JP2002065660A - Method to form tomographic image - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method to form tomographic images that enables a change of sliced thickness of the tomographic images without reducing processing speed and also to solve archfacts generated at the edges of individual projection data. SOLUTION: This invention relates to a method to form tomographic images of a substance from the projection data gathered by a helical scanning using a helically scanning CT scanner that is equipped with an X-ray detector 3 composed of a single row. When a tomographic image is produced from the projection data array, a summation factor the size and shape of which can be optionally changed is produced based on a predetermined facing data summation factor index and a summation factor index. In addition, by adapting the summation factor to such a phase that copes with the range of a sliced position to be desired, a summation project data array can be produced. At the same time, by reusing the summation projection data array, a tomographic image can be formed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a computed tomographic image, and more particularly to a method for producing a tomographic image from projection data obtained by spiral scanning CT having an X-ray detector having a single row. About.

[0002]

2. Description of the Related Art When a tomographic image is created from projection data acquired by spiral scanning by spiral scanning CT having a single conventional detector, a weighting function determined for each correction method is applied to the projection data. Then, weighted projection data is obtained, and a tomographic image is created by reconstructing the obtained weighted projection data.

When displaying a three-dimensional image, a plurality of obtained tomographic images are stacked to reconstruct a three-dimensional image, and the image is projected onto a projection surface by a projection method capable of obtaining a desired image. Then, a method of displaying the image as a pseudo three-dimensional image by performing the shading is generally adopted.

On the other hand, in helical scanning CT, an X-ray beam passing through the center of the helical trajectory apparently draws a helical trajectory with respect to the object to be imaged, which may cause inconsistencies (errors). Since errors also occur in the obtained tomographic images, in order to correct these errors, spiral correction is conventionally performed by a 360-degree interpolation method or a 180-degree opposite interpolation method.

[0005] In a correction method such as a 360-degree interpolation method or a 180-degree opposite interpolation method used for correcting an error due to spiral scanning, a coefficient array called a weighting function is weighted for each view and each channel. It is known that the error is corrected, and the weighting function used here has a spread in the view direction and the channel direction, and that the larger the size in the view direction, the lower the body axis resolution.

[0006] The body axis resolution is a measure of the limit at which an object can be decomposed (discriminated) in the body axis direction. If all other conditions are the same, the body axis resolution is higher (the smaller the object is, the smaller the object is). Can be discriminated). It is also known that the larger the view direction size in the weighting function is, the more perfect the correction can be.

[0007]

However, in the conventional 180-degree opposite interpolation method and 360-degree interpolation method, the shape of the weighting function for each correction and the size in the view direction are fixed, and the tomographic image is not fixed. Slice thickness (body axis resolution)
Cannot be arbitrarily changed, so that there is a problem that only an image having a slice thickness determined for each interpolation method can be created.

The 360-degree interpolation method has a high degree of completeness of correction, but has a problem in that the slice thickness is so large that the resolution of the body axis is low. Although the slice thickness is small, the completeness of the correction is low due to the small size in the view direction. Therefore, when a tomographic image is stacked to form a three-dimensional image and displayed as a pseudo-three-dimensional image, it is unique to spiral scanning CT. There are problems such as occurrence of artifacts.

As a method for reducing artifacts unique to the spiral scanning CT, a method using a Z_filter (USP-5301108) has been proposed.
In this method, there is a problem that the processing man-hour is increased by using the Z_filter, so that the processing speed is reduced.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned conventional problems, and an object of the present invention is to make it possible to arbitrarily change the size and shape of a weighting function by using an opposite data weighting coefficient index and weighting coefficient shift index. This makes it possible to change the slice thickness of the tomographic image without lowering the processing speed,
By providing a method for creating a tomographic image that eliminates artifacts that occur at the end of each projection data by correcting errors by using the same phase data before and after the circumference, a high-quality tomographic image with few artifacts is provided. And a three-dimensional image can be obtained.

[0011]

According to the present invention, there is provided a method for producing a tomographic image, comprising the steps of:
A creation method for creating a tomographic image of an object from projection data collected by spiral scanning using a spiral scan CT having a line detector, wherein when a tomographic image is created from a projection data array, a preset facing image is created. Creating a weighting function whose size and shape can be arbitrarily changed based on a data weighting coefficient index and a weighting coefficient shift index, and applying the weighting function to the projection data to a phase corresponding to a desired slice position range. To create a weighted projection data array, and to reuse the obtained weighted projection data array to create a tomographic image.

According to the above method, when a tomographic image is created from projection data obtained by a spiral scanning X-ray CT scanner having an X-ray detector consisting of a single row, and three-dimensional images are created from the created tomographic image. When creating an image, more complete spiral correction can be performed by setting the weighting function parameters, so that a high-quality tomographic image and a three-dimensional image with less artifacts can be created. Further, since the slice thickness can be changed linearly by the weighting coefficient parameter, a tomographic image with high body axis resolution can be obtained, and the number of processing steps can be reduced as compared with the processing performed by Z_filter.
As compared with the method using er, a tomographic image having an arbitrary slice thickness can be formed at a higher speed.

In order to achieve the above object, a tomographic image generating apparatus according to the present invention converts a tomographic image of an object from projection data acquired by helical scanning by helical scanning CT having an X-ray detector having a single row. A creation apparatus for creating projection data, wherein means for obtaining projection data from a detector by irradiating X-rays, and arbitrarily changing the utilization efficiency of opposed data at a reconstruction slice position of projection data based on preset conditions. Means for generating a weighting function that can set the slice thickness arbitrarily and reduce artifacts unique to spiral scanning CT, and means for generating a weighted projection data array by applying a weighting function to each of the projection data arrays. Means for generating a slice image from the weighted projection data array, wherein the predetermined condition is a finger for determining a shape of a weighting function. And a weighting coefficient shift index Wshift that can change the weight amount without error, and a weighting coefficient shift index Wshift that determines the ratio of the weighting coefficient at the countering data position to the weighting function reconstruction position. is there.

According to the above arrangement, when a tomographic image is created from projection data obtained by a spiral scanning X-ray CT scanner having an X-ray detector consisting of a single row, and three-dimensional images are created from the created tomographic image. When creating an image, more complete spiral correction can be performed by setting the weighting function parameters, so that a high-quality tomographic image and a three-dimensional image with less artifacts can be created. Further, since the slice thickness can be changed linearly by the weighting coefficient parameter, a tomographic image with high body axis resolution can be obtained, and the number of processing steps can be reduced as compared with the processing performed by Z_filter.
As compared with the method using er, a tomographic image having an arbitrary slice thickness can be formed at a higher speed.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of a tomographic image creating method and a tomographic image creating apparatus according to the present invention will be described below in detail with reference to the drawings. FIG. 1 shows a helical scanning CT apparatus for executing a method for producing a tomographic image according to the present invention. The host computer 1 controls the whole, an X-ray generator 2 for generating X-rays, and the generated X-rays. X-ray detector 3 consisting of a single column for detecting, a scanner 4 capable of continuous scanning by a slip ring, an image processing apparatus 5 for performing preprocessing, image reconstruction processing and various analysis processing, and applying a high voltage to X-rays. It comprises a high voltage generator 6 to be supplied, a patient table 7, and a display device 8.

FIG. 2 is an explanatory view showing a method of photographing the subject 9 using a spiral scanning X-ray CT scanner. The X-ray detector 3 at a diagonal to the X-ray generator 2 is rotated about the subject 9. Let
The subject 9 is irradiated with X-rays, and the subject 9 is moved in the rotation axis direction, and the obtained projection data is taken into the image processing device 5 as a projection data array. Also, a weighting function creation parameter is input from the host computer, a weighting function is created based on the parameters, and the weighting function created in the previous process is applied to the obtained projection data array to obtain a weighted projection data array. At the same time, the obtained weighted projection data array is reconstructed to obtain a tomographic image.

FIG. 3 is a diagram showing an example of a weighting function. The weighting function is a center symmetric function having a different shape depending on the set values of the opposing data weighting coefficient index and the weighting coefficient shift index. The coefficient shift index can be arbitrarily set in the host computer 1 when creating a tomographic image.

When the value of the counter data weighting coefficient index is increased, the slice thickness of the tomographic image to be created can be increased, and at the same time, the artifacts that occur when displaying a three-dimensional image can be reduced. When the value of the weighting coefficient shift index is increased, the slice thickness of the tomographic image to be created can be reduced.

The weighting coefficient shift index is added to a weighting coefficient intermediate between the reconstructed slice position and the opposite data position, and the weight of the intermediate position between the opposite data position and the in-phase position which is about one revolution around the reconstructed slice position is calculated. The coefficients are subtracted. As shown in FIG. 3, the weighting function can be determined by linearly changing between the set weighting coefficients in the view direction.

Next, a method for creating a tomographic image according to the present invention will be described with reference to the flowchart of FIG. When a tomographic image is created, first, Woppo (opposite data weighting coefficient index) and Wshift (weighting coefficient shift index) are input and stored in the memory from the input device as parameters for determining the parameters of the imaging function and the shape of the weighting function. Then, the parameters are read out from the memory (step 2), and a weighting function is calculated based on the parameters (step 3).

On the other hand, the imaging parameters are read out (step 4), and the data is spirally scanned based on the imaging parameters using a CT scanner to obtain projection data (step 5). Then, the weighting function obtained by the above process is weighted to the projection data to obtain weighted projection data (Step 6), and the obtained weighted projection data is back projected (Step 7) to obtain a tomographic image. Is displayed on the display device (step 8).

When calculating the weighting function, the parameters for the weighting function are adjusted so that the shape of the weighting function can be adjusted to a desired slice thickness and correction. However, an error is effective at a thinner slice thickness (high body axis resolution). It is desirable to create a weighting function that resolves

FIG. 5 is a block diagram showing a tomographic image forming apparatus equipped with the tomographic image forming method according to the present invention. The tomographic image forming apparatus collects a target portion of the subject 9 by X-ray CT. A central processing unit (CPU) 10 for controlling the operation of each component, a main memory 11 in which a control program for the device is stored, and a pre-created medical image data. A weighting function memory 21 storing a weighting function, a scanner controller 12 for controlling an X-ray CT scanner, and a data capturing circuit 13 for capturing data detected by the scanner.
A storage medium 14 in which a plurality of tomographic images and image reconstruction programs are stored; a display memory 15 for storing reconstructed image data; and a display memory 15
A CRT monitor 16 as display means for displaying image data from the mouse, a mouse 17 as position input means, a mouse controller 18 for detecting the state of the mouse 17 and outputting it to the CRT 16, and a keyboard for inputting various operation commands and the like. 19 and a common bus 20 for connecting the above-mentioned components.

FIG. 6 is a diagram showing a detailed method of creating a weighting function used in the method of creating a tomographic image according to the present invention. Next, FIG. 6A will be described. FIG. 6B is a diagram for explaining symbols, β represents a rotation angle when the scanner rotates, α represents an opening angle from the X-ray tube to the channel, and FIG. 6, L1 to L9 shown in FIG. 3 are represented as follows.

[0025]

[Table 1]

The tomographic image forming method and the tomographic image forming apparatus described in the above embodiment are not limited to the description and the examples. For example, in the above embodiment, the weighting factor shift index is added to the weighting factor in the middle between the reconstructed slice position and the opposite data position, and the intermediate between the opposite data position and the in-phase position that is around one revolution around the reconstructed slice position. The position weighting factor is subtracted, but is not limited to this.
The weighting factor shift index is added to the weighting factor at one-third of the position from the reconstructed slice position to the opposite data position, and one-third of the position from the opposite data position to the same phase position around one revolution from the reconstructed slice position. May be subtracted.

That is, the weighting coefficient shift index is applied to a position where the weighting coefficient can be shifted without error, and in the formula for generating the weighting function, the weighting coefficient is shifted using only one weighting coefficient shift index. Although the coefficients are shifted, the present invention is not limited to this, and the weighting coefficients may be shifted by a plurality of independent weighting coefficient shift indexes.

Further, in the above embodiment, the weighting function is calculated based on the weighting function parameter. However, the present invention is not limited to this. The weighting function created in advance is stored in the weighting function memory. The stored weighting function may be stored and the corresponding weighting function may be read out from the weighting function memory based on the arbitrarily set weighting function parameter.

[0029]

As has been described in detail above, the present invention relates to a method and apparatus for forming a tomographic image from projection data obtained by a helical scanning X-ray CT scanner having an X-ray detector having a single row. When creating a three-dimensional image from the tomographic image that has been obtained, by setting the parameters for the weighting function, more complete spiral correction can be performed, so that it is possible to create a high-quality tomographic image and a three-dimensional image with less artifacts. become able to.

Further, since the slice thickness can be changed linearly by the weighting coefficient parameter, a tomographic image with a high axial resolution can be obtained, and the number of processing steps can be reduced as compared with the processing performed by the Z_filter.
Compared with the method using the ilter, a tomographic image having an arbitrary slice thickness can be formed faster.

[Brief description of the drawings]

FIG. 1 is an overall configuration diagram of a tomographic image creating apparatus according to an embodiment of the present invention.

FIG. 2 is an explanatory diagram showing an imaging method using an X-ray CT scanner of the tomographic image creating apparatus according to the embodiment of the present invention.

FIG. 3 is an example diagram showing a weighting function used in a method for creating a tomographic image according to an embodiment of the present invention.

FIG. 4 is a flowchart illustrating a processing procedure of a tomographic image creation method according to the embodiment of the present invention.

FIG. 5 is a configuration diagram showing a main part of the tomographic image creating apparatus according to the embodiment of the present invention.

FIGS. 6A and 6B are explanatory diagrams showing a method of creating a weighting function used in the method of creating a tomographic image according to the embodiment of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Host computer 2 X-ray generator 3 X-ray detector 4 Scanner 5 Image processing device 6 High voltage generator 7 Patient table 8 Display device 9 Subject 10 Central processing unit (CPU) 11 Main memory 12 Scanner controller 13 Scanner data capture Circuit 14 Storage medium 15 Display memory 16 CRT monitor 17 Mouse 18 Mouse controller 19 Keyboard 20 Common bus 21 Memory for weighting function

Claims (1)

[Claims] 1. A method for producing a tomographic image of an object from projection data acquired by helical scanning by helical scanning CT having an X-ray detector comprising a single column, comprising: When creating a photographic image, a weighting function whose size and shape can be arbitrarily changed based on a preset facing data weighting coefficient index and weighting coefficient shift index is created, and the weighting function is applied to the projection data. A weighted projection data array is created by applying to a phase corresponding to a desired slice position range, and a tomographic image is created by reusing the obtained weighted projection data array. How to make.