JP3352215B2 - SPECT apparatus and SPECT image reconstruction method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an SPE for detecting a gamma ray emitted from a radioactive substance (hereinafter, referred to as "RI") injected and distributed into a subject and obtaining a three-dimensional distribution image thereof.
The present invention relates to a method for reconstructing a CT image.

[0002]

2. Description of the Related Art Conventionally, as an apparatus for reconstructing an image by detecting radiation, there are an X-ray CT apparatus and a SPECT apparatus. The X-ray CT apparatus is provided with an X-ray source and a detection element group in a geometrical positional relationship where the X-ray source is the focal position of the X-ray fan beam, and the X-ray source is set with the body axis of the subject as the center of rotation. And X-rays that have transmitted through the subject linearly from the X-ray source while rotating the detection element group are detected by the detection element group to collect projection data, and the X-ray absorption in the object is performed based on the collected projection data. The distribution is image-reconstructed.

On the other hand, a SPECT apparatus detects a γ-ray emitted from RI distributed in a subject, and converts a γ-ray selectively passed through the fan-beam collimator into a virtual focus into light. A gamma camera comprising a scintillator detector for detecting the distribution of RI itself in a subject based on projection data collected using the gamma camera while ignoring absorption of γ-rays. .

[0004] In any of the apparatuses, when reconstructing an image, a filtered backprojection method (filtered back project) is used.
ion) is often applied. An example to which the filter-corrected backprojection method is applied is shown in "Document 1".

Reference 1: CONVOLUTION RECONSTRUCTION TEC
HNIQUES FOR DIVERGENT BEAMS.comput.Biol, Med, 1976, V
OL.6, PP259-271 In this document 1, X-ray fan beam is bombarded from the X-ray source rotating around the subject toward the subject, and the X-ray transmitted through the subject is converted into a large number of detection elements by a straight line. After the detected projection data is corrected by a filtering process, the detected projection data is corrected by filtering, and then back-projected along the X-ray beam into the entire effective field of view to reconstruct an X-ray CT image. It is shown. In this case, since the X-ray source is in focus,
The information amount from any direction of 360 ° at a certain point on the image can be handled in a relatively equivalent relation.

However, when an image is reconstructed by applying the filtered back projection method to a SPECT apparatus using a fan beam collimator, the spatial resolution near the collimator is good, but the spatial resolution decreases as the distance from the collimator increases. Therefore, the RI distribution far from the collimator is not sufficiently reflected in the information of the projection data. In other words, the resolving power of the reconstructed image is significantly deteriorated.

[0007]

As described above, when an image is reconstructed by applying the filtered back projection method to a SPECT apparatus using a fan beam collimator, the spatial resolution near the collimator is good. Since the spatial resolution decreases as the distance from the collimator increases, the RI distribution at a distance from the collimator is not sufficiently reflected in the information of the projection data. As a result, the resolving power of the reconstructed image is significantly deteriorated.

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and an object of the present invention is to provide a SPECT image reconstruction apparatus and method capable of improving the resolution of a SPECT image.

[0009]

According to one aspect of the present invention, there is provided a SPECT apparatus for reconstructing a distribution image of a radioisotope administered to a subject along a tomographic plane of the subject. In, detecting means for detecting γ-rays emitted from the radioisotope in the form of a fan beam and outputting projection data, between the subject and the detecting means to obtain projection data from different directions Scanning means for giving a relative rotational movement to the projection data, means for convolving the projection data with a first convolution function, and back-projecting only the area of the reconstruction area close to the detection means, Reconstructing means comprising means for convolving with a second convolution function and back-projecting the entire reconstruction area, and radiation reconstructed by the reconstructing means. Characterized by comprising a display means for displaying the distribution image of isotopes.

According to a fourth aspect of the present invention, there is provided a method for reconstructing a SPECT image which obtains a SPECT image along a tomographic plane of a subject from a distribution image of the radioisotope administered to the subject. Obtaining projection data over 360 ° around the subject by detecting means for detecting γ-rays emitted from the object in a fan beam shape;
Convolving the projection data with a first convolution function, back-projecting only a region of the reconstructed region that is close to the detection means, convolving the projection data with a second convolution function,
Back-projecting the entire reconstruction area;
And obtaining a SPECT image based on the data back-projected with the second convolution function.

[0011]

According to the SPECT apparatus and the SPECT image reconstruction method of the present invention, the projection data is convolved with the first convolution function and the second convolution function, respectively, and the convolution is performed with the first convolution function. Only the area close to the collimator in the reconstructed area is back-projected with respect to the set portion. The portion convolved by the second convolution function is back-projected over the entire reconstruction area. Then, a SPECT image is obtained based on these backprojection results.

[0012]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a diagram schematically showing a SPECT image reconstruction SPECT apparatus 100 according to the present invention.

The SPECT apparatus 100 includes a gantry 1, a data acquisition unit 2, an image reconstruction unit 3, and a display unit 4, and performs an image reconstruction process performed by the image reconstruction unit 3 by a method described later.

The gantry 1 rotates a gamma camera 6 equipped with a fan beam collimator 5 by 360 ° around the subject P to detect γ-rays emitted from the RI distributed on the subject P from a 360 ° direction. Can be done.
Projection data from the gamma camera 6 of the gantry 1 is collected by the data collection unit 2.

At this time, two or more gamma cameras may be used.

Projection data obtained from the gamma camera 6 rotatably mounted on the gantry 1 is collected by a data collection unit. The projection data obtained by the gamma camera 6 is first filtered in a data collection unit. This filtering process is separated into two separate processes. These two separated filtering processes will be described later. Next, the two data that have been separately filtered, that is, convolved by two separate convolution functions (also referred to as reconstruction filter functions), are respectively back-projected by the reconstruction unit 3 and their inverses. A reconstructed image is obtained by adding the projection data to each other.

[0018] Figure 2 is a geometric effective visual field (EF) and the focal point V _F of the virtual fan beam collimator 5 and fan beam when reconstructing a SPECT image by the projection data in the case of using the fan beam collimator The positional relationship is shown. In FIG. 2, β is a gamma camera 6
, P (β, s) is a projection image in the l−1 ′ direction. S
Indicates the distance between the rotation center and the intersection point where a line passing through the rotation center and parallel to the scintillator effective light-emitting surface intersects l ′ ′.
R represents a distance between the rotation center of the focal point V _F and gamma camera 6 of the collimator 5.

Under the relationship shown in FIG. 2, the SPECT value f (x, y) as a result of image reconstruction is given by the following (7).
It is represented by an expression.

[0020]

(Equation 1)

[Outside 1] A central vertical line which, projected images P (beta from the virtual focal point V _F,
S) shows the angle between the line and the line. Further, L is shown a virtual focal point _{V F, f (x, y} ) the distance between the intersection of the vertical drawn from the center line.

[0021] At a 3, the projection rays toward a circular field of view EF is drawn from the focal point V _F of the virtual, gamma rays generated within the effective field of view are detected by a gamma camera. Since the fan beam collimator has the characteristic that the resolution decreases as the distance from the collimator surface increases and the resolution increases as the distance from the collimator surface increases, the higher the data detected from a portion closer to the gamma camera, the higher the quality of data can be obtained. To obtain high-resolution reconstructed image of taking advantage of this feature, the reverse only a limited region of the portion close to the circle of the area outside of the collimator through the virtual focal point V _F and the detector rotation center O of the collimator as shown in Figure 3 Projection is performed, and back projection is not performed on a portion far from the collimator where the resolution is deteriorated. As a result, since reconstruction is performed using only data having a high resolution, a high-resolution image can be obtained. Even if limited back projection is performed, 18
By utilizing the 0-degree symmetry, backprojection can be performed without inconsistency mathematically. That is, only the data within the effective field of view EF is back-projected outside the circle having a radius of (ga) / 2 (the hatched portion in the figure). The in hatched portion in FIG. 3, the apex and the focal point V _F of the virtual fan beam collimator, a bottom connecting the BASE1 and BASE2 At a the isosceles triangle to the detection surface of the collimator, the rotation of the fan beam collimator A portion where a second circle having a diameter between the focal point of the fan beam collimator and the rotation center overlaps with the region of the first circle in a region of the first circle inscribed in the isosceles triangle about the center. Is the area of the first circle excluding. Where g is the vertical distance to the effective light emitting surface from the focal point V _F of virtual, a is a vertical distance to the effective light emitting surface from the center of the EF. Line m-m 'indicates the perpendicular line grated from the focal point V _F to the collimator detection surface.

Next, the mathematical meaning of the back projection area as the hatched portion in FIG. 3 will be briefly described.

Considering the case where projection data is collected by a parallel beam collimator, if an area close to the camera is back-projected with a straight line passing through the center of rotation of the gamma camera and parallel to the detection surface of the gamma camera as a boundary, it takes one round. After the backprojection, the backprojection of 180 ° has been performed at any point in the effective visual field. That is, since no extra overlap occurs in the backprojection area, no artifact occurs in the obtained SPECT image. The parallel beam is considered to have brought the virtual focus of the fan beam to the point at infinity.
In this case, the area of the hatched portion may be back-projected.

By using the above-described method, a SPECT image with very high resolution can be obtained. However, in the above-described method, when the obtained projection data has 180 ° symmetry, an accurate SPECT image can be obtained. Is obtained. But,
As described later, the projection data obtained by using the fan beam collimator does not have 180 ° symmetry. Therefore,
As described later, the convolution function is separated, a backprojection process is performed on each part, and separately obtained backprojection data is finally added to obtain a final backprojection result.

First, the properties of projection data using a parallel beam collimator and a fan beam collimator will be described below.

As shown in FIG. 7, when the projection data is obtained using the parallel beam collimator, the projection data f (x,
θ) and the projection data f (−x,
θ + π) are equal. After the convolution with the reconstruction filter h (x), the following equations (2) and (3) are obtained.

[0027]

## EQU2 ## q (x, θ) = f (x, θ) * h (x) (2)

Q (−x, θ + π) = f (−x, θ + π) * h (x) (3) Accordingly, q (x, θ) = q (−x, θ + π), and after convolution Are also kept at 180 ° symmetry.

On the other hand, when projection data is obtained by using a fan beam collimator as shown in FIGS. 8 and 9, f (x, θ) ≠ f (−x, θ + π), and has 180 ° symmetry. Absent. Then, when convolution is performed by the reconstruction filter h (x), the following equations (4) and (5) are obtained.

[0029]

## EQU4 ## q (x, θ) = f (x, θ) * h (x) (4)

Q (−x, θ + π) = f (−x, θ + π) * h (x) (5) Accordingly, q (x, θ) ≠ q (−x, θ + π), and after convolution Data has no symmetry.

That is, when a fan beam collimator is used, and when one path of projection data is viewed,

There is always another path where F (x _a , θ ₁ ) = F (x _b , θ ₂ ) (6) (see FIG. 9). However, unlike when using a parallel beam collimator,
Equation (6) above does not hold for all x. After convolution with the reconstruction function,

Q (x, θ ₁ ) = F (x, θ ₁ ) * h (x) Q (x, θ ₂ ) = F (x, θ ₂ ) * h (x) For all x, F (x, θ ₂ ) * h (x) , Θ ₁ ) = F (x, θ ₂ ) does not hold, so Q (x, θ ₁ ) is not equal to Q (x, θ ₂ ). A portion where h (x) can be expressed by a constant multiple of the delta function (see FIG. 4 (b), hereinafter referred to as a first convolution function) and a portion which is not (see FIG. 4 (c), hereinafter a second combo function) And a convolution with the projection data, respectively, the portion convolved with the first convolution function can have 180 ° symmetry as described below. Here, the definition equation of the convolution is shown in the following equation (7).

(Equation 7) The convolution with the delta function δ (x) is given by the following equation (8).

[0032]

(Equation 8) However, δ (x) = 1 (x = 0) = 0 (x ≠ 0) That is, from equation (8), it can be understood that even if a function is convolved with a delta function, the function returns to the original function. Therefore, if the shape of the reconstruction filter approaches the delta function,
Convolution can be performed without changing the shape of the projection data.

Now, for example, the characteristic h of the reconstruction filter
When (x) has a shape as shown in FIG. 4 (a), the vicinity of the origin can be approximated to a value obtained by multiplying the delta function by a constant, so that the first region as shown in FIGS. 4 (b) and 4 (c). Separation into a convolution function h ₁ (x) and a second convolution function h ₂ (x). That is, h (x) = h
₁ to become _{(x) + h 2 (x} ). Since the convolution satisfies the law of distribution, the reconstruction filter function h
Even if (x) is calculated separately for h ₁ (x) and h ₂ (x), the result is the same. That is, the following equation (9) is obtained.

[0034]

P (x, θ) * h (x) = P (x, θ) * {h ₁ (x) + h ₂ (x)} = P (x, θ) * h ₁ (x) + P ( x, θ) * h ₂ (x) = A · P (x, θ) + P (x, θ) * h ₂ (x), where A is a constant... (9) Since the first term has 180 ° symmetry, it is possible to backproject only the area near the collimator described above, and the second term on the right side has no 180 ° symmetry, so if only the area near the collimator is backprojected, Since an accurate SPECT image cannot be obtained, back projection may be performed over the entire effective field of view.

Finally, the final back-projected result is obtained by mathematically adding back-projected results based on the convolution functions of the first and second terms of equation (9). And a SPECT image is reconstructed based on the backprojection result.

As described above, according to the present invention, the reconstruction filter function is separated into the first convolution function and the second convolution function, and the projection data is convolved with each other. Since the result of the convolution with the convolution function is back-projected only in the area close to the collimator, and the result of the convolution with the second convolution function is back-projected over the entire effective visual field, the resolution is high and accurate. A SPECT image is obtained.

In the embodiment described above, the reconstructed filter function divided into two parts is shown in FIG.
To 5 (c). FIG.
The filter shown in (a) is a RAMP filter, which has good resolution, but is susceptible to noise and makes it difficult to stabilize an image. FIG.
The filter shown in (b) is a Shepp & Logan filter.
The high frequency component is dropped from the AMP filter, and the image is stable against noise. The filter shown in FIG.
An esler filter that removes the high-frequency components from the Shepp & Logan filter, and is used for extremely noisy data.

Further, since the data actually collected by the gamma camera includes some blur, if the method of dividing the reconstruction filter is changed in consideration of the blur, the image becomes more stable. FIG. 6 shows examples of filter values of the reconstruction filter in consideration of the blur of the gamma camera.
₁ (x)) and a sub-filter as shown in FIG. 3 (b) (corresponding to h ₂ (x)).

The present invention is not limited to the embodiment described above, and various modifications can be made without changing the gist of the present invention.

[0040]

As described above, in the present invention, an accurate SPECT image having a high resolution can be obtained.

[Brief description of the drawings]

FIG. 1 is a diagram showing an overall configuration of a SPECT apparatus according to an embodiment of the present invention.

FIG. 2 SPECT using a fan beam collimator
The figure which shows the reconstruction method of an image.

FIG. 3 is a view for explaining how to determine a region when reconstructing a SPECT image using a delta function.

FIG. 4 is an explanatory diagram showing an example in which a reconstruction filter is separated into a delta function part and other function parts.

FIG. 5 is a diagram showing an example of three types of reconstruction filters.

FIG. 6 is an explanatory diagram showing an example of an actual filter value.

FIG. 7 is a diagram illustrating an example in which projection is performed using a parallel beam.

FIG. 8 is a diagram showing an example in which projection is performed by a fan beam.

FIG. 9 is a diagram showing a specific example of performing projection by a fan beam.

──────────────────────────────────────────────────続 き Continuation of front page (56) References JP-A-4-370784 (JP, A) JP-A-3-225292 (JP, A) JP-A-4-120492 (JP, A) JP-A-63- 172983 (JP, A) JP-A-57-69268 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) G01T 1/161 G06T 1/00

Claims (6)

(57) [Claims] 1. A SPECT apparatus for reconstructing a distribution image of a radioisotope administered to a subject along a tomographic plane of the subject, wherein a γ-ray emitted from the radioisotope is detected in a fan beam shape. Detecting means for outputting projection data; scanning means for providing a relative rotational movement between the subject and the detecting means in order to obtain projection data from different directions; Means for performing convolution with a convolution function and back-projecting only the area close to the detection means in the reconstructed area, and convolving the projection data with a second convolution function and back-projecting the entire reconstructed area And a display means for displaying a distribution image of the radioisotope reconstructed by the reconstructing means. SPECT device. 2. The SPECT apparatus according to claim 1, wherein the first convolution function is an approximate expression of a delta function. 3. The area where the projection data convolved by the first convolution function is back-projected is defined by an isosceles triangle whose vertex is the focal point of the detecting means and whose base is the detecting means surface. A second circle having a diameter between the focal point of the detection means and the rotation center within the first circle area inscribed in the isosceles triangle about the rotation center of the detection means, The SPECT apparatus according to claim 1, wherein the SPECT apparatus is a first circle area excluding a portion overlapping the circle area. 4. Obtaining a SPECT image from a distribution image of a radioisotope administered to a subject along a tomographic plane of the subject.
In a method of reconstructing a PECT image, a detection means for detecting a γ-ray emitted from the radioisotope in a fan beam form, by using a detecting means for detecting 360 degrees around the subject.
Obtaining projection data over an angle, convolving the projection data with a first convolution function, and back-projecting only a region of the reconstructed region close to the detection means; Convoluting with a convolution function and back-projecting the whole reconstruction area; and obtaining a SPECT image based on the data back-projected with the first and second convolution functions. A characteristic SPECT image reconstruction method. 5. The SPECT image reconstruction method according to claim 4, wherein the first convolution function is an approximate expression of a delta function. 6. The area where the projection data convolved by the first convolution function is back-projected is an isosceles triangle whose vertex is the focal point of the detecting means and whose base is the detecting means surface. A second circle having a diameter between the focal point of the detection means and the rotation center within the first circle area inscribed in the isosceles triangle about the rotation center of the detection means, claim, characterized in that except for the region with the overlapping portions of the circle is a region of the first circle 4 or
Or a method for reconstructing a SPECT image according to item 5 .