Scintillation camera with improved output means

In a scintillation camera system, the output pulse signals from an array of photomultiplier tubes are coupled to the inputs of individual preamplifiers. The preamplifier output signals are coupled to circuitry for computing the x and y coordinates of the scintillations. A cathode ray oscilloscope is used to form an image corresponding with the pattern in which radiation is emitted by a body. Means for improving the uniformity and resolution of the scintillations are provided. The means comprise biasing means coupled to the outputs of selected preamplifiers so that output signals below a predetermined amplitude are not suppressed and signals falling within increasing ranges of amplitudes are increasingly suppressed. In effect, the biasing means make the preamplifiers non-linear for selected signal levels.

BACKGROUND OF THE INVENTION 
This invention relates to scintillation cameras which are commonly called 
gamma cameras. The invention is particularly concerned with improving the 
uniformity and resolution of scintillation cameras. 
In nuclear medicine, scintillation cameras are used to detect gamma ray 
photons emitted from a body in which a radioisotope has been infused. The 
photons are emitted in correspondence with the extent to which the isotope 
is absorbed by the tissue under examination. With proper processing, 
signals corresponding with the photons may be used to develop a 
point-by-point image, corresponding with the emission pattern, on a 
cathode ray oscilloscope. A common camera system in use today is based on 
the camera of Anger as disclosed in U.S. Pat. No. 3,011,057. The Anger 
camera comprises an array of photosensitive devices such as 
photomultiplier tubes, usually hexagonally arranged, having their input 
ends adjacent a light conducting plate or disc. Beneath the disc is a 
scintillation crystal which converts incoming gamma photons into light 
photons or scintillations. A collimator is interposed between the 
scintillator and the body so that photons emitted by the body will impinge 
perpendicularly on the planar scintillation crystal. 
The scintillations are detected by the array of individual photomultiplier 
tubes which view overlapping areas of the crystal, and well-known 
electronic circuits are used to convert the outputs of the photomultiplier 
tubes into x and y coordinate signals which are used to control a cathode 
ray oscilloscope in such manner that each point source of light formed on 
the oscilloscope tubes correspond with a point at a similar location in 
the crystal or on the body. The output signals are also used to develop a 
z signal which turns on the oscilloscope tube in accordance with the 
computed coordinates. The z signal is developed only if the energy of the 
scintillation event falls within a predetermined energy window. A 
photographic film may be used as an integrator of the large number of 
light spots appearing on the screen of the oscilloscope. A substantial 
number of scintillation events is required to make up the final picture of 
radioactivity distribution in the body tissue. 
One problem in existing scintillation camera systems is that when a source 
of radioactivity having uniform distribution is placed close to the 
crystal disc and a photograph is made of the oscilloscope, the photograph 
will show non-uniformity which is characterized by "hot spots" under each 
photomultiplier tube and cold spots between the tubes. In other words, a 
spot or scintillation event actually occurring between the photomultiplier 
tubes is sensed as being partially shifted under the tubes, causing a 
decrease in spot density between the tubes and an apparent increase in 
spot density under the tubes. This phenomenon can be mitigated by moving 
the photomultiplier tubes further from the disc, but this decreases the 
ability of the camera to resolve small details. Hence, if small details 
are to be resolved and if uniformity or correspondence between the 
generated and displayed image patterns is to be maintained, the normal 
electric signals that exist in the system must be modified or corrected. 
One method of obtaining correction with non-electronic means is illustrated 
in U.S. Pat. No. 3,774,032 which is assigned to the assignee of the 
present invention. In this patent the distribution of light as perceived 
by the photomultiplier tubes is altered by placing masks between the 
crystal and photomultiplier tubes so that light from certain areas of the 
scintillator crystal cannot go directly to the photomultiplier tubes. This 
reduces the output of the tubes for scintillations occurring directly 
under them but it permits light from other areas, that is, from between 
the tubes to go directly to the tubes. The result is better resolution and 
uniformity in the image. 
It has been proposed heretofore to achieve the results obtained in the 
cited patent by use of electronic correction means. Without electronic or 
other correction, scintillations occurring in areas between the tubes 
appear to be, by inherent geometric phenomena, nearer to the tubes. This 
is manifested by what is called nonuniformity of the displayed image. More 
particularly, the image derived from a uniformly distributed isotope 
source is more dense or concentrated immediately under and near the tubes 
than in between the tubes. Electronic correction is further based on 
recognition that if the input and output signals of the preamplifiers are 
linearly related, the disproportionality between brightness and distance 
remains, but if the output is modified so that low level signals 
corresponding with noise are eliminated and high level signals 
corresponding with the scintillation event occurring at or near the center 
of the tube are suppressed, more uniform distribution of the light spots 
on the display will be accomplished. It has been proposed and demonstrated 
in the prior art that if the output of the preamplifiers is properly 
biased, high level signals can be clipped or suppressed which, in effect, 
amounts to reducing the gain of the preamplifiers for high amplitude 
signals or signals above a predetermined amplitude. Thus, the plot of 
preamplifier input signal versus preamplifier output signal is linear for 
a first comparatively low level signal range and it has a break point in 
it after which gain is reduced for higher level input signals. 
A system using the single break point concept has been made and tested and 
found to produce better results than were obtainable with linear 
amplification over the entire input signal range. However, uniformity and 
resolution were still not optimized for there was still some evidence of 
nonuniformity or concentration of light spots where they should have been 
uniformly distributed. In other words, there were still localized "hot" 
and "cold" spots which appeared randomly throughout the crystal, varying 
from system to system and depending upon the individual characteristics of 
the components of the system. 
SUMMARY OF THE INVENTION 
A preferred embodiment of the present invention is based on the recognition 
that more than one change in slope of the input to output transfer 
characteristic of the preamplifiers can eliminate the small localized hot 
and cold spots which still existed when known techniques for eliminating 
them were employed. Thus, in accordance with the invention, two or more 
selected bias voltages are applied to the output of selected preamplifiers 
to optimize uniformity and resolution. 
It is a general object of this invention to improve the uniformity or 
accuracy of correspondence between the light dots that comprise a 
displayed image with the positions of the scintillations which correspond 
with radiation absorption events. 
Another object is to improve the resolution of a scintillation camera. 
How these and other more specific objects of the invention are achieved 
will appear in the course of the ensuing description of a preferred 
embodiment of the invention which will now be set forth in reference to 
the drawings.

DESCRIPTION OF A PREFERRED EMBODIMENT 
FIGS. 2 and 3 show a schematic transverse section and an elevation view of 
a scintillation camera with which the new circuitry may be used. In FIG. 3 
the scintillation camera is generally designated by the reference numeral 
20. It is disposed over a body 21. The body or an organ thereof is assumed 
to have absorbed a radioactive isotope and that the distribution of the 
isotope and, hence, the configuration of the tissue that absorbed it is to 
be imaged. The isotope emits gamma ray photons which are intercepted by 
the gamma camera 20. The camera comprises a radiation opaque housing 22. 
Fastened to the bottom of the housing is a collimator 23 comprised of an 
array of gamma radiation permeable tubes with impermeable material between 
them. Inside of the housing is a closed container 24 which has a gamma ray 
photon permeable bottom 25. Immediately above bottom 25 is a planar disc 
26 made of crystalline material such as thallium activated sodium iodide 
which produces a scintillation at any point where it absorbs a gamma ray 
photon. An array of photosensitive devices such as photomultiplier tubes 
1-19 are situated above scintillator crystal 26. The photomultiplier tubes 
are coupled to crystal 26 with a light pipe 27 which may consist of a 
glass plate. Scintillations in crystal 26 are detected by the tubes which 
each produce pulse output signals for each scintillation event. 
As can be seen in FIG. 2, an array consisting of 19 photomultiplier tubes 
are used in this example and they are arranged hexagonally about a central 
tube 10. As is well-known, a common number of tubes used in a 
scintillation camera is 19 but cameras with 37 photomultiplier tubes are 
also used. The present invention has application in systems using 19 or 37 
or other numbers of tubes. 
FIG. 1 is a block diagram of the main components of a scintillation camera 
system in which the invention may be used. As indicated, the 19 
photomultiplier tubes which, in this FIGURE, are indicated collectively by 
the number 30, cooperate to detect each scintillation and their 19 outputs 
31 are separately coupled to individual preamplifier circuits 32. The 19 
preamplifier outputs 33 are coupled to a resistor matrix and summing 
amplifier circuit 34 which develops from the preamplifier outputs, four 
coordinate output signals +x, -x, +y, and -y on lines 35-38. These four 
output signals are fed to line amplifiers and gated stretchers 39 and to a 
z pulse former and pulse height analyzer (PHA) 40. The z pulse former 
combines the four input signals into a z signal which corresponds with the 
energy of a scintillation event. The z signal is supplied by means of line 
41 to difference amplifier and ratio circuits 42. The pulse height 
analyzer 40 gates the gated stretchers if the energy of a scintillation 
event falls within a selected energy window so that stretched +x, -x, +y, 
and -y signals on lines 43-46 may be fed to the difference amplifiers and 
ratio circuits 42 where the +x and -x signals and +y and -y signals are 
subtracted and the results ratioed with the z pulses as the denominator to 
produce x and y coordinate signals on line 47 and 48. The pulse height 
analyzer 40 also produces an unblanking signal on line 50 which is fed to 
a display cathode ray oscilloscope (CRO) 51 when the analyzer has 
determined that a scintillation event falls within a selected energy 
window and the display CRO then produces a spot of light on its screen 52 
at the x and y coordinates which have been computed. 
The system just outlined is basically well-known to those involved in 
design and use of scintillation camera systems. 
One channel for developing signals corresponding with the x and y 
coordinates of the scintillations detected by one photomultiplier tube is 
depicted in FIG. 4. Some parts of this circuitry are well-known in the 
art. Typically a channel comprises an input photomultiplier tube 30 of a 
well-known multiple dynode type. The anode of photomultiplier tube 30 is 
connected through a resistor 53 to a high voltage source terminal 55 
which, as indicated by the arrowhead line 56 also connects to all of the 
other photomultiplier tubes in the 19 tube array 30. The output pulse 
signals from photomultiplier tubes 30 are supplied to the noninverting 
input of a signal converting means which comprises an operational 
amplifier 57 that serves as a preamplifier. The signals are coupled to 
preamplifier 57 from the intermediate point 58 of a capacitive voltage 
divider comprised of capacitors 59 and 60. The input of amplifier 57 is 
protected against excess voltage with a diode 61. A bias resistor 62 for 
establishing zero offset is also provided. The preamplifier has a feedback 
circuit comprised of resistors R2 and an input resistor R1 and is ac 
coupled to ground with a capacitor 63. 
As is known, the amplitudes of the pulse signals from photmultipliers 30 
depends on the distance of a particular scintillation event in the 
scintillation crystal 26 from the photomultiplier tube under 
consideration. In the absence of the new features of the circuitry, input 
signals to the amplifier 57 are amplified linearly which, as was explained 
earlier, results in giving weight to the signal in correspondence with the 
apparent distance of the scintillation event from the photomultiplier 
tube, but this does not correspond with the real distance and leads to 
error. As was also explained, there is a bunching effect in direct 
alignment with the photomultiplier tubes which is manifested by hot spots 
in a CRO display even though the isotope source is uniform. Hot spots 
still exist although the highest amplitude signals are suppressed with the 
use of prior art single step suppression techniques. 
Whether or not the new features of the circuitry which will be explained 
are used, the output signals from the preamplifiers 57 are conducted 
through a resistor R3 to a resistor matrix 64 which is involved in 
computing the x and y coordinates of the scintillation event that produced 
the pulse signal. A typical resistor matrix is illustrated. It comprises 
four voltage dividers. The dividers are comprised of resistors +RX, R6, 
-RX, R7; +RY, R8; and -RY, R9. As is well-known by those using matrixes of 
this kind in scintillation camera systems, the resistors in the RX and RY 
pairs are weighted so that they are representative of or correspond with 
the reciprocal of the distance of the particular photomultiplier tube from 
the Y or X axes of the photomultiplier tube array 30. As shown in FIG. 2, 
the Y axis passes through the center point of tubes 2, 10 and 18 and the X 
axis passes through the center points of tubes 8-12. The midpoints of the 
dividers are connected to common lines 35-38. Each of the photomultiplier 
tube channels has an associated resistor matrix 64 with its particularized 
value resistors connected to common lines 35-38. For instance, the ends 65 
of the common lines will connect to resistor matrixes associated with each 
of the other photomultiplier tubes in the array 30. The output signals on 
lines 35-38, which are indicative of the x and y coordinates of the 
scintillations, are supplied to the amplifier and pulse stretchers which 
are symbolized by the block 39 in FIG. 1. Then, in accordance with known 
practice, coordinates of a scintillation are used to create a light spot 
at a corresponding position on the screen 52 of the CRO. 
The new means for obtaining non-linear amplification or output from 
preamplifiers 57 for photomultiplier tube pulse output signals falling 
within two or more amplitude ranges will now be discussed. The lowest 
level signals are amplified linearly by preamplifiers 57 and pass without 
modification to the resistor matrixes 64. A diode, not shown, could be 
inserted between the outputs of preamplifiers 57 and R3 so that noise 
signals below the diode forward voltage threshold are blocked. In 
accordance with the invention, the output lines 66, connecting the outputs 
of preamplifiers 57 to the resistor matrixes have bias voltages 
selectively applied to them. In this example, two biasing means 67 and 68 
are shown. Biasing means 67 comprises diode D2 and resistor R4 which are 
connected in series between the lines 66 and the output terminal 69 of a 
bias or threshold voltage source 70. A filter capacitor 71 is connected 
between the bias voltage source 70 and ground. The voltage at the output 
terminal 69 of bias voltage source 70 is set at a threshold value which is 
slightly negative with respect to ground in this example. Diode D2 is, 
thus, normally reverse biased. Incoming pulse signals from the 
photomultiplier tubes 30 drive the output of preamplifiers 57 negatively. 
Output pulse signals which are less than the negative bias or threshold 
voltage applied through diode D2 are applied by way of line 66 at their 
full amplitude to resistor matrix 64. However, signals at the output of 
preamplifiers 57 which are sufficiently negative to exceed the bias source 
threshold voltage cause diode D2 to become forward biased to thereby limit 
the amplitude of the signal or, to effectively reduce the gain of the 
preamplifiers for signals that are more negative than the negative voltage 
applied through D2. 
The other biasing means 68 comprised of R5, diode D3 and a filter capacitor 
72 is connected to the output terminal 73 of a second bias voltage source 
which establishes a second threshold voltage. Source 74 is set so that the 
bias voltage appearing on its output 73 is slightly more negative than the 
bias voltage on output terminal 69 of the other voltage source 70. Diode 
D3 is reversed biased to a greater extent than diode D2. Hence, diode D3 
becomes forward biased when the output signals from the preamplifiers 57 
are somewhat more negative than is required to forward bias diode D2. 
The bias voltage sources 70 and 74 are similarly constructed but, as has 
been explained, their outputs are set at different threshold voltage 
levels. Batteries or other stable voltage sources could be used in place 
of sources 70 and 74. In this example, source 74 comprises a transistor 80 
having its collector connected to ground or the midpoint of a dual voltage 
power supply, not shown. The bias voltage is developed across an emitter 
resistor 81. Fixed base-emitter bias is supplied through resistor 82 and 
this bias is made adjustable for controlling output voltage by use of a 
potentiometer 83. The output bias voltage of the other source 70 is set 
with a potentiometer 84. 
Those skilled in the art will appreciate that in cases where the pulse 
output signals from the preamplifiers 57 are positive going, diodes D2 and 
D3 would be poled oppositely and bias sources 70 and 74 would be arranged 
to supply positive bias voltages on their output terminals 69 and 73, 
respectively instead of negative voltages as in this example. 
FIG. 5 shows graphically how the gain of the preamplifiers is affected by 
use of the two biasing means 67 and 68. It will be seen that a first low 
range of input signals from the photomultiplier tubes will be amplified 
linearly as evidenced by the first segment 86 of the transfer 
characteristic curve having a constant slope up to a first break point 87 
where the bias voltage applied to diode D2 is less negative or more 
positive than the output signals. Hence, input signals such as 88 in a 
first range up to break point 87 are amplified linearly as shown by the 
corresponding output signal 89. The break point at which the bias of diode 
D3 is overcome is marked 90 on the graph. Thus, any input voltage such as 
91 having a peak in a second range between break points 87 and 90 will 
undergo decreased or non-linear amplification as can be seen from tracing 
the dashed lines extending from the input signal 91 to the corresponding 
output signal 92. When diode D3 becomes forward biased, corresponding with 
break point 90, the slope of the transfer characteristic or the effective 
gain of the preamplifier again changes to a lesser value. Thus, any input 
signal such as 93 having a peak in a third range greater than the break 
point 90 will undergo less amplification as evidenced by the corresponding 
output signal 94. Additional steps of bias voltage could be used such that 
there would be more than two break points in the transfer characteristic, 
but good results have been obtained with only two break points. If a 
diode, not shown, were placed in series with the output of preamplifier 57 
to eliminate response to low level noise, line 86 would start at a small 
offset from the zero axis in FIG. 5. 
Specific values of the bias voltages applied to diodes D2 and D3 would 
depend on the characteristics of a particular circuit. However, in a 
practical embodiment, the desired results have been obtained by setting 
the lower bias voltage on output terminals 69 at about one-half of the 
peak voltage of the outputs from the preamplifiers 57. The second bias 
voltage on terminal 73 was set at about 0.7 of peak signal voltage. Peak 
signal voltage may be determined by opening the D2 and D3 series circuits 
and making a measurement of the maximum signal that is obtainable at the 
outputs of preamplifiers 57 or by biasing diodes D2 and D3 to a larger 
value than peak voltage. Then the diodes circuits are closed and the 
potentials of the biasing sources are adjusted with potentiometers 83 and 
84 until a uniform display on the display tube 52 with a uniform 
radioisotope source is obtained. 
The relatively higher of the two bias voltages, that is, the voltage from 
source 74 is applied only to the preamplifier outputs 57 for the central 
cluster of photomultiplier tubes in FIG. 2, in other words, to the 
preamplifiers for tubes 10, 5, 6, 11, 15, 14 and 9 in a nineteen tube 
array. The lower bias voltage supplied by source 70 is applied to the 
preamplifier outputs of all of the remaining tubes. However, for tubes 1, 
3, 8, 12, 17 and 19 the value of R4 is about 30% lower than the value of 
R4 in the circuit to preamplifiers 57 associated with photomultiplier 
tubes 2, 4, 7, 13 and 18. Thus, voltage division resulting from action by 
the divider R4 and R3 is slightly different from one group of tubes than 
for another. In a practical embodiment, 1 kilohm resistors were used for 
R4 and R5 and most of the tubes and a value of 1.3 kilohms was used for R4 
in preamplifier circuits associated with photomultiplier tubes 2, 4, 7, 13 
and 18. 
The field of view of the scintillation camera shown in FIG. 2 falls 
substantially within a circle that is slightly inside of the centers of 
tubes 4, 13 ,18, 16, 7 and 2. Other tubes at the edge of the array do not 
contribute very much signal. Scintillations are only detected if they 
occur on the inside of the outer tubes. This accounts for the manner in 
which the bias voltages are applied as discussed in the last paragraph. In 
any event, it will be evident that the photomultiplier tubes which make 
the greatest contribution to signal are suppressed most, which means that 
hot spots are reduced and resolution is improved. In scintillation cameras 
using 37 or some number of photomultiplier tubes in excess of 19, the 
central clusters of tubes within the hexagonal array would have their 
preamplifier 57 outputs subjected to the higher bias or threshold voltage 
and rings of peripheral tubes would be biased less. Although the foregoing 
selective connection arrangement is preferred, good results are also 
obtained if all of the tubes are treated in the same manner. 
Although a particular scheme for producing multiple break points in the 
transfer characteristics of the photomultiplier tube preamplifiers has 
been described in considerable detail, such description is inteded to be 
illustrative rather than limiting for the attainment of multiple break 
points may be variously achieved. Hence, the scope of the invention is to 
be determined only by construing the claims which follow.