail. Energy analyzer 21 comprises a sampling circuit 21a and an 
addition circuit 21b. As described previously, sampling circuit 21a 
receives signal Z which may have a value from 0 to 4096 and divides a 
range from 2700 to 3300 centered at 3000 into 0 to 31 channels. The output 
signal indicating one of 0 to 31 channels is supplied to memory 20 as an 
address signal. As a result, data Z(x,y,k), which represents the energy 
detection frequency in the kth channel, is read out from the memory 
location accessed by address data X, Y, and k to addition circuit 21b, and 
is incremented by one so that data Z(x,y,z)+1 is returned to the same 
location of memory 20. This means the count of energy detection frequency 
in the kt channel at a position defined by position signals X and Y. 
Energy spectrum peak detector 22 comprises emory control circuit 22a and 
spectrum peak value calculation circuit 22b. Memory control circuit 22a 
receives address data X, Y, and k from CPU 17, supplies them to memory 20, 
and then receives data Z(x,y,k) therefrom. Circuit 22a supplies data 
Z(x,y,k) read out from memory 20 to calculation circuit 22b. Circuit 22b 
calculates the peak value of gc of energy spectrum distribution as 
follows: 
##EQU1## 
Peak value data gc thus calculated is supplied to CPU 17 through control 
circuit 22a. CPU 17 incorporates an internal memory 17a, which stores 
previously calculated peak value data gc'(x,y). When the shifting of the 
center of the energy spectrum distribution occurs, CPU 17 calculates new 
compensation coefficient data a'(x,y) from the following expression: 
EQU a'(x,y)=a(x,y).times.(gc/gc') 
CPU 17 rewrites RAM 19 in accordance with the thus calculated compensation 
data. In this manner, even if a change with time in gains of 
photomultipliers occurs, since the Z compensation table can be 
automatically rewritten, precise scintigram data of an object to be 
examined can always be stored in image data memory 22. 
During the above-mentioned compensation operation, if the shifting of the 
center of the energy spectrum is too large to be compensated for, the CPU 
may be arranged to produce an alarm to an operator without performing the 
compensation operation.