Abstract:
A processing circuit, which carries out coincidence counting, acquires calibration data so that time delays of γ-ray detection signals from radiation detectors coincide with one another. A technique for acquiring calibration data faster and easily is provided to attain high time precision and respond to multi-channeling of detectors. A signal from a test signal generator is sent to signal processing apparatuses and coincidence count events are generated as a test. The events generated are processed by a delay time control apparatus and a variable delay circuit is controlled to improve the accuracy of coincidence counting.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to a radiological imaging apparatus and a timing correction method therefor, and more particularly, to a radiological imaging apparatus and a timing correction method therefor suitable for use in a Positron Emission Tomography (hereinafter referred to as “PET”) apparatus.  
         [0002]     A PET inspection is an inspection carried out by administering radio pharmaceuticals (hereinafter referred to as “PET pharmaceuticals”) containing positron emitters ( 15 O,  13 N,  11 C,  18F , etc.) and having the nature of accumulating in a specific region (e.g., cancer cells) to an examinee and detecting γ-rays emitted from the affected area of the examinee by being provoked by the PET pharmaceuticals accumulated in the region using radiation detectors. When a positron emitted from the positron emitter contained in the PET pharmaceuticals encounters with neighboring electrons and annihilates, a pair of γ-rays having energy of 511 keV are emitted in substantially diametrically opposite directions. It is possible to identify locations where the PET pharmaceuticals are accumulated, that is, the affected area of cancer of the examinee based on the respective detection signals outputted from a pair of radiation detectors which have detected this pair of γ-rays.  
         [0003]     To identify the affected area of cancer, it is necessary to identify the respective positions of the pairs of radiation detectors which have detected the pairs of γ-rays generated by annihilation of positrons and it is necessary to take a coincidence count of detection signals outputted from these radiation detectors. This requires time resolution with high precision. However, even when γ-rays enter two radiation detectors simultaneously, there is a variation in signal transmission from the respective radiation detectors to a coincidence circuit and there are differences in the times at which signals arrive at the coincidence circuit. For this reason, it is necessary to adjust transmission delays of signals from the respective radiation detectors so that the times at which signals arrive at the coincidence circuit coincide with one another.  
         [0004]     Conventionally, timing correction of signals detected by radiation detectors is realized by acquiring calibration data using a calibration radiation source and adjusting a time variation of signal transmission based on the calibration data. This timing correction method is described, for example, in JP-B-6-19436 and Japanese Patent No. 3343122.  
         [0005]     The signal timing correction method described in JP-B-6-19436 will be explained. First, γ-rays from the radiation source are detected by a radiation detector, a timing signal is created based on an output signal of the radiation detector and this timing signal is inputted to the coincidence count apparatus through a delay adjusting circuit. The sensitivity of the signal outputted from the coincidence count apparatus is measured. Next, the sensitivity of γ-rays from the radiation detector is measured using the same method as that described above while changing an amount of delay to be set in the delay time adjusting circuit. This is the method of correcting a signal delay time by setting the amount of delay corresponding to the highest measured sensitivity in the delay time control apparatus.  
         [0006]     Next, the signal timing correction method described in Japanese Patent No. 3343122 will be explained. This method corrects timings of signal transmission by setting a calibration radiation source within a field of view of the PET apparatus, creating timing data indicating a time difference measured value of a coincidence event which occurs between a pair of radiation detectors, calculating a time delay value corresponding to each radiation detector and setting this time delay value in the corresponding radiation detector channel.  
         [0007]     However, the above described conventional technologies obtain calibration data necessary for timing correction using a radiation source and γ-rays emitted for one event form a pair, and therefore it is possible to obtain calibration data for only the circuits connected to the two radiation detectors into which the respective γ-rays are introduced. For this reason, it takes a long time to obtain calibration data corresponding to all radiation detectors.  
         [0008]     It is an object of the present invention to provide a radiological imaging apparatus and a timing correction method therefor capable of reducing a time required to acquire timing calibration data to be used for timing correction of output signals of radiation detectors.  
       SUMMARY OF THE INVENTION  
       [0009]     A feature of the present invention to attain the above described object is to input a test signal outputted from a test signal generator to a plurality of signal processing apparatuses and generate timing calibration data based on the outputs of the plurality of signal processing apparatuses. Since the present invention can input a test signal to the respective signal processing apparatuses reliably, the time required to acquire timing calibration data necessary for timing correction of output signals of the radiation detectors can be reduced.  
         [0010]     It is preferable to carry out timing correction of γ-ray detection signals from the radiation detectors based on the calibration data during an inspection of an examinee.  
         [0011]     According to the present invention, the time required to acquire timing calibration data necessary for timing correction of output signals of the radiation detectors is shortened.  
         [0012]     Other objects, features and advantages of the invention will become apparent from the following description of the embodiments of the invention taken in conjunction with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]      FIG. 1  is a block diagram of a PET apparatus which is an embodiment of the present invention;  
         [0014]      FIG. 2  is a detailed block diagram of the signal processing unit shown in  FIG. 1 ;  
         [0015]      FIG. 3  is a block diagram of a PET apparatus which is another embodiment of the present invention;  
         [0016]      FIG. 4  is a detailed block diagram of the signal processing unit shown in  FIG. 3 ; and  
         [0017]      FIG. 5  is block diagram of a signal processing unit of a PET apparatus which is a further embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS  
       [0018]     With reference now to the attached drawings, embodiments will be explained below.  
       Embodiment 1  
       [0019]     A radiological imaging apparatus that is a preferred embodiment of the present invention will be explained below using  FIG. 1  and  FIG. 2 . The radiological imaging apparatus of this embodiment is a PET apparatus.  
         [0020]     As shown in  FIG. 1 , the PET apparatus  40  of this embodiment is provided with a bed  39  for holding an examinee (test subject), a plurality of radiation detectors  1 , a plurality of signal processing units  26 , a time correction apparatus  10 , a coincidence circuit  6 , a delay time control apparatus  7  and a test signal generator  15 . The plurality of radiation detectors  1  are arranged around the bed  39  in a ring shape surrounding the bed  39 . The radiation detectors  1  are also arranged in a plurality of rows in the longitudinal direction of the bed  39 . The radiation detector  1  is a semiconductor radiation detector and approximately 100,000 radiation detectors  1  are provided for the PET apparatus  40 . The signal processing unit  26  which is provided for each radiation detector  1  is provided with a signal processing apparatus  20  in the front stage and a packet data generator  28  in the posterior stage as shown in  FIG. 2 . The signal processing apparatus  20  is provided with a preamplifier  2 , and a timing signal generator  3  and a pulse height signal generator  11  connected to the preamplifier  2 . The preamplifier  2  is connected to the radiation detector  1 . A switch (opening/closing device)  16  is connected to the signal processing apparatus  20 , that is, the preamplifier  2 . The switch  16  is provided for each signal processing apparatus  20 . All the switches  16  are connected to the test signal generator  15 . The packet data generator  28  is provided with a time measuring apparatus  9  and a pulse height measuring apparatus  12 . The time measuring apparatus  9  is connected to the timing signal generator  3 . The pulse height measuring apparatus  12  is connected to the pulse height signal generator  11  on one hand and connected to the time measuring apparatus  9  on the other. The time measuring apparatus  9  is connected to the time correction apparatus  10 . The coincidence circuit  6  is connected to the time correction apparatus  10 . The delay time control apparatus  7  is connected to the coincidence circuit  6  through a switch  21  and also connected to the time correction apparatus  10  through a switch  35 . A data collection apparatus  31  connected to the coincidence circuit  6  is connected to an image reconstruction apparatus  33 . A data saving apparatus  32  is connected to the data collection apparatus  31 . A display device  34  is connected to the image reconstruction apparatus  33 . In this embodiment, the delay time control apparatus  7  is the calibration data generator which generates calibration data.  
         [0021]     For example, before starting a PET inspection everyday, it is possible to acquire calibration data necessary for timing correction of the PET apparatus  40  using a test signal outputted from the test signal generator  15 . The test signal is an electric signal, and more specifically, a charge signal. It is difficult to send a charge signal from the test signal generator  15  to the preamplifier  2  and it is desirable to convert a voltage signal to a charge signal through the switch  16  or the preamplifier  2  using a capacitor. It is desirable to realize equi-length or equi-electric length wiring from the test signal generator  15  to the preamplifier  2 , but it is also possible to obtain an amount of delay from the test signal generator  15  to the preamplifier  2  through a calculation or comparison with measurement of the delay time using a radiation source to perform correction when creating calibration data. The method of acquiring calibration data in this embodiment will be explained more specifically below. In this embodiment, calibration data is acquired using a test signal outputted from the test signal generator  15 .  
         [0022]     When acquiring the calibration data, the operator (radiological technician and medical doctor, etc.) operates buttons provided on an operator console (not shown) whereby a data acquisition start signal is outputted from the operator console to the test signal generator  15  and a switch control apparatus  18 . Furthermore, this data acquisition start signal is inputted to the delay time control apparatus  7  and all the calibration data saved in the delay time control apparatus  7  when previous calibration data was acquired is thereby set to zero (or a specific value). The test signal generator  15  generates a test signal through an input of the data acquisition start signal. This test signal is generated asynchronously to a measuring clock of a time counter inputted to the time measuring apparatus  9 . Acquiring calibration data requires the test signal to be inputted to each signal processing apparatus  20  connected to a pair of radiation detectors  1 . The switch control apparatus  18  starts a corresponding ON, OFF operation of the switch  16  through the input of the data acquisition start signal. That is, the switch control apparatus  18  closes the two switches  16  connected to the two signal processing apparatuses  20  together. Furthermore, the switch control apparatus  18  also closes the switch  21 . This switch  21  remains closed during a calibration period. The test signal outputted from the test signal generator  15  is inputted to the pair of signal processing apparatuses  20  through the respective switches  16 . When inspecting the examinee, the signal processing apparatus  20  inputs a γ-ray detection signal outputted from the radiation detector  1  and when acquiring calibration data, the signal processing apparatus  20  inputs a test signal through the switch  16 .  
         [0023]     Here, the opening/closing operation of the switch  16  will be explained. The opening/closing of the switch  16  is controlled by a command signal from the switch control apparatus  18 . In this embodiment, in order to acquire calibration data, it is necessary to select a pair of signal processing apparatuses  20 ; a signal processing apparatus  20  which serves as a reference (hereinafter referred to as “reference signal processing apparatus”) and a signal processing apparatus  20  carrying out calibration (hereinafter referred to as “calibration signal processing apparatus”) and input a test signal to each signal processing apparatus  20 . The test signal is preferably inputted to the pair of signal processing apparatuses  20  simultaneously. Pairs of reference signal processing apparatus  20  and calibration signal processing apparatus  20  are preset and information on many combinations of reference signal processing apparatus  20  and calibration signal processing apparatus  20  that form a pair is stored in a memory (not shown) of the switch control apparatus  18 . The reference signal processing apparatus  20  and calibration signal processing apparatus  20  that form a pair correspond to a pair of signal processing apparatuses  20  connected to a pair of radiation detectors  1  located in diametrically opposite directions which detect a pair of γ-rays during an inspection of the examinee. Other combinations of reference signal processing apparatus  20  and calibration signal processing apparatus  20  are also stored in the above described memory. Some reference signal processing apparatuses  20  also serve as calibration signal processing apparatuses  20 . The switch control apparatus  18  repeats “close (ON)” and “open (OFF)” operations of the switch  16  connected to a certain reference signal processing apparatus  20  based on the information on the combinations of the reference signal processing apparatus  20  and calibration signal processing apparatus  20  stored in the memory until inputs of test signals to the reference signal processing apparatus  20  and all calibration signal processing apparatuses  20  to be paired therewith are completed. Next, the switch control apparatus  18  sequentially carries out “close” and “open” operations of the calibration signal processing apparatus  20  with respect to the respective switches  16  connected to the reference signal processing apparatus  20 . The test signal outputted from the test signal generator  15  is inputted to the calibration signal processing apparatus  20  when the switch  16  is closed and the input of the test signal to the calibration signal processing apparatus  20  is stopped when the switch  16  is opened. That is, the test signal outputted from the test signal generator  15  is inputted to the pairs of reference signal processing apparatus  20  and calibration signal processing apparatus  20  sequentially and reliably. When inputs of a test signal to one reference signal processing apparatus  20  and all calibration signal processing apparatuses  20  to be paired therewith are completed, the switch control apparatus  18  opens the switch  16  connected to the aforementioned reference signal processing apparatus  20  and stops the input of the test signal. Then, the switch control apparatus  18  performs control of turning ON/OFF the respective switches  16  connected to another reference signal processing apparatus  20  and calibration signal processing apparatuses  20  paired therewith until the input of the test signal to all the reference signal processing apparatuses  20  is completed. This is the opening/closing operation of the switch  16 .  
         [0024]     Next, the method of inputting a test signal to the reference signal processing apparatus  20  and calibration signal processing apparatus  20  and acquiring calibration data using this test signal will be explained more specifically. For convenience, the time measuring apparatus  9  connected to the reference signal processing apparatus  20  will be called a “time measuring apparatus  9 A” and the time measuring apparatus  9  connected to the calibration signal processing apparatus  20  will be called a “time measuring apparatus  9 B.” 
         [0025]     The test signal inputted to the reference signal processing apparatus  20  is amplified by the preamplifier  2  and inputted to the timing signal generator  3 . The timing signal generator  3  creates a timing signal based on the test signal and outputs the timing signal to the time measuring apparatus  9 A. The time measuring apparatus  9 A measures the time at which the timing signal arrives and outputs time information (hereinafter referred to as “first time information”). The time measuring apparatus  9 B outputs time information (hereinafter referred to as “second time information”) based on the timing signal outputted from the timing signal generator  3  of the calibration signal processing apparatus  20  to which the test signal has been inputted. When the pulse height measuring apparatus  12  receives the time information from the time measuring apparatus  9 , it obtains a detector ID to identify the radiation detector  1  connected to the time measuring apparatus  9 . That is, the pulse height measuring apparatus  12  stores the detector ID corresponding to each time measuring apparatus  9  connected to the pulse height measuring apparatus  12  and when time information is inputted from a certain time measuring apparatus  9 , it is possible to identify the detector ID corresponding to the time measuring apparatus  9 . This is possible because the time measuring apparatus  9  is provided for each radiation detector  1 . When the first time information is inputted to the pulse height measuring apparatus  12 , the pulse height measuring apparatus  12  identifies the corresponding detector ID (detector ID of the radiation detector  1  connected to the time measuring apparatus  9 A, hereinafter referred to as “first detector ID”) and outputs it together with the first time information. The pulse height measuring apparatus  12  identifies the corresponding detector ID (detector ID of the radiation detector  1  connected to the time measuring apparatus  9 B, hereinafter referred to as “second detector ID”) based on the second time information and outputs it together with the second time information. The coincidence circuit  6  receives the first time information, first detector ID, and second time information and second detector ID outputted from the pulse height measuring apparatus  12 . The coincidence circuit  6  calculates a time difference between the first time information which becomes a reference for the pair of radiation detectors  1  and second time information (hereinafter referred to as “arrival time difference”). Even when a test signal is inputted to the same reference signal processing apparatus  20  and calibration signal processing apparatus  20 , a variation occurs in the time required for signal transmission, and therefore the test signal is inputted repeatedly to the same pair of signal processing apparatuses  20  and the coincidence circuit  6  calculates the arrival time difference in each case. The delay time control apparatus  7  calculates an average of the arrival time difference obtained from the coincidence circuit  6 , calculates the calibration data to be set in all the radiation detectors  1  mounted in the radiation detection apparatus based on the average and causes the calibration data obtained to be stored in the memory of the delay time control apparatus  7 . Furthermore, the delay time control apparatus  7  also stores the detector ID corresponding to the calibration data. The test signal is inputted to pairs of opposing signal processing apparatuses  20  until calibration data is created for all signal processing apparatuses  20 . The calibration data for each signal processing apparatus  20  and the detector ID corresponding to the calibration data are saved in the delay time control apparatus  7  and this calibration data is used during a PET inspection of the examinee. After the acquisition of calibration data for all the signal processing apparatuses  20  is completed, the switch control apparatus  18  opens the switch  21 .  
         [0026]     The test signal outputted from the test signal generator  15  is a signal having a sawtooth wave or square wave voltage signal. This test signal is converted to a pulse-shaped charge signal by a capacitor. To obtain time information using the test signal, the amplitude of the waveform may be constant, but by changing the amplitude of the signal waveform, it is possible to calibrate information other than time information, for example, the relationship between the energy of γ-rays and the amplitude of an electric signal outputted from the signal processing apparatus  20 .  
         [0027]     A CFD (Constant Fraction Discriminator) circuit or leading edge trigger circuit is used for the timing signal generator  3  provided for the signal processing apparatus  20 .  
         [0028]     This embodiment, which uses a test signal from the test signal generator, can select a reference signal processing apparatus  20  and calibration signal processing apparatus  20  reliably, and can thereby input a test signal to a plurality of reference signal processing apparatuses  20  or a plurality of calibration signal processing apparatuses  20  reliably to acquire calibration data.  
         [0029]     Next, the operation of the PET apparatus when carrying out a PET inspection of the examinee will be explained. During a PET inspection, a timing of a γ-ray detection signal is corrected using calibration data obtained using a test signal.  
         [0030]     Before starting a PET inspection, PET pharmaceuticals are administered to the examinee by injection, etc., beforehand. The PET pharmaceuticals are selected according to the purpose of the inspection. The PET pharmaceuticals administered to the examinee are concentrated on the affected area of cancer of the examinee. The examinee administered the PET pharmaceuticals are laid on the bed  39 .  
         [0031]     When starting a PET inspection, the operator operates buttons provided on an operator console (not shown) and outputs an inspection start signal to a centralized control section (not shown). When the inspection start signal is inputted, the centralized control section outputs information on the inspection target range of the examinee and a bed movement start signal to a bed movement control section (not shown). The bed movement control section, which has received the bed movement start signal, moves the bed so that the inspection target range of the examinee enters a γ-ray detection area of the PET apparatus  40  based on the inputted information. The centralized control section, which has received the inspection start signal, transfers the calibration data from the delay time control circuit  7  to the memory (not shown) of the time correction apparatus  10 . A PET inspection starts in this state.  
         [0032]     Many pairs of γ-rays provoked by the PET pharmaceuticals are emitted in all directions from within the body of the examinee who lays on the bed  39 . A pair of γ-rays are emitted in substantially opposite directions and detected by a pair of radiation detectors  1 .  
         [0033]     When these radiation detectors  1  detect γ-rays, they output pulse-like electric signals (hereinafter referred to as “γ-ray detection signals”) according to the energy of γ-rays. Since this γ-ray detection signal is faint, the signal is amplified by the preamplifier  2  and inputted to the timing signal generator  3 . The timing signal generator  3  generates a timing signal indicating the time of detection of γ-rays based on the γ-ray detection signal and outputs the timing signal. The time measuring apparatus  9  calculates the arrival time of the timing signal and outputs the time information obtained to the time correction apparatus  10  through the pulse height measuring apparatus  12 .  
         [0034]     In this embodiment, the time correction apparatus  10  corrects the inputted time information with the calibration data, adjusts the delay in signal transmission and inputs the corrected time information to the coincidence circuit  6 . The method of correcting the time information (timing correction method) using the time correction apparatus  10  will be explained below.  
         [0035]     Time information corresponding to a γ-ray detection signal of the radiation detector  1  is inputted to the time correction apparatus  10  together with a detector ID which identifies the radiation detector  1 . As with the time of acquisition of calibration data, the detector ID is added by the pulse height measuring apparatus  12 . The time correction apparatus  10  identifies the radiation detector  1  which outputted the γ-ray detection signal from which the inputted time information derives using the detector ID. The time correction apparatus  10  reads calibration data from the memory based on the detector ID. The time correction apparatus  10  corrects time information based on the calibration data and outputs the corrected time information to the coincidence circuit  6 .  
         [0036]     When the corrected time information signal is inputted to the coincidence circuit  6 , the coincidence circuit  6  decides based on the time information whether the inputted signal is a γ-ray detection signal derived from a pair of γ-rays emitted from the affected area of the examinee provoked by PET pharmaceuticals or not. The coincidence circuit  6  compares time information of two signals out of the time information corresponding to γ-ray detection signals inputted successively and calculates the time difference. The coincidence circuit  6  coincidence-counts γ-ray detection signals corresponding to the calculated time difference which falls within a set time (e.g., 10 nsec) (a pair of γ-ray detection signals produced by annihilation of one positron). The coincidence circuit  6  outputs the detector IDs of the respective radiation detectors which have detected a pair of coincidence-counted γ-rays and information on the coincidence count to the data collection apparatus  31 .  
         [0037]     The information inputted to the data collection apparatus  31  is saved in the data saving apparatus  32  and after all measurements are completed, data is outputted to the image reconstruction apparatus  33 . The image reconstruction apparatus  33  creates information on the tomogram including the affected area of the examinee based on the information. The tomographic image is displayed on the display device  34 .  
         [0038]     This embodiment is intended to correct the difference in the transmission time of γ-ray detection signals between detector channels (including the signal processing apparatuses  20 ) using the calibration data obtained using the aforementioned test signal.  
         [0039]     This embodiment allows the following effects to be obtained.  
         [0040]     (1) In this embodiment, a test signal outputted from the test signal generator  15  is inputted to the signal processing apparatus  20 , and therefore it is possible to input a test signal to all the signal processing apparatuses  20  reliably and acquire calibration data of timings corresponding to detection signals of all the radiation detectors included in the PET apparatus  40  in a short time. Especially, this embodiment inputs a test signal to the preamplifier  2 , and therefore it is possible to obtain more accurate calibration data which reflects propagation times of a signal at the preamplifier  2  and timing signal generator  3 . It is also possible to obtain calibration data in a shorter time than the conventional example by connecting the test signal generator  15  between the preamplifier  2  and timing signal generator  3  and inputting a test signal, which is an electric signal, to the timing signal generator  3 . However, in this case, the accuracy of calibration data is reduced compared to the case where a test signal is inputted to the preamplifier  2  because the time for signal transmission at the preamplifier  2  cannot be reflected. When a test signal is selectively inputted to the respective radiation detectors  1  using the test signal generator  15 , it is necessary to use radiation or a very short light pulse signal as the test signal. However, it is difficult to realize the input of such a test signal to the radiation detector  1 . Therefore, it is desirable to input the test signal to the signal processing apparatus  20  without passing through the radiation detector  1 .  
         [0041]     (2) In this embodiment, the electric signal used as a test signal is easier to handle than a radiation source used in the conventional example.  
         [0042]     (3) In this embodiment, when calibration data is acquired, the switch control apparatus  18  turns ON/OFF many switches  16  sequentially to input a test signal to the corresponding pair of signal processing apparatuses  20 , and therefore it is possible to input the test signal to all the signal processing apparatuses  20  reliably. Furthermore, compared to a case where the signal processing apparatus  20  is selected manually, it is possible to drastically reduce time and trouble.  
         [0043]     In this embodiment, a test signal is inputted to a pair of signal processing apparatuses  20  respectively, but the number of signal processing apparatuses  20  to which a test signal is inputted is not always 2 and it is possible to input a test signal to three or more signal processing apparatuses  20  to acquire calibration data. That is, the switch control apparatus  18  turns ON, three or more, for example, ten switches  16  connected to ten signal processing apparatuses  20 . A test signal from the test signal generator  15  is inputted to the ten corresponding signal processing apparatuses  20 . The coincidence circuit  6  calculates an arrival time difference for each combination of two out of ten signal processing apparatuses  20 . Information on combinations of signal processing apparatuses  20  for which an arrival time difference is calculated is preset and stored in a memory (not shown) of the coincidence circuit  6 . Thus, by inputting a test signal to three or more signal processing apparatuses  20  simultaneously, it is possible to further shorten the time required to acquire calibration data corresponding to all radiation detectors  1 .  
         [0044]     (4) This embodiment outputs a test signal outputted from the test signal generator  15  asynchronously to the clock for measuring an arrival time. Such asynchronous outputting eliminates any correlation between a pseudo-signal generation system and coincidence count system, and can thereby perform accurate calibration.  
         [0045]     (5) This embodiment converts a signal detected by the radiation detector  1  and a test signal from the test signal generator to digital values and processes the digitized time data. Using such a digital calculation facilitates the setting of a time window. Furthermore, the digital circuit can be integrated more easily than a digital/analog circuit.  
       Embodiment 2  
       [0046]     A radiological imaging apparatus which is another embodiment of the present invention will be explained using  FIG. 3  and  FIG. 4  below. The radiological imaging apparatus of this embodiment is a PET apparatus.  
         [0047]     The PET apparatus  40 A of this embodiment has a configuration with the time correction apparatus  10  in the PET apparatus  40  of Embodiment 1 replaced by a variable delay circuit (delay adjustment apparatus)  4 . The PET apparatus  40 A provides a signal processing unit  26 A for each radiation detector  1 . The signal processing unit  26 A is provided with a signal processing apparatus  20 A which is the signal processing apparatus  20  provided with the variable delay circuit  4 . The variable delay circuit  4  has its input end connected to a timing signal generator  3  and its output end connected to a coincidence circuit  6 . A pulse height measuring apparatus  12  is connected to a pulse height signal generator  11  and the coincidence circuit  6 . The rest of the structure of the PET apparatus  40 A is the same as that of the PET apparatus  40 .  
         [0048]     When calibration data is acquired, a data acquisition start signal is inputted to a test signal generator  15  and a switch control apparatus  18  as in the case of Embodiment 1. The switch control apparatus  18  closes a pair of switches  16  connected to the corresponding pair of reference signal processing apparatus  20  and calibration signal processing apparatus  20 . A test signal outputted from the test signal generator  15  is inputted to the reference signal processing apparatus  20  and calibration signal processing apparatus  20  through the respective switches  16 .  
         [0049]     The opening/closing operation of the switch  16  is the same as that of Embodiment 1, and therefore explanations thereof will be omitted.  
         [0050]     A test signal inputted to the respective signal processing apparatuses  20  is amplified by a preamplifier  2  and then inputted to the timing signal generator  3 . The timing signal generator  3  generates a timing signal based on the test signal and outputs the. timing signal to a variable delay circuit  4 . When calibration data is acquired, a delay time control apparatus  7  sets an amount of delay of the variable delay circuit  4  (hereinafter referred to as “reference variable delay circuit  4 A”) connected to the reference signal processing apparatus  20  to a constant value (e.g., a median value within the variable range). Furthermore, the amount of delay of the variable delay circuit  4  (hereinafter referred to as “calibration variable delay circuit  4 B”) connected to the calibration signal processing apparatus  20  is set to a minimum value. The variable delay circuit  4  delays and outputs the timing signal based on the set amount of delay. The pulse height measuring apparatus  12  calculates a pulse height based on the pulse height signal outputted from the pulse height signal generator  11  and identifies the corresponding detector ID. The coincidence circuit  6  calculates sensitivity based on the delayed timing signal and pulse height information. When the amount of delay set in the calibration variable delay circuit  4 B is changed, the sensitivity to be calculated by the coincidence circuit  6  also changes. The amount of delay time set in the calibration variable delay circuit  4 B is gradually increased from the initially set value and the amount of delay time corresponding to the maximum sensitivity is calculated. The information of the amount of delay corresponding to the maximum sensitivity is transmitted to the delay time control apparatus  7  and stored in a memory (not shown) of the delay time control apparatus  7 . In this way, it is possible to obtain calibration data corresponding to all signal processing apparatuses  20 , and the calibration data and the corresponding detector IDs are stored in the memory of the delay time control apparatus  7 .  
         [0051]     The operation of the PET apparatus during a PET inspection of the examinee will be explained using  FIG. 3 .  
         [0052]     Differences from Embodiment 1 will be explained. When an inspection start signal is inputted to an overall control section (not shown), the overall control section sends a delay amount setting signal to the delay time control apparatus  7 . The delay time control apparatus  7  which has received the delay amount setting signal sends a command signal to the corresponding variable delay circuit  4  so as to set the amount of delay (calibration data) to be set based on the detector ID (stored in the memory) of the radiation detector  1  connected to the variable delay circuit  4 . That is, before a PET inspection, the delay time control apparatus  7  sets an amount of delay in all the variable delay circuits  4  to obtain maximum sensitivity based on the calibration data saved in the memory. Furthermore, during a PET inspection, a switch  21  set between the coincidence circuit  6  and the delay time control apparatus  7  is opened so that no γ-ray detection signal is inputted to the delay time control apparatus  7 .  
         [0053]     A pair of γ-rays emitted from within the body of the examinee lying on the bed  39  by being provoked by PET pharmaceuticals are detected by a pair of radiation detectors  1 . Based on the γ-ray detection signals outputted from the radiation detectors  1 , the timing signal generated by the timing signal generator  3  is inputted to the coincidence circuit  6  through the corresponding variable delay circuit  4 . That is, the variable delay circuit  4  outputs the timing signal (time information) corrected based on the set amount of delay to the coincidence circuit  6 . The coincidence circuit  6  carries out coincidence counting similar to that in Embodiment 1 based on the timing signal. The coincidence circuit  6  outputs detector IDs of the respective radiation detectors which have detected a pair of coincidence-counted γ-rays and information on the coincidence count value to a data collection apparatus  31 .  
         [0054]     The signal inputted to the data collection apparatus  31  is saved in a data saving apparatus  32  and data is outputted to an image reconstruction apparatus  33  after all measurements are completed. An image of the affected area of the examinee is created based on the data processed by the image reconstruction apparatus  33  and the image is displayed on a display device  34 .  
         [0055]     A time window of the coincidence circuit  6  set when acquiring calibration data of the PET apparatus  40 A is preferably wider than during a PET inspection. Before calibration data is acquired, even if a test signal is inputted to the reference signal processing apparatus  20  and calibration signal processing apparatus  20  simultaneously, the signal arrives at the coincidence circuit  6  with a time variation. When the time window of the coincidence circuit  6  is narrow, even the test signals which have been inputted simultaneously may be processed as non-coincidence signals by the coincidence circuit  6 . For that reason, it is preferable to set a wide time window for the coincidence circuit  6  when calibration data is acquired. In addition to the widening of the set value of the time window, it is possible to prevent a test signal from being processed as a non-coincidence signal by setting a time period after a test signal is inputted to the calibration signal processing apparatus  20  until a test signal is inputted to the next calibration signal processing apparatus  20  to a value greater than a certain value.  
         [0056]     This embodiment also inputs a test signal (electric signal) outputted from the test signal generator  15  to the signal processing apparatus  20 , and therefore it is possible to obtain the effects (1), (2) produced in Embodiment 1.  
       Embodiment 3  
       [0057]     A radiological imaging apparatus which is a further embodiment of the present invention will be explained using  FIG. 5 . The radiological imaging apparatus of this embodiment is a PET apparatus  40 B and different from the PET apparatus  40  in Embodiment 1 in that a plurality of signal processing units  26 B are provided. The structure of the PET apparatus  40 B other than the signal processing unit  26 B is the same as that of the PET apparatus  40 .  
         [0058]     The signal processing unit  26 B is provided with a signal amplifier  17 , a plurality of analog ASICs  23 , a plurality of digital ASICs  27  and a data merge IC  25 . Several tens of signal processing units  26 B are arranged in the PET apparatus  40 B.  90  analog ASICs  23  are arranged for one signal processing unit  26 B. Furthermore, a CFD (Constant Fraction Discriminator) circuit or leading edge trigger circuit is used for the timing signal generator  3 . The signal amplifier  17  is connected to a test signal generator  15 . The analog ASIC  23  is provided with a plurality of signal processing apparatuses  20  and the same number of switches  16 . Each signal processing apparatus  20  is provided with a preamplifier  2 , and a timing signal generator  3  and a pulse height signal generator  11  connected to the preamplifier  2 . The preamplifier  2  is connected to a radiation detector  1 . Each switch  16  provided for the analog ASIC  23  is connected to the preamplifier  2  of each signal processing apparatus  20 . These switches  16  are connected to the signal amplifier  17 . In this embodiment, the signal amplifier  17  is connected to the respective switches  16  of all the analog ASICs  23  provided within the signal processing unit  26 B. The digital ASIC  27  includes a plurality of packet data generators  28 A and a data acquisition IC  24 . The packet data generator  28 A is connected to each analog ASIC  23  and provided with a time measuring apparatus  9  which is individually connected to each timing signal generator  3  of one analog ASIC  23 . The respective time measuring apparatuses  9  of the packet data generator  28 A are connected to one pulse height measuring apparatus  12 . The pulse height measuring apparatus  12  is connected to each pulse height signal generator  11  of one corresponding analog ASIC  23 . The pulse height measuring apparatus  12  of each packet data generator  28 A is connected to the data acquisition IC  24 . The data integration IC  25  connected to a coincidence circuit  6  is connected to the pulse height signal generator  11  of each packet data generator  28 A.  
         [0059]     The pulse height measuring apparatus  12  receives time information on the time at which γ-rays are detected from the time measuring apparatus  9  and identifies the detector ID. Furthermore, the pulse height measuring apparatus  12  measures pulse height information of a γ-ray detection signal proportional to the energy of γ-rays based on the output from the pulse height signal generator  11  connected to the pulse height measuring apparatus  12 . The pulse height measuring apparatus  12  also functions as an information integration apparatus that integrates time information, detector ID information (detector position information) and pulse height information. The information integration apparatus outputs the integrated information (packet information) which is digital information including those three types of information to the data acquisition IC  24 . The packet data (including time information, detector ID and pulse height information) outputted from the pulse height measuring apparatus  12  of each packet data generator  28 A is outputted to the coincidence circuit  6  (see  FIG. 1 ) in the following stage through the data merge IC  25 .  
         [0060]     This embodiment can obtain the effects (1) to (5) produced in Embodiment 1 and can also obtain the following effects.  
         [0061]     (6) Since this embodiment sets each switch  16  in the analog ASIC  23 , wiring which transmits a test signal to the respective switches  16  can be shared. For this reason, it is possible to drastically reduce the number of wires connecting the test signal generator  15  and the respective switches  16 . Therefore, when the test signal generator  15  is provided and wiring of the PET apparatus  40 B is carried out, it is possible to simplify the wiring work. The switches  16  are connected to the respective signal processing apparatuses  20  included in the analog ASIC  23 , but it is also possible to achieve the same effect even if the switches  16  are set between the signal processing apparatus  20  and time measuring apparatus  9 .  
         [0062]     It should be further understood by those skilled in the art that although the foregoing description has been made on embodiments of the invention, the invention is not limited thereto and various changes and modifications may be made without departing from the spirit of the invention and the scope of the appended claims.