SCINTILLATION CAMERA APPARATUS INCLUDING AUTOTUNING

Abstract: A scintillation camera apparatus (100) comprises a plura­ lity of photomultipliers (20A, 20N) a plurality of high voltage control circuits (2A, 2N), and a comparator (8). The photomul­ tipliers (20A, 20B) generate radiation energy signals upon re­ ceipt of incidence light. The comparator (8) compares the peak pulse heights of the radiation energy signals with the corre­ sponding reference peak values to derive deviation signals. Based upon the deviation signals, the supply of the high volt­ ages to the photomultiliers (2A, 2N) is independently control­ led.

Description: The present invention generally relates to a scintillation camera apparatus, and more particularly to an autotuning circuit for automatically controlling the high voltage applied to a photomultiplier and the output characteristics of the photomultiplier. In a scintillation camera apparatus as one of the medical diagnostic imaging apparatuses, to adjust the high voltage applied to a photomultiplier, an operator sets an object in a predetermined position with respect to a scintillation camera, and then manually adjusts the high voltage while observing the positional relationship between a spectrum displayed on a monitor television and a window displayed thereon. In a scintillation camera apparatus operable with a computer, the CPU of the computer processes the spectrum to be displayed in a multi-channel and automatically recognizes the peak value of the spectrum. The CPU controls the stepping motor interlocked with a potentiometer for fine adjustment of high voltage, on the basis of a deviation of the peak value from a target value. In this way, the peak value of the spectrum can be automatically tracked. The output of the photomultiplier of each channel is controlled with manual adjustment of a potentiometer, which is coupled in series between the cathode of each photomultiplier and ground. Alternatively, the output of the preamplifier of each photomultiplier is automatically controlled by an attenuator with a feedback system. The manual adjustment of the high voltage supply causes a complicated and time-consuming work, and is a lack of qualitatively precise controlling. The adjustment using the stepping motor arrangement requires a large and expensive apparatus which is also poor in durability. With controlling the preamplifier output, a control circuit cannot be accommodated within the photomultiplier or its socket. This results in an increase in the size of the amplifier. Prior art document FR-A-2 286 393 discloses a system for calibrating a scintillation camera which comprises an automatic gain control and has features similar to the scintillation camera apparatus as summarized in the precharacterizing part of claim 1. It is an object of this invention to provide a scintillation camera apparatus with an arrangement which is small in size, good in durability, reliability and cost performance, and further with an autotuning means capable of automatically setting the high voltage and output of a photomultiplier. To solve this object the present invention provides a scintillation camera apparatus as specified in claim 1. For a better understanding of the above-described object and various features of the invention, reference is made to read the description of the specification in conjunction with the accompanying drawings, wherein: Fig. 1 is a schematic block diagram of a scintillation camera apparatus useful for explaining the invention; Fig. 2 is a circuit diagram of the high voltage fine control circuit employed in the apparatus shown in Fig. 1; Fig. 3 is a circuit diagram of the practical counter circuit of Fig. 2; Fig. 4 is a schematic block diagram of a scintillation camera apparatus according to the invention; Fig. 5 is a circuit diagram of the gain control circuit employed in the apparatus shown in Fig. 4; and Fig. 6 is a schematic block diagram of the practical gain control circuit used in the apparatus shown in Fig. 4. The basic idea of the apparatus useful for explaining the invention is characterized by employing an autotuning means having a comparator for comparing a peak pulse height of an energy signal output from a photomultiplier with a predetermined reference peak value so as to produce a deviation signal representing the differences, a high voltage fine adjustment circuit connected between the cathode of the photomultiplier and ground for precisely adjusting high voltage of the photomultiplier, and a control means for setting adjusting conditions for the high voltage fine adjustment circuit on the basis of the deviation signal from the comparator, whereby the high voltage and output of the photomultiplier in the respective channels are automatically and independently set. Fig. 1 shows an autotuning means employed in a scintillation camera apparatus 100. The autotuning means is comprised of a CPU 1, a plurality of high-voltage fine adjustment circuits 2A to 2N (N being more than 2), a plurality of preamplifiers 3A to 3N (N being more than 2), a plurality of photomultipliers 20A to 20N (N being more than 2), a light source changer 4, a first channel selector 5A, a second channel selector 5B, a channel controller 6, an A/D (analog-to-digital) converter 7, and a comparator 8. CPU 1 has input setting means 1A and controls the autotuning means. CPU 1 serves as a system control means. High voltage fine adjustment circuits 1A to 2N are connected between ground, on the one hand, and the cathodes of a plurality of photomultipliers 20A to 20N, on the other hand. Preamplifiers 3A to 3N amplify the peak pulse heights of the spectrum of energy signals output from N sets of photomultipliers 20A to 20N. That is to say, N sets of signal processing channels are provided in the apparatus 100. Light source changer 4 selects one of reference light sources Ra to Rn such as LEDs. First channel selector 5A selects one of high voltage fine adjustment circuits 2A to 2N. Second channel selector 5B selects one of preamplifiers 3A to 3N. CPU 1 controls channel controller 6, which in turn controls light source changer 4, and first and second channel selectors 5A and 5B. An A/D (analog-to-digital) converter 7 A/D-converts the output of any of preamplifiers 3A to 3N, which is output from second channel selector 5B, and detects a peak pulse height of the spectrum. Comparator 8 compares this peak pulse height of the spectrum transferred from A/D converter 7 through CPU 1 with the reference peak value, and transfers, as a deviation signal, the difference between them to CPU 1. CPU 1 transfers a channel control signal to channel controller 6. In response to the channel control signal, controller 6 controls light source changer 4, first channel selector 5A, and second channel selector 5B, to select one of these channels and to specify, for example, the light source Ra, the high voltage fine adjustment circuit 2A and the preamplifier 3A in the selected first signal processing channel. CPU 1 transfers various control signals other than the channel control signal to high voltage fine adjustment circuits 2A to 2N through first channel selector 5A. These control signals are prepared on the basis of the deviation, i.e. the difference between the peak values. High voltage fine adjustment circuits 2A to 2N, which are major circuits of the autotuning circuit, will be described in more detail referring to Fig. 2. The high voltage fine adjustment circuit 2A will typically be described since these circuits 2A to 2N have the same circuit arrangements. High voltage fine adjustment circuit 2A is comprised of high voltage selection analog switches 9A to 9C, first, second and third resistor groups 10A to 10C, an up-down counter 11, and an up-down changer 12. First to third analog switches 9A to 9C each has one primary terminal and eight secondary terminals. First resistor group 10A includes (8-1) resistors (of which resistance values are equally set at 6.4 Kilo ohms) each connected between the adjacent secondary terminals of first analog switch 9A. Second resistor group 10B includes (8-1) resistors (of which resistance values are equally set at 0.8 Kilo ohms) each connected between the adjacent secondary terminals of second analog switch 9B. Third resistor group 10C includes (8-1) resistors (resistance 0.1 Kilo ohms) each connected between the adjacent secondary terminals of third analog switch 9C. Up-down counter 11 can preset a select signal of 3 bits, or 3-bit binary data indicated by MSB to LSB, to be supplied to third high voltage selection analog switches 9A to 9C. Up-down changer 12, under control of CPU 1, is connected to the up-and-down terminals UP and DW of counter 11. The primary terminal of first high voltage analog switch 9A is connected to the high voltage adjustment terminal (not shown in detail) of the cathode of photomultiplier 20A. The last terminal of the secondary terminals (i.e. the lowest terminal) is connected to the primary terminal of second high voltage select analog switch 9B. Of the secondary terminals of second high voltage selection analog switch 9B, the last terminal is connected to the primary terminal of third high voltage select analog switch 9C. Of the third terminals of second high voltage select analog switch 9C, the last terminal is connected to the ground line. The up-and-down terminals of up-down counter 11 are used for incrementing and decrementing up-down counter 11, and are controlled by up-down changer 12. The preset terminal PL is provided for receiving a command signal from CPU 1. A preset value prepared on the basis of this command signal is loaded in up-down counter 11. In Fig. 2, arrow 30 indicates that terminals of up-down counter 11 for loading the preset value into up-down counter 11 are each connected between junctions of the adjacent variable resistors (not shown). The operation of the high voltage fine adjustment circuit 2A thus arranged will be described. A command to set a peak value (i.e. pulse height) of a spectrum for the photomultiplier 20A is input from input means 1A to CPU 1. In response to the command, CPU 1 transfers a control signal to channel controller 6. Upon receipt of the control signal, channel controller 6 sends a selection signal to light source changer 4, and first and second channel selectors 5A and 5B, and selects the signal processing channel in which the peak value (peak pulse height) is to be preset. That is, the channel including first light source Ra, photomultiplier 20A, high voltage fine adjustment circuit 2A and preamplifier 3A is selected in this case. A command is supplied from input means 1A, whereby another command for loading a preset value is entered to the preset terminal PL of up-down counter 11 provided in high voltage fine adjustment circuit 2A. As a result, the preset value is loaded on up-down counter 11. Counter 11 sends a selection signal to first to third high voltage select analog switches 9A to 9C. The resistor, or its resistance, of groups 10A to 10C, which corresponds to the preset value, is selected, thereby setting a desired high voltage. The peak value (peak pulse height) of the output spectrum of the photomultiplier 20A at this time is sent through preamplifier 3 to A/D converter 7. The peak value is sent to comparator 8 through CPU 1, after converted into a digital signal by converter 7. Comparator 8 compares the peak value with a predetermined reference value stored therein. The deviation, or shift, as the result of comparison effected by comparator 8, is sent to CPU 1. CPU converts it into a given number of clock pulses, and selects a counting mode of the up-down counter, count up or count down. The clock pulses are applied to the clock terminal CK of up-down counter 11. A control signal representative of the count-up or count-down is input to up-down changer 12. Changer 12 inputs a command representing the count-up or count-down to the up terminal UP or down terminal DW of counter 11 respectively. Upon receipt of this command, counter 11 starts counting of up or down the pulses based on the preset value. The result of the counting is supplied as a 3-bit selection signal to first to third high voltage selection analog switches 9A to 9C respectively. According to the input selection signal, the analog switches 9A to 9C provided in high voltage fine adjustment circuit 2A shown in Fig. 2, turn their contacts to the positions of first to third resistor groups 10A to 10C, as shown. These groups are arranged in that order of decreasing resistance so that the resistance is changed gradually, or stepwisely, from the first resistance when the contact position changes. This is one of the features of this invention, i.e., a gradual resistance change. In this way, the resistance of the resistor groups connected to the high voltage fine adjustment terminal of photomultiplier 20A can be set to determine the desirable high voltage for the photomultiplier 20A. Under this condition, a peak value of the spectrum is again detected to see if it has still been deviated from the reference peak value, by comparator 8 through second channel selector 5B, A/D converter 7 and CPU 1. Any deviation can be reduced in the same adjustment way. If no deviation is detected, the setting work of the high voltage for photomultiplier 20A is accomplished. The overall adjustment of high voltage fine adjustment circuits 2A to 2N is performed prior to the actual measurement by the scintillation camera. By this adjustment, the peak pulse height, i.e., sensitivity of each of photomultipliers 20A to 20N can have a proper value. In other words, the high voltage adjustment is independently completed for photomultipliers 20A to 20N before the actual measurement by scintillation camera commences. Alternatively, the other time period than that for acquiring image data is used for performing the above procedural operation, for each signal processing channel. By executing it for an appropriate period of time, the autotuning can be achieved with a minimum change amount of the sensitivity for each channel. As described above, the autotuning means, first mode of the invention can make a fine tuning of the high voltage and the output of photomultipliers. Therefore, it realizes a long life and a reduced size. If the high voltage fine adjustment circuits are assembled into a module package, it may be accommodated into photomultipliers. This feature further gives size reduction of the apparatus. The up-down counter is employed and a number of resistors with equal resistances are used. A plurality of resistor groups containing series-connected resistors are provided with different resistors. These resistor groups are switched for high voltage adjustment. Therefore, the high voltage can be continuously adjusted, realizing an autotuning of a less fluctuation of high voltage. In the case of the autotuning for multiple signal processing channels, the channel selector can easily connect the channels to the control means. Furthermore, use of the UP, DW and CK terminals of the up-down counter reduces the number of control terminals. For example, only three control signals, i.e., up/down control, load command and clock signals, are required for the control of 9 bits (= 3 bits × 3) of three analog switches 9A to 9C. Referring back to the circuit shown in Fig. 1, the scintillation signals under the normal scintillation camera measurement are output from preamplifiers 3A to 3N, and supplied to a signal processor 30 where these are appropriately processed. Fig. 3 shows a practical arrangement of up-down counter 11 in the high voltage fine control circuit 2A. The up-down counter (Fig. 2) is comprised of three resettable up-down counters 11A to 11C. The up/down changer 12 is comprised of two NAND gates LS10 and one NOR gate LS04. First analog switch 10A is of the high voltage type. This switch 10 is connected to first and second up-down counters 11A and 11B via level shifter 11D having variable resistors (not shown in detail). Second and third analog switches 10B and 10C are of the same type. The up/down control, clock and load command signals, which are supplied from CPU 1, are supplied to related circuit components with the circuit arrangement as shown. Turning now to Fig. 4, there is shown a scintillation camera apparatus 200 according to an embodiment of the invention. Like reference numerals are used for designating like circuit portions in Fig. 1. In this embodiment, the effects like those of the apparatus of Fig. 1, i.e. appropriate setting of the sensitivity of each photomultiplier, can be attained by automatically controlling the gains of preamplifiers 3A to 3N for amplifying the output signals of photomultipliers 20A to 20N. In brief, this embodiment is featured by providing automatic gain control circuits 50A to 50N in preamplifiers 3A to 3N. Automatic gain control circuits 50A to 50N have the same circuit arrangements, and hence a basic circuit of first automatic gain control circuit 50A will typically be given in Fig. 5. As apparent from Fig. 5, this circuit is essentially identical to the high voltage fine control circuit 2A shown in Fig. 2. The circuit 2A of Fig. 2 is for adjusting the high voltage applied to the photomultipliers, whereas the control circuit 50A of this embodiment is for gain adjustment of the preamplifier. In this respect, the electrical characteristics are different from each other. Therefore, there is no need for use of the high voltage switch for the first analog switch 9A'. Roughly speaking, the basic operation of the automatic gain control circuit 50A is equal to that of the control circuit 2A of Fig. 2, because the resistances of the series resistors of the resistor groups 10A to 10C connected to first to third analog switches 9A' to 9C are varied depending on the operation of up-down counter 11. Fig. 6 shows an actual automatic gain control circuit using the basic circuit of the AGC circuit 50A of Fig. 5. As already mentioned, automatic gain control circuits 50A-1 and 50A-2 have exactly the same arrangements. A buffer amplifier 52 is provided for separating preamplifier 3A (Fig. 3) from these gain control circuits 50A-1 and 50A-2. An inverter 54 is connected to the up-down counter terminal UP/DW of circuit 50A-1 to maintain both the gain control circuits 50A-1 and 50B-2 in their inverse operation modes. Buffer amplifier 56 is connected to a junction between these gain control circuits 50A-1 and 50B-2, thereby separating these circuits from the following channel selector. Parallel resistor R1 is an impedance converting resistor, which may compensate for the output characteristic of input buffer amplifier 52. Resistor R21 determines a variable range of the gain as determined by automatic gain control circuits 50A-1 and 50A-2. As can be seen from the foregoing description, the apparatus of the present embodiment can automatically adjust the gains of the preamplifiers for amplifying the output signal of photomultiplier. Therefore, this apparatus, similar to the apparatus of Fig. 1, can automatically and independently correct a variation of the characteristic of each photomultiplier, i.e. a sensitivity variation. As can be seen from the foregoing description, the autotuning circuit constructed on the basis of the electrical arrangement, provides a scintillation camera apparatus which is small, durable, reliable and inexpensive, and can automatically tune the high voltage and output of photomultiplier.

Claims: A scintillation camera apparatus (200) comprising: a plurality of scintillation detecting units (20A, 20N) for detecting radiation containing radiation information of an object under medical examination to derive a plurality of radiation energy signals; comparison means (8), and a plurality of amplification means (3A-3N), wherein: said comparison means (8) compare the pulse heights of each of the radiation energy signals with a reference value so as to derive a deviation signal; and said plurality of amplification means (3A-3N) each include a gain control circuit (50A-50N), for amplifying said plurality of radiation energy signals at predetermined amplification gains independently controlled by the deviation signals, each of said gain control circuits (50A-50N) including: first, second and third resistor groups (10A, 10B, 10C), first, second and third analog switches (9A', 9B, 9C) series-connected to each other via said first, second and third resistor groups (10A, 10B, 10C), said first, second and third analog switches (9A', 9B, 9C) and resistor groups (10A, 10B, 10C) forming one series circuit; and an up/down counter (11), connected to said first, second and third analog switches (9A', 9B, 9C), for controlling said first, second and third analog switches (9A', 9B, 9C), thereby changing a total resistance of the series-connected first, second and third resistor groups (10A, 10B, 10C), characterized in that: each of said first, second and third resistor groups (10A, 10B, 10C) is constructed of a plurality of resistors having same resistance values. An apparatus (200) as claimed in claim 1, characterized in that the resistance of the resistor of said first resistor group (10A) is greater than that of said second resistor group (10B), and the resistance value of the resistor of said second resistor group (10B) is greater than that of said third resistor group (10C). An apparatus (200) as claimed in claim 1, characterized in that said up/down counter (11) derives a 3-bit binary preset signal to each of said first, second and third analog switches (9A', 9B, 9C). An apparatus (200) as claimed in claim 1, characterized in that said first resistor group (10A) is constructed of seven resistors having the same resistance value of 6.4 kΩ, said second resistor group (10B) is constructed of seven resistors having the same resistance value of 0.8 kΩ and said third resistor group (10C) is constructed of seven resistors each having the same resistance value of 0.1 kΩ.

Inventors: YAMAKAWA, TSUTOMU C/O PATENT DIVISION
Applicants: KABUSHIKI KAISHA TOSHIBA
Application year: 1986
Main classification: G01T1
IPC codes: G01T1