CA1052012A - Delay line clipping in a scintillation camera system - Google Patents

Abstract

Abstract of the Disclosure A scintillation camera system includes compensated delay line clipping circuitry for eliminating base line under-shoot and a restoring circuit for minimizing base line restora-tion of a data signal. The compensated circuitry includes a single delay line circuit for clipping an incoming data pulse having a relatively long trailing edge. A variable resistor is coupled with the circuit for controlling amplitude and polarity of the unclipped, incoming data pulse. A value of this unclipped pulse is controllably summed with the clipped pulse for correcting delay line attenuation and base line under-shoot. The restoring circuit is combined with the single delay line clipping circuit for restoring the base line of the single delay line clipped pulses. The restoring circuit produces double delay line clipped timing pulses from the single delay line clipped waveform pulses. The circuit includes a restoring gate connecting a data line coupling capacitor to circuit ground.
The gate is responsive to the double delay line clipped timing pulses for allowing selective discharging of the coupling capacitor in anticipation of a data pulse for maintaining the base line of the data pulse.

Description

105~01;~
This invention relates to scintillation camera systems and more specifically to scintillation camera systems having high speed pulse shaping circuitry for minimizing base line fluctuation and voltage undershooting.
In the diagnosis of certain illnesses, radioactive isotopes are administered to the patients. These isotopes have the characteristic of concentrating in certain types of tissue.
The degree of concentration in the tissue is dependent upon tissue type. For example, iodine 131 generally collects or concentrates in the tissue of the thyroid gland. Upon detec- -tion of the level of radioactive isotope concentration and presentation of this detected information on a suitable read-out device, such as an oscilloscope, it is frequently possible to diagnose the condition of the tissue under examination.
One well known type of device for detecting levels of radioisotope concentration is the scintillation camera system.
Scintillation cameras generally incorporate a relatively large disc-shaped scintillation crystal which is positioned so that the crystal intercepts gamma radiation emitted by a patient under study. The crystal scintillates in response to impinging gamma ray energy to provide pulses of light energy. A thallium activated sodium iodide crystal is typically employed as the scintillation crystal.
A plurality of phototubes are positioned adjacent the crystal so that a scintillation occurring in the crystal is normally detected by several of the phototubes. Each of the detecting phototubes develops in response to the scintillation an electrical signal having an amplitude proportional to the intensity of the light energy received by it. The signals developed by the phototubes are then amplified and applied to appropriate electronic computing circuitry for developing elec-trical signals representative of the position of the light pulse 1. ~ .

lOSZ012 or scintillation. The intensity of the signal is threshold de-tected by a process commonly referred to as pulse height analy-sis to determine whether the signals represent photopeak scin-tillations resulting from a gamma ray originating from the isotope which has been administered to the patient. The signals developed by the phtotubes are typically preamplified to pro-vide signals having relatively narrow pulse widths to enable as high a processing rate as possible. One such gamma ray imaging camera system is disclosed in U.S. Patent No. 3,683,180 issued August 8, 1972 to the Picker Corporation of New York.
At high counting rates this type of camera system has several difficulties which have hindered high speed oper-ation. Base line coupling capacitors used for transmission of the data pulses are intermittently charged and discharged by the pulse bursts passing through the capacitors. This results in considerable base line bouncing and lessens the fidelity of data detection. Because these pulses are pulse height analysed by threshold detectors, base line bouncing tends to introduce pulse height detection errors.
Scintillation camera systems now commonly utilize delay line clipping techniques for narrowing the pulse width of the data pulses for obtaining higher counting rates with minimum distortion of the pulse amplitudes.
Known delay line clipping circuits utilize a delay line to couple the pulse transmission line to circuit ground.
One input of a differential amplifier is coupled to the pulse transmission line and to the delay line while the other input of the differential amplifier is coupled to circuit ground. When using an ideal delay line, ideal resistors, and ideal amplifiers, a step input pulse is shortened to a square pulse of width twice the delay time of the delay line. Since delay lines have un-avoidable attenuation and are not ideal, the base lines of lOSZO~Z
the clipped pulses do not return to the original value. After the clipping they exhibit an offset proportional to the original step input voltage and to the delay line attenuation. Further-more, since the input pulse is finite with a decaying trailing edge rather than a step, the resultant clipped pulse frequently undershoots the base line voltage.
Attempts have been made in systems other than scin-tillation camera systems to stabilize the base line of single delay line clipped pulses. One such base line restoring circuit is that often referred to as an amplified diode restorer. Due to the finite forward breakover voltage of a diode restorer, even upon amplification, the restorer is unable to work properly below a threshold voltage of approximately 30 millivolts. This relatively large threshold voltage is unacceptable in radiation imaging systems as accurate pulse intensity data is required down to substantially zero volts. The accurate detection of such small voltages is required if the situs of respective scintillations is to be accurately reconstructed.
Double delay line clipping networks have been utilized to narrow pulses which have relatively long exponential fall times in nuclear spectroscopy systems. It is known that such double delay line clipped pulses are advantageous in certain respects over comparable single delay line clipped pulses. That is, the total charge deposited on any coupling capacitor in the data transmission line due to passage of each pulse is zero because the double delay line pulse exhibits equal energies above and below the base line. Accordingly, the average base line value is relatively constant; whereas, the single delay line clipped pulse exhibits a base line which is astable at high rates due to random time variations in the average charge passing through coupling capacitors.

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Double delay line clipped pulses have their disad-vantages when used as data pulses in a scintillation camera system. ~or example, for a given pulse duration the effective integration time of the photomultiplier tube generating the pulse is reduced. Conversely, if the integration time is kept equal to that of a single delay line clipped pulse, the total pulse duration is lengthened, resulting in a slower operating system.
On the other hand, double delay line data pulses are ideal timing pulses. The pulse widths may be sufficiently narrow to accommodate high frequency data pulses separated by as little as 1.5 microseconds. Because the base line of a double delay line pulse is stable, restoring circuitry is un-needed.
The present invention overcomes the above noted and other disadvantages by providing a novel base line restoring circuit and a novel delay line clipping circuit in a scintil-lation camera system. Single and double delay line clipped signal waveforms are generated for increasing the operational frequency and fidelity of data detection of the camera system by base line distortion such as undershooting, overshooting, and capacitive build-up.
The camera system includes a set of photomultiplier tubes and associate amplifiers which generate sequences of pulses. These pulses are pulse-height analyzed for detecting a scintillation having an energy level which falls within a predetermined energy range. Data pulses which have the pre-determined energy are said to represent photopeak events of the isotope which has been administered to the patient. These data pulses are combined to provide x+, x-, y+, y- coordinate data presentative of the situs of a photopeak event and to provide Z energy data representative of the energy of the photopeak event. The pulses are characteristically produced having a relatively long decaying trailing edge by dynamic biasing of the amplifiers. The amplifiers are biased out of saturation over all ranges of pulse energy levels and count rates.
Single delay line clipping circuitry is provided for narrowing the pulse width of the decaying electrical data pulses which increase operating speed without the occurrence of data loss. According to one aspect of the invention, the clipping circuitry is compensated and includes a variable resistance element which controls magnitude and polarity of portions of an unclipped data pulse and adds these portions to a delay line clipped data pulse for substantially eliminating undershooting base line voltage.
In another embodiment, a novel base line restorer circuit is provided which is advantageously used prior to any thresholding of the data pulses to allow precise amplitude analysis. The restorer circuit is used in combination with the single delay line clipping circuitry for providing double delay line type pulses without the requirement of a second delay line.
The delay line of the single delay line clipping circuitry is used for generating the double delay line pulses. The double delay line pulses are substantially synchronized in time with the single delay line clipped data pulses and are used as timing pulses.
The base line restorer circuit includes a gating element and a coupling capacitor connected in series with a data line over which the data pulses are transmitted. The gating element is responsive to the double line clipped timing pulse for selectively coupling the data line to a reference potential such as circuit ground. This discharges the coupling capacitor lOSZ~lZ
in anticipation of a data pulse and thereby provides the single delay line shaped data pulses with improved base line voltage characteristics.
It accordingly is a general object of this invention to provide a scintillation camera system having novel and impxoved pulse-shaping clipping circui~ry for maximizing system speed without incurring data loss.
Other objects and advantages and a fuller understand-ing of the invention may be obtained by referring to the follow-ing detailed description when read in conjunction with the accompanying drawings.
FIGURE 1 is a part pictorial, partially functional representation of a scintillation camera system which embodies the invention;
FIGURE 2 is a schematic of single and double delay line pulse-shaping clipping circuitry in accordance with an aspect of the invention for restoring the base line voltage of single delay line clipped data pulses;
FIGURE 3 is a schematic of pulse-shaping clipping cir-cuitry for eliminating base line voltage undershoot and base line voltage offset in accordance with another aspect of the invention;
FIGURE 4 represents waveforms associated with the pulse-shaping circuitry of FIGURE 2; and, FIGURE 5 represents typical waveforms associated with the pulse-shaping circuitry of FIGURE 3.
Referring now to FIGURE 1, a radiation imaging detec-tion system is depicted which embodies the present invention.
The radiation system generally includes a radiation detector unit 2 coupled to data processing circuitry 3. Radiation impinging upon the detector head 2 causes the generation of signals which are converted by the data processing circuitry 3 . 105~012 - ? -into meaningful pulse data relating to the ~tudy A computer 5 and a display mechanism 7 are also provided. The computer 5 converts the pulse data into a form required by the display mechanism 7. The display mechanism 7 produces an image repre-sentative of the radiation striking the detector head 2. The computer 5 and the display mechanism 7 are conventional and are described in U.S. Patent No. 3,697,753, Scintillation Camera Having a Variable Gain Plural Amplifier System, issued october 10, 1972.
The detector head 2 includes a large scintillation crystal of thallium activated sodium iodide and a collimator 4 which is positioned betweena subject under s~udy (not shown) and the crystal. The detector head 2 further includes a plur-ality of photomultiplier tubes positioned adjacent the crystal.
Conventionally there are nineteen tubes, and they generate elec-trical pulses in response to crystal scintillations that are ; generated by radiation impinging upon the crystal. The detector ; head 2 i8 explained in detail in united States Patent No.
3,784,819, issued January 8, 1974 to the Picker Corporation of Ohio.
The signals from the phototubes in the detector head

2 are coupled to the data processing circuitry.3. The data pro-cessing circuitry 3 includes an amplifier section 10, a decoder matrix 14, and clip and restore circuitry 16. The amplifier section 10 includes a charge preamplifier section 8 and a variable gain stage, line driver section 12, both of which are conventional. The sections 8, 12 are of the charge amplifier/
RC differentiation type. They generate pulses having typically a 220 n~nosecond rise time constant and a 10 microsecond fall time constant. Dynamic biasing circuitry 9 is provided for biasing the amplifiers for unsaturated operation over all ranges of energy levels and pulse rates.

105Z01'~

It has been found advantageous to increase the fall time constant of the pulses to approximately 10 microseconds from the conventional one microsecond fall time. These pulses h~ving a relatively long trailing edge are subsequently clipped to provide pulses having width of approximately 2 microseconds.
The described pulse-shaping arrangement allows approximately 99% of the scintillation energy to be captured by the photo-multiplier tubes and transmitted.
The 10 microsecond data pulses from the amplifier section 10 are input into the decoder matrix 14 whereby a plur-ality of coordinate channel signals x+, x-, y+, y- and an energy channel Z are generated in decoders 14a-14e respectively. More specifically, the nineteen photomultiplier tubes in the detec-tor head 2 generate section 10, whose outputs are coupled to the decoder matrix 14. The decoder matrix 14 appropriately combines the data pulses according to location of the phototubes to provide the coordinate and energy information. This infor-mation characterizes the situs and strength of the photopeak event. The decoder matrix 14 is now well known in the art, and its operation is readily understood.
The data representing coordinate and energy channel information from the decoder matrix 14 is input into the clip and restore circuitry 16 for wave-shaping and base line restor-ing, according to an important embodiment of the invention. The data pulses having a ten microsecond fall time are clipped to provide pulses having an approximate two microsecond width.
This has advantageously proven to minimize pulse pile-up, but has also proven to maintain data fidelity. The clip and restore circuitry 16 is shown and explained in detail in FIGURE 2.
The data pulses output from the clip and restore circuitry 16 are input into the computer circuitry 5 where various stages of threshold analysis of the data pulses is lOSZOlZ
performed. The circuitry 5 includes delay circuitry, pulse-stretching circuitry, and peak detectors, and ratio circuits (all of which is not shown). This circuitry is conventional and conforms the data for eventually reconstructing an image of the object under study on the display mechanism 7, which is an oscilloscope or recorder.
Referring now to FIGURE 2, a preferred embodiment of the clip and restore circuitry 16 is shown. The circuit 16 includes a single delay line clipping circuit 17 and a restoring circuit 18. The clipping circuit 17 is of a generally conven-tional design for a single delay line clipping circuit, and comprises a delay line 19 coupled to a data input line of a differential amplifier 22. A pair of resistors 21, 23 of value twice the magnitude of the characteristic impedance Zo, of the delay line 19 are serially connected in the data input line of the amplifier. The delay line 19 is coupled between circuit ground and the common connection of the resistors 21, 23. A
feedback resistor 25 of value 8 Zo is connected between the output of the amplifier 22 and the data input line.
2.0 The wave-shaping effect of the clipping circuit 17 is seen when viewing FIGURE 4. Waveform I in FIGURE 4a depicts a conventional prior art data pulse having a 220 nanosecond rise time constant and one microsecond fall time constant. The disadvantage of a pulse having this waveform is that such a pulse requires a relatively large integration time to sufficient-ly characterize the data. Not only does such a pulse having a large width reduce system speed, but it induces base line instability.
Waveform II in FIGURE 4a has a ten microsecond fall time constant, as produced by the amplifier section 10 in FIGURE
1. Data pulses having this waveform are input into the clipping circuit 17 which generates a single delay line clipped pulse, 105Z~2 waveform III. As seen in FIGURE 4b, waveform III exhibits a sufficiently ~ide pulse width to characterize the data from the photomultiplier tubes, yet it exhibits a sufficiently narrow pulse width to enable high speed system operation.
Because a single delay line clipped pulse, such as waveform III, causes base line fluctuation and instability, the restoring circuit 18 is utilized. The restoring circuit 18 assures maintenance of a steady base line for allowing precise amplitude determination by the thresholding circuitry in the computer circuitry 5.
Referring again to FIGURE 2, the restoring circuit 18 includes a feedback amplifier 24, a comparator 26 and a logic gate 27 which are serially connected to an output terminal of the comparator 26. A set of base line restoring elements in the form of restoring transistors 29a-29d is also provided.
The amplifier 24 has a feedback resistor 31 connecting the output terminal to a data input terminal. The data input terminal is directly connected to the shield of the delay line 19. This causes a double delay line clipped waveform to be generated by the amplifier 24.
A delay mechanism 28, a coupling capacitor C, and an output buffer amplifier 30 are serially connected to the output of the amplifier 22 for providing the single delay line clipped waveform. The delay mechanism 28 delays the arrival of the single delay line clipped data pulse at the coupling capacitor C. It is delayed until the double delay line clipped pulse generated by the restoring circuit 18 arrives at the restoring transistors 29a-29d. The coupling capacitor C removes high speed, A.C. base line fluctuations of the data pulses.
The amplifier 24 functions as a current to voltage converter for generating the double clipped pulse. The input 10 .

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current to the amplifier 24 is the return current in the delay line shield. Shield current is "out" of the delay line shield for the first half of the delay line shield output pulse, and is "into" the delay line shield during the second half of the output pulse. This causes the amplifier 24 to generate double clipped pulse shown in FIGURE 4C.
Because amplitude information is not important at the output of the amplifier 24, a double delay line clipped signal is ideal. The base line of a double clipped pulse is self-restoring and does not deviate with pulse occurrence. That is, as seen from FIGURE 4C, the voltage waveform exists an equal amount of time above the base line as it does below the base line, so that the net charge on the line is substantially zero.
If the restoring transistors 29 are not employed, variations in the average charge passing through the coupling capacitor C generate base line variance due to the randomness of pulses. This base line variation introduces detection errors in the processing circuitry and produces false output informa-tion describing the organ under study.
The restoring transistors 29a-29d are gated conductive by the double delay line timing pulses to overcome this problem.
When the transistors 29 are selectively gated conductive, they clamp the output side of the series coupling capacitor C to ground. If there is any residual charge accumulated in the capacitor C from the passage of a previous pulse, accumulation is discharged by the connection to circuit ground. When a pulse arrives at the output of the delay mechanism 28, a correspond-ing double clipped pulse also arrives at the logic gate 27 for rendering the restoring transistors inoperative and removing the connection to circuit ground.
Nonconductance of any restoring transistor 29 momen-tarily leaves the capacitor discharged (or negatively charged to lOSZ012 the base line voltage immediately preceding the data pulse).
The pulse therefore passes through the capacitor rapidly without additional charging of the line, and the base line is stable.
Buffer amplifier 30 presents a very high load impedance to the capacitor for insuring that the pulses pass through capacitor C
without further charging it. Since the output side of serially connected coupling capacitor C has been clamped at circuit ground, the buffer amplifier 30 faithfully reproduces the pulse starting from zero volts, regardless of the base line value of the single at the output of the delay mechanism 28.
The restoring circuit 18 is employed only in the clip and restore circuitry 16 of the Z energy channel. The x+, x-, y+, y- coordinate channels only have the restoring transistors 29b-29d connected to their respective clipping circuits 17. The Z energy channel is chosen to drive the restoring transistor 29 because photopeak events of interest generate pulses in the energy channel above a minimum, detectable level such as 100 millivolts. This level is a threshold for triggering the re-storing transistors 29b-29d in the coordinate channels and accurately determining the arrival of data pulses having a near zero magnitude. As is understood in radiation impinging systems, such small level data pulses in the coordinate channels must be accurately detected if precise situs reconstruction of the scin-tillation event is to be realized. Therefore, the Z energy channel in a radiation detection system is particularly well suited for driving the base line restoring circuitry 18.
Referring now to FIGURE 3, there is shown an improved single delay line clipping circuit which features compensation for voltage undershoot. The _mproved single delay line clipping circuit is similar to that referenced at 17 in FIGURE 2, but features a compensating circuit 32. Similar functional elements in the clipping circuits of FIGURE 2 and FIGURE 3 have like 12.

10520~Z
numbers and are not further described.
The compensating circuit 32 comprises a variable resistance element coupling the unclipped data signal on an input line 33 to one input terminal of the amplifier 22. In the illustrated embodiment variable resistance element is a voltage dividing potentiometer 34 which is coupled to the input ter-minals of the amplifier 22, and is coupled to receive the un-clipped signal on its wiper terminal. A resistor 36 is coupled to the wiper terminal and has a value determined by the imped-ance and the attenuation of the delay line. The potentiometer 34 provides a single control for uniformly correcting delay line attenuation and base line voltage undershoot of the clipped data pulse in one continuous adjustment.
More specifically, because delay lines, resistors and - 15 differential amplifiers are nonideal, and because the input data pulses are not purely step function pulses, i.e., they do exhibit decaying tails, the clipped output data pulse waveform is less than ideal. Due to attenuation by the delay line, the clipped pulse fails to return to the base line and exhibits an offset. This offset is proportional to the original step input voltage and to the delay line attenuation. Because the input pulse is not a step and exhibits a decaying tail, the resultant clipped pulse exhibits undershoot. The compensating circuit 32 corrects for both conditions in one continuous adjustment.
Adjustment of the potentiometer 34 modifies the un-clipped data pulse sc that output base line undershooting and ; offset of the clipped data pulse is substantially eliminated.
If the potentiometer 34 is set to one extreme position, a maxi-mum portion of the unclipped signal input via the wiper terminal is summed with the clipped signal passing through the resistor 23 to correct undershooting. By way of example, the unclipped - pulse on the input line 33 has a relatively long fall time lOS'~O~Z
constant, such as 10 microseconds and exhibits a decaying pulse above the base reference potential of the line. ~ecause the undershoot is of a similar but opposite magnitude, addition of the two signals results in an output signal having substantially zero voltage offset. By setting the potentiometer 34 to the other extreme position, a maximum portion of the input signal, determined by the resistance element 36, is summed in the opposite polarity with the clipped pulse to correct for delay line attenuation.
Although the invention has been described in its pre-ferred form with a certain degree of particularity, it is under-stood that such particularity has been for example only. Numer-ous changes in the details of construction and changes in the combinations and arrangements of the parts will be hereafter obvious without departing from the spirit and scope of the invention.

14.

Claims (14)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. In a scintillation camera system having scintillator means for producing scintillations in response to impinging radiation, light responsive means responsive to the scintil-lations for generating sequences of data signals representative of radiation impinging on a respective portion of said scintil-lator means, signal processing circuitry including pulse shaping circuitry responsive to the data signals for generating a plurality of data pulses defining the situs of respective scintillations and a display mechanism for producing images representing the radiation impinging on the scintillator means, the improvement wherein the pulse-shaping circuitry comprises a compensating circuit means including circuitry coupled to produce and combine first signals representing data signals and second signals representing data pulses for decreasing base line fluctuation of the data pulses and thereby improving pulse detection.

2. The scintillation camera system according to claim 1 wherein the pulse-shaping circuitry includes a clipping circuit having an input coupled to a data line and an output for clipping the data pulses.

3. The scintillation camera system according to claim 2 wherein said compensating circuit means further includes:
(a) differential amplifier means having first and second input terminals, said first input terminal coupled to a first reference potential; and, (b) resistor means coupling said second input terminal to the output of said clipping circuit and to said data line.

4. The scintillation camera system according to claim 3 wherein said clipping circuit includes a delay line resistively coupling said data line and said second input terminal to a second reference potential.

5. The scintillation camera system according to claim 3 wherein said resistor means further includes a potentiometer having its fixed resistance terminals coupling said first and second input terminals, and having its wiper terminal coupled to said data line.

6. The scintillation camera system according to claim 5 and further including resistor coupling said wiper terminal to said data line.

7. The scintillation camera system according to claim 1 wherein said compensating circuit means includes a capacitive element serially connected to a data line and a gating element selectively coupling said capacitive element to a reference potential.

8. The scintillator camera system according to claim 3 wherein said compensating circuit means includes:

(a) means coupled to said clipping circuit for providing a double delay line clipped timing signal;
(b) a capacitive element serially coupled to said data line; and (c) a gating element responsive to said timing signal for selectively coupling said capacitive element to a reference potential.

9. In a scintillation camera system having scintillator means for producing scintillations in response to impinging radiation, a plurality of amplifier drivers responsive to the scintillations for generating electrical pulses having ampli-tudes representative of radiation impinging on a respective portion of said scintillator means, and signal processing circuitry responsive to the electrical pulses for generating a plurality of data signals, said signal processing circuitry containing a plurality of coordinate signal amplifier driven lines for indicating coordinate information of the scintillation, and an energy signal amplifier driven line for indicating the energy magnitude of the scintillation, the combination including base line amplitude restorer means coupled to the energy signal and coordinate signal amplifier drivers, said base line amplitude restorer means comprising:
(a) a timing signal generator responsive to at least one of the energy and coordinate signal amplifier driven lines to produce a timing signal in response to the occurrence of a radiation scintillation; and (b) gating means coupling at least one of said coordinate signal lines to a reference potential in response to said timing signal for conditioning the signal processing circuitry for operation in anticipation of receipt of data representing a scintillation.

10. The scintillation camera system according to claim 9 wherein said gating means includes transistor switching means selectively coupling said energy channel line to said reference potential in response to said timing signal.

11. The scintillation camera system according to claim 10 wherein said transistor switching means includes a plurality of transistors selectively coupling said coordinate channel lines to said reference potential, in response to said timing signal.

12. A method of operating a radiation camera system having a detector for producing radiation signals in response to impinging radiation, a plurality of amplifier drivers responsive to the radiation signals for generating electrical pulses having amplitudes representative of radiation impinging on a restric-tive portion of said detector, and signal processing circuitry responsive to the electrical pulses for generating a plurality of data signals, said data processing circuitry including a plurality of coordinate signal amplifier driven lines for indicating coordinate information of the radiation and an energy signal amplifier driven line for indicating the energy magnitude of the radiation, and a base line amplitude restorer coupled to the energy signal and coordinate signal amplifier drivers, said method comprising the steps of:
(a) producing a timing signal in response to the occurrence of a radiation event as sensed by signals transmitted by at least one of the energy and coordinate signal amplifier driven lines, and (b) gating at least one of the coordinate signal lines to couple said coordinate signal line to a reference potential in response to said timing signal for conditioning the signal processing circuitry for operation in anticipation of receipt of data representing a radiation event.

13. In a radiation camera system having radiation sensitive structure for producing radiation indicating signals in response to radiation impinging on the radiation sensitive structure, and circuitry responsive to the radiation indicating signals for generating sequences of data signals representative of radiation impinging on the radiation sensitive structure, signal proces-sing circuitry including pulse shaping circuitry responsive to the data signals for generating a plurality of data pulses defining the situs of respective radiations and a display apparatus for producing images representing the radiation impinging on the radiation sensitive structure, the improvement wherein the pulse-shaping circuitry comprises a compensating circuit including circuitry coupled to produce and combine first signals representing data signals and second signals representing data pulses for decreasing base line fluctuations of the data pulses and thereby improving pulse detection.

14. In a radiation camera system having radiation sensitive structure for producing radiation indicating signals in response to radiation impinging on the radiation sensitive structure, a plurality of amplifier drivers responsive to the radiation indicating signals for generating electrical pulses having amplitudes representative of radiation impinging on said radiation sensitive structure, and signal processing circuitry responsive to the electrical pulses for generating a plurality of data signals, said signal processing circuitry containing a plurality of coordinate signal amplifier driven lines for indicating coordinate information of the impinging radiation, and an energy signal amplifier driven line for indicating the energy magnitude of the radiation event, a combination including base line amplitude restorer means coupled to the energy signal and coordinate signal amplifier drivers, said base line amplitude restorer means comprising:
(a) a timing signal generator responsive to at least one of the energy and coordinate signal amplifier driven lines to produce a timing signal in response to the occurrence of radiation impinging on the radiation sensitive structure; and (b) gating means coupling at least one of said coordinate signal lines to a reference potential in response to said timing signal for conditioning the signal processing circuitry for operation in anticipation of receipt of data representing a radiation indicating signal.