JP2001281341A - Positron CT system - Google Patents

Abstract

(57) [Summary] [PROBLEMS] To perform relatively accurate scattered radiation correction in a short time by reducing complicated calculation processing. SOLUTION: Projection data, which is a count value distribution on a projection plane having a body axis direction and an in-slice plane direction, is created from measurement emission data, and the same projection plane is obtained from a three-dimensional TCT image reconstructed from measurement transmission data. Scattered medium distribution is calculated, the scattered ray distribution on the projection plane is calculated from these, and the scattering angle, scattered energy and scattered ray trajectory are calculated using a three-dimensional TCT image, and the effect of absorption on scattered rays is calculated. Is calculated and subtracted from the scattered radiation distribution, then subtracted from the projection data,
An operation for rearranging the projection data into a sinogram and reconstructing a three-dimensional ECT image is performed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an emission computer tomography apparatus (positron CT apparatus) using a positron emitting nuclide.

[0002]

2. Description of the Related Art In a positron CT apparatus, R
When positrons emitted from I (radio isotope) disappear, two gamma rays radiating in 180 ° opposite directions are simultaneously counted to collect data (emission data), and this data is calculated to calculate the distribution image of RI. Is what you want. That is, since RI exists on a straight line connecting the detection positions of these two gamma rays, radiation is counted for each position information representing the straight line, data is collected, and an image (RI) is obtained using a reconstruction algorithm.
Distribution image).

However, gamma rays are radiated to the outside through the inside of the subject and are scattered in the body. If the scattered radiation is detected at the same time, the RI is counted as being on a straight line connecting the detection positions. Therefore, the scattered radiation data is always included in the measured emission data. This scattered radiation data is estimated to exceed 50% of the total data collected by a 3D (three-dimensional) positron CT apparatus using a cylindrical radiation detector.

Therefore, the scattered radiation data is removed from the emission data collected including the scattered radiation data,
A correction method for extracting only true emission data has been proposed. The following two methods are representative correction methods. One correction method is to superimpose and integrate the scattered radiation response function on the measured data of the two-dimensional projection plane to obtain the scattered radiation distribution, and subtract this on the two-dimensional projection plane (DLBaiely, et. Al.). al., IEEE Trans. Me
d. Imag., vol. 11, pp. 560-569, 1992).

[0005] Another correction method uses the assumed radiation distribution (an image reconstructed from measurement data including scattered radiation data) and the actually measured scattering medium distribution (an image reconstructed from transmission data) to calculate the physical scattering. This method calculates each scattered radiation based on the model and finds its distribution (CC Watson, et al., IEEE Trans. Nucl. Sci., V
ol.44, No.1, pp.90-97, 1997).

[0006]

However, each of the conventional scattered radiation correction methods has a problem. In the former method, the correction can be performed in a relatively short time. However, when the scattering medium is non-uniform, the assumed scattered radiation response function cannot be applied, so that the estimation of the scattered radiation data becomes inaccurate. In the latter method, a relatively accurate correction can be made even with an uneven scattering medium, but the process of each scattered ray (emission, scattering and absorption of gamma rays) is pursued one by one in a three-dimensional space. Calculation time increases.

SUMMARY OF THE INVENTION In view of the above, it is an object of the present invention to provide a positron CT apparatus improved so that correction for removing scattered radiation data can be accurately performed in a short calculation time.

[0008]

In order to achieve the above object, in a positron CT apparatus according to the present invention, a slice plane of a count value obtained by simultaneously counting two gamma rays from a positron emitting nuclide in a subject is used. A distribution relating to position information specified by the inner angle and the position in the slice plane is collected as a sinogram at a different position in the body axis direction using a detection unit and a line source placed outside the subject. Means for reconstructing a three-dimensional scattering medium distribution for the subject from the obtained three-dimensional transmission data, means for extracting count value distributions in various projection directions from the large number of sinograms as projection data, Means for calculating a scattering medium thickness distribution in various projection directions for each projection plane from a two-dimensional scattering medium distribution, and projection for each of the projection planes By chromatography data and scattering medium thickness distribution, means for determining the distribution of the scattered radiation counts at each projection surface,
Means for calculating the effect of absorption by the scattering medium by obtaining the trajectory of the scattered radiation in the scattering medium from the three-dimensional scattering medium distribution, and adding the effect of the absorption to the distribution of the scattered ray count value on each projection plane. A means for subtracting from the projection data,
Means for repeating these processes for each projection direction and rearranging the obtained count values as a number of sinograms, and means for reconstructing the three-dimensional distribution of positron-emitting nuclides in the subject from the obtained sinograms are provided. It is characterized by being provided.

[0009] Emission data at different positions in the body axis direction of the subject are collected as a sinogram by using a cylinder type detector or the like, and a line source is rotated around the subject to obtain a sinogram. Collect dimensional transmission data. A three-dimensional absorption coefficient distribution is reconstructed from three-dimensional transmission data. This absorption coefficient distribution is also the distribution of the scattering medium. By extracting the data in the same angular direction from the emission data collected as a sinogram, count value distributions in various projection directions are created as projection data. Further, the distribution of the thickness of the scattering medium projected in various directions from the three-dimensional scattering medium distribution, that is, the thickness distribution on the projection plane is obtained. Considering the projection data of a certain point on the projection surface, this includes the scattered radiation count value. The scattered radiation count value is calculated based on the distribution of the count value (projection data) and the scattering medium for the same projection surface. It can be seen that it depends on the thickness distribution. Therefore, using these projection data and the scattered radiation medium thickness distribution, the scattered radiation count value can be obtained at one point. From this, the scattered radiation count value distribution on the projection plane is obtained. In each of the scattered radiation count values, the effect of absorption when the scattered radiation flies is neglected. Therefore, the trajectory of the scattered radiation in the scattering medium is obtained from the above three-dimensional scattering medium distribution, and the absorption of the scattering medium is determined. The effect is calculated, and this is added to the scattered radiation count value, and then the scattered radiation count value is subtracted from the projection data. By repeating this process for each projection plane,
This means that the scattered radiation has been corrected for the projection data in all the projection directions. Therefore, if the projection data is rearranged into a sinogram and a three-dimensional image is reconstructed, a three-dimensional distribution of positron-emitting nuclides in the subject, to which scattered radiation has been corrected, can be obtained. Attention is paid to the two-dimensional projection plane in a certain direction, and the distribution of the scattered radiation count value is calculated on the projection plane using the emission data distribution and the scattering medium distribution of the projection plane. The number of such calculation processes becomes small, and the time required for the scattered radiation correction process can be reduced.

[0010]

Next, embodiments of the present invention will be described in detail with reference to the drawings. In the present invention, as shown in FIG. 1, data which is a projection in a specific direction of measurement emission data obtained by collecting a coincidence count of two gamma rays as a sinogram is taken out, and projection data arranged in the body axis direction is created. Since this projection data is data distributed on a plane (projection plane) perpendicular to the projection direction, position information (coordinates of the projection plane) on the projection plane includes a position in the body axis direction and a right angle to the body axis direction. And a position perpendicular to the projection direction, that is, a distance from the center on a plane perpendicular to the body axis.

On the other hand, a three-dimensional image is reconstructed from the measured transmission data collected as a sinogram by rotating a line source around the subject to reconstruct a three-dimensional TCT (transmission CT) image.

The above projection data includes scattered radiation data. By considering how the scattered radiation is incident, the amount of scattered radiation data is represented by the distance from the center. It can be seen that the position depends on the nuclide concentration distribution and the scattering medium distribution distributed in the body axis direction. Therefore, based on these, the body axis direction distribution of the scattered radiation data is obtained for each position.

The reconstructed three-dimensional TCT image is
Since the image is a three-dimensional distribution image of the scattering medium, it is possible to calculate the angle, the scattering energy, and the trajectory of the scattered radiation that generate the scattered radiation data included in the projection data. From this, the distance through which the scattered radiation passes through the scattering medium is determined, and the absorption is calculated. By subtracting this absorption from the distribution of the scattered radiation in the body axis direction, scattered radiation data after the absorption can be obtained. By subtracting the scattered radiation data after the subtraction from the original projection data, ECT data excluding the scattered radiation data can be obtained. By rearranging the scattered radiation corrected projection data, returning to the original sinogram format, and performing a three-dimensional image reconstruction operation, a three-dimensional ECT image based on the true emission data from which the scattered radiation data has been removed can be obtained. Obtainable.

More specifically, as shown in FIG. 2, a subject 19 is inserted into the cylinder type detector 11. The cylinder type detector 11 is a cylinder type in which a number of radiation detector elements are arranged in a ring shape and a ring type detector is laminated in multiple layers. A radiopharmaceutical is administered to the subject 19, and positron is released from radionuclides distributed in the body, and when the positron disappears, 2
Emits gamma rays in 180 ° opposite direction. This gamma ray is incident on each element of the cylinder type detector. When a signal generated from each element when the radiation is incident is input to the coincidence counting device 12 and it is determined that two gamma rays are simultaneously incident, positional information indicating a combination of the incident elements in the data acquisition memory 13. Is counted at the address corresponding to.

The position information indicates a straight line connecting elements to which two gamma rays have been simultaneously incident.
It is represented by an angle θ from a reference angle in a plane perpendicular to the body axis 0 and a distance r from a center point (the position of the body axis 0). In the case of one layer of the ring detector, when a signal is generated simultaneously by two elements in the one layer of the ring detector, a line connecting the two elements that generated the signal at the same time is represented by (θ , R) is counted as “1” (data collected for each address based on such position information is called a sinogram). Since such counting of radiation is performed by each of a large number of ring type detectors stacked in the direction of the body axis 0 (Z direction), a large number of sinograms as shown in FIG. 6 are collected at each position in the Z direction. Will be done. Further, since gamma rays emitted at an angle other than a right angle with respect to the body axis 0 may simultaneously enter two elements across two or more ring detectors, each combination of the ring detectors A sinogram is collected.

Thus, gamma rays emitted from nuclides in the subject 19 are counted, and emission data (distribution of count values of the gamma rays emitted from the subject 19) is collected on many sinograms.

On the other hand, transmission data for the same subject 19 is also collected. Before administration of the radiopharmaceutical, a linear source is rotated around the subject 19,
The two gamma rays transmitted through the subject 19 are detected by the cylinder type detector 11 and their coincidence is performed. In the case of this transmission data, similarly to the case of the above-mentioned emission data, for each ring type detector stacked in the Z direction of the cylinder type detector 11 (that is, for each different position in the Z direction), and for the cylinder type detection, Data is collected as a number of sinograms for each combination that straddles each ring detector of the detector 11. These transmission data are collected in a different area of the data collection memory 13 from the above emission data.

Thus, after the data acquisition memory 13 acquires the emission data and the transmission data having not only the three-dimensional position information in the plane perpendicular to the body axis 0 but also the Z direction, the data is acquired. The processing device 14 performs an operation for correcting the scattered radiation as shown in FIG. From the three-dimensional transmission data, a three-dimensional TCT image, that is, a three-dimensional distribution of absorption coefficients is reconstructed.

First, projection data in a certain direction of the collected emission data is created. For example, projection data is created by projecting onto a projection plane 18 as shown in FIG. 3 in a direction perpendicular to that plane (directly upward in the figure). That is, when viewed from the Z direction (a direction parallel to the body axis 0), the result is as shown in FIG. 4, and the r direction distribution of the data of the gamma rays emitted from the reference angle in the angle direction of φ is obtained (gamma ray The nuclides that emit are indicated by black circles). This distribution is obtained for each position in the Z direction. That is, for example, as shown in FIG. 6, from many sinograms perpendicular to the body axis 0 obtained for each position in the Z direction, 1
By extracting and arranging dimension distribution data,
Obtain the measured emission data distribution on the projection plane 18 in the (r, Z) coordinate system.

It is assumed that the distribution of count values in the Z direction for a certain r in the projection data created in this way is represented by a curve B shown in FIG. 5B. FIG. 5A is a cross-sectional view of the cylinder type detector 11 and the subject 19 taken along a plane (parallel to the Z direction) defined by r and φ described above. Considering the count value at the position 02 in the Z direction in the above curve A, this includes not only the two gamma rays emitted from the nuclide (indicated by a black circle) at the position 02 but also the Z direction. It can be seen that gamma rays emitted from nuclides (black circles) present at the position, for example, position 03 include those due to scattered radiation scattered at the position 04 or the like.

Then, of the data at the position of Z = 02, the scattered radiation contribution of the nuclide located at Z = 03 is the distribution of the scattering medium (subject 19) located behind (to the right in the figure). You can also see that it depends on That is, for example, Z =
In 04, since the scattering medium is relatively large, more scattered rays are taken in as data of Z = 02,
At Z = 05, since the scattering medium is small, less scattered radiation is generated, the scattered radiation captured as data at Z = 02 is also small, and furthermore, a region where the subject 19 does not exist (the right side of Z = 06). In the (region), no scattered radiation occurs.

From the above, regarding the radiation emitted from the nuclide located at Z = 03, the clock values at each position in the Z direction of the scattered radiation by the scattering medium at the Z = 04 position (Z direction distribution of scattered radiation data) ) Is large as shown by the curve C in FIG. 5D, and the clock value at each position in the Z direction of the scattered radiation by the scattering medium at the Z = 05 position is small as shown by the curve d in FIG. It turns out that it becomes. Where Z
= 04, 05 The size of the scattering medium at the position is r and φ
Z = 0
The thicknesses d4 and d5 of the subject 19 at the positions 4 and 05 are obtained.

Further, these scattered radiation distribution curves c and d are
If the nuclide concentration at the Z = 03 position is high,
If the count value c3 at the position = 03 is large, the count value is large, and if the count value c3 is small, the count value is small. As a result, the distributions c and d of the scattered radiation contribution correspond to the count value c3 at the Z = 03 position and Z =
It depends on the thicknesses d4 and d5 of the scattering medium at the positions 04 and 05.

Therefore, the distribution in the Z direction on the left side of the scattered radiation contribution of the nuclide at the Z = 03 position is Z = 03.
The sum of everything from the scattering medium on the right side of
That is, the scattered radiation distribution based on the thickness d at each position on the right side of Z = 03 is obtained by integrating (including all distributions C, D, etc.). The scattered radiation distribution on the right side of Z = 03 is obtained by integrating the scattered radiation distribution based on the thickness d at each position on the left side. Since the distribution of the thickness d is as shown by the curve B in FIG. 5, it is sufficient to integrate such a curve B in the Z direction from Z = 01 to Z = 06. The distribution of the thickness d is obtained by reconstructing the three-dimensional TC
It is determined from the T image.

By such an integration, a scattered radiation distribution is obtained for the nuclide at the position Z = 03.
Is distributed in the Z direction as shown by the curve a. Therefore, if the integration as described above is performed for each position in the Z direction of the count value distribution as shown by the curve a and they are added (for each curve a) By integrating in the Z direction), the entire scattered radiation distribution (all of the Z direction distribution) on the specific r of the projection plane 18 can be obtained. That is, the distribution of the scattered radiation data in the Z direction for the specific r of the projection plane 18 is a function of the thickness d at each position in the Z direction on the r and the count value c at each position in the Z direction. The scattered radiation distribution can be obtained by integrating the thickness d (curve b) and the count value c (curve a) in the Z direction.

Thus, for a specific r of the projection plane 18,
The Z-direction distribution of the scattered radiation data obtained by simultaneously counting the gamma rays emitted from all the nuclides in the Z direction on the r by being scattered by all the scattering media in the Z direction on the r is obtained. Thus, for convenience of explanation, the projection surface 18
For example, the data at the Z = 02 position includes data obtained by scattering a gamma ray generated from a nuclide at another r position by a scattering medium at another r position. Therefore, the above integration needs to be performed also in the r direction. By such two-dimensional integral calculation on the two-dimensional projection plane, scattered radiation data at each coordinate position on the projection plane 18 is obtained. Further, this process is repeated for all the projection planes, and scattered radiation data at each coordinate position of each projection plane is obtained. Therefore, this calculation is a two-dimensional calculation and can be performed at high speed.

However, the scattered radiation data thus obtained is
It is merely an assumption that scattered radiation is not absorbed. In practice, the scattered radiation is absorbed in the flight process, so it is necessary to determine this absorbed component. Therefore, the trajectory of the scattering angle, the scattered energy, and the scattered ray are calculated from the three-dimensional TCT image, and the influence of the absorption by the subject (scattering medium) 19 is calculated for each of these trajectories. If this absorption is subtracted from the previous scattered radiation data, scattered radiation data that takes into account the absorption is also obtained.By subtracting this from the previous projection data, the absorption is also taken into account. True emission data can be obtained excluding the scattered radiation data.

Such a correction operation is performed by the data processor 1
4 (see FIG. 2), only the calculation of the absorption of the scattered radiation is performed in the three-dimensional space, and the distribution of the scattered radiation data ignoring the influence of the absorption is calculated for each coordinate of the two-dimensional projection plane 18. , The calculation time can be greatly reduced.

Since the corrected projection data can be obtained, the data is rearranged and returned to the original sinogram form, and a three-dimensional image reconstruction operation is performed by the image reconstruction device 15 (see FIG. 2). Obtain a two-dimensional ECT image.

In the above description, the projection direction is a direction perpendicular to the body axis 0.
The same applies to directions other than the right angle, such as coincidence counting between adjacent rings of the multilayer ring detector in No. 1.
Regarding coincidence counting data between adjacent rings of the multilayer ring detector, between the first ring and the second ring, between the second ring and the third ring, the third ring and the fourth From the sinogram of each of the..., The data arranged in the r direction on the same θ
Obtain projection data by arranging in the directions. On the projection data,
Since the distribution of the scattered radiation data is considered to be approximately the same as the distribution of the scattered radiation data perpendicular to the body axis 0 obtained above, it is possible to divert this. Further, the same processing can be repeated again for the data on which the scattered radiation correction has been performed as described above. In addition, the above description relates to one embodiment of the present invention, and it goes without saying that the specific configuration and the like can be variously changed.

[0031]

As described above, according to the positron CT apparatus of the present invention, instead of tracking scattered radiation three-dimensionally one by one, attention is paid to a two-dimensional projection plane in a certain direction, and Using emission data distribution and scattering medium distribution (integral value of emission point and scattering point in the above direction),
Since the distribution of scattered radiation data is calculated on the projection plane, there are few complicated and time-consuming calculations in three-dimensional space, and relatively accurate scattered radiation correction is performed with a short calculation and processing time. Thus, an ECT image with excellent image quality can be obtained.

[Brief description of the drawings]

FIG. 1 is a flowchart showing the operation of an embodiment of the present invention.

FIG. 2 is a block diagram showing the configuration of the embodiment.

FIG. 3 is a perspective view showing a positional relationship between projection surfaces.

FIG. 4 is a front view viewed from the body axis direction to show the positional relationship of the projection plane.

FIG. 5 is a cross-sectional view taken along a plane parallel to the projection direction and the body axis direction and a positional relationship therewith for explaining the Z-direction distribution of the count value, the subject thickness, and the scattered radiation data on the projection surface. FIG.

FIG. 6 is a diagram showing a sinogram at each position in the Z direction.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 11 Cylinder type detector 12 Simultaneous counting device 13 Data acquisition memory 14 Data processing device 15 Image reconstruction device 18 Projection surface 19 Subject

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Yuichiro Narita 5-25-1 Oyumino, Midori-ku, Chiba City, Chiba Prefecture A-207 (72) Inventor Naoji Nakamura 2-19-5-104, Nakamachi, Musashino City, Tokyo (72) ) Inventor Keiji Kitamura 1-term, Kuwabaracho, Nishinokyo, Nakagyo-ku, Kyoto, Kyoto Prefecture F-term in Shimadzu Corporation (reference) 2G088 EE02 FF04 FF07 JJ07 KK07 LL06 LL09

Claims (1)

[Claims] 1. A distribution relating to position information specified by an angle in a slice plane and a position in a slice plane of a count value obtained by simultaneously counting two gamma rays from a positron-emitting nuclide in a subject as a sinogram. Detecting means for collecting at different positions in the body axis direction, and reconstructing a three-dimensional scattering medium distribution for the subject from three-dimensional transmission data collected using a radiation source placed outside the subject Means for extracting count value distributions in various projection directions from the plurality of sinograms as projection data, and means for calculating the scattering medium thickness distributions in various projection directions from the three-dimensional scattering medium distribution in each projection plane. Means for calculating the distribution of the scattered radiation count value on each projection plane, based on the projection data and the scattering medium thickness distribution for each of the projection planes, Means for calculating the trajectory of the scattered radiation in the scattering medium from the three-dimensional scattering medium distribution and calculating the effect of absorption by the scattering medium, and the distribution of the scattered radiation count value on each projection plane taking into account the effect of the absorption. Above, means for subtracting from the above-mentioned projection data, means for repeating these processes for each projection direction, means for rearranging the obtained count values as a number of sinograms, and positron emission in the subject from the obtained sinograms Means for reconstructing a three-dimensional distribution of sexual nuclides.