JPH11299769A - X-ray ct apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To shorten the reverse projection processing time in obtaining a tomographic image from a subject. SOLUTION: A X-ray tube 1 is placed so as to face a fan-shaped X-ray detector 2 through a subject 21, both of them are continuously revolved and the transmitted X ray is processed by a raw data generator 4 to obtain projection data. The first revolution sends this projection data to a reverse projection data generator 7, the reverse projection data obtained from the reverse projection data generator are added to an image addition memory 9 by a reverse projection data apparatus 8 to obtain an image. The second revolution and more calculate the difference between the first revolution data and the second revolution data at the same angle and the same location, the difference data are added at each pixel corresponding to an image addition memory in the reverse projection apparatus 8 to obtain a continuously scanned image from the subject 21.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an X-ray CT apparatus, and more particularly to an X-ray CT apparatus capable of reducing the processing amount of backprojection data and increasing the speed of arithmetic processing.

[0002]

2. Description of the Related Art An X-ray CT apparatus obtains tomographic images of a subject, collects projection data of X-rays transmitted through the subject from different angles, processes the data using a computer, and performs back projection. By doing so, the image is reconstructed. In order to obtain a tomographic image for one slice plane, 360-degree or 180-degree projection data for the slice plane is required.

As a scanning method for collecting projection data from each of these required angular directions, a rotation / rotation type in which both the X-ray tube and the X-ray detector are rotated, and a rotation / stationary type in which only the X-ray tube is rotated. A type, a stationary / stationary type in which neither the X-ray tube nor the X-ray detector rotates and only the X-ray focal point rotates is known.

In these methods, in order to obtain projection data from different angles, it is necessary to rotate at least only the X-ray source around the subject. If the X-ray source is continuously rotated a plurality of times and projection data of the angle range necessary for image reconstruction for one slice plane is sequentially obtained in time series, the projection data changes in time series for that one slice plane. Many tomographic images are obtained.

In this case, in the prior art, one projection image of an angle range necessary for reconstructing a single tomographic image is sequentially back-projected into a back-projection memory for each angle, thereby obtaining one tomographic image. After reconstructing, this memory was initialized in preparation for the next backprojection. In this case, there is a problem that a long processing time is required to obtain one tomographic image.

As a conventional example for solving this problem, there is JP-A-8-24253. FIG. 2 shows such a conventional example.
Shown in Figure 2 shows rotation / rotation type X-ray CT
This is an example applied to a device. X-ray tube 101 and X-ray detector 1
The X-ray tube 101 and the X-ray detector 102 are integrally rotated around the subject 121 by a rotating mechanism. It has become.

The X-ray transmitted through the subject 121 is incident on the X-ray detector 102, and a signal indicating the transmitted X-ray intensity is obtained. Thereby, data indicating the X-ray absorption in the subject 121 is obtained. The X-ray detector 102 is arranged in a fan shape, and a large number of detection elements are arranged in an arc direction. The data obtained from the X-ray detector 102 is profile data indicating X-ray absorption by X-rays emitted from the X-ray tube 101 in a fan shape.

[0008] The profile data is sent to the projection data creation device 103 and converted into back projection data.

When the X-ray tube 101 and the X-ray detector 102 rotate from the reference position at an angle of 0 °, projection data for each angle is obtained.
04 backprojects onto any of the four memories 106, 107, 108, 109 specified by the selector 105. These four memories are back projection memories, each of which is one plane.

That is, the four memories 106, 107, 1
Each of 08 and 109 has a 256 × 256 address corresponding to the pixel if the reconstructed image has a pixel represented by a 256 × 256 matrix.

When the X-ray tube 101 and the X-ray detector 102 rotate continuously, the angle range of 0 to 90 degrees is A, 90 to 18 degrees.
The range of 0 degrees is B, and the range of 180 to 270 degrees is C, 270.
D is defined as a range of up to 360 degrees. By continuous rotation, projection data is sequentially obtained as ranges A, B, C, D of the first rotation and ranges A, B, C, D of the second rotation.

The selector 105 designates the memory 106 when the projection data of the range A is obtained.
In 06, the back projection of the projection data in the range A is performed, and in the ranges B, C, and D, the memories 107, 108, and 109 are used.
Is specified, and back projection is performed for each.

Thus, when the first rotation is completed, 90-degree projection data is back-projected into each of the memories 106, 107, 108, and 109, and 360-degree back-projection data is back-projected as a whole. State.

Therefore, these 106, 107, 108,
When the corresponding pixels 109 are added by the adder 110, an image # 1 of 360-degree projection data from 0 to 360 degrees is obtained.

Next, the first rotation is completed, and the following １ ／
Immediately upon entering the rotation, the memory 106 is initialized,
The new projection data in the range A, which is the quarter turn, is back-projected. At the time when (1 + 1/4) rotation is completed counting from the beginning, adders 110, 106, 107, 10
90-45 if 8,109 identical pixels are added together
A second image # 2 is obtained from 360 degrees of projection data in the range of 0 degrees.

In this case, the initialization of the back projection memory and the rewriting thereof may be performed for the memory 106 corresponding to 1/4 of the entire memory for 360 degrees, and the memories 107 and 108 corresponding to the other 3/4. , 109, no operation is performed, and the contents stored up to that point may be simply read as they are and added by the adder 110. That is,
Since the backprojection operation only needs to correspond to 1/4 of the whole,
The time from obtaining the first image # 1 to obtaining the second image # 2 can be shortened.

When the next 1/4 rotation is completed, a third image #
3 is obtained, and when the next 1/4 rotation is completed, a fourth image # 4 is obtained. These images are sequentially stored in the image memory 11.
1 is displayed on the display device 102.

[0018]

The first of the above-mentioned prior arts is to project back projection data of an angle range necessary for constructing one tomographic image onto a memory for each angle. After reconstructing one tomographic image, all of the memories are initialized and the next backprojection is performed. Therefore, there is a problem that a long processing time is required to obtain one tomographic image.

The conventional technique described with reference to FIG. 2 is an improvement of the above technique, but (a) an arithmetic unit or the like for adding images is separately required. (B) A memory buffer for storing a plurality of tomographic images is required. And the number of images per measurement unit (or unit time) is limited.

It is an object of the present invention to minimize the increase in the number of arithmetic units and memories, and to perform X-ray C-ray projection at high speed, which is a bottleneck in tomographic image reconstruction processing time.
T device.

[0021]

SUMMARY OF THE INVENTION An object of the present invention is to rotate an X-ray source around a subject and collect projection data at a plurality of angles of the subject with an X-ray detector having a plurality of X-ray detection element channels. Means for back-projecting the projection data for a predetermined rotation of the X-ray source into first back-projection data, and a tomographic image of the subject at a first time from the first back-projection data. In an X-ray CT apparatus provided with a means for reconstructing an image, projection data newly acquired beyond a predetermined rotation of the X-ray source is measured by a predetermined rotation of the X-ray tube. Means for obtaining a difference from the projection data at the same angle and adding the difference to obtain updated projection data; means for back-projecting the updated projection data to convert it to updated back-projection data; Differential performance from rotation projection data The synthesizes the backprojection data consisting minus those subjected the updating backprojection data 2
X-ray CT comprising: means for obtaining backprojection data of the first and second sections; and means for reconstructing a tomographic image at a second time after a lapse of a predetermined time from the first time from the second backprojection data. Achieved by the device.

[0022]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows an embodiment in which the present invention is applied to a rotation / rotation type X-ray CT apparatus. This device is used for displaying a measurement image in a spiral scan and for continuously observing a temporal change of the same measurement site. An X-ray tube 1 and a fan-shaped X-ray detector 2 in which a large number of detection elements are arranged in an arc shape are arranged to face each other, and a subject 21 is arranged therebetween. The X-ray tube 1 and the X-ray detector 2 are integrally rotated around the subject 21 by a rotation mechanism.

X-rays emitted from the X-ray tube 1
Attenuated by this, transmitted through and incident on the X-ray detector,
A signal representing the transmitted X-ray intensity is measured. This X-ray measurement data is transferred to the raw data creation device (arithmetic unit) 4 and converted into projection data (called raw data P) by calculation such as logarithmic conversion. This raw data is two-dimensional raw data in the direction of spread of the detector and in the angle direction of the X-ray tube. This raw data P is written to the first raw data memory 5.

The raw data P has a two-dimensional array and can be represented by a matrix as shown in (Equation 1).

(Equation 1) Here, m is the number of angles measured in one rotation, and n is the number of data taken per angle. Next, the raw data is sent to a second raw data memory 6 attached to the back projection data generating device (arithmetic unit) 7.

[0025] In backprojection data creating device 7, the raw data using a P for each angle measuring X-ray output, create a function f _K expressed by the following equation for obtaining an inverse projection image.

(Equation 2) Here, f _K indicates the back projection result at the K-th angle. Function data f _K created in device 7 is sent to the back projection apparatus (calculator) 8. The image addition memory 9, backprojector 8 the function data f _K for each angular backprojected sum, the image F (p) is constructed as follows.

(Equation 3) At the time of the addition, the content F (p) is transferred to the image display device 10 to display a tomographic image. This tomographic image is obtained by imaging the inside of the subject 21 as a distribution of X-ray absorbance.

As described above, in the first rotation, the image is reconstructed based on the projection data (raw data) P developed based on the X-ray measurement data of the X-ray detector and the back projection data f _K.
Is back-projected (added) by the back-projection device 8 by the number of operation lines (usually several hundred to several thousand) to obtain one tomographic image.

One feature of the present invention is in the second and subsequent rotations. After the second rotation, instead of the raw data of the second and third rotations, for example, the raw data of the first, second, third,... Raw data of the second rotation)-(Raw data of the first rotation)
Alternatively, a raw data difference (ΔP) such as (raw data of the third rotation) − (raw data of the second rotation) is obtained, and backprojection data f _K (ΔP) is created from the difference raw data by the backprojection data creation device 7. I do.

Here, the difference raw data (ΔP) obtained by subtracting the raw data of the first rotation from the raw data of the second rotation is the difference between the data at the same measurement angle and the same position (channel) of each raw data. Means the raw data created. FIG. 1 shows an example in which the difference raw data creation device (difference means) 22 is provided between the raw data creation device 4 and the back projection data creation device 7. However, the difference data creation function can be provided in the raw data creation device 4 or the back projection data creation device 7.

The creating device 22 creates difference raw data from the raw data of the first and second rotations input from the first raw data memory 5 and sends it to the back projection data generating device 7 via the second raw data memory 6. send. By creating a back projection data f _K from the difference raw data, always be the tomographic image made of one rotation to the most recent operation line entering the summing memory 9 of the backprojection device 8.

Incidentally, the above-mentioned prior art JP-A-8-242 is disclosed.
In No. 53, when one image is completed and displayed, the memory is initialized and the process proceeds to the processing of the next tomographic image. However, in the present invention, the initialization process is canceled, and the above addition is repeated, so that the line unit is obtained. New tomographic images that can be sectioned are reconstructed one after another. According to this method, if data is taken out of the addition memory 9 at any time after completion of the tomographic image for one rotation (after completion of an arbitrary operation line), the latest tomographic image at that time is obtained. Therefore, the time for a series of processes from raw data collection to display can be greatly reduced.

The contents of the present invention will be described in more detail with reference to specific examples. For example, it is assumed that back projection data of 1000 lines is created from raw data for one rotation, and back projection is performed to create one tomographic image. Since the back projection data is created from the difference data (possible both positive and negative) in the second and subsequent rotations, the first line of the second rotation, that is, the 1001th line from the beginning, is the (1001th line-1st line) difference calculation. It is. Differential backprojection data for the 1001 line can tomographic images G ₁ equivalent up to previous 1001 line from the second line do it backprojected added to the tomographic image (1 from line to 1000 line addition result). Thereafter, if it is recursively repeated, the addition area always has tomographic images (addition results) G ₂ , G ₃ , and 1000 for the 1000 lines up to the latest line.
... will be included. That is, it becomes as follows. The first tomographic image G _{1 from} the first to 1000th lines, the first to 2nd to 1000th lines and the second to 10th line 10
01, the second tomographic image G ₂ , 3 to 1000 lines in the first rotation and 1001 in the second rotation
From 1002 to 1002, a third tomographic image G ₃ ,...,..., Images G ₄ , G ₅ . With respect to such tomographic images obtained for each line, the interval of taking out tomographic images can be set arbitrarily. For example, 1
To make 10 tomographic images per rotation (1000/1
0) = A take-out interval every 100 lines.
In this case, it is also possible to freely set the start line such as whether the start line of the 100 lines is the line 1 or the line 50. Obviously, if the next image is taken out when the previous image display is completed, the image can be displayed at intervals of one line.

According to the present embodiment, 2 is output from the image addition area after the second rotation and displayed on the image display device.
The tomographic image after the rotation is a corrected image in which only the portion covered by the backprojection data added to the addition area created based on the raw data after the second rotation with respect to the tomographic image after the previous rotation is used as the corrected image. become.

It is also possible to transfer the raw data for two rotations and to calculate the difference by the arithmetic unit. If it is not possible to secure a memory for storing raw data for two consecutive rotations, the raw data may be divided and transferred, and a process of cyclically using the memory for one rotation may be performed. Except that the raw data is difference data, the newest image always enters the addition area by the same processing as before. Naturally, the first half of the first rotation, 1
By calculating from raw data (or difference data) corresponding to a half rotation, such as the second half of the first rotation and the first half, the first half of the second rotation and the second half of the first rotation, an image equivalent to one half rotation can be output in real time. It is.

[0034]

In the X-ray CT apparatus according to the present invention, the amount of calculation for backprojection processing is reduced, so that the time required to reconstruct one image is significantly reduced as compared with the prior art, and a large number of time-series tomographic images can be obtained at high speed. Can be obtained.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram of an embodiment of the present invention.

FIG. 2 is an explanatory diagram of a conventional technique.

[Description of Signs] 1 X-ray tube 2 X-ray detector 4 Raw data creation device 5 First raw data memory 6 Second raw data memory 7 Back projection data creation device 8 Back projection device 9 Image addition memory 10 Image display device 21 Subject 22 Difference data creation device

Claims (1)

[Claims] A means for rotating an X-ray source around an object to collect projection data at a plurality of angles of the object with an X-ray detector having a plurality of X-ray detection element channels; Means for back-projecting the projection data for a predetermined number of rotations into first back-projection data, and means for reconstructing a tomographic image of a subject at a first time from the first back-projection data. In the X-ray CT apparatus, the difference between the projection data newly acquired beyond the predetermined rotation of the X-ray source and the projection data at the same angle in the projection data measured and held by the predetermined rotation of the X-ray tube. Means for obtaining updated projection data by adding the difference, means for back-projecting the updated projection data into updated back-projection data, and calculating a difference from the projection data for a predetermined rotation of the X-ray source. Excluding those provided for Means for obtaining second backprojection data by combining the backprojection data comprising the backprojection data and the updated backprojection data, and a second time at which a predetermined time has elapsed from the first time from the second backprojection data. An X-ray CT apparatus comprising means for reconstructing a tomographic image.