JPH02263279A - Discriminating method for static grid direction - Google Patents

Abstract

PURPOSE:To automatically and accurately discriminate the direction of a static grid and to prevent the occurrence of a moire pattern by applying the Fourier transform to picture signals for picture element trains of two directions and discriminating the direction where the picture element train having the smaller peak electric power is extended as the direction where a radiation absorbing material is extended. CONSTITUTION:A radiation picture of a subject is photographed with use of a grid having many radiation absorbing materials extending in the fixed direction. Then the picture signals are obtained from a recording medium, then, the picture signals of the picture element trains of two directions which are approximately orthogonal to each other on the recording medium with either one of both trains set parallel to the direction of a static grid are converted by Fourier transform part 33 and 34 respectively. The peak detecting parts 35 and 36 obtain the peak electric power on a space frequency axis for the paired Fourier transformation data. Then the direction where one of two picture element trains that has the smaller peak electric power is discriminated as the direction of the static grid. Thus each block is shifted in the direction rectangular to the static grid direction in order to prevent the occurrence of a low frequency moire pattern.

Description

[Detailed Description of the Invention] (Industrial Application Field) The present invention obtains an image signal indicating radiographic image information of the subject by performing a reading process on a recording medium onto which radiation that has passed through the subject is irradiated via a stationary grid. The present invention relates to a method for determining the arrangement direction of the stationary grid based on this image signal.

(Prior Art) It is used in various fields to read a radiation image recorded on a recording medium to obtain an image signal, perform appropriate image processing on this image signal, and then reproduce and record the radiation image based on the signal. It is carried out in

For example, an X-ray image is recorded on an X-ray film with a low gamma value designed to be compatible with later image processing, and a microphotometer or the like is used to read the X-ray image from this XIl film and convert it into an electrical signal. After image processing is applied to this electrical signal (image signal), contrast sharpness is achieved by subjecting the signal to visible image reproduction of a photocopy or the like.

A system that can obtain reproduced radiographic images with excellent graininess has been developed (see Japanese Patent Publication No. Shofil-5193).

The applicant has also proposed radiation (X-rays, α-rays).

When irradiated with β rays, γ rays, electron beams, ultraviolet rays, etc., a part of this radiation energy is accumulated, and then when irradiated with excitation light such as visible light, stimulable fluorescence exhibits stimulated luminescence depending on the accumulated energy. A radiation image of a subject such as a human body is photographed and recorded on a sheet of stimulable phosphor using a stimulable phosphor, and this stimulable phosphor sheet is scanned with excitation light such as a laser beam. The resulting stimulated luminescence is read photoelectrically to obtain an image signal, and based on this image signal, a radiation image of the subject is recorded on a recording material such as a photographic material, or a CRT, etc. A radiation image recording and reproducing system has also been proposed in which the radiation image is output as a visible image on a display means (Japanese Patent Application Laid-open No. 12429/1983).

5B-11395, 55-163472, 5
No. G-104645.

No. 55-118340, etc.).

By the way, when a subject is irradiated with radiation to record radiographic image information of the subject, scattering (Comp) of the radiation occurs due to elastic collisions and electromagnetic interactions between the radiation and the subject material.
ton scattering and Thomson scattering). Scattered radiation generated by this scattering propagates in three-dimensional random directions and also irradiates a recording medium such as an X-ray film or a stimulable phosphor sheet. In this way, the recording medium
When the above-mentioned scattered radiation is irradiated in addition to the main transmitted radiation (which is responsible for the transmitted radiation image of the subject) that should be irradiated, the contrast and sharpness of the radiographic image recorded by the main transmitted radiation deteriorates. It ends up.

Therefore, various attempts have been made to eliminate the influence of the above-mentioned scattered radiation. As one such attempt,
For example, as shown in FIG. 2, a method is known in which a grid 4 that absorbs scattered radiation 2b is placed between the subject 1 and the recording medium 5 when the recording medium 5 is irradiated with the radiation 2 that has passed through the subject 1. It is being As shown in the tBB diagram, this grid 4 is made up of radiation absorbing materials 4a such as lead plates with a thickness of about 0.1 m or less, which are combined in a row with radiation transparent materials 4b such as wood or aluminum placed between them. This is what happens. By arranging the grid 4 as described above, the scattered radiation 2b traveling in random directions is absorbed by the radiation absorbing material 4a, and only the main transmitted radiation is irradiated onto the recording frame 5.

On the other hand, image signals obtained by reading a recording medium such as an X-ray film or a stimulable phosphor sheet on which radiation image information is recorded are digitized and then recorded and stored on, for example, an optical disk or a magnetic disk. There are many. In that case, it is common to compress the image data before recording and storing it. As such an image data compression method, for example, a? 1 encoding and orthogonal transformation are widely known, but in this case, for example,
As shown in Japanese Patent No.-181576, preprocessing to reduce the number of data may be performed.

This process of reducing the number of image data involves, for example, dividing all the pixels constituting the image into blocks for each of a plurality of adjacent pixels.
Representative value of image data for each pixel in one block (
(average value, median value, etc.) is used as the image data of that block. To explain specifically, 2×2-4 pixels are 1
When a block is formed and the representative value of the image data for these four pixels is used as the image data for the block, the number of image data is reduced to 1/4.

After undergoing such preprocessing, compressed image data is read out, and a radiographic image is reproduced based on that image data. A data count restoration process is performed.

(Problem to be Solved by the Invention) However, when the image data is processed to reduce or increase the number of pixels as described above, radiation image information is recorded on this image data through the aforementioned grid. If the radiation image is obtained from a recording medium that has been photographed (photographed), a low-frequency moiré pattern may occur in the radiation image reproduced based on the image data. Such a moire pattern naturally impairs the diagnostic performance of the reconstructed radiographic image.

In order to prevent the occurrence of this type of moiré pattern, a so-called Butsky device has been provided that moves the grid slightly back and forth in a direction parallel to the recording medium during radiographic imaging, but such devices are expensive. It becomes.

Therefore, several methods have been proposed in the past in which radiographic imaging is performed with the grid kept stationary, and then some processing is performed on the read image signal to prevent the occurrence of moiré patterns. As one such method, a method has been considered in which the phase of the aforementioned block division is shifted for each block line. This method is effective when the stationary grid is as shown in FIG. .

One square shown in Figure 8 (1) is taken as one pixel, 2
- Find an average value that is a representative value for each block X of every 4 pixels to reduce the number of data.

At this time, if the radiation absorbing material 4a of the stationary grid 4 shown in FIG. 3 extends in the vertical direction of FIG. 8 (1), the direction in which this radiation absorbing material extends (this is referred to as the stationary grid direction) Each block line extending at right angles to L1 r L2 r L3 * ・Old... Continuously and with a phase shift of half a block length for each block line, blocks X,...゜・・・・・・
・,X, ,X,□,X2. ......,Xl,,X
,,,X,,.

......, and the average value of the image data of 4 pixels in each block is set as the representative surface image data a of each block.
ll+ a12+ a13+ “””+ 821+ a
22+ a23°°゛...+ 831+ 832
h a 3m+ ...... and sample as shown in Figure 8 (2).Here, a lln
A specific example of a l□ is as follows.

a-(A+B+C+D)/4 a12-(E+F+G+H)/4 By shifting each block in the direction perpendicular to the stationary grid direction in this manner, the generation of the low-frequency moiré pattern described above is prevented.

However, when implementing this method, it is troublesome that the stationary grid direction must be set in a predetermined direction during radiographic imaging, and if the stationary grid direction is incorrectly set, the effect of preventing moire pattern generation may be affected. There is also the problem of not being able to get it.

If the stationary grid direction at the time of radiographic imaging can be determined from an image signal representing a single radiographic image, the above-mentioned problem can be resolved by setting the direction in which the blocks are shifted according to the determined direction. It will be possible. Therefore, it is an object of the present invention to provide a method that can automatically determine the stationary grid direction during radiographic imaging as described above.

(Means for Solving the Problem) The first stationary grid direction determination method of the present invention, as described above, performs radiographic imaging of a subject using a grid having a large number of radiation absorbing materials extending in a certain direction, Next, after performing a reading process on this recording medium to obtain image signals carrying radiation image information, the image signals are substantially orthogonal to each other on the recording medium, and one of the image signals is parallel to the stationary grid direction. Fourier transform the image signals for pixel rows in two directions, calculate the peak power on the spatial frequency axis for each of these two sets of Fourier transform data, and then fix the direction in which the pixel row with the smaller peak power extends. This feature is characterized in that it is determined that the orientation is in the grid direction.

Further, in the second stationary grid direction determination method of the present invention, one-dimensional prediction is performed on image signals regarding pixel columns in two directions similar to that in the first method, and the one with the smaller sum of the absolute values of prediction errors The direction in which the pixel rows extend is determined to be the stationary grid direction.

Note that a recording medium such as an X-ray film or a stimulable phosphor sheet is usually rectangular, and image information is read in the directions of two adjacent sides of the rectangular recording medium as the main scanning direction and the sub-scanning direction, respectively. Therefore, when radiographic imaging is performed, the stationary grid direction should be parallel to any side of the recording medium, and the pixel rows in two directions for performing Fourier transform or one-dimensional prediction should be divided into the main scanning direction pixel row and the sub-scanning direction pixel row. It is convenient to use pixel columns.

(Function) When a grid having a large number of radiation absorbing materials extending in a certain direction is used, a striped pattern by the grid is recorded on the recording medium. Therefore, when an image signal for a pixel row extending in a direction perpendicular to this striped pattern is subjected to Fourier transform, the spatial frequency component of this striped pattern appears as a uniquely large peak in the Fourier transformed data. Therefore, the direction in which this specific peak does not appear, that is, the direction in which the pixel row with the smaller peak power extends, can be regarded as the stationary grid direction.

Furthermore, for pixel rows extending in a direction perpendicular to the striped pattern, rapid density changes occur at both ends of each knitted portion of the pattern. Therefore, if one-dimensional prediction Δ-1 is performed on the image signal regarding this pixel column, extremely large prediction errors will occur frequently. Therefore, the direction in which the pixel row in which this does not occur, that is, the pixel row in which the sum of the absolute values of the pre-1j11 errors is smaller, can be regarded as the stationary grid direction.

(Embodiments) Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

FIG. 2 shows a stimulable phosphor sheet 5 using a stationary grid.
2 shows a state in which a radiation image of the subject 1 is recorded (photographed). As shown in the figure, a stimulable phosphor sheet 5 is placed so as to face a radiation source 3 such as an X-ray tube. A subject (subject) 1 is placed between the stimulable phosphor sheet 5 and the radiation source 3. This stimulable phosphor sheet 5 is as described in detail above. Furthermore, a stationary grid 4 is arranged between the stimulable phosphor sheet 5 and the subject 1. As shown in FIG. 3, this stationary grid 4 is made up of a large number of radiation absorbing materials 4a extending in one direction, which are combined in a row with radiation transmitting materials 4b such as wood or aluminum placed between them. In this stationary grid 4, the direction in which the radiation absorbing material 4a extends (grid direction) is
It is arranged so as to be parallel to any one side of the rectangular stimulable phosphor sheet 5.

In this state, the radiation source 3 is driven and the subject 1 is irradiated with radiation 2 such as X-rays. Then, the radiation (main transmitted radiation) 2a that has passed through the subject 1 is irradiated onto the stimulable phosphor sheet 5, and the sheet 5 stores a part of the energy of this main transmitted radiation 2a.

That is, on the stimulable phosphor sheet 5, transmitted radiation image information of the subject 1 carried by the main transmitted radiation 2a is stored and recorded. At this time, part of the radiation 2 is scattered as described above, but this scattered radiation 2b is absorbed by the radiation absorbing material 4a. Therefore, radiation image information is recorded on the stimulable phosphor sheet 5 mainly by the main transmitted radiation 2a.

Next, the stimulable phosphor sheet 5 is subjected to image information reading in the apparatus shown in FIG.

The stimulable phosphor sheet 5 is transported in the direction of arrow Y for sub-scanning by a sheet transport means 21 such as an endless belt. Further, the laser beam 23 as excitation light emitted from the laser light source 22 is deflected by an optical deflector 24 such as a galvanometer mirror, and the stimulable phosphor sheet 5 is mainly directed in the direction of the arrow X, which is substantially perpendicular to the sub-scanning direction Y. scan. The sheet 5 irradiated with the laser beam 23 in this way
Stimulated luminescent light 25 is emitted from the location corresponding to the radiation image information that has been stored and recorded, and this stimulated luminescent light 25 is collected by a condenser 26 and sent to a photomultiplier serving as a photodetector. It is photoelectrically detected by a plier (photomultiplier tube) 27.

The light collector 2B is made by molding a light guiding material such as an acrylic plate, and has a linear incident end face 2 [ta extending along the beam scanning line on the stimulable phosphor sheet 5. The light receiving surface of the photomultiplier 27 is coupled to the output end surface 26b formed in an annular shape. The stimulated luminescent light 25 that enters the light condenser 26 from the incident end face 20a travels through the light condenser 26 through repeated total reflection, exits from the output end face 26b, and is received by the photomultiplier 27. , the amount of the stimulated luminescent light 25 carrying the radiation image information is determined by this photomultiplier.
27.

The output signal S of the photomultiplier 27 is logarithmically amplified in a logarithmic amplifier 28 and then sent to an A/D converter 29.
Digitized by. The digitized read image signal S is sent to the data compression device 31 which performs data compression by the aforementioned pre-7I-1 encoding. The compressed image data Sf is sent to the image filing device 32, where it is recorded and stored on a recording medium such as an optical disk.

The data compression device 31 also performs preprocessing to reduce the number of image data described above, and in this preprocessing, the phase of block division is changed to prevent the occurrence of the low frequency moiré pattern described above. is shifted for each block line. This process is performed by the grid direction determination signal Sd sent from the stationary grid direction determination device 30.
Based on , the blocks are shifted in a direction perpendicular to the stationary grid direction.

Hereinafter, a first part showing the stationary grid direction discriminating device 30 in detail will be described.
Determination of the stationary grid direction will be described in detail with reference to the drawings. The main scanning direction Fourier transform unit 33 of the stationary grid direction discriminating device 30 converts the input image signal S into
An image signal for one pixel row Lx (see FIG. 5) extending in the main scanning direction X is extracted, and the image signal is Fourier transformed. The Fourier transform data FX obtained thereby is sent to the peak detection section 35. On the other hand, the sub-scanning direction Fourier transform unit 34 extracts an image signal for one pixel column Ly (see FIG. 5) extending in the sub-scanning direction Y from the input image signal S, and converts the image signal into a Fourier transform. The Fourier transform data Fy obtained thereby is sent to the peak detection section 36.

The above two types of Fourier transform data FW and Fi1 are as shown in FIGS. 6(1) and (2), for example.

That is, if the stationary grid 4 is arranged so that the radiation absorbing materials 4a are parallel to the sub-scanning direction Y at the time of radiographic imaging, the pixel row Lx in the direction across these radiation absorbing materials 4a is Fourier transform data F
As shown in FIG. 6(1), a peak component is generated in x due to the regularly arranged radiation absorbing materials 4a. The Fourier transform data Fx in FIG. 6(1) is, for example,
This is a case where the arrangement pitch of the radiation absorbing material 4a is 40/C1, and correspondingly, the spatial frequency component of 4 cycles/-1 shows a sharp peak. On the other hand, in the Fourier transform data Fy for the pixel row Ly in the direction parallel to the radiation absorbing material 4a, no sharp peak component due to the above-mentioned cause appears.

The peak detection units 35 and 3B each calculate a peak power value P for the input Fourier transform data Fx and Fy.
r and Py are detected, and a signal indicating it is sent to the grid direction determining section 37. This grid direction determination unit 37 compares the peak power values Px and Py, and if Px > Py, the stationary grid direction is the sub-scanning direction 71, whereas if Px<Py, the stationary grid direction is the main scanning direction X. Decide that there is. A signal S indicating the stationary grid direction determined in this way
d is input to the data compression device 31 as described above.

Next, another embodiment of the present invention will be described with reference to FIG. FIG. 7 shows a stationary grid direction determining device 40 that can be used in place of the stationary grid direction determining device 30 in the system shown in FIG. Main scanning direction one-dimensional element Δ11 error accumulating unit 41 of this discriminator 40
extracts the image signal for the pixel row LX shown in FIG. 5 from the input image signal S, sequentially calculates the one-dimensional prediction error regarding the image signal, and then
The total sum ΣεX of absolute values of 1 error is determined. On the other hand, the sub-scanning direction one-dimensional prediction error accumulating unit 42 extracts the image signal for the pixel array Ly shown in FIG. 5 from the input image signal S, and sequentially calculates the one-dimensional prediction error regarding the image signal. Then, the total sum Σεy of the absolute values of the prediction errors is determined.

The one-dimensional prediction error accumulating units 41 and 42 each send a signal indicating the sum of absolute values of the prediction errors Σεx1Σεy to the grid direction determining unit 43. This grid direction determining section 43
compares the value of ΣεX and the value of Σεy, and ΣεX〉Σεy
If so, the stationary grid direction is Σε opposite to the sub-scanning direction 71
If x x Σεy, the stationary grid direction is determined to be the main scanning direction X.

As mentioned above, by doing so, the stationary grid direction can be correctly determined.

Note that instead of calculating the sum of absolute values of prediction errors Σεx1Σεy, it is also possible to calculate the sum of square values of prediction errors and determine the stationary grid direction based on the magnitude relationship thereof. In this case as well, since the magnitude relationship of the total sum of absolute values of prediction errors is indirectly investigated, such a method is also included in the method of the present invention.

Furthermore, in the above two embodiments, the image signals for the pixel rows Lx and Ly near the center of the stimulable phosphor sheet 5 are extracted and used for determining the direction of the stationary grid. In order to ensure that the specificity of the Fourier transform data or prediction error caused by the radiation absorbing material 4a appears more reliably than that of the image signal that is not affected by the radiation absorbing material 4a, the original It is preferable to extract two pixel columns for an image region in which there is little fluctuation in the low frequency component of the image signal representing the subject. To do this, for example, divide the image into several blocks and extract two pixel sequences from the block with the smallest variance of the image signal, or from the block with the smallest difference between the maximum and minimum values of the image signal. good.

Furthermore, in the present invention, each image data for two pixel columns may be thinned out at a predetermined ratio, and after reducing the number of data, the Fourier transform data or prediction error may be obtained.

Furthermore, in the embodiments described above, a stimulable phosphor sheet is used as a recording medium for radiographic image information, but the method of the present invention is capable of producing radiation from conventionally known silver halide photographic film for X-ray photography. The same method can be used when reading an image to obtain an image signal.

(Effects of the Invention) As explained in detail above, according to the method of the present invention, the direction of the stationary grid can be automatically and accurately determined, so the installation direction of the stationary grid can be strictly controlled when radiographic images are taken. Even if the stationary grid is not used, it is possible to prevent the occurrence of a moire pattern through various subsequent processes, and therefore radiographic imaging using a stationary grid can be performed more efficiently and easily than in the past.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing an example of an apparatus for carrying out the first method of the present invention, FIG. 2 is a schematic diagram showing radiographic imaging using a static grid according to the present invention, and FIG. 3 is a static diagram FIG. 4 is a partial perspective view showing an example of a grid, FIG. 4 is a schematic perspective view showing an example of a device for reading image information from a stimulable phosphor sheet, and FIG. 5 is an explanation explaining extraction of two pixel columns according to the present invention. Figure 6 is a graph showing an example of Fourier transform data obtained in the first method of the present invention, Figure 7 is a block diagram showing an example of an apparatus implementing the second method of the present invention, and Figure 8 is a graph showing an example of Fourier transform data obtained in the first method of the present invention. FIG. 3 is an explanatory diagram illustrating image data number reduction processing according to the present invention. S... Image signal Sd... Stationary grid direction discrimination signal 1... Subject 2... Radiation 3... Radiation source 4... Stationary grid 4a... Radiation absorbing material 5... Accumulability Phosphor sheet 21... Sheet transport means 22... Laser light source 23... Laser beam 24... Optical deflector 27... Photo multiplier 28... Logarithmic amplifier 29... A/D Converter 30.40... Stationary grid direction determination device 33...
Main scanning direction Fourier transform section 34...Sub-scanning direction Fourier transform section 35.3B...Peak detection section 37.43...Grid direction determination section 41...Main scanning direction one-dimensional prediction error calculation section 42 ...
Sub-scanning direction one-dimensional preparatory table 71-1 error accumulating section Fig. (2 cycles/mm) Fig. d Fig.

Claims (2)

[Claims] (1) Radiation that has passed through the subject is irradiated onto a recording medium through a stationary grid having a large number of radiation absorbing materials extending in a certain direction, and from the recording medium on which radiation image information of the subject is recorded, an image signal carrying the image information is transmitted. After obtaining, among the image signals, substantially orthogonal to each other on the recording medium,
Image signals for pixel rows in two directions, one of which is parallel to the fixed direction, are Fourier-transformed, and the peak power on the spatial frequency axis is calculated for each of the two sets of Fourier-transformed data obtained in this way. and determining that the direction in which the pixel column with the smaller peak power extends is the direction in which the radiation absorbing material extends. (2) Radiation that has passed through the subject is irradiated onto a recording medium through a stationary grid having a large number of radiation absorbing materials extending in a certain direction, and an image signal carrying the image information is obtained from the recording medium on which radiation image information is recorded. After that, among the image signals,
One-dimensional prediction is performed on image signals for pixel columns in two directions on the recording medium that are substantially orthogonal to each other, one of which is parallel to the certain direction, and the one with the smaller sum of absolute values of prediction errors. A method for determining a stationary grid direction, characterized in that the direction in which the pixel row extends is determined to be the direction in which the radiation absorbing material extends.