JPH0116493B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an image reconstruction method in a computer tomography apparatus that is capable of correcting distortions caused by misalignment of a radiation detector in a computer tomography apparatus on a reconstructed image.

A so-called computerized tomography (CT) device, which is known as a cross-sectional inspection device using X-rays, etc., is an A radiation source 1 and an X-ray detector 2 formed of a plurality of X-ray detection elements arranged in parallel for detecting the X-rays are faced to each other with the subject P in between, and the X-ray source 1 and the X-ray detector 2 to the subject P
X-ray projection data is collected in various directions on the cross-section of the object by moving the circuit in the same direction at the same angular velocity, and after collecting sufficient data, this data is analyzed with an electronic computer. The X-ray absorption rate corresponding to each position in the cross section of the object is calculated, and a gradation level corresponding to the absorption rate is given to reconstruct image information in the cross section of the object, Clear tomographic images can be obtained from soft tissues to hard tissues.

The X-ray detector 2 is configured, for example, as a radiation detector that includes a large number of radiation detection cells each forming an ionization chamber, and is filled with a high-pressure gas such as Xe (xenon). The energy of the rays is detected as ionization current, and this is output as detection data by X-ray projection.

That is, when collecting this X-ray projection data, X-rays incident on the path (referred to as the "X-ray path") connecting each radiation detection cell constituting the ionization chamber and the X-ray source 1 are used. The energy of the ray is detected as an ionization current, this is integrated for a predetermined time, the integrated value is discharged in a discharge circuit with a predetermined time constant, and the discharge time value is used as the X-ray projection data for each X-ray pass. It is something to do. When data collection for all X-ray passes at one angular position is completed, data collection for each X-ray pass at the next angular position is completed. etc. completely disappear, and if it is not completed, it will appear as an error in the next data collection. The resolution of the reconstructed image depends on the sensitivity of the X-ray detector 1,
Since it is determined by resolution, a radiation detector with fast recovery time, high sensitivity, and high resolution must be used to obtain excellent CT image information.

FIG. 2 shows an example of a conventionally used radiation detector. In the figure, reference numeral 3 denotes a container body that houses an electrode group, and 4 a lid that closes the opening of the container body 3. The container body 3 and lid 4 are designed to maintain sufficient strength and airtightness against the high pressure gas (for example, Xe gas) filled inside. This radiation detector is a fan beam X-ray
Corresponding to the spread angle θ of Fx, the side wall 3 on the incident side
A portion 3b of a is made thinner than other portions so that sufficient X-ray energy can reach the internal electrode group. FIG. 3 shows a cross section along the plane taken along the line A-A' shown in FIG.
It turns out that there is.

FIG. 4 shows a state in which the electrode plates 5 and 6 are inserted into the electrode support plate 7, which is provided with grooves for arranging the electrode plates 5 and 6 at predetermined pitches. The signal electrode plate 6 is a bias electrode plate for high voltage. 8 is a lead wire for applying high voltage to the bias electrode plate 6, and 9 is a lead wire for applying high voltage to the bias electrode plate 5.
This is a lead wire for extracting the signal current from the terminal to the outside. These are the signal electrode plate 5 and the adjacent 2
The bias electrode plates 6 constitute a set of radiation detection cells.

FIG. 5 shows B-B shown in FIG. 2 when the electrode plates 5 and 6 shown in FIG. 4 are placed inside the container body 3.
This figure shows a cross section taken along a plane taken along line B' in the direction of the arrow, and it can be seen that electrode plates 5 and 6 are arranged alternately inside the container body 3.

By the way, in order to obtain excellent CT images, the X-ray source and radiation detector must be accurately placed at predetermined positions. That is, radiation detectors are arranged to cover the spread range of the fan beam of the X-ray source, and each
The radiation detection cell must be oriented toward the focal point of the fan beam x-rays so that it can detect radiation in the line path.

That is, in the mathematical method of image reconstruction of a CT apparatus, it is usually assumed that the geometrical conditions of the X-ray source and the radiation detector are all ideal. For example, assume that the center of the Fan beam x-ray is aligned with the center channel of the radiation detector.

However, in reality, it is impossible to align the center of the beam with the center channel of the radiation detector, and the beam center usually deviates to within a decimal point of the channel width.

If this misalignment occurs, an error will occur in the positional correspondence of the collected X-ray projection data, and this error will cause a problem in that the image will be distorted during image reconstruction.

The present invention was made in view of the above circumstances, and uses a pin phantom with a thin wire at the center.
A pin phantom is arranged so that the wire is located at the center of rotation of the radiation source and the radiation detector in a direction perpendicular to the object slice plane by the X-ray beam,
The projection data of this pin phantom is detected by a radiation detector, and based on the detected data, the amount of deviation between the center position of the pin phantom and the channel at the center of the radiation detector is determined, and this is recognized as the amount of alignment deviation of the radiation detector. However, by adding correction corresponding to the amount of misalignment during image reconstruction, the above defects can be removed, and a reconstructed image without distortion can be obtained without adjusting the position until the amount of misalignment is completely reduced to zero. The computer that was
The present invention aims to provide an image reconstruction method in a tomography device.

Hereinafter, one embodiment of the present invention will be described with reference to FIG.

FIG. 6 is a diagram showing a method of measuring the amount of alignment deviation in the apparatus of the present invention, in which 1 indicates an X-ray source;
2 is a multi-channel radiation detector. O is the rotation center of these X-ray source 1 and radiation detector 2, and a pin fantom Pf made of a straight wire with a finite diameter is positioned at this rotation center and the slice plane (projection) by the fan beam X-rays is (plane) perpendicular to the surface. The exposure point of X-ray source 1 (focal point in the case of an X-ray tube) and the radiation detection cell of radiation detector 2
The center of the Ds group is adjusted at the time of manufacture of the CT apparatus so that it faces on a straight line A via the rotation center O, but since it is difficult to make them completely coincident, the adjustment is only made to a certain extent. Therefore, misalignment remains.

When X-ray radiation is performed from the X-ray source 1 in this state, the radiation detector 2 obtains X-ray projection data of the pin phantom Pf. That is, since this X-ray projection data spreads because the X-ray beam from the X-ray source 1 is a fan beam Obtained by cells. Therefore,
Among the radiation detection cells of all the channels constituting the radiation detector 2, the X-ray projection data detected by the radiation detection cells of the channels on the X-ray path passing through the pin fantom Pf undergoes X-ray attenuation as shown in FIG. The radiation detection cells on the other X-ray paths do not receive X-ray attenuation, so they exhibit a high value N.

Therefore, based on these collected data, we CT
Calculated by the device's calculator. Calculation accuracy is 1/5 of the accuracy of channel width (width of radiation detection cell)
~1/10 is obtained.

Since there must be two channels that exhibit half value, the center channel of the two channels is said to be the true alignment point (channel).
Generally, this type of radiation detector has at least several hundred channels, and the width of each channel is constant at a few millimeters or less, so if the center channel is found with the above accuracy, the amount of misalignment of the channel that should be the original center can be determined. Determined numerically.

The amount of alignment deviation obtained in this way is stored in the memory of the computer, and is stored during image reconstruction processing, which proceeds from preprocessing of X-ray projection data collected at the time of imaging → convolution calculation → back projection. The amount of misalignment thus determined is read out and corrected accordingly. In other words, there is a process in which the geometrical conditions of the X-ray beam are taken into consideration during the back projection process in the image reconstruction process, so at this time the amount of alignment deviation is input and corrected to correct the alignment deviation. Remove the distortion of the reconstructed image based on .

In this way, a wire with a finite diameter is placed at the center of rotation of the X-ray source that emits Fan beam X-rays and the multi-channel radiation detector that detects the X-rays, in a direction perpendicular to the slice plane of the subject by the Fan beam X-rays. An object to be examined is placed, and the X-ray projection data of this object is detected and collected by the radiation detector.
A radiation detector located on a line passing through the X-ray source and the center of rotation is obtained by determining the distribution state of the data of the object from the line projection data based on the correspondence with the channel in which the data was detected. In addition, in the calculation process for this channel and image reconstruction processing, the amount of deviation from the channel passing through the X-ray source and the center of rotation, which is set in advance, is determined and saved as correction data, and image reconstruction is performed. During processing, this correction data is used to correct the actual positional deviation of each channel from the relationship between the Fan beam X-ray and the normal position of each channel of the radiation detector determined in the image reconstruction process. Since image distortion is removed, it is possible to prevent any misalignment from affecting the image, and once the correction data is obtained, it can be used as is from now on, making it easy to obtain high-quality images. It is possible to provide an image reconstruction method for a computer tomography apparatus that can perform the following steps.

Note that the present invention is not limited to the embodiments described above and shown in the drawings, and can be implemented with appropriate modifications within the scope of the gist thereof. For example, in the above explanation, a pin phantom is used to determine the amount of alignment deviation. However, it is also possible to carry out the experiment using other subjects that can serve as a reference.

[Brief explanation of drawings]

Figures 1a and b are principle diagrams for explaining an example of a CT device, Figures 2 to 5 are diagrams for explaining an example of the structure of a radiation detector, and Figure 6 is an alignment diagram for explaining an example of the structure of a radiation detector. FIG. 2 is a diagram for explaining a quantity data collection method. 1...X-ray source, 2...Radiation detector, Ds...
Radiation detection element per channel, Pf...pin phantom.

Claims (1)

[Claims] 1 A radiation source that emits radiation in the form of a fan beam and a multi-channel detector that detects the radiation are arranged facing each other across the tomographic plane of the object, and they are moved at the same angular velocity around the tomographic plane of the object. In this apparatus, the apparatus rotates and moves in the same direction, collects radiation projection data from various directions, calculates radiation absorption values at each position of the tomographic plane from this, and reconstructs an image of the tomographic plane. , a linear object with a finite diameter is arranged at the center of rotation in a direction perpendicular to the plane of radiation projection of the object by the fan beam-like radiation, and projection data of this object is collected and based on these projection data. The distribution state of the data of the object to be inspected is determined based on the correspondence between the data and the detected channel, and from this, the channel of the radiation detector located at the center line of the fan beam-like radiation is determined, and from this, In the calculation process for image reconstruction processing, the amount of deviation from the normal position of the channel that should face the radiation source via the rotation center set in advance is determined, and this is stored as correction data, and the image is reconstructed. An image reconstruction method for a computer tomography apparatus, characterized in that during processing, the correction data is used to correct the amount of positional deviation so that the image is the one that should be obtained when each channel is at a normal position.