JPS6113169A - Position ct apparatus - Google Patents

Abstract

PURPOSE:To enable the accurate correction of counting omission even if an object to be inspected or a collimator changes, by providing a detector, a means for collecting real and accidental simultaneous counting data, a means for obtaining counting omission correction factor and a means for an image re-construction by using real coincidence counting. CONSTITUTION:When gamma-rays are simultaneously incident on detectors 11, 1n, the outputs of the detectors 11, 1n are sent to an AND circuit 3 through shaping circuits 21, 2n. Real and accidental countings are contained in the output of this circuit 3. In the case of real coincidence counting, if the other one is delayed by a delay circuit to be sent to an AND circuit 4, no output is generated from the circuit 4 and, in the case of the accidental one, the coincidence with the other pulse is generated and output is generated. The output of the circuit 4 is the coincidence counting of an OFF-time and, if the coincidence counting of the OFF-time is subtracted from that of an ON-time, real coincidence counting is obtained. By this method, the data relating to the real coincidence counting with respect to each of many detectors 11, 12... is measured by a data collection apparatus 6 and the algorithm treatment in the re-constitution of an image is performed.

Description

DETAILED DESCRIPTION OF THE INVENTION (a) Field of Industrial Application This invention relates to a positron CT apparatus.1. In particular, the present invention relates to a positron CT apparatus that can accurately correct for missing counts of γ-rays.

(b) Prior Art A positron CT apparatus is an emission CT apparatus that uses a positron-emitting nuclide as a radiopharmaceutical to be administered into a patient's body. When the positron disappears 2
Since the gamma rays of wood are simultaneously emitted in 180° directions, if these are detected simultaneously by two detectors, a positron exists on the straight line connecting the two detectors. We need to collect a large amount of information about straight lines on which radioactive materials exist, and then process the data using image reconstruction techniques similar to those used in X@CT devices (computed tomography devices). For example, a concentration distribution image of radioactive substances can be obtained.

By the way, in this positron CT apparatus, it is inevitable to omit counting due to hardware constraints, such as omitting gamma rays when two events occur at an extremely short time interval.

Therefore, it is necessary to correct this omission, but
In the simplest case, a certain phantom is prepared in advance, separate from the subject, and the phantom is measured to determine the actual percentage of missing counts, and then the subject is measured based on that percentage. It is possible to correct the measured value when In other words, since only true coincidence counts are useful data, we can actually measure the true coincidence rate for a certain phantom and obtain true coincidence rate data regarding the concentration of radioactive substances as shown in B in Figure 2. obtain. In an ideal state with no omissions, the true coincidence rate for concentration is A in Figure 2.
Since it is known that the difference between line A and line B is used, a table of correspondence between the actual measurement value of the counting rate and the proportion of missed counts is created. And when measuring the object, 6
The true count rate measurement value obtained is corrected using this correspondence table. Note that true coincidence here means two γ beams that occur when one positron annihilates.
It refers to simultaneous counting of lines, and is used as a term for accidental simultaneous counting. Accidental coincidence refers to a coincidence that does not occur at the same time when a single positron annihilates, but occurs when two unrelated and separate gamma rays coincidentally inject at the same time.

However, in actual practice, the percentage of missed counts varies depending on the size and shape of the subject or phantom, the shape of the collimator, etc.

Therefore, if the phantom diameter and the subject diameter are different, correct correction cannot be made, and even if the collimator is replaced, correct correction cannot be made.

(C) Objective The object of the present invention is to provide a positron CT apparatus that can accurately correct counting errors even if the diameter of the object to be examined or the collimator changes.

(D) Structure The positron CT apparatus according to the present invention includes a large number of detectors arranged in a ring shape, a means for collecting true coincidence data between each of these detectors, and a means for collecting true coincidence data between these detectors. and a means for collecting accidental coincidence data between each of them, and by measuring the phantom in advance, a counting omission correction coefficient corresponding to each of the accidental coincidence data and the accidental coincidence data is calculated. Obtain true coincidence data and accidental coincidence data at the time of subject measurement, find the correction coefficient corresponding to the accidental coincidence data from the above correction coefficient, and apply this correction coefficient to the above correction coefficient. The counting omission correction is applied to the true coincidence data, and the corrected true coincidence data is used to reconstruct an image.

(E) Example This invention is based on the fact that, as a result of many experiments conducted by the present inventor, it was discovered that the rate of missed counts related to the accidental coincidence rate is a constant value regardless of the subject, collimator, etc. It has its roots in From this, the reason why the count loss rate related to the accidental coincidence rate is a constant value regardless of the subject or collimator is that the value depends on the count loss characteristics inherent to the positron CT device. This is thought to be because Therefore, if a correction coefficient for missed counts related to the accidental coincidence rate can be obtained, that correction coefficient can not only be applied to any subject or collimator to correct the data of accidental coincidences. This means that it can be used for any type of object or collimator, even for correcting true/coincidence counting data. Therefore, if you have calculated the missed-count correction coefficient ° for the accidental coincidence rate in advance and obtained the true coincidence data by measuring the subject, it is simply a matter of how much the accidental coincidence data at that time is. By simply checking whether the correction coefficient is correct, finding the corresponding correction coefficient, and applying this correction coefficient to the data of the true simultaneous coefficient, it is possible to obtain highly accurate data regardless of the object or collimator. You can make corrections for missing counts.

In FIG. 1, a number of detectors 11. ．． 12, .
. . . true coincidences and accidental coincidences between each of them are detected. For example, detector 1
If gamma rays are incident on detector 11.1 and detector In at the same time, then detector 11. Each output of in is sent to the AND circuit 3 via a pulse shaping circuit 21.2n. The output of this AND circuit 3 indicates that simultaneous incidence has occurred on the detector 11.1n (this is called on-time coincidence counting).
This includes true coincidences and accidental coincidences. In case of true coincidence.

If one of the pulses is delayed by a delay circuit and sent to the ANDN0 circuit, no output will be generated from these ANDN0 circuits, but in an accidental case, even if it is delayed, simultaneity with other pulses will occur, and the A
Outputs are generated from the NDN0 circuit. The ANDN0 circuit output can be said to be an off-time coincidence, and this off-time coincidence corresponds to an accidental coincidence. The true coincidence count can be found by subtracting the off-time coincidence count from the on-time coincidence count. In this way, data regarding true coincidence is measured by the data acquisition device 6 for each of the many straight lines connecting each of the many detectors 11, 12, . Ru.

Therefore, first, we measure the accidental coincidence rate for the phantom. In this case, the phantom diameter may be arbitrary. Then, data as shown by D in FIG. 2 is obtained. Since this data includes counting errors, it differs from C, which represents data in an ideal state with no counting errors. Since this difference corresponds to omitted counts, the proportion of omitted counts with respect to the actual measured value of the accidental coincidence rate is calculated from this difference, and this is used as a correction coefficient. Find each one. Alternatively, a curve that best approximates the change in this correction coefficient may be calculated and obtained.

Next, the subject is measured. At this time, both the true coincidence rate and the accidental coincidence rate are determined. That is, the off-time output obtained from the ANDN0 circuit of FIG. 1 is counted and the counted value is stored. Then, the above-mentioned correction coefficient corresponding to this off-time counting rate is determined, and the actual measured value of the true coincidence rate is multiplied by this correction coefficient. In this way, accurate counting correction of the actual measured value of the true coincidence rate is performed.

By processing the data of the true coincidence rate that has been subjected to such counting omission correction using an algorithm for image reconstruction by the CPU 7, an image of the concentration distribution of radioactive substances inside the subject can be reconstructed.

In an actual experiment, by correcting the counting loss as described above, the counting loss, which was about 5% at 30Kcps before correction, could be reduced to about 1% up to 1oOKcps. It was also experimentally confirmed that the accuracy of this counting correction is hardly affected by the phantom diameter or collimator shape.

In addition, to correct for missed counts using a correction coefficient determined in advance, data on true coincidence is collected at the time of measuring the object, and off-time coincidence data is also collected, and corrections are made corresponding to this off-time data. A specific procedure may be used in which a coefficient is determined and true coincidence data is corrected during image reconstruction using this correction coefficient, or another procedure may be used. Also,
Various specific circuit configurations can be considered.

(f) Effects According to the present invention, accurate counting correction can be performed even if the diameter of the object to be examined or the type of collimator changes, and no special hardware is required.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of an embodiment of the present invention, and FIG. 2 is a graph showing the relationship between the counting rate and the concentration of radioactive substances. 11.12, ln...detector

Claims (1)

[Claims] (1) A large number of detectors arranged in a ring, a means for collecting true coincidence data between each of these detectors, and an accidental coincidence between each of these detectors. A means for collecting count data, a means for obtaining accidental coincidence data and a count correction coefficient corresponding to each of the accidental coincidence data by measuring a phantom in advance, and measuring a subject. Obtain true coincidence data and accidental coincidence data by 1. A positron CT apparatus comprising: a means for performing count-off correction; and a means for reconstructing an image using true coincidence data after the correction.