JPH056459B2 - - Google Patents

Description

Detailed Description of the Invention (Industrial Application Field) The present invention rotates an X-ray tube and a detector as one body around a subject, and collects X-ray absorption data from various directions of a cross section of the subject. X-ray CT equipment (CT is
Abbreviation for Computer Tomography).

(Conventional technology) Conventional X-ray CT equipment has a sufficiently narrow time width in each view direction in the cross section of the subject (a time width that does not degrade image resolution due to blurring and allows the imaging position to be regarded as a point). Exposing the subject to an X-ray pulse of
Imaging is being performed.

(Problems to be Solved by the Invention) In the above-mentioned X-ray CT apparatus, in order to prevent motion artifacts of the subject and to shorten the time required for diagnosis, the scanning speed is increased to capture images. If you try to shorten the time required and secure a sufficient number of views, the following various problems arise.

(a) If the scan time is shortened while keeping the number of views and the X-ray pulse width unchanged, the rotational momentum of the X-ray tube and detector per one pulse of X-rays will increase, and resolution deterioration due to vibration in the rotational direction will become a problem. Become.

(b) In order to prevent resolution deterioration due to this blurring, if the width of the X-ray pulse is narrowed in proportion to the rotation speed, the total irradiation time expressed as (X-ray pulse width) x (number of views) will be shortened, and the signal intensity will be reduced. Because it becomes weaker, it becomes more susceptible to noise.

(c) In order to solve the problems mentioned in (b), it is possible to increase the number of views or increase the X-ray exposure dose per unit time, but the exposure dose to the subject increases. In addition, scanning time and calculation time required to produce an image are increased, and the cost of the device increases.

The present invention was developed in view of the various problems with conventional devices, and its purpose is to minimize resolution deterioration without increasing the number of views or the X-ray exposure dose per unit time. The object of the present invention is to realize an X-ray CT apparatus that can perform high-speed scanning and obtain high-quality tomographic images with little noise.

(Means for Solving the Problems) The present invention solves these problems by integrating an X-ray tube and a detector to scan the area around the object, and detecting cross-sections of the object from various directions. In an X-ray CT device that obtains X-ray absorption data and processes each of these data to obtain a tomographic image of the subject, data of each channel obtained by the scanning and a restoration function are used to calculate data in the viewing direction. The present invention is characterized in that a means for applying correction only in the viewing direction is provided, and a tomographic image of the subject is obtained using data that has been corrected only in the viewing direction.

(Example) Hereinafter, an example of the present invention will be described in detail using the drawings. FIG. 1 is an explanatory diagram showing an embodiment of an X-ray CT apparatus according to the present invention. In Figure 1, 1
is an X-ray tube that emits continuous X-rays. Reference numeral 2 denotes a subject, and 3 a detector for detecting the X-rays after passing through the subject 2. Together with the X-ray tube 1 with the subject 2 in between, it constitutes a gantry. Rotate (scan) around the subject around the patient's body axis. 4 is a data collection means for inputting X-ray absorption data obtained from the detector 3 from various directions of the cross section of the object;
By 360° rotation, data corresponding to 576 views, for example, is obtained. 5 is a means for sensitivity correction of the data from the data collection means 4; 6 is a means for interpolating one point between each view to obtain data equivalent to double 1152 views; 7 is a means for applying correction only in the view direction; 8
is 576 from the corrected data from correction means 7.
This is a means of extracting data equivalent to a view. The above-mentioned means 6, 7, and 8 constitute means for correcting the data of each channel only in the viewing direction on the view channel plane. 9 is a log conversion means for inputting data from the data extracting means 8; 10 is a means for performing other preprocessing; 11 is a means for reconstructing an image; 12 is a display for displaying the reconstructed tomographic image. It is a means. Each of the means 10, 11, and 12 described above is realized using conventionally known techniques. Moreover, each means 5 to 11 is
It shall be realized by software.

With this configuration, it is possible to obtain high-quality tomographic images with less noise by shortening the scan time or widening the X-ray pulse width while keeping the scan time constant while minimizing blur caused by the X-ray pulse width. That's what I do.

This point will be explained in more detail below.
If the scan time is shortened while keeping the number of views and the X-ray pulse width unchanged, the time interval between X-ray irradiations becomes 0, resulting in continuous X-rays. So, for example, if we consider the case of collecting data for 576 views at 360° using continuous X-rays, the data for each channel of each view will be
It is the integral of the measured value while moving through 576 divided angles. Therefore, these data can be considered to be data with no blur, synthesized and integrated in the viewing direction using an X-ray waveform. If we look at this in frequency space, it becomes the frequency component of the original data without blur multiplied by the frequency component (transfer function) of the X-ray waveform. In order to estimate original data without blurring from data whose frequency characteristics have changed due to this X-ray waveform, it is necessary to use a transfer function T() which is the frequency component of the X-ray waveform that is known in advance.
It is obtained by multiplying the frequency characteristic of the obtained data by the inverse function I()=1/T(), and then performing an inverse Fourier transform on it. I() is called a restoration function. In the embodiment of the present invention, the same thing is done by performing inverse Fourier transform on the above-mentioned restoration function as it is by each means 6, 7, and 8, and synthesizing and integrating the data using the obtained function (only in the viewing direction). This is achieved without determining the frequency characteristics. Here, if there is a point where the restoration function diverges, complete restoration is not possible, and a restoration function that does not approximately diverge is used.

On the other hand, on the other hand, the correction may be made by using another similar function (for example, an appropriate high-frequency emphasis function) without actively considering the restoration function.

The interpolation of means 6 will be explained as follows. Continuous X-rays can be thought of as a square wave with a pulse width equal to one view time for each view. Therefore, the width of the rectangular X-ray pulse is set to 1
Considering sine wave data having a period, integrating this data over one period always yields 0, so the transfer function is 0. If the frequency in between is taken as the transfer function T by the rectangular X-ray pulse
() becomes as shown in Figure 2.

Therefore, although the inverse function I() diverges as shown by the broken line in FIG. 3, an appropriate restoration function must be selected to prevent this from happening.

As a result, the frequency component cannot be restored, so this frequency is called the limit frequency for restoration. Here, considering restoration up to this limit frequency, two points of data are required in one cycle to express this limit frequency.
Therefore, one view requires two points of data, but since the actual data is one view and one data, it can only be expressed up to a frequency of /2, where one cycle is two views wide. This frequency is called the Nyquist frequency. Therefore, in order to enable frequency expression up to the limit frequency, it is necessary to increase data by interpolating one more point between views. This interpolation is performed using the data of the opposing view.
Opposite view data refers to the case where X-rays pass through the subject from direction a to b as shown in Figure 4, and vice versa.
It refers to the relationship between two data when passing from b to a as shown in FIG. 5, and if even the opposite view data is included, it is possible to restore up to a frequency close to the limit frequency.

(Effects of the Invention) As explained above, according to the present invention, image deterioration due to blurring can be minimized, high-speed scanning can be performed, and high-quality tomographic images with less noise can be obtained using X-rays. A CT device can be realized.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram showing an example of an X-ray CT apparatus according to the present invention, and FIGS. 2 to 5 are operational diagrams. 1... X-ray tube, 2... Subject, 3... X-ray detector, 4... Data collection means, 5... Sensitivity correction means, 6... Inter-view interpolation means, 7... View direction correction means , 8...Data extraction means, 9...Log conversion means, 10...Preprocessing means, 11...Reconstruction means, 12...Display means.

Claims (1)

[Claims] 1 An X-ray tube and a detector are integrated to scan the area around the subject, obtain X-ray absorption data from various directions of the cross section of the subject, and calculate the cross-section of the subject by processing each of these data. In an X-ray CT device that obtains an image, a means is provided to apply correction only in the viewing direction using the data of each channel obtained by the scanning and a restoration function, and the data corrected only in the viewing direction is used. An X-ray CT apparatus characterized in that it obtains a tomographic image of a subject.