JPH0721349A - Image reconstituting method - Google Patents

Abstract

PURPOSE:To provide the image reconstituting method which can obtain an image of a good quality by improving the interpolation accuracy at the time of reconstitution in the case a helical scan is used, and also, adjusting arbitrarily thickness of a slice. CONSTITUTION:By using a two-dimensional (surface) detector, data are collected at equal intervals in a slice position and its vicinity, or the data of an equal interval are utilized from among the data taken out in sufficient density, and from this obtained data of an equal interval, data in a slice position is interpolated by using a sampling function, and by using the data obtained by this interpolation, the image is reconstituted.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image reconstruction method for interpolating X-ray projection data collected by performing a spiral scan (hereinafter referred to as a helical scan) in an X-ray CT apparatus.

[0002]

2. Description of the Related Art In an X-ray CT apparatus, an X-ray tube and a one-dimensional detector arranged so as to sandwich the subject are integrally rotated around the subject while the subject is fixed on a bed. While irradiating the X-ray beam, X-ray projection data (so-called projection data) over the 360 ° direction of the subject
Are acquired, and a tomographic image (slice image) along the X-ray irradiation surface can be obtained by performing image processing and image reconstruction based on the projection data.

However, in the scanning method in which the X-ray beam is irradiated from the periphery of the subject while the subject is fixed in this way to collect X-ray projection data, tomographic images at different slice positions of the subject are obtained. In that case, the X-ray scanning operation by the X-ray tube is temporarily stopped, the subject is moved to position the planned slice position on the X-ray irradiation path, and then the scanning operation is performed again. It is necessary to repeat the same operation according to the number of sheets. For this reason, the scan time for obtaining a plurality of tomographic images from the same subject becomes long, and the operating efficiency of the X-ray CT apparatus deteriorates.

For this reason, a helical scanning method has been provided as a scanning method for improving such drawbacks. However, although this helical scan method improves the above-mentioned drawbacks, one rotation 360
Since the position of the X-ray beam on the object over a 90 ° shifts little by little, the slice position is constantly changing.
Since there is a mismatch between the projection data at 60 ° and the projection data at 360 °, an artifact is generated in the reconstructed tomographic image, and correct diagnostic information cannot be obtained. For this reason, the helical scan method employs a method of interpolating data at an arbitrary slice position based on the collected projection data and then performing image reconstruction. As the interpolation method, the applicant of the present application has proposed a 360 ° interpolation method and an opposite beam interpolation method.

The method called the 360 ° interpolation method is to interpolate data to be used for image reconstruction at an arbitrary slice position, by data obtained at the same rotation phase before or after that. FIG. 5 schematically shows this. Arrows in the figure indicate projection data of the same rotation phase, and the arrow indicates the projection detector and the original X-ray tube. The interpolation is performed by linear interpolation (linear interpolation), and the distance from the slice position (reconstruction center in the figure) to be interpolated is used as a weighting coefficient. In the case of performing such interpolation, since data of the same rotation phase on both sides of an arbitrary rotation phase on an arbitrary slice position is used, a maximum of 180 ° is provided on both sides of the main data area of 360 °. Interpolation data area is required. Therefore, as a whole, the interpolation process is performed using the data for 720 °.

In the method called the opposed beam interpolation method, when data of an arbitrary rotation phase on an arbitrary slice position is interpolated, the data of this rotation phase is present at the closest distance in the same phase. And the opposite beam data is used for interpolation, and FIG. 6 schematically shows this. In the figure, the arrow indicates projection data, and there is a projection detector at the tip of the arrow and the existence of an X-ray tube at the origin. Also in this method, interpolation is performed by linear interpolation (linear interpolation), and the distance from the slice position (reconstruction center in the figure) to be interpolated is used as a weighting coefficient. In the case of performing such interpolation, 360 ° is obtained by interpolating data of an arbitrary rotation phase on an arbitrary slice position by using both beams having a relationship of opposite beam data.
Data interpolation can be performed by providing an interpolation data area narrower than the interpolation data area of the 360-degree interpolation method on both sides of the main data area.

[0007]

However, in the conventional interpolation method of this type, both the 360 ° interpolation method and the opposed beam interpolation method use data collected by using a one-dimensional detector, and the linear interpolation method is also used. Since the interpolation is performed by (linear interpolation), the accuracy of the interpolation is low. Further, in the 360 ° interpolation method, since the interpolation process is performed using the data for 720 ° including the data for interpolation of 360 °, there is a problem that the effective slice thickness becomes thick. As a result, even data that is relatively distant from the slice position is used, and since data that is far from the true data is used, the accuracy of interpolation becomes low. Although this point is a slight improvement in the opposed beam interpolation method, it is a difference in degree and has a similar problem.

The present invention has been made in view of the above circumstances, and an object of the present invention is to improve the interpolation accuracy at the time of reconstruction in the case of using a helical scan and to adjust a slice thickness arbitrarily to obtain a high quality image. An object of the present invention is to provide an image reconstruction method capable of obtaining the image.

[0009]

In order to achieve the above-mentioned object, the present invention provides an X-ray irradiating an X-ray tube while rotationally moving around a subject continuously moving in the body axis direction. In the image reconstruction method for reconstructing an image at an arbitrary slice position in the body axis direction of the subject by using the collected X-ray projection data detected by a two-dimensional (plane) detector, the two-dimensional (plane)
Collect data at equal intervals at the slice position and its vicinity using a detector, or use evenly-spaced data from data taken with sufficient density, and obtain a sampling function from this evenly-spaced data. Is used to interpolate the data at the slice position, and the data obtained by this interpolation is used for reconstruction.

Further, according to the present invention, the two-dimensional (plane) detector detects the X-rays emitted from the X-ray tube while rotationally moving around the subject continuously moving in the body axis direction, Collected X
In an image reconstruction method for reconstructing an image at an arbitrary slice position in the body axis direction of a subject using line projection data, the two-dimensional (plane) detector is used to divide the slice position and its vicinity at equal intervals. Collect data or use evenly spaced data from data taken with sufficient density, interpolate slice position data with a sampling function from this evenly spaced data, and obtain by this interpolation The feature is that the section width is determined from the obtained data, the average is obtained to obtain data of an arbitrary slice thickness, and the reconstruction is performed using this data.

[0011]

According to the above-mentioned image reconstruction method, since the X-ray projection data by the helical scan is collected by using the two-dimensional (plane) detector, the projection data can be obtained at a higher density. Then, since the data at the slice position is interpolated using the sampling function using this high-density data, the interpolation accuracy is high, and when the image is reconstructed using this interpolation data, it is more practical than that by linear interpolation (linear interpolation) An image close to is obtained.

Further, since it is possible to obtain the data of an arbitrary slice thickness by determining the section width and obtaining the average from the interpolation data with high interpolation accuracy, the slice thickness can be made sufficiently thin.

[0013]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows an X implementing the image reconstruction method of the present invention.
It is a block configuration diagram showing an example of a line CT apparatus. In FIG. 1, reference numeral 1 is a gantry unit, and in the gantry unit 1,
An X-ray tube 3 for irradiating the subject P placed on the bed 2 with X-rays, a two-dimensional (plane) detector 4 arranged to face the X-ray tube 3, and this plane detector. A data collecting unit 5 and the like for collecting the projection data detected in 4 are provided.
The bed 2 is controlled by the bed drive control device 6 so as to be continuously movable along the body axis direction of the subject P (direction perpendicular to the plane of the drawing). The X-ray tube 3 is controlled by the high voltage generator 8 via the X-ray controller 7 and the surface detector 4 is controlled by an X-ray tube drive controller (not shown).
It is controlled so as to rotate around the subject P integrally. Further, the projection data collected by the data collection unit 5 is converted into proper reconstruction data by the correction calculation device 9 and sent to the image reconstruction device 10. Image reconstruction is performed based on the reconstruction data sent by the image reconstruction device 10. The image-reconstructed data is temporarily stored in the image storage device 12 via the computer 11 and displayed on the image display device 13 as necessary. The computer 11 is a system control device
The entire control operation including U is performed. In the case of this embodiment, the two-dimensional (plane) detector 4 is different from the conventional one-dimensional detector, that is, a plurality of single detectors (one channel) arranged in an array, as shown in FIG. In addition, a plurality of single detectors are arranged in a plurality of stages to detect the transmitted X-rays of the subject P irradiated from the X-ray tube 3. Therefore, the projection data collected by the surface detector 4 will be dense and thinner slices of data can be obtained with high density. The thus obtained product can have a slice thickness of 1 mm or less.

Further, in the case of the present embodiment, under the control of the computer 11 in the correction arithmetic unit 9, interpolation processing of helical scan data as described later is performed. FIG. 3 schematically shows the method of this interpolation processing. In the figure, the arrow indicates projection data, and there is a projection detector at the tip of the arrow and the existence of an X-ray tube at the origin. The procedure of this interpolation processing method will be described below. First of all, the transmitted X-ray of the subject P irradiated from the X-ray tube 3 by the helical scan is detected by using the surface detector 4.
The projection data thus obtained can collect high density data as described above. As a method of collecting data, data is collected at equal intervals at a slice position (reconstruction center in the drawing) and its vicinity, or data at equal intervals is used from data obtained with sufficient density. As data,
It may be the same direction or the opposite direction (data in which the positions of the X-ray tube 3 and the surface detector 4 are exchanged). That is, a plurality of data around the slice position is collected by using the counter beam data. A sampling function is applied to each of the plurality of equally-spaced data thus obtained. Interpolation data can be obtained by calculating the sum of each data multiplied by these sampling functions. When the slice is thin enough, the interpolation data is used as it is to reconstruct the image. When thinning the slice, the section width (slice thickness) is determined from the obtained interpolated data, and data of an arbitrary slice thickness can be obtained by averaging. Therefore, this data is used to reconstruct an image. . An image can be reconstructed using the projection data obtained in this way from each direction on the same plane.

Incidentally, when collecting data, if the data is collected with a fineness twice as large as the fineness of the structure of the human body to be diagnosed, the interpolated data at each slice position can be accurately obtained. Here, the structure of the human body is a tissue that a doctor or the like wants to see and, for example, a thin blood vessel or a tumor is considered.

FIG. 4 is a diagram showing an interpolation result by the method of the present invention, which is an interpolation between X and Y. According to the method of the present invention, since the surface detector cuts out a thin slice, the slice thickness of the helically interpolated data can be thinned. Then, the data in the vicinity of the slice position is taken sufficiently densely, and this data is interpolated using a sampling function, so that the value after interpolation is closer to the correct value than in conventional linear interpolation. Further, in FIG. 4, an interpolated image having an arbitrary slice thickness can be obtained by taking an average of a certain section (for example, A) of the interpolated data.

[0017]

As described above, according to the present invention, the data at the slice position and the vicinity thereof are taken by the surface detector sufficiently densely, and this data is interpolated using the sampling function, so that the interpolation accuracy is good. Interpolated data can be obtained.
Further, since the section width (slice thickness) is determined from the obtained interpolation data and the average is obtained, data of an arbitrary slice thickness can be obtained, so that an interpolation image with a sufficiently thin slice thickness can be obtained.

[Brief description of drawings]

FIG. 1 is a block configuration diagram showing an example of an X-ray CT apparatus for carrying out an image reconstruction method of the present invention.

FIG. 2 is a diagram showing an example of a two-dimensional (plane) detector.

FIG. 3 is a diagram schematically showing an interpolation processing method of the present invention.

FIG. 4 is a diagram showing an interpolation result according to the method of the present invention.

FIG. 5 is a diagram schematically showing a conventional interpolation processing method.

FIG. 6 is a diagram schematically showing a conventional interpolation processing method.

[Explanation of symbols]

 1 gantry unit 2 bed 3 X-ray tube 4 two-dimensional (plane) detector 5 data acquisition unit

Claims (4)

[Claims] 1. An X-ray collected by detecting a X-ray emitted from an X-ray tube by a two-dimensional (plane) detector while rotationally moving around a subject continuously moving in a body axis direction. In the image reconstruction method for reconstructing an image at an arbitrary slice position in the body axis direction of a subject using projection data, the two-dimensional (plane)
Collect data at equal intervals at the slice position and its vicinity using a detector, or use evenly-spaced data from data taken with sufficient density, and obtain a sampling function from this evenly-spaced data. An image reconstructing method characterized in that the data at the slice position is interpolated by using, and reconstructed using the data obtained by this interpolation. 2. X-rays collected and detected by a two-dimensional (plane) detector of X-rays emitted by an X-ray tube while rotationally moving around a subject continuously moving in the body axis direction. In the image reconstruction method for reconstructing an image at an arbitrary slice position in the body axis direction of a subject using projection data, the two-dimensional (plane)
Collect data at equal intervals at the slice position and its vicinity using a detector, or use evenly-spaced data from data taken with sufficient density, and obtain a sampling function from this evenly-spaced data. Interpolate the data of the slice position using, and obtain the data of any slice thickness by determining the section width from the data obtained by this interpolation and averaging,
An image reconstruction method characterized by performing reconstruction using this data. 3. The data according to claim 1 or 2, wherein the data at which the positions of the X-ray tube and the detector are interchanged (data obtained by the opposite beam) is also used as the data at equal intervals in the slice position and its vicinity. Image reconstruction method. 4. The image reconstructing method according to claim 1 or 2, wherein interpolation data is obtained by multiplying each data at equal intervals by a sampling function and obtaining a sum thereof.