JPS61132887A - Correcting method of accidental simultaneous counting in pct device - Google Patents

Abstract

PURPOSE:To minimize an S/N by collecting in a memory a fact that two detecting signals have been generated simultaneously, and on the other hand counting the simultaneity of a signal delayed by a prescribed time and an undelayed signal, collecting it in the second memory, performing a smoothing, and thereafter, subtracting it from the first memory. CONSTITUTION:Many radiation detectors 1, 1... are provided in a ring type in one plane, and a radiant ray radiated in the direction of 180 deg. when a positron annihilates is made incident simultaneously on two detectors 1. As a result, an output is generated from a simultaneous detecting circuit 3, and '1' is added to the contents of an address corresponding to a straight line 21 of a memory 5. On the other hand, when a gamma ray is made incident by a time difference of, for instance, 50nsc on the detectors 1, 1, respectively, an output is generated from a delay simultaneous detecting circuit 4, the contents of an address corresponding to a straight line of a memory 6 are added by '1', and counting of an off-time coincidence is executed. This counting data is brought to smoothing, and an accidental simultaneous counting data is subtracted from a simultaneous counting data. In this way, an image whose S/N is excellent can be obtained.

Description

Detailed Description of the Invention (a) Industrial Application Field This invention relates to a method for correcting accidental coincidence in a PCT device, that is, an ECT device (emission-computed tomography device) that uses a positron-emitting nuclide as a radionuclide. Regarding.

(b) As is well known, the conventional PCT device utilizes the fact that 2 (!l) of gamma rays are emitted in the 180' direction when positro/ disappears. It is based on the fact that by detecting two gamma rays directed toward each other, it is known that positron-emitting nuclides are present on the line connecting the two sets of detectors.In an actual device, many γ-rays are detected in one plane radiation detectors are arranged in a ring shape, and when a positron disappears inside this ring shape, 180
Taking advantage of the fact that radiation emitted in six directions is simultaneously incident on two of the detectors, this coincidence is used to determine on which straight line the positron disappears,
Obtain data regarding coincidence counts for each of every straight line in the plane. Then, pieces of count data whose straight lines have the same direction are collected as one profile data, and the profile data in each direction is processed by an image reconstruction processing algorithm (for example, after applying a correction filter to the profile data in each direction, (back projection, etc.)
As a result, a concentration distribution image of positron-emitting nuclides within the plane is reconstructed.

When calculating this coincidence, a problem arises in that separate gamma rays, which are not caused by a single positron-emitting nuclide, happen to be incident on two detectors at the same time and are counted simultaneously. This is called accidental coincidence and results in an error in the profile data.

Therefore, various methods have been proposed to correct this accidental coincidence. As one of them.

, Delay φ coincidence (Dela7 Co1n
A method of counting delay simultaneity (delay simultaneity) is known as a highly accurate correction method. This is due to radiation incident events that actually occurred at the same time (on-time coincidence), including the accidental coincidences mentioned above.
On the other hand, we deliberately delay the signal from one detector by a suitable amount of time (e.g. 50 r+sec) so that the delayed signal and the undelayed signal from the other detector are counted. Simultaneity (off-time coincidence; Off Ti layer eCa-
Then, since off-time coincidences occur only by chance, if you subtract the counted value of off-time coincidences from the counted value of on-time coincidences, it will be included in the counted value of on-time coincidences. Accidental coincidences can be removed and corrected. That is what it is.

Specifically, there are ways to add +1 to the corresponding memory address for on-time coincidences and -e for off-time coincidences, and to collect the counted values for on-time coincidences and the counted values for off-time coincidences in separate memories. However, in either case, the statistical fluctuation included in the off-time coincidence count value is calculated by subtracting it.
This is added to the final data, and as a result, statistical fluctuations remain in the final image, as if there were twice as many coincidental coincidences, deteriorating the S/N ratio of the image.

In particular, in the case of transmission data used for absorption correction, it is extremely problematic that the S/N ratio thereof deteriorates due to statistical fluctuations in off-time coincidence counts.

(c) Purpose This invention provides a method for correcting errors caused by accidental coincidences by counting off-time coincidences using delay coincidences and subtracting this counted value from the counted value of on-time coincidences. It is an object of the present invention to provide a correction method that can minimize the deterioration of S/N by eliminating the influence of statistical fluctuations included in .

(D) Structure The method of correcting accidental coincidence in a PCT device according to the present invention collects on-time coincidence data and off-time coincidence data in separate memories, and performs smoothing on the off-time coincidence data. The feature is that it is subtracted from the on-time coincidence data after that.

(E) Embodiment In this invention, random coincidences are generally distributed over the entire field of view, and even in the case of a point source, they are spatially distributed gently. The focus is on the fact that there is almost no change in , and only the effect of correcting and removing statistical fluctuations by smoothing can be obtained.

In the figure, a number of radiation detectors 1. l, . It is designed so that it can be incident on both at the same time. The subject 2 is placed in this ring-shaped arrangement, and a drug containing a positron-emitting nuclide has been administered into the body of the subject 2, and when this positron disappears, two γ rays are emitted and are simultaneously incident on the 2 gs detectors 1, l lying on one straight line 21, for example.

Here, a number of detectors 1. A coincidence detection circuit 3 and a delayed coincidence detection circuit 4 are provided for each combination of two of l, . . . . The coincidence detection circuit 3 is for counting on-time coincidences, and is composed of an AND circuit 31 and a 7-lip-flop 32, and is connected to two detectors 1.
produces an output when the outputs of are input at the same time. Also,
The delayed coincidence detection circuit 4 is for off-time coincidence counting, and outputs from the AND circuit 41, the flip-flop 42, and one of the detectors 1 for a certain period of time, e.g.
It has a delay circuit 43 that delays by n5ec,
The flip-flop 42 produces an output when the delayed output of one detector l and the undelayed output of the other detector l are simultaneously input to the AND circuit 41. The respective outputs of the coincidence detection circuit 3 and the delayed coincidence detection circuit 4 are sent to the memory 5.6, and the content of the address corresponding to the set of detectors 1 is incremented by 1.

As mentioned above, the two detectors 1. on the straight line 21. When gamma rays are simultaneously incident on the line 21, an output is generated from the simultaneous detection circuit 3 to which the outputs of the detectors 1 and 1 are sent, and the contents of the address corresponding to the straight line 21 in the memory 5 are incremented by 1. The on-time coincidence count obtained in this way includes not only the count of two gamma rays generated from one positron-emitting nuclide as described above, but also the count of incidental gamma rays that happened to be incident at the same time. Concurrency values are also included.

On the other hand, straight! When γ-rays are incident on each of the two detectors 1.1 on 121 with a time difference of 50 nsec.

An output is generated from the delay coincidence detection circuit 4, and this output is sent to the memory 6, so that the contents of the address corresponding to the straight line 21 are +
1, and off-time coincidences are counted.

The fact that an output is generated from the delayed simultaneous detection circuit 4 in this way occurs when the gamma rays happen to be incident on the two detectors 1.1 with a time difference of 50 n5ac. That is, this is a case that would never occur in the case of two gamma rays from one positron-emitting nuclide, and can be said to occur only accidentally. Therefore, this memory 6
Data on random coincidences is collected.

The off-time coincidence count data is smoothed by a smoothing circuit 7 and then sent to a subtraction circuit 8 where it is collected into a memory 6 from the coincidence data collected in a memory 5. A subtraction is performed to subtract the data of random coincidences.

Here, it can be said that accidental simultaneous incidence occurs with equal probability at any point in time. In other words, in the case of accidental γ-rays, the probability of simultaneous incidence on the two detectors 1, 1 and 50n+ec
The probability of incidence with a time difference of is the same. Therefore, by subtracting the off-time coincidence data collected in memory 6 from the on-time coincidence data collected in memory 5, the accidental coincidence value included in the count value in memory 5 can be removed. It is possible. Since the off-time coincidence count value data is smoothed, statistical fluctuations included in the off-time coincidence count value data are removed. Incidental coincidences are generally distributed over the entire field of view, and even in the case of point sources, they are spatially distributed gently, so even if smoothing is applied to the off-time coincidence count data, the distribution hardly changes. Therefore, there is no problem even if smoothing is applied.

Therefore, the statistical fluctuations of the off-time coincidence counts will not remain in the data after this subtraction, and by reconstructing the image using this data, the increase in statistical fluctuations due to the correction of accidental coincidences can be suppressed. Minimize, S/
N excellent final images can be obtained. This effect is almost equivalent to halving the temporal detection width during simultaneous detection.

Note that the present invention can be applied not only to emission data for reconstructing images as described above, but also to correction of accidental coincidence in transmission data, blank data, or normalization data. The S/
This is effective in improving N.

(f) Effects According to the present invention, it is possible to reduce statistical fluctuations in off-time coincidence counting data, and it is possible to reduce statistical fluctuations in data after correction of accidental coincidences performed using this off-time coincidence counting data. S/N of reconstructed concentration distribution image of positron emitting tower species
can be improved. In particular, when correcting accidental coincidences in transmission data, the S/N of the data can be improved, which is extremely effective.

[Brief explanation of drawings]

The figure is a block diagram of an embodiment of the present invention. l... Radiation detector 2... Subject 3... Simultaneous detection circuit 4... Delayed simultaneous detection circuit 5.6...
・Memory 7...Smoothing circuit 8...Subtraction circuit

Claims (1)

[Claims] (1) A large number of radiation detectors are arranged in a ring shape within one plane, and when a positron disappears inside this ring-shaped arrangement, the radiation emitted in a 180° direction is from two of the above detectors. By using the simultaneous incidence of
Using this coincidence, it is possible to determine on which straight line the positrons have disappeared, and from the data regarding the coincidence for each straight line in the plane, the concentration of positron-emitting nuclides in the plane is determined using an image reconstruction method. In a PCT device reconstructing a distribution image, the simultaneous occurrences of the signals from each pair of said detectors are counted and collected in a first memory, while the signals from each pair of said detectors are counted and collected in a first memory; The signal from the detector is delayed for a predetermined time, the simultaneity of this delayed signal and the undelayed signal from the other detector is counted and collected in a second memory, and the second memory A method for correcting accidental coincidence by smoothing data collected in the first memory and subtracting the data from the data in the first memory.