DATA ACQUISITION CIRCUIT

Abstract: A data acquisition circuit having arithmetic units (22) installed in a data bus and adapted to carry out correction such as dead-time correction and decay correction, and also having an address modification storage (12) installed in an address bus and adapted to carry out the process of changing raw data into a sinogram. As the size of memory matrices for positron ECT grows larger in recent years, software dependent on a CPU has tended to require more time for data processing and prevent image formation from taking place in real-time. The aim of the present invention is to allow data reduction and correction to be carried out in real-time.

Description: The present invention relates generally to the field of imaging devices for use in medical diagnosis as well as to the field of digital data acquisition. The present invention relates to a data acquisition circuit, and more particularly to a data acquisition circuit for use in positron ECT. In ECT a positron emitted from a positron emitter combines with an electron and is annihilated. The result is that two gamma rays, each of which acquires 0.511 MeV of energy, are emitted in opposite directions making an angle of approximately 180° with each other. In positron ECT, therefore, two gamma rays detected simultaneously are regarded as indicative of a positron emitted and annihilated. For the purpose of such detection, it is most common to connect a coincidence circuit to each of a pair of facing detectors. Every time a coincidence circuit is actuated, an additional 1, which indicates that an event has occured, is written into a particular location in a memory circuit allotted to the pair of detectors connected to the actuated coincidence circuit. Consequently, the readout from this particular location after the end of the data acquisition process indicates the number of positrons emitted and annihilated along the line extending from one of the abovementioned pair of detectors to the other. Fig. 3-(1) shows the construction of an ordinary memory matrix used in the above-described data acquisition. This memory matrix has an address capability of the number of detectors falling under one set multiplied by the number of detectors falling under the other set. In Fig. 3-(1), the ordinates denote the numbers i given to the detectors falling under one set, and the abscissas denote the numbers j given to the detectors falling under the other set. A point (i,j) in this orthogonal coordinate system indicates that one of two gamma rays, which have been discharged as a result of the annihilation of a positron, has been incident on a detector i falling under one set, while the other of the two gamma rays has been incident on a detector j falling under the other set. Because (i,j) and (j,i) indecate one and the same pair of detectors, the accumulation of data occurs only in the region provided with oblique lines in Fig. 3-(1). It is most common to reconstruct the image by convolution and back projection. Data to be subjected to these algorithms, i.e., a plurality of points (i,j) in Fig. 3-(1), are collected in a straight direction, and this direction is ever-changing. These data have to be rearranged for the reconstruction of the image so that an angle ϑ (Fig. 4) made with the positive direction of a reference axis by a line perpendicular to the above-mentioned straight direction may be plotted on an ordinate against the distance of the straight direction from the origin of the reference axis on an abscissa. The data plotted in this manner are called a sinogram, which is shown in Fig. 3-(2). These data are subjected to convolution by means of a reconstruction filter and then to back projection so as to be reconstructed into a tomographic image. So far, high resolution has been attained by increasing the number of detectors, with the result that the address capability of a memory matrix has been enlarged from 64 x 64 to 128 x 128, then to 256 x 256, and then to 512 x 512. Consequently, more time has come to be required for the above-described processing from raw data to a sinogram, and the final image formation is apt to be prevented from taking place in real time. An example of a position emission computal tomograph (ECT) is described in an article entitled "A computer assisted ring detector position camera system for reconstruction tomography of the brain" IEEE Transactions on Nuclear Science, Vol. NS-25 No. 1, February 1978, pages 624-637, C. Bohm et al. This system has a detection stage the output of which is connected to a coincidence and counting system. Address information is supplied to an address bus and data to a data bus, both busses being connected to a buffer storage means. The address bus includes address modification storage means for cirectly taking addresses from said coincidence and counting system. The system does not employ any data correction circuitry. Especially when an object to be imaged is in motion, dead-time correction has to be carried out on the basis of mean values, by which a large error is introduced. This holds true for the decay correction as well. Another problem is that the conventional positron ECT requires software for allowing the processing to proceed from raw data shown in Fig. 3-(1) to a sinogram shown in Fig. 3-(2), except that hardware such as a microcomputer, for which the software support is a must, is used as the case may be. An example of such a correction arrangement is described in an article entitled "A dedicated hardware architecture for data acquisition and processing in a time-of-flight emission tomography system (Super-Fett)", IEEE Transactions on Nuclear Science, Vol. NS-30, No. 1, February 1983, pages 170-174. SUMMARY OF THE INVENTION In view of the above-described problems involved in the conventional positron ECT, it is an object of the present invention to provide a data acquisition circuit for use in positron ECT, this data acquisition circuit being characterised in that hardware alone is enough to allow the data to be rearranged in the above-described manner and that this hardware accommodates arithmetic units so that operation efficiency per unit time may be improved to such an extent that dead-time correction and decay correction can be carried out nearly in real-time. According to the present invention there is provided a data acquisition circuit in a positron ECT in which uncorrected data is produced by coincidence circuits upon anninilation of positrons as claimed in Claim 1. A preferred embodiment of the present invention is hereinafter described with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic block diagram of an embodiment of the present invention; Fig. 2 is a view illustrating an example of a buffer storage for use in the circuit in accordance with tie present invention; Fig. 3 is a schematic illustration to help explain the process with which a prior art ECT proceeds. Fig. 4 is a drawing to help explain the principle of tomographic scanning; Fig. 5 is a graphical representation of a calibration curve, illustrating how an amount for dead-time correction is determined; and Fig. 6 is a drawing to help explain the process of dead-time correction to be carried out in accordance with the present invention. DESCRIPTION OF A PREFERRED EMBODIMENT An embodiment of the present invention shown in Fig. 1 includes temporary storages 10 connected to a coincidence circuit (not shown), an address modification storage 12 installed in an address bus and adapted to carry out the process of changing raw data into a sinogram, an adder 14 for the base address of each frame region, a multiplier 16 for multiplying each data by a coefficient which can be changed with time, an adder 18 for adding a new data to a preceding data, and a buffer storage 20 having a plurality of frame regions. Preferably, quick acting SRAMs are used as the temporary storages 10, and a large-capacity DRAM is used as the buffer storage 20. Each data is stored first in the temporary storage 10A and transmitted to the buffer storage 20 every 0.5 to 2 seconds. On its way to the buffer storage 20, the data is transferred to the temporary storage 10B and stored therein. The transmission takes place substantially at the rate of a word per microsecond. Addresses given to the data in the temporary storages 10 are modified and permuted in the storage 12. The data per se are subjected to operations together with a constant (a coefficient), fed to the buffer storage 20, and added to the preceding data so that data stored in the buffer storage 20 may be renewed. The buffer storage 20 has a capacity of 1 MW or more, and is capable of arbitrary switchover from the base address of one frame region to the base address of another frame region so that a data may be stored in any memory location in the buffer storage 20. Reference is now specifically made to the decay correction. In positron ECT, it is desirable to use a radioisotope (RI) having a longer half-life, because such an RI permits even its metabolites to be traced in the tissues. However, a given amount of an RI is reduced to one-half by radioactive decay when the half-life has elapsed. Therefore, data collected after the lapse of the half-life do not give a clue to the distribution of the RI in vivo unless these data are multiplied by 2. In fact, decay correction is carried out by allowing the data to be multiplied by at extremely short intervals of time, where T = half-life, and t = time which has elapsed after the start of data acquisition. This multiplier is derived from the theorem that the number of atoms of the original radionuclide remaining after time t is given by where N(t) = number remaining N(o) = original number Reference is now specifically made to the dead-time correction. It is a frequent occurrence that counting dead time cannot be given by a simple equation but can be found only from a calibration curve such as the one shown in Fig. 5. For the preparation of such a calibration curve, an RI is allowed to mix at various concentrations with water in a water tank, and this water is scanned. The counting rate is plotted on an ordinate against the concentration of the RI on an abscissa. Ideally, i.e., if an electronic circuit is free from counting loss, the counting rate is in a direct proportion to the concentration of the RI as shown by a line OA in Fig. 5. Actually, however, it is no exaggeration to say that no electronic circuits are free from counting loss, and the relationship between the counting rate and the concentration of the RI is represented by a curve OB in Fig. 5. Let is be supposed that a counting rate measured at a given concentration of the RI can be represented by a point C which lies on the curve OB. Then the line segment CC′ represents an amount by which the counting rate should be corrected. Fig. 6 is a drawing to help explain the process of dead-time correction to be carried out in accordance with the present invention. Outputs taken from respective coincidence circuits (not shown) are summed up, and the sum total of these outputs are stored in a counter (Fig. 6). Let it be supposed that data are taken out of this counter every one second. Then a difference between the data taken out at a given moment and the data taken out at the preceding moment represents the number of events which have occurred during the past one second. In other words, this difference corresponds to the concentration of the RI mentioned in the preceding paragraph as a criterion for determining the amount by which the counting rate should be corrected. This difference is set in the temporary storages 10 as a constant which changes every one second, in case of the above-mentioned supposition, so as to allow the dead-time correction to be carried out nearly in real-time. Actually, however, the temporary storage 10A has a capacity of as large as 256 KW. The implication is that 0.256 second is enough to transmit each data even if the transmission takes place at the rate of a word per microsecond. Therefore, one can draw nearer to real-time if the concentration of the RI is monitored at intervals of 0.256 second, i.e., under the condition of synchronism with the transfer of data. The constant to be set in the temporary storages 10 is the product of two amounts determined for decay correction and dead-time correction respectively, i.e., and the amount represented by the line segment CC′ in Fig. 5. Alternatively, a parameter or parameters other than the above-mentioned two amounts may be used as a constant to be set in the temporary storages 10. For example, a data is multiplied by a constant α, allowed to have a base address of O, and transmitted to the buffer storage 20. Then the same data is multiplied by a constant β,allowed to have a base address of 2,000,000, and transmitted to the buffer storage 20. The result is that two differently weighted data are made from one data. Adding these two data to each other is tantamount to convolution. Consequently, the time which would otherwise be required for convolution can be done without. An arithmetic unit 22 shown in Fig. 2, which will be described later, is the very device for serving such a time-saving purpose. The address modification storage 12 functions also as a means for picking out necessary data from among the whole of data supplied, or as a means for suppressing the data. For dynamic framing, it is preferable to make a switchover from the base address of one frame region to the base address of another frame region every time a data is transmitted. Interpolating operations are performed by an arithmetic unit 22, which is attached to the buffer storage 20 as shown in Fig. 2. In an arithmetic unit 22a, a data stored in the buffer storage 20 and provided with a first base address can be added to, subtracted from, or multiplied by a data provided with a second base address so that a data provided with a third base address may be obtained from such addition, subtraction or multiplication. Such operations can be combined with each other. For example, a plurality of frames can be added to each other, or a constant can be subtracted from each of them, or each of them can be multiplied by a constant so that they may be normalized, or one of them can be masked by another. These operations can be performed without the aid of a CPU. Consequently, operation efficiency per unit time can be improved. A device 22b for setting the addresses and gates 22c are controlled by a CPU. In case where the data acquisition circuit in accordance with the present invention is used in ECG gated Ect, an address taken from a coincidence circuit (not shown) is fed to an auxiliary input terminal shown in Fig. 1, and the multiplier 16 is allowed to have a constant of 1. Then, ordinary data acquisition can be carried out, wherein the count is increased by one every time an event occurs in a modified address in the buffer storage 20. Data divided into n parts can be collected in the buffer storage 20 if a switch-over from the base address of one frame region to the base address of another frame region is made at the rate of one n th of the period of the R-R wave, where n = 2 to 32. Although the processing such as dead-time correction and decay correction can be carried out without the aid of a CPU, the CPU may be used for the time sequencing of the operation of the arithmetic units installed in the address bus and the data bus. The temporary storages 10 may be monitored so that a constant or a coefficient may be set in the multiplier 16 every time a new data is fed to the temporary storages 10. A means for such monitoring may be provided as a function of a CPU. Thus the present invention has an effect of making the data throughput higher so as to meet a demand for higher resolution and dynamic positron Ect studies. Processing such as dead-time correction and decay correction can be carried out nearly in real-time. The present invention has another effect of improving the quantitative properties of measurement in positron ECT, and can be used for data acquisition in ECG gated ECT as well.

Claims: A data acquisition circuit in a positron ECT in which uncorrected data is produced by coincidence circuits upon annihilation of positrons, said circuit comprising buffer storage means (20) coupled to an address bus and a data bus, and capable of storing data derived from said coincidence circuit means in a plurality of addressable data frames; and address modification storage means (12) arranged in said address bus connected to said buffer storage means (20); characterised in that said circuit includes temporary storage means (10A, 10B), inserted between said coincidence circuit means and said address and data buses, for temporarily storing data output from said coincidence circuit means; adding means (14) arranged in said address bus and adapted to specify any of the base addresses given respectively to frame regions provided in said buffer storage means (20); and arithmetic units (16, 18) arranged in said data bus connecting said temporary storage means to said buffer storage means and adapted to carry out correction such as deadtime correction and decay correction and also adapted to subject the data to other arithmetic operations. A data acquisition circuit as claimed in claim 1, wherein a CPU for image reconstruction processing and hardware for the exclusive use therein are connected to said buffer storage means (20), data being transmitted from said buffer storage means (20) to said CPU and said hardware each time data is transmitted from said temporary storage means (10A, 10B) to said buffer storage means (20). A data acquisition circuit as claimed in claim 1, wherein addresses taken from detectors are adapted to be directly fed to said address modification storage means (12), and an integrator (16) having an initial value of 1 is arranged in said data bus, so as to adapt said data acquisition circuit for use in ECG or gated ECT. A data acquisition circuit as claimed in claim 1, wherein either constants computed beforehand or readouts from a memory, which can take the form of a nomograph permitting quick access in real-time, are used as inputs to be fed to said arithmetic units in (16, 18) said data bus. A data acquisition circuit as claimed in any preceding claim, wherein the temporary storage means comprise first (10A) and second (10B) temporary storages which are arranged such that one such storage collects data whilst the other outputs data.

Inventors: HIROSE, YOSHIHARU
Applicants: SHIMADZU CORPORATION
Application year: 1988
Main classification: G06F15
IPC codes: G06F17, A61B6, G01T1