METHOD FOR CORRECTING SENSITIVITY OF GAMMA CAMERA

Abstract: A gamma camera (15) sensitivity correction device for a camera having a detector and a collimator (16), in which correction coefficients are first derived for the collimator, and then the detector's characteristic uniformity data are found with the collimator removed. From the collimator's characteristic sensitivity data and detector's characteristic uniformity data, the uniformity data in the condition with the collimator mounted on the detector are derived. Correction coefficients are then derived from these derived uniformity data. In this way, the uniformity correction matrix for sensitivity correction can be found with a small radiation source in a short time.

Description: This invention relates to a gamma camera sensitivity correcting method. When such a gamma camera is used, the subject is dosed with a radio-isotope (hereinbelow abbreviated to RI), and the amount of this RI that is selectively accumulated in a specific organ or disease site is detected, and imaged as an accumulation image. The main parts of a gamma camera are shown in Figure 1. Detector 15 detects radiation (gamma rays) from the RI. It consists of a scintillator 17, light guide 18, photoelectron multiplier tubes 19, and pre-amplifiers 20. Gamma rays cannot be refracted and focused by a lens as in an optical camera, so a collimator 16 is arranged ahead of detector 15. Collimator 16 is built around a lead sheet provided with a large number of parallel holes. By this means an image of the same size as the subject is created at the surface of scintillator 17. In scintillator 17, photoelectron multiplier tubes (PMT) 19 are arranged in hexagonally close-packed fashion on the other side of light-guide 18, which is constructed of a transparent substance (lucite or the like). A large number (19, 37 or 61) of PMTs 19, of 2-inch or 3-inch diameter, are used, arranged in a hexagonal pattern. When gamma rays are incident on scintillator 17, their energy is absorbed, causing fluorescence at the point of absorption. This light is input via light guide 18 to PMTs 19, where it is subjected to a photoelectric conversion, so that output pulses proportional to the incoming light are output from each PMT 19. If light is emitted directly below a given PMT, the largest output will be obtained from that PMT; or, alternatively if light is emitted at a point between three PMTs, equal output will be obtained from those three PMTs. Thus the point of luminescence, i.e. the position of gamma ray incidence, can be found from the output pulses of PMTs 19. Using the values of X and Y of an orthogonal coordinate system whose origin is at the centre of scintillator 17, adder 21 multiplies the outputs of the respective PMTs 19 by coefficients (called "weights") that depend on the positional coordintes of the respective PMT and adds them, to obtain signals for the various directions X⁺, X⁻, Y⁺, and Y⁻. In addition to this, if the outputs of all the PMTs 19 are multiplied by a constant coefficient and then added, the resultant signal will be proportional to the total luminescence of the scintillator, i.e. to the energy of the gamma ray. This is therefore termed the energy signal or Z signal. This Z signal is amplified by amplifier 22, then input to pulse wave height analyser 25, so that only gamma rays of the required preset energy are selected. The X⁺, X⁻, Y⁺, and Y⁻ position signals and the Z signal are each amplified and then the signals of the selected gamma rays only are input to a waveform expanding circuit 23, from the output of which signals: are obtained. The reason for dividing by Z is to make the magnitude of the image independent of the gamma ray energy. These X and Y signals are applied to the deflecting input of a cathode ray oscilloscope (CRT). The output of pulse wave height analyser 25 is applied to a spot generating circuit 26, so that each incoming gamma ray is displayed as a luminous point on CRT 27. A 2-dimensional image can be obtained by recording and accumulating these luminous points on polaroid film or X-ray film by means of lens. It is also possible to record and accumulate the frequency with which gamma rays arrive in an IC memory corresponding to X and Y to obtain a functional diagnostic image by carrying out data processing on the data stored in this IC. Figure 2 shows a case where such a functional diagnostic image is obtained. Position signals X and Y are supplied via A/D converters 2 and 3 to correction table 5 and linear correction part 6. From correction table 5, correction values ai(X,Y), bi(X,Y), Ci(X,Y), and di(X,Y); (where i = 1, 2, ...) corresponding to the X and Y signals are produced, and these are then supplied to linear correction part 6. This linear correction part 6 finds X′ and Y′ by calculating X′ = a₁X + b₁X + c₁XY + d₁ Y′ = a₂X + b₂X + c₂XY + d₂ . The Z signal is also supplied to correction part 7, where it is corrected by performing the calculation. Z′ = e(X,Y).Z This corrected Z′ is supplied to window circuit 9, constituting the next stage, and, if this input Z′ is a prescribed energy value, causes the pixel value of the corresponding address of image data memory 10 to be incremented by +1. The memory content of image data memory 10 is input to main CPU (central processing unit 8) through line 11 and the values of Z, X, and Y are input to main CPU 8 through lines 12 and 13, where calculation and rewriting of the correction coefficients are performed. Main CPU 8 performs a sensitivity correction on the contents of image data memory 10 in accordance with correction coefficients f(i,j). In collection modes such as single photon emission CT (hereinbelow termed "ECT") where strict uniformity etc. is required, main CPU 8 performs sensitivity correction in accordance with: N′ (i,j) = f(i,j) X N(i,j) where (i,j) are the memory addresses corresponding to (X,Y). It may be remarked that, since there is a timewise variation of the output wave height of photoelectron multiplier tube 19 due to optical coupling of scintillator 17 and light guide 18, or to variation of the characteristic of photoelectron multiplier 19 etc., it is necessary to rewrite the correction data of Z correcting part 7 periodically, to match these changed circumstances. Furthermore, being made of a metal, such as lead, of high gamma ray screening ability, collimator 16 shows differences in the proportion of gamma rays that it lets through, depending on the precision with which it was machined and on the location (X,Y). Consequently, in the case of ECT, there were the problems that ring-shaped artifacts were formed in the reconstructed image due to statistical scatter in the uniformity of the sensitivity of the gamma camera, or that artifacts were produced with lapse of time (however, such artifacts are not produced if the correction data are re-written to match time-wise variation, as mentioned above). This correction of uniformity of sensitivity for ECT collection purposes was carried out by creating a correction table (matrix) from the uniformity data of detector 15 (scintillator + light guide + photoelectron multiplier) + electrical system + collimator 16, and implementing a sensitivity correction as (collimator + detector) by multiplying the data in image data memory 10 by these coefficients. In this case, a change in the sensitivity of the whole system is seen due to the time-wise variation of the sensitivity of detector 15. Conventionally, to obtain correction data, steps (1) to (3) are necessary: (1) find the correction coefficient of the Z correction part 7 by removing the collimator; (2) find the correction coefficient of linear interpolation part 6 by mounting a phantom for X-Y correction (linear correction) on the detector; (3) find the uniformity correction matrix by collecting uniformity data with (collimator + detector) using a uniform linear source, by mounting collimator 16 on detector 15 after (1) and (2) above. It may be remarked that the data collection required for sensitivity correction of the detector with collimator fitted (step (3) above) usually takes 2 to 3 hours, and it is difficult to perform this data collection frequently, since there are a number of different types of collimator. Japanese Laid-Open specification no. 57-72087 discloses one method of dealing with variations in types of collimators. A collimator kind sensor 103 senses the collimator every time after the collimator is changed. A kind sensing signal 211 changes switches constituting a compensation matrix selecting circuit 102, and selects the corresponding matrix out of XY compensation coefficient matrixes 9.1 and 9.2. The selected XY compensation coefficient matrix is connected to an output 401 from an XY operating circuit 6. The compensation for the uniformity with respect to the collimator after the change is correctly performed, and the compensated data is displayed on a display 11. The object of this invention is to provide a gamma camera sensitivity correction device whereby the uniformity correction matrix for sensitivity correction can also be obtained rapidly. Accordingly the present invention provides a method for correcting uniformity of a gamma camera including a collimator for collimating radiation from a radioisotope administered to an object and a scintillation detector for detecting the radiation passing through the collimator to image an accumulation of the radioisotope of the object, said method including a first step of obtaining collimator characteristic sensitivity data that are characteristic of said collimator and do not vary as a function of time and storing said collimator characteristic sensitivity data in a memory; said method being characterised by:a second step of obtaining uniformity data that are characteristic of said detector and vary as a function of time in a condition with said collimator removed by using a radiation source; a third step of deriving uniformity data in the condition in which the collimator is fitted on said detector, based on said uniformity data of said second step and said collimator characteristic sensitivity data read from said memory of the first step; and a fourth step of updating uniformity correction coefficients for radioisotope accumulation image correction to compensate for said uniformity data that are characteristic of said detector and vary as a function of time, based on the uniformity data from said third step. Since the correction coefficients that are characteristic of the collimator do not vary with time, this sensitivity data that is characteristic of the collimator is stored beforehand in the memory means. The correction coefficients to be finally applied are found from the uniformity data that are derived in the condition in which the collimator is mounted on the detector, based on uniformity data that are characteristic of the detector (these being freshly collected with the collimator removed) and on the aforementioned sensitivity data that is characteristic of the collimator. Thus, in the compilation of the final uniformity correction matrix, the uniformity data that are characteristic of the detector can be collected with the collimator is removed; this makes it possible to do this in a short time, using a small radiation source. A preferred embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings, wherein: Figure 1 is a block diagram showing the main parts of a conventional gamma camera; Figure 2 is a block diagram showing the main parts of a gamma camera used to obtain a functional diagnostic image; Figure 3 is a block diagram of the layout of a gamma camera sensitivity correction device according to an embodiment of this invention. Figure 4, Figure 5, and Figure 6 are flow charts given in explanation of the operation of the device of Figure 3. A specific description of the invention is given below with reference to an embodiment. Figure 3 shows an embodiment of a gamma camera sensitivity correction device according to this invention. As shown in Figure 3, the device 30 of this embodiment is provided with: collimator characteristic sensitivity calculating means 31, collimator characteristic sensitivity memory means 32, self diagnostic means 33, uniformity data derivation means 34, correction coefficient calculating means 35, correction coefficient memory means 36, and sensitivity correction means 37. Collimator characteristic sensitivity calculating means 31 finds the sensitivity data Ucol(i,j) that are characteristic of the collimator from uniformity data F(i,j) that are collected in the condition with collimator 16 (See Figure 1) mounted on detector 15 and from uniformity data U(i,j) that are collected with collimator 16 removed, by performing the calculation of equation (1): Ucol(i,j) = F(i,j)/U(i,j) (1) The data Ucol(i,j) that are found by this collimator characteristic sensitivity calculating means 31 are written to collimator characteristic sensitivity memory means 32, which is located in a subsequent stage. Thus a table of the collimator's sensitivity characteristic is formed in this memory means 32. To compile the uniformity correction matrix of the detector with collimator mounted, self diagnostic means 33 finds the difference between the data U(i,j) freshly collected with collimator removed and the previously collected U(i,j) data. Depending on the magnitude of the difference between these two, it discriminates whether compilation of a uniformity correction matrix is appropriate or not. If it decides that it is appropriate, it sends the U(i,j) data to uniformity data derivation means 34. This uniformity data derivation means 34 derives the uniformity data in the condition where detector 15 is fitted with collimator 16 (i.e. "detector with collimator mounted"), based on the freshly collected data U(i,j) and data Ucol(i,j) in collimator's characteristic sensitivity memory means 32. If this uniformity data is represented as F′(i,j), this F′(i,j) is derived by performing the calculation of equation (2) below. F′(i,j) = U(i,j) . Ucol(i,j) (2) This data F′(i,j) derived by uniformity data derivation means 34 is then sent to correction coefficient calculating means 35 that is provided in a subsequent stage. Based on these input derived data F′(i,j), this correction coefficient calculating means 34 finds the uniformity correction coefficients for correcting the RI accumulated image. If these uniformity correction coefficients are represented as f(i,j), these f(i,j) are derived by performing the calculation of equation (3) below. where x.y means the total number of pixels in one RI accumulation image frame. Thus the found correction coefficients f(x,y) are written to correction coefficient memory means 36 provided in a subsequent stage, so that a correction table is formed in this correction coefficient memory means 36. This table constitutes the uniformity correction matrix of the detector with collimator mounted. Sensitivity correction means 37 performs sensitivity correction as (collimator + detector) using this uniformity correction matrix. This sensitivity correction is performed by multiplying the image data, i.e. the RI accumulated image data, in image data memory 10 by the correction coefficients held in correction coefficient memory means 36. This image data memory 10 corresponds to that indicated by the same reference numeral in Figure 2. Collimator characteristic sensitivity calculating means 31, self diagnostic means 33, uniformity data derivation means 34, correction coefficient calculating means 35, and sensitivity correction means 37 are functionally implemented by a CPU (central processing device). The operation of the device of the embodiment constructed as above will now be described with reference to the flow charts of Figure 4 through Figure 6. In compiling the uniformity correction matrix in the device of this embodiment (correction table in correction coefficient memory means 36), the sensitivity correction table that is characteristic of collimator 16 is already formed in collimator characteristic sensitivity memory means 32. This sensitivity correction table is found by collimator characteristic sensitivity calculating means 31. Specifically as shown in Figure 4, the uniformity data F(i,j) in the condition in which collimator 16 is fitted on detector 15 in the gamma camera are collected (S1), then, the uniformity data U(l,j) in the condition in which this collimator 16 is removed are collected (S2). Thereupon, these F(i,j) and U(i,j) are input to collimator characteristic sensitivity calculating means 31, where Ucol(i,j) are found by carrying out the calculation of equation (1) above. In cases where a sensitivity correction table of the Ucol(i,j) of the collimator in question has been already formed, carrying out steps S1 to S3 may be unnecessary (for example, it is planned that the manufacturer may provide a recording medium containing the sensitivity correction table Ucol(i,j), when such a collimator is shipped). The uniformity correction matrix is compiled as follows. (For convenience in the description, steps relating too self-diagnosis are omitted). First of all, the uniformity data U(i,j) that are characteristic of detector 15 in the condition in which collimator 16 is not fitted to detector 15 in the gamma camera are collected (S4). These data U(i,j) are input to uniformity data derivation means 34. This uniformity data derivation means 34 derives the uniformity data F′(i,j) in the condition in which collimator 16 is fitted to detector 15 (S5). This derivation is performed by carrying out the calculation of equation (2) given above. Next, when these F′(i,j) are output to correction coefficient calculating means 35, the calculation of equation (3) given above is performed by this calculating means 35, to find uniformity correction coefficients f(i,j) (S6). The thus-found f(i,j) are then written to correction coefficient memory means 36, forming the uniformity correction matrix. In compilation of the above uniformity correction matrix, the collimator's characteristic sensitivity data are treated as known, and only the uniformity data U(i,j) that are characteristic of the detector with collimator 16 removed are collected. Since collimator 16 is not fitted, this collection of uniformity data that are characteristic of the detector can be completed in a short time (about 10 to 20 minutes) using a small radiation source. The time required for data collection can therefore be greatly shortened as compared with the time of 2 to 3 hours that was previously necessary for the collection of data to compile the uniformity correction matrix (data collection with collimator mounted). This shortening of the data collection time means that compilation of the uniformity matrix (re-writing of the correction table of correction coefficient memory means 36) can be performed frequently. This makes it possible to carry out sensitivity correction by sensitivity correction means 37 with high accuracy. For example, compilation of the uniformity matrix of the device of this invention can be carried out before the daily clinical examination, thereby increasing the stability of the gamma camera. Self-diagnosis by self diagnostic means 33 will now be described. A flow chart of this operation is shown in Figure 6. First of all, the uniformity data U(i,j) that are characteristic of detector 15 with collimator 16 not mounted are collected (S7). Next, the difference (i.e. the fluctuation) SB is found (S8) between these data U(i,j) and the most recent previously collected data U(i,j). A decision is then made (S9) as to whether or not this difference SB is larger than a pre-set value m that was preset to take into account statistical variation. In this decision process, if a decision NO (SB > m does not hold) is made, the U(i,j) are collected (S13), F′(i,j) are derived (S14), and f(i,j) are found (S15). These steps S13, S14, and S15 correspond to steps S4, S5, and S6 of Figure 5. Also, in the decision process of step S9, if a decision YES (SB > m does not hold) is made, a decision is now made (S10) as to whether or not this difference SB is larger than a pre-set value M that was pre-set as the threshold value for correction to be performed. If, in this decision process, a decision NO (SB > m does not hold) is made, this implies that this difference is not yet large enough for correction to be performed, so the Z correction data of Z correction part 7 (see Figure 2) is re-calculated and the Z correction table re-formed (S11). Processing then shifts to execution of step S13. If, in the decision process of this step S10, a decision YES (SB > M holds) is made, in view of the possibility that this may be due to a malfunction of the gamma camera, a "correction not possible" message is output, with a request for service by a maintenance engineer (S12). Self-diagnosis of the nature of the abnormality may then be performed. Of the above steps, steps S8 to S12 are carried out by self-diagnostic means 33. Carrying out a self-diagnosis in this way makes it possible to perform the routine inspection and self-servicing of the gamma camera in an appropriate way. One embodiment of this invention has been described above, but this invention is not restricted to the above embodiment but may of course be put into practice in various modified ways, within the scope of the appended claims.

Claims: A method for correcting uniformity of a gamma camera including a collimator for collimating radiation from a radioisotope administered to an object and a scintillation detector for detecting the radiation passing through the collimator to image an accumulation of the radioisotope of the object, said method including a first step of obtaining collimator characteristic sensitivity data that are characteristic of said collimator and do not vary as a function of time and storing said collimator characteristic sensitivity data in a memory; said method being characterised by:a second step of obtaining uniformity data that are characteristic of said detector and vary as a function of time in a condition with said collimator removed by using a radiation source; a third step of deriving uniformity data in the condition in which the collimator is fitted on said detector, based on said uniformity data of said second step and said collimator characteristic sensitivity data read from said memory of the first step; and a fourth step of updating uniformity correction coefficients for radioisotope accumulation image correction to compensate for said uniformity data that are characteristic of said detector and vary as a function of time, based on the uniformity data from said third step. Method according to claim 1, wherein said uniformity correction coefficients are expressed as f(i,j), said uniformity data as F(i.j), and the total number of pixels per frame of the radioisotope accumulation image as x,y, and the uniformity correction coefficients for radioisotope accumulation image correction are determined by the following relationship: Method according to claim 2, further comprising: storing said uniformity correction coefficients determined in said fourth step to form a correction table in a correction coefficient memory. Method according to claim 3, further comprising: performing sensitivity correction using said correction table, wherein the radioisotope accumulation image data are multiplied by the correction coefficients in said correction coefficient memory. Method according to claim 1, further comprising: determining whether a sensitivity correction is necessary by determining the difference between recently collected uniformity data and previously collected uniformity data and using this difference to make a decision as to whether said correction table is appropriate or a correction is required.

Inventors: ICHIHARA,TAKASHI C/O PATENT DIVISION
Applicants: KABUSHIKI KAISHA TOSHIBA
Application year: 1988
Main classification: G01T1
IPC codes: A61B6, G01T1