JP2796112B2 - Fan beam CT system - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a polygonal radiation obtained by approximating an element arrangement of an arc-shaped or ring-shaped radiation detector by a combination arrangement of a plurality of linear radiation detection element blocks. Fan beam CT using the detector as an arc or ring radiation detector
It concerns the device.

[Conventional technology]

The development of high-resolution CT devices to detect minute lesions is underway, but the resolution is limited to radiation (usually X-rays).
Line) greatly depends on the element spacing of the detector. If the element spacing is narrowed, in a conventional gas detector, for example, an ionization box detector, the number of detectable X-ray photons decreases, and the detection efficiency decreases.
Photon noise is greatly increased. For this reason, a solid state detector using a scintillator and a photodiode with high detection efficiency has been used at present.

There are various manufacturing procedures for this type of solid state detector,
For example, as shown in FIG. 2, grooves 20 are formed at equal intervals on a flat plate 20 in which a scintillator 201 and a photodiode (SPD) 202 are joined.
In many cases, 3 is cut and a separator 204 for dividing the radiation detection element into each radiation detection element is inserted into the groove 203 in many cases.

When configuring a multi-element detector for a fan beam CT apparatus using a fan-shaped radiation beam as a radiation beam from the linear radiation detection element block made through such a procedure, the linear radiation detection is performed. A plurality of element blocks are arranged corresponding to a plurality of sides of the polygonal shape to approximate the element arrangement of an arc-shaped or ring-shaped radiation detector.

Incidentally, it is preferable that the dimensions of the radiation detecting elements constituting the linear radiation detecting element block are made equal to each other in terms of manufacturing cost. Therefore, not only the width of the radiation detecting elements but also the length in the arrangement direction are made equal as described above. It is customary to do so.

[Problems to be solved by the invention]

However, such a conventional technique has the following problems. That is, as described above, the polygonal radiation detector obtained by approximating the element arrangement of the arc-shaped or ring-shaped radiation detector with the combination arrangement of a plurality of linear radiation-detection element blocks is referred to as the arc-shaped or ring-shaped radiation detection. In the case of using as a detector, in the fan-beam CT apparatus in which the lengths in the arrangement direction of the radiation detection elements constituting the linear radiation detection element block are each made equal, the arc-shaped or ring-shaped radiation detector In comparison with the case of the radiation detection element of No. 5, a geometric displacement occurred in the radiation detection element of the polygonal radiation detector.

This geometric displacement will be described in detail below. The number of radiation detection elements (hereinafter simply referred to as elements) constituting one linear radiation detection element block (hereinafter simply referred to as element block) is N, and the number of element blocks used in combination is M.
Then, the total number of elements is N × M.

FIG. 3 is a diagram showing a schematic configuration of a polygonal radiation detector comprising eight element blocks, in which 30 is a polygonal radiation detector (hereinafter abbreviated as a polygonal detector), and 301 is its element block. , 302 is a radiation source, α is a fan beam angle, θo is an opening angle of one element block, αp is an opening angle per element, xo is a length of one element block in the X-axis direction, and O is a rotation center of the detector. .

FIG. 4 is a diagram schematically showing an element block (one element block) composed of six elements together with a corresponding portion of an arc-shaped or ring-shaped, here arc-shaped, radiation detector, and FIG. The same reference numerals indicate the same or corresponding parts. 401,402
Represents a radiation beam directed to the center in the element length direction.

In FIG. 4, a radiation source 302 and an element block 301 are shown.
Is defined as a Y-axis, and an axis orthogonal to the Y-axis is defined as an X-axis. Here, the following conditions are set as conditions for approximating an arc-shaped radiation detector (hereinafter abbreviated as an arc-shaped detector) 40 by the polygonal detector 30.

(A) The element block center BO points to the radiation source 302, (B) The element block 301 is arranged in parallel to the X axis, and (C) The total number of elements of the approximate arc detector 40 and polygonal detector 30 (D) The fan beam angles α of the arc detector 40 and the polygon detector 30 are equal.

In the above description, assuming that the opening angle per element of the arc detector is αp, the opening angle θo of the one element block 301 can be obtained by the following equation (1) from the condition (A).

θo = αp · N = α / M (1) Further, the length Δx in the X-axis direction of one polygonal detector is 1
From the length xo in the X-axis direction of the element block 301, the following equation (2),
Required in (3).

xo = 2d · tan (θo / 2) (2) Δx = xo / N (3) where d is the distance from the radiation source 302 to the element block 301.

X-axis distance x to each element i of element block 301
(I) and opening angle θ with respect to Y axis viewed from radiation source 302
(I) is: x (i) = Δx · (i− (N + 1) / 2) (4) θ (i) = arctan (x (i) / d) (5) (i = 1 2,... N).

On the other hand, in the arc detector 40, the opening angle θf (i) with respect to the Y axis as viewed from the radiation source 302 is as follows: θf (i) = αp · (i− (N + 1) / 2) (6) The difference γ between θ (i) and θf (i) results in a geometric displacement as shown in FIG. This misalignment is caused by one element block 301 in order to reduce the total number of element blocks.
When θo increases by increasing the number N of elements per unit, the value further increases.

Here, the opening angle αs (i) of each element i of the approximate detector (polygonal detector 30) corresponding to the opening angle αp per element of the arc detector 40 is x ′ (i) = Δx · ( i−N / 2−1) (7) θ ′ (i) = arctan (x ′ (i) / d) (8) αs (i) = θ ′ (i + 1) −θ ′ (i) ... (9)

This αs (i) is plotted as shown in FIG. That is, in the elements which are arranged linearly and have the same length in the arrangement direction, the angle of view of the X-ray source 302 becomes smaller by η as the distance from the element block center BO increases. This means that the element i far from the element block center BO
As the number of detected X-ray photons decreases, the signal output decreases according to the ratio between αp and αs (i).

As described above, the polygonal detector 30 obtained by approximating the element array of the arc (or ring) detector 40 with the plurality of combination arrays of the element blocks 301 was used as the arc or ring radiation detector. In the fan beam CT apparatus in which the X-axis lengths (lengths in the array direction) Δx of the elements i constituting the element block 301 are made equal to each other, the above θ (i) and θf (i ), The output of the measurement projection data fluctuates according to the above αs (i),
There is a problem that distortion occurs in the reconstructed image.

An object of the present invention is a polygonal detector obtained by approximating an arc-shaped or ring-shaped detector element array by a combination arrangement of a plurality of linear element blocks, and an array of elements constituting the element block. An object of the present invention is to provide a fan-beam CT apparatus that can obtain a good reconstructed image without distortion even when the lengths in the directions are equal.

[Means for solving the problem]

The above object is to use a polygonal radiation detector obtained by approximating an element array of an arc-shaped or ring-shaped detector with a plurality of arrangement arrays of linear radiation detection element blocks as an arc-shaped or ring-shaped radiation detector. In the fan beam CT device, the geometric positions of the radiation detection elements of the polygonal radiation detector are generated by making the lengths of the radiation detection elements constituting the linear radiation detection element block equal in the arrangement direction. This is achieved by providing a position shift correcting means for correcting the shift.

[Action]

The positional deviation correcting means is configured to make the lengths of the radiation detecting elements constituting the linear radiation detecting element block equal in the arrangement direction, and the geometrical positional deviation of the radiation detecting elements of the polygonal radiation detector is caused. Is corrected.
As a result, a good reconstructed image without distortion can be obtained as if it were detected by an arc-shaped or ring-shaped radiation detector.

〔Example〕

Hereinafter, embodiments of the present invention will be described with reference to the drawings,
Prior to the description of the specific configuration example, first, the outline of the present invention will be described.

The present invention is provided with the above-described misregistration correction means. Here, the misregistration correction means converts the projection data obtained by the polygonal detector, which are arranged nonuniformly in the direction of the opening angle θ from the Y axis. The distortion of the image is corrected by converting the projection data into projection data uniformly arranged at intervals αp in the θ direction, that is, projection data obtained by an arc detector.

Now, assume that the measured projection data is P (j). If the number in the element block of the element of element number j is i, the element j
Is determined by the following equation.

i = mod (j−1, N) +1 (10) (→ mod is a built-in function of Fortran) γ (i) = θf (i) −θ (i) (11) From γ (i) If the projection data P ′ (j) (corrected measurement data) at θf (i) is obtained by interpolation,
By primary interpolation, for j of i ≦ N / 2, δ (i) = γ (i) / (θ (i + 1) −θ (i)) (12) P ′ (j) = (1−δ) ) · P (j + 1) −δ · P
(J) (13) For j>i> N / 2, δ (i) = γ (i) / (θ (i) −θ (i−1)) (14) P ′ (j) = (1−δ) · P (j) −δ · P (j +
1)… (15) is required.

The corrected measurement data P ′ (j) is, for example, a known filtered back projection method (filtered back projection method).
d: Hereinafter referred to as the FBP method. Regarding the FBP method, for example,
Published by Asakura Shoten Co., Ltd., supervised by Hirokazu Kimura, “Recent Medical Diagnostic Imaging System”, pp. 108-110), and can be easily reconstructed by the same processing means as conventional arc detectors. No good images can be reconstructed.

FIG. 1 is a block diagram showing a specific configuration example of a fan beam CT apparatus according to the present invention. In the figure, reference numeral 1 denotes a radiation source (usually an X-ray source) for generating a fan-shaped radiation beam (this is called a fan beam). Reference numeral 2 denotes a high voltage generator for supplying a high voltage power to the radiation source 1. Numeral 3 denotes a multi-element detector oriented with the radiation source 1 with an object to be inspected (not shown) in the imaging region 4 interposed therebetween. A polygonal detector which is made equal and approximates the element arrangement of the arc detector by a plurality of arrangement arrangements of the element blocks, and is used as an arc detector. Reference numeral 5 denotes a rotating plate on which the radiation source 1 and the detector 3 are mounted, and 6 denotes a rotating plate or a bed (not shown) for rotating the rotating plate 5 to measure projection data from a plurality of angular directions.
A bed and a bed driving device, a measuring circuit for measuring radiation detected by the detector 3 as digital data at a desired measuring interval, and a measuring process for buffering measured data from the measuring circuit 7. The apparatus 9 is a preprocessing apparatus for performing preprocessing such as detector sensitivity correction, radiation source fluctuation correction, beam hardening correction, and logarithmic conversion on the digital data from the measurement processing apparatus 8. Reference numeral 10 denotes a position shift correction processing device 10a for performing the above-described position shift correction processing,
An image reconstruction processing device comprising a filter processing device 10b for performing a filtering process according to the method and a backprojection processing device 10c for performing a backprojection process on the projection data subjected to the filtering process, 11 is reconstructed by the image reconstruction processing device 10. A display device that displays the tomographic image of the inspected object as a visual image.

 Next, the operation of the device of the present invention will be described.

Rotating plate 5 equipped with radiation source 1 and polygonal detector 3
Is rotated by the scanner and bed driving device 6. At this time, radiation according to the voltage and current from the high voltage generator 2 is emitted from the radiation source 1 toward the subject. Radiation that has not been absorbed in the cross section of the test object is converted into an electric quantity by the polygonal detector 3 and converted into digital data by the measurement circuit 7. The digital data measured at many angles is sequentially sent to the pre-processing device 9 while being buffered by the measurement processing device 8. The preprocessing device 9 sends the projection data obtained by performing the preprocessing to the position shift correction processing device 10a. The positional deviation correction processing device 10a corrects the geometrical positional deviation of the polygonal detector 3 composed of linear element blocks with respect to the transmitted projection data. The corrected projection data is sent to the back projection processing device 10c after correcting the blur caused by the back projection process in the filter processing device 10b in advance, and is reconstructed as a tomographic image of the inspection object. The display device 11 displays the reconstructed tomographic image.

Here, the operation of the displacement correction processing device 10a will be described in detail. The misregistration correction processing device 10a has a δ table of N words corresponding to the number N of elements constituting the element block of the polygonal shape detector 3. This δ table is obtained from the above equations (10), (11), (12) and (14). When the projection data is input, the misregistration correction processing device 10a calculates the value δ (i of the δ table of the address i (the number in the element block of the element of the element number j) i obtained from the element number j by the above equation (10). ). Then, the processing for correcting the geometric displacement of the polygonal shape detector 3 is performed for all the elements according to the above equation (13) or (15), and projection data uniformly arranged at an interval αp without distortion is output. I do.

In the above-described embodiment, the misregistration correction processing device is provided in the image reconstruction processing device. However, the misregistration correction processing device may be provided after the measurement processing device or the preprocessing device. Further, even in the case of being provided in the image reconstruction processing device, it may be provided not in the first stage of the image reconstruction processing device but in the subsequent stage of the filter processing device.

In the above-described embodiment, the present invention is applied to the RR (Rotate-Rota).
te) type fan beam CT apparatus has been described as an example. However, for example, an SR (Stationary-type) using a ring-shaped detector having a radiation source rotation center as the center of the array is used.
The present invention may be applied to a fan beam CT apparatus of the (Rotate) type. In this case, it is needless to say that the displacement can be corrected in the same manner as in the above-described embodiment.

〔The invention's effect〕

According to the present invention, an easy-to-create, polygonal detector that approximates the element arrangement of an arc-shaped or ring-shaped detector with a combination arrangement of linear element blocks having the same element dimensions,
Even when used as an arc-shaped or ring-shaped detector, there is an effect that a good reconstructed image without distortion can be obtained.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a specific configuration example of the device of the present invention,
FIG. 2 is a cross-sectional view of a solid state detector, FIG. 3 is a view showing a schematic configuration of a polygonal detector, FIG. 4 is a view schematically showing a linear element block together with a corresponding part of an arc detector. FIG. 5 and FIG. 6 are diagrams for explaining the cause of distortion in the reconstructed image in the conventional apparatus. 1,302 radiation source 2, high voltage generator 3,301
Polygonal radiation detector, 4 ... imaging area, 5 ... rotating plate, 6 ... scanner, bed driving device, 7 ... measurement circuit,
8 Measurement processing device, 9 Preprocessing device, 10 Image reconstruction processing device, 10a Position displacement processing device, 10b Filter processing device, 10c Back projection processing device, 11 Display Apparatus, 301 ... Polygonal radiation detector element block, 40
…… A circular radiation detector.

Claims (1)

(57) [Claims] A polygonal radiation detector obtained by approximating an element arrangement of an arc-shaped or ring-shaped radiation detector with a plurality of combination arrangements of linear radiation-detection element blocks is used as an arc-shaped or ring-shaped radiation detector. In the fan beam CT apparatus used, the geometrical shape of the radiation detecting element of the polygonal radiation detector is generated by making the lengths of the radiation detecting elements constituting the linear radiation detecting element block equal in the arrangement direction. A fan beam CT apparatus comprising: a position shift correcting unit that corrects a position shift.