JPH0792825B2 - Radiation image data generation method - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a radiation image data generating method for obtaining image data by reading a radiation image having a striped pattern corresponding to the grid obtained by imaging using a grid.

(Prior Art) It is performed in various fields to read a recorded radiation image to obtain image data, perform appropriate image processing on the image data, and then reproduce and record the image. For example,
An X-ray image is recorded by using an X-ray film having a low gamma value designed to be suitable for the subsequent image processing, and the X-ray image is read from the film on which the X-ray image is recorded and converted into an electric signal. , A system that can sell reproduced images with good image quality such as contrast, sharpness, and graininess by reproducing this as a visible image on a copy photograph after performing this electric signal (image data) image processing (See Japanese Patent Laid-Open No. 61-5192).

In addition, according to the applicant, radiation (X-ray, α-ray, β-ray, γ
Ray, electron beam, ultraviolet ray, etc.) causes a part of this radiation energy to be accumulated, and then irradiation with excitation light such as visible light causes stimulated emission depending on the accumulated energy. Radiation image of a subject such as a human body is temporarily photographed and recorded on a sheet-shaped stimulable phosphor by using a luminescent phosphor, and this stimulable phosphor sheet is scanned with excitation light such as laser light to stimulate emission. To generate image data by photoelectrically reading the resulting stimulated emission light, and based on this image data, output a radiation image of the subject as a visible image on a recording material such as a photographic light-sensitive material or a CRT. A radiation image recording / reproducing system has already been proposed (JP-A-55-12429).
Nos. 56-11395, 55-163472, 56-104645
No. 55-116340).

This system has the practical advantage of being able to record images over a very wide radiation exposure area compared to conventional radiographic systems using silver halide photography. In other words, in terms of radiation exposure dose, it has been recognized that the amount of emitted light that is stimulated and emitted by excitation after storage is proportional to the amount of radiation such as radiation over a very wide range, and therefore radiation exposure under various imaging conditions. Even if the amount fluctuates considerably, the amount of stimulated emission light emitted from the stimulable phosphor sheet is set to an appropriate reading gain, read by photoelectric conversion means, and converted into an electrical signal (image data). Then, by using this image data to output a radiation image as a visible image on a display device such as a photographic light-sensitive material or a CRT, it is possible to obtain a radiation image that is not affected by variations in radiation exposure.

(Problems to be Solved by the Invention) When capturing and recording a radiation image of a subject on a recording sheet such as the X-ray film or the stimulable phosphor sheet, the radiation scattered by the subject is prevented from being applied to the recording sheet.
There is a case in which a grid that does not transmit radiation at a fine pitch of about book / mm, for example, lead, and a grid that easily transmits radiation, such as aluminum or wood, are alternately arranged between the subject and the recording sheet. When the image is captured using the grid, the radiation scattered by the subject is less likely to irradiate the recording sheet, so the contrast of the radiation image of the subject can be improved, but on the other hand, a grid image with a fine striped pattern along with the subject image Is recorded.

On the other hand, a radiation image reading device that reads a radiation image to obtain image data photoelectrically reads light carrying radiation image information, and obtains the highest spatial frequency f required as image information.
Sampling is performed at sampling intervals Δx = 1 / 2fss corresponding to ss to obtain digital image data. The image data obtained in this manner includes not only useful information representing the subject image but also the above-mentioned grid even if the spatial frequency of the striped pattern (grid image) is higher than the required spatial frequency fss. There may be noise due to the image.

FIG. 6A is a diagram showing a graph showing a spatial frequency characteristic of a radiation image (including a striped grid image) recorded on a recording sheet in a direction orthogonal to the striped pattern.

Here, we shoot using a grid of 4.0 lines / mm,
Assuming that the spatial frequency of the grid image is 4 cycles / mm,
It is assumed that the highest spatial frequency fss required for good reproduction of the radiation image of the subject is 2.5 cycle / mm.

FIG. 6B shows a sampling interval Δx corresponding to fss = 2.5 cycle / mm in order to obtain information on the highest spatial frequency fss = 2.5 cycle / mm or less required for good reproduction of the radiation image of the subject. = 1/2 fss = 0.2 mm, that is, a diagram for explaining a mixed state of noise when sampling is performed 5 times per 1 mm.

In this case, the so-called aliasing noise is mixed in at the position where the graph shown by the solid line in the figure (the same as the graph in FIG. 6A) is folded back at fss = 2.5 cycle / mm. Therefore, in this case, the aliasing of the grid image with the spatial frequency of 4 cycle / mm occurs at 1 cycle / mm.

Fig. 6C shows the sampling interval Δx = 1 / 2fss = 0.2mm.
It is the figure which showed the spatial frequency characteristic of the radiographic image which the image data obtained by sampling with.

When the noise corresponding to the above striped pattern is mixed at the position of 1 cycle / mm and a visible image of the same size is reproduced and recorded, 1c is added to the visible image.
The result is a striped pattern of ycle / mm. in this way,
When observing a radiation image, even if the spatial frequency of the grid image is in the inconspicuous spatial frequency band, a striped pattern appears in the spatial frequency band that is easily noticeable when the image data is obtained by sampling, and based on this image data In some cases, the visible image that was replayed was very difficult to see.

In view of the above circumstances, an object of the present invention is to generate image data carrying high-quality image information from a radiographic image captured and recorded using a grid, in which noise due to the grid is inconspicuous. It is a thing.

(Means for Solving the Problems) A radiation image data generating method of the present invention is: a radiation image having a striped grid image obtained by imaging using a grid is read, and at least the striped pattern on the radiation image is read. To obtain a large number of image data corresponding to a large number of discrete points on the radiographic image, which have irregular intervals in a direction transverse to
It is characterized by sampling the read radiation image, image data finally obtained,
If it corresponds to an irregular sampling interval, when sampling the analog signal that represents the read radiation image into a digital signal, by setting the sampling period to an irregular period, discrete points with irregular intervals It is also possible to obtain a large number of image data corresponding to, or at the time of sampling for digitizing into a digital signal, sampling is performed at a sampling interval at equal intervals and a very large number of data corresponding to discrete points at equal intervals are obtained. It is possible to obtain a large number of image data corresponding to discrete points at irregular intervals by obtaining the image data and then re-sampling the image data.

(Operation) In the present invention, the above-mentioned aliasing has the sampling interval Δ.
Since x is kept constant, a specific spatial frequency (for example,
It occurred at 1 cycle / mm shown in Fig. 6B), and this was made paying attention to the fact that this deteriorates the image quality of the visible image.

The radiation image data generating method of the present invention is adapted to obtain image data corresponding to a large number of discrete points each having an interval in which irregular fluctuation is added to a predetermined interval in at least a direction crossing a striped pattern. Therefore, aliasing does not occur at a specific spatial frequency, and thus image data carrying high-quality image information with reduced aliasing noise is generated.

Therefore, the "irregular fluctuation" may be an irregular fluctuation generated by using, for example, a random number or the like, but even if the irregular fluctuation is not completely irregular as described above, it is reproduced based on the image data. When observing a visible image, there may be regularity that repeats in a sufficiently long cycle so that aliasing is prevented from concentrating on a specific frequency and standing out.
The "irregular fluctuation" in the present invention is a concept including these.

(Examples) Examples of the present invention will be described below with reference to the drawings.

FIG. 1 is a diagram schematically showing an example of a radiation image capturing apparatus. Here, an example of using the above-mentioned stimulable phosphor sheet as a recording sheet will be described.

The radiation 2 emitted from the radiation source 1 passes through the subject 3 and further through the grid 4, and the stimulable phosphor sheet.
Irradiate 11. In the grid 4, lead 4a and aluminum 4b are alternately arranged at a pitch of 4 lines / mm. Radiation 2
Is shielded by the lead 4a and penetrates the aluminum 4b to irradiate the sheet 11. Therefore, a striped pattern of 4 lines / mm is accumulated and recorded on the sheet 11 together with the radiation image of the subject 3. The radiation 2a scattered in the subject 3 is obliquely incident on the grid 4 so that the radiation is blocked by the grid 4 or reflected by the grid 4 and is not irradiated on the sheet 11, so that the sheet 11 is not irradiated with scattered radiation. A few clear radiation images are stored and recorded.

FIG. 2 is composed of a stripe pattern (vertical stripes in the figure) caused by the grid in a subject image (shaded area in the figure) accumulated and recorded in the stimulable phosphor sheet 11 by photographing using the grid. It is the figure which showed the radiographic image. In this way, the radiation image in which the subject image 5 and the striped pattern 6 are superimposed is recorded on the sheet 11.

FIG. 3 is a perspective view showing an example of a radiation image reading apparatus using an embodiment of the radiation image data generating method of the present invention.

The stimulable phosphor sheet 11 on which the radiation image set at a predetermined position is recorded is conveyed at a constant speed in the arrow Y direction by the sheet conveying means 15 such as an endless belt driven by a driving means (not shown) (sub scanning). ) Will be done. The stimulable phosphor sheet is set at a predetermined position so that the striped pattern (see FIG. 2) of the grid image of the radiation image extends parallel to the arrow Y direction. On the other hand, a light beam 17 emitted from a laser light source 16 is reflected and deflected by a rotary polygon mirror 18 driven by a motor 24 and rotating at a high speed in the direction of an arrow, passes through a converging lens 19 such as an fθ lens, and then a mirror.
The optical path is changed by 20 to enter the sheet 11 and a constant velocity Vx is applied in an arrow X direction substantially perpendicular to the sub-scanning direction (arrow Y direction).
The main scanning is performed at (mm / sec). From the position where the light beam 17 of the sheet 11 is irradiated, the stimulated emission light 21 of a light amount corresponding to the stored and recorded radiation image information is diverged, and this stimulated emission light 21 is guided by the light guide 22. It is detected photoelectrically by a photomultiplier (photomultiplier tube) 23. The light guide 22 is formed by forming a light guide material such as an acrylic plate, and is arranged so that the linear incident end face 22a extends along the main scanning line on the stimulable phosphor sheet 11. The light receiving surface of the photomultiplier 23 is coupled to the exit end surface 22b formed in a ring shape. The stimulated emission light 21 that has entered the light guide 22 from the incident end face 22a is
The photomultiplier 23 receives the emitted light from the exit end face 22b by repeating total reflection inside the photomultiplier 23, and the photomultiplier 23 converts the stimulated emission light 21 representing the radiation image into an electric signal.

The analog output signal S output from the photomultiplier 23 has the highest spatial frequency fss = 2. Of the spatial frequency bands necessary for reproducing and outputting a good radiation visible image.
Information on the spatial frequency band higher than 5 cycles / mm, especially information on the grid image 6 shown in FIG. 2 is also included. The information of this grid image is one of the factors that make the visible image difficult to see when observing the visible image, and is information that needs to be reduced or removed.

The analog output signal S is logarithmically amplified by the log amplifier 26, and then the analog clock S is amplified by the random clock generator 28 in the A / D converter 27.
Is sampled and digitized at the timing of each pulse of the random clock C _R output from the digital image data S _D. This image data S _D is temporarily stored in the storage unit 29.

FIG. 4 is a diagram for explaining the random clock C _R output from the random clock generator shown in FIG. The horizontal axis t is the time axis (sec).

FIG. 4 (A) shows a pulse train having a constant time interval Ts. This time interval Ts is image data carrying image information of the maximum spatial frequency fss = 2.5 (cycle / mm) or less required for good reproduction of the subject image from the stimulable phosphor sheet 11 (see FIG. 3). Is a time interval corresponding to a sampling interval Δx = 1 / 2fss = 0.2 (mm) for obtaining the following. When the speed Vx (mm / sec) of the main scanning is set as described above, Is.

FIG. 4B is a diagram showing a pulse train of the random clock C _R output from the random clock generator 28 (see FIG. 3). The timing of generation of each pulse is randomly distributed before and after the fixed time interval Ts.

As described above, the random clock C _R is input to the A / D converter 27 shown in FIG. 3, and is sampled at the timing of each pulse of the random clock C _{R in} the A / D converter 27 to obtain the image data S _{R. D} is generated.

Since this image data S _D is obtained at each timing that is randomly modulated as described above, aliasing of the grid image 6 (see FIG. 2) does not occur at a specific spatial frequency and high image quality is achieved. The image information is carried.

The above embodiment is an example in which the striped pattern of the grid image 6 is set in the radiation image reading apparatus of FIG. 3 so as to extend in parallel with the sub-scanning direction (direction of arrow Y in FIG. 3).
When this striped pattern is set so as to extend in the main scanning direction (arrow X direction), the sampling interval in the sub scanning direction needs to be randomly modulated. The random modulation in the sub-scanning direction is realized by, for example, randomly moving the mirror 20 so that the main scanning line randomly changes in the sub-scanning direction, or randomly changing the transport speed in the Y direction. It If the extending direction of the striped pattern does not coincide with either the main scanning direction or the sub scanning direction,
Alternatively, when the extending direction of the striped pattern is unknown, the sampling intervals of both main scanning and sub scanning are randomly modulated.

The image data S _D generated as described above is stored in the storage unit 29.
After being stored in the image processing / reproducing apparatus 50, The image processing / playback device 50 performs appropriate image processing on the transferred image data S _D , and then plays back and displays a visible image based on this image signal. Since the transferred image data s _D is a signal in which the influence of the grid 4 is reduced or removed as described above, it is possible to obtain a visible image with good image quality performance.

FIG. 5 is a diagram for explaining another embodiment of the radiation image data generating method of the present invention.

FIG. 5 (A) is a diagram showing a pulse train having a constant time interval ΔTs, which is considerably smaller than the sampling interval Ts required to reproduce the subject image. Further, FIG. 5 (B) is provided with a clock generator for generating a clock composed of a pulse train having a time interval ΔTs instead of the random clock generator 28 of FIG. 3, and the A / D converter 27 uses this clock. This shows that after sampling a large number of image data at the interval ΔTs to obtain a large number of image data, image data at random timing is extracted from the image data.

In this way, the sampling interval may be constant (ΔTs), as long as the finally obtained image data corresponds to a random sampling interval. Further, in addition to simply extracting from a large number of image data obtained at the sampling interval ΔTs, the average value of a plurality of image data before and after that is corresponded to a random interval as shown by the dashed line in FIG. It may be new image data. In the sub-scanning direction, this small sampling interval ΔTs can be realized by lowering the conveyance speed of the sheet 11 (see FIG. 3).

Although the embodiment using the stimulable phosphor sheet has been described here, the radiation image data generating method of the present invention is not limited to the system using the stimulable phosphor sheet, and the X-ray image recorded on the X-ray film may be used. The present invention can be widely applied to a system for reading a radiation image having a striped grid image obtained by imaging using a grid, such as a system for obtaining X-ray image data.

(Effects of the Invention) As described in detail above, the radiation image data generation method of the present invention has at least a plurality of intervals in which irregular fluctuations are added to the predetermined intervals in the direction crossing the striped pattern of the grid image. Since the image data corresponding to each discrete point is obtained, aliasing of the grid image does not occur at a specific spatial frequency. Therefore, this image data carries high-quality image information with reduced aliasing noise. It will be what you did.

[Brief description of drawings]

FIG. 1 is a diagram showing an outline of an example of a radiographic image capturing apparatus, and FIG. 2 is a diagram showing a subject image recorded in a stimulable phosphor sheet on a grid by performing image capturing using the grid. FIG. 3 is a perspective view showing an example of a radiation image reading apparatus using an embodiment of the radiation image data generating method of the present invention, and FIG. FIG. 6A is a diagram for explaining a random clock output from the random clock generator shown in FIG. 3, and FIG. 5 is a diagram for explaining another embodiment of the radiation image data generating method of the present invention. The figure shows a graph showing the spatial frequency characteristic of the radiation image (including the striped pattern) in the direction orthogonal to the striped pattern. FIG. 6B shows the sampling interval Δx = 1 / corresponding to the graph of FIG. 6A. Sampled at 2 x 2.5 = 0.2 (mm) Illustrates the aliasing of the image data, FIG. 6C is a diagram image data sampled at the same sampling interval 0.2mm and Figure 6B showed the spatial frequency characteristics of the radiation image bearing. 1 ... Radiation source, 3 ... Subject 4 ... Grid, 5 ... Subject image 6 ... Striped pattern, 11 ... Storable phosphor sheet 17 ... Light beam 23 ... Photomultiplier 26 ... Log amplifier , 27 …… A / D converter 28 …… Random clock generator 29 …… Storage section, 50 …… Image processing device

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Claims (1)

[Claims] 1. A radiographic image having a striped grid image obtained by imaging using a grid is read, and the radiographic image has irregular intervals at least in a direction crossing the striped pattern on the radiographic image. To obtain a large number of image data corresponding to a large number of discrete points above,
A method for generating radiation image data, comprising sampling the read radiation image.