JPH04176446A - Grid - Google Patents

Abstract

PURPOSE:To enable generation of an image data carrying information on an image of high picture quality without noticeable noises attributed to a grid used in photographing by arranging radiation shielding matters at irregularly varying intervals. CONSTITUTION:Radiation 2 radiated from a radiation source 1 is applied to a storage phosphor sheet 11 via an object 3 and further via a grid 4. The grind lead 4a and aluminum 4b arranged alternately at intervals varied irregularly centering around four lines/mm. The radiation 2 is interrupted by the lead 4a and is applied to the sheet 11 being transmitted through the aluminum 4b. As a result, stripe patterns of irregular intervals centering around four lines/mm are recorded in the sheet 11 together with a radiation image of the object 3. This enables the prevention of aliasing noises from concentrating on a specified spatial frequency even when a radiation image taken and recorded using the grid 4 is read out at fixed sampling intervals thereby generating an image data carrying information on an image of high picture quality with aliasing noises lowered at least apparently.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a grid used in radiography for eliminating or reducing scattered radiation.

(Prior Art) It is practiced in various fields to read a recorded radiation image to obtain image data, perform appropriate image processing on this image data, and then reproduce and record the image. for example,
X-ray images are recorded using an X-ray film with a low gamma value designed to be compatible with subsequent image processing, and the X-ray images are read from the film on which they are recorded and converted into electrical signals. A system has been developed that can sell reproduced images with good image quality performance such as contrast sharpness and graininess by processing this electrical signal (image data) and then reproducing it as a visible image in a photocopy or the like. (Refer to Japanese Patent Publication No. 61-5193).

Also, by the applicant, Radiation! (X-rays, α-rays.

When irradiated with β rays, γ rays, electron beams, ultraviolet rays, etc., a portion 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 photostimulated luminescence is read in a charging manner to obtain image data, and based on this image data, a radiation image of the subject can be recorded on recording materials such as photographic materials, CRTs, etc. A radiation image recording and reproducing system that outputs visual images has already been proposed (Japanese Patent Laid-Open No. 5
No. 5-12429, No. 56-11395.

No. 55-163472, No. 5B-104845, No. 55-118340, etc.).

This system has the practical advantage of recording images over a much wider range of radiation exposure compared to conventional radiographic systems using silver halide photography. In other words, it is recognized that the amount of radiation exposure is proportional to the amount of emitted light that is stimulated to emit light due to excitation after accumulation, over a very wide range. Therefore, depending on various imaging conditions, radiation exposure Even if the amount varies considerably, the amount of stimulated luminescent light emitted from the stimulable phosphor sheet is read by the photoelectric conversion means by setting the reading gain to an appropriate value and converted into an electrical signal (image data). However, by using this image data to output a radiation image as a visible image on a display device such as a photographic material or a CRT, a radiation image that is not affected by fluctuations in radiation exposure can be obtained.

(Problem to be Solved by the Invention) When photographing and recording a radiation image of a subject on a recording sheet such as the above-mentioned X-ray film or stimulable phosphor sheet, four measures must be taken to prevent radiation scattered by the subject from irradiating the recording sheet. ，
A grid made of alternating materials such as lead that does not transmit radiation and materials that transmit radiation such as aluminum or wood at a fine pitch of about 0 lines/ml may be placed between the subject and the recording sheet. When taking images using a grid, the radiation scattered by the object is less likely to be irradiated onto the recording channel, so it is possible to improve the contrast of the radiation image of the object. is recorded.

On the other hand, a radiation image reading device that obtains image data by reading a radiation image photoelectrically reads light carrying radiation image information and uses the highest spatial frequency f required for image information.
It is configured to obtain digital image data by sampling at a sampling interval Δx-1/2fss corresponding to ss. The image data obtained in this way includes
In addition to useful information representing the subject image, noise caused by the grid image may be included even if the spatial frequency of the striped pattern (grid image) is higher than the necessary spatial frequency fss.

FIG. 5A is 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, imaging is performed using a grid of 4.0 lines/+101, and the spatial frequency of the grid image is 4 cycles/f.
The highest spatial frequency fss required for good reproduction of the radiation image of the subject is 2.5 Cyele.
/l! 111.

Figure 5B shows the highest spatial frequency f 5s-2,5cyele/++m necessary for good reproduction of the radiation image of the subject.
In order to obtain information on the following spatial frequency bands, the sampling interval Δx corresponding to fss-2°5cycle/IIm
-1/2 f 5s - 0,2mm, i.e. lll
FIG. 7 is a diagram for explaining the state of noise mixing when sampling is performed five times per l11.

In this case, noise called aliasing is mixed in at the position where the graph shown by the solid line in the figure (same as the graph in FIG. 5A) is folded back at f5s=2.5cycle/a+m. Therefore, in this case, the aliasing of the grid image with a spatial frequency of 4 cycles/■ is 1 cycle/ma
+ occurs.

FIG. 5C shows the sampling interval ΔX-1/2fss
It is a figure showing the spatial frequency characteristic of the radiographic image carried by the image data obtained by sampling at -0, 2-1.

Noise corresponding to the striped pattern is mixed in at a position of 1 cycle/si, and when a visible image of the same size is reproduced and recorded, a striped pattern of 1 cycle/aIm appears in the visible image. In this way, when observing radiographic images,
Even if the spatial frequency of the grid image is in an inconspicuous spatial frequency band, when image data is obtained by sampling, a striped pattern will appear in the conspicuous spatial frequency band, and the visible image reconstructed based on this image data will be very In some cases, the image became difficult to see.

In view of the above circumstances, it is an object of the present invention to provide a grid capable of generating image data carrying high-quality image information in which noise caused by the grid used during photography is not noticeable. It is.

(Means for Solving the Problems) A grid of the present invention for achieving the above object is a radiographic grid in which radiation shielding objects extending in a predetermined direction and radiation transmitting objects extending in the predetermined direction are repeatedly and alternately arranged. In a grid that is placed between a subject and a recording sheet on which a radiographic image of the subject is recorded and that removes or reduces scattered radiation directed toward the recording sheet, the radiation shields are arranged at irregularly varying intervals. It is characterized by the fact that

Here, the maximum interval of the irregularly fluctuating intervals or 172 or less of the sampling interval at which the radiation image recorded on the recording sheet is read at a predetermined sampling interval to obtain an image signal is determined. It is preferable to configure the above-mentioned grid at 10 (Function) In the present invention, the conventional grid is one in which radiation shielding materials such as lead are arranged at equal intervals, and the radiation shielding material photographed and recorded using this grid is Because images were read at regular sampling intervals, certain spatial frequencies (for example, the 6th
This was done with the focus on the fact that aliasing noise occurs at 1 cycle/mm) shown in Figure B and is noticeable, deteriorating the quality of the visible image.

In the grid of the present invention, radiation shielding materials such as lead are repeatedly arranged at irregularly varying intervals. Concentration of aliasing noise only in frequencies is prevented, and thus image data carrying high quality image information with at least apparently reduced aliasing noise is generated.

Therefore, the above-mentioned "irregular fluctuations" may be completely irregular fluctuations, but even if they are not completely irregular fluctuations, aliasing may occur when a visible image reproduced based on the above-mentioned image data is observed. There may be a regularity in which the frequency repeats at a sufficiently long period to avoid concentrating on a specific frequency and conspicuously conspicuous.

In the present invention, "irregular fluctuations" refers to a concept that includes these.

Here, as for the degree of "irregularly fluctuating intervals" in the grid of the present invention, in practical terms, for example, the maximum interval of the "irregularly fluctuating intervals" is recorded on the recording sheet. 1/1 of the sampling interval when obtaining an image signal by reading a radiation image at a specific sampling interval
The interval is selected to be 2 or less.

In addition, instead of performing imaging using a grid in which radiation shielding objects are formed at irregular intervals, imaging is performed using a conventional grid in which grids are formed at equal intervals, and the subsequent reading of the radiographic image is It is also possible to reduce aliasing noise by sampling at irregular intervals (see Japanese Patent Application No. 1-91240).

However, in this case, a random clock generator or a recording sheet conveyance speed modulator or the like is required, resulting in a problem that the entire system becomes complicated. Another problem is that it is necessary to always and reliably associate the direction of the striped pattern caused by the grid with the direction of irregular sampling, which requires time and attention.

Because the grid itself has irregularities, the present invention does not complicate the entire system, does not require special attention or effort, and can reliably make aliasing noise less noticeable. .

(Example) Hereinafter, an example of the present invention will be described with reference to the drawings.

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

Radiation 2 emitted from a radiation source 1 passes through a subject 3 and further passes through a grid 4 to irradiate a stimulable phosphor sheet 11 . In the grid 4, lead 4a and aluminum 4b are alternately arranged with irregularly varying pitches centered around 4 lines/a+n. Radiation 2 is lead 4
a is blocked, aluminum 4b is transmitted through sheet 1
Irradiate 1. Therefore, along with the radiographic image of the subject 3, striped patterns at irregular intervals centered around 4 lines/mm are accumulated and recorded on the sheet IF. The radiation 2a scattered within the subject 3 is obliquely incident on the grid 4 and is therefore blocked by the grid 4 or reflected by the grid 4, and the sheet 1
Therefore, a clear radiation image with less scattered radiation is accumulated and recorded on the sheet 11.

FIG. 2 shows a subject image (
FIG. 3 is a diagram showing a radiographic image consisting of a striped pattern (vertical stripes in the diagram) at irregular intervals centered on 4 stripes/a+a+ caused by a grid (shaded area in the diagram);

In this way, a 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 graph showing the spatial frequency characteristics of the radiation image shown in FIG. 2 in a direction perpendicular to the striped pattern (left-right direction in FIG. 2).

The striped pattern shown in the radiographic image shown in Figure 2 consists of 4 stripes and 10 stripes.
Since it is a striped pattern with irregular intervals centered at +m, the spatial frequency of this striped pattern does not appear only at a specific spatial frequency, but appears vaguely over a wide range centered at 4 cycles/mm. .

The broken line in the figure is the spatial frequency spectrum if there were no striped pattern.

FIG. 4 is a perspective view of an example of a radiation image reading device.

The stimulable phosphor sheet 11 on which a radiation image has been recorded is set at a predetermined position and is transported by a sheet conveyance means 15 such as an endless belt driven by a drive means (not shown).
It is transported (sub-scanning) in the direction of arrow Y at a constant speed. On the other hand, a light beam 17 emitted from a laser light source 16 is reflected and deflected by a rotating polygon mirror 18 that is driven by a motor 24 and rotates at high speed in the direction of the arrow. After passing through a focusing lens 19 such as an fθ lens, the light beam 17 is changed into an optical path by a mirror 20. Instead, it enters the sheet 11 at a constant speed Vx (n+m/se
e) to perform main scanning. Stimulated luminescent light 21 is emitted from the portion of the sheet 11 that is irradiated with the light beam 17, and the amount of stimulated luminescent light 21 corresponds to the radiographic image information that has been stored and recorded, and this stimulated luminescent light 21 is guided by a light guide 22. It is photoelectrically detected by a photomultiplier (photomultiplier tube) 23. The light guide 22 is made by molding a light-guiding material such as acrylic steel, and has a linear entrance end surface 2.
2a is arranged to extend along the main scanning line on the stimulable phosphor sheet 11, and is formed in an annular shape.
The light-receiving surface of the photomultiplier 23 is coupled to. The stimulated luminescent light 21 that enters the light guide 22 from the input end surface 22a travels through the interior of the light guide 22 through repeated total reflection, exits from the exit end surface 22b, is received by the photomultiplier 23, and is converted into a radiographic image. Expressing stimulated luminescence light 2
1 is converted into an electrical signal by the photomultiplier 23.

The analog output signal S output from the photomultiplier 23 has the highest spatial frequency f s in the spatial frequency band necessary to reproduce and output a good radiographic visible image.
Information in a spatial frequency band higher than s -2,5cycle/ +u+, especially information in the grid image 6 shown in FIG. 2, is also included, but the information in this grid image does not appear only in a specific spatial frequency; It is included in a fairly wide spatial frequency band centered around 4 cycles/am.

The analog output signal S is logarithmically amplified by a log amplifier 26, and then sampled at predetermined sampling intervals and digitized by an A/D converter 27 to obtain digital image data SD.

This image data SD is temporarily stored in the storage section 29.

This image data SD does not contain aliasing noise only at a specific spatial frequency, but includes its influence vaguely across a fairly wide spatial frequency band, so this aliasing noise is not noticeable on the visible image. Therefore, this image data SD carries high-quality image information.

The image data SD generated as described above is temporarily stored in the storage section 29 and then transferred to the image processing/reproduction device 50. The image processing/reproduction device 50 performs appropriate image processing on the transferred image data SD, and then reproduces and displays a visible image based on this image signal. Since the transferred image data SD is a signal in which the influence of the grid 4 is reduced as described above, a visible image with good image quality performance can be obtained.

Although an example in which the grid is applied to a system using a stimulable phosphor sheet has been described above, the grid of the present invention is not limited to a system using a stimulable phosphor sheet.
It can also be widely applied to systems that obtain X-ray image data from X-ray images recorded on X-ray film.

(Effects of the Invention) As described in detail above, the grid of the present invention is one in which radiation shielding objects are repeatedly arranged at irregularly varying intervals. When image data is obtained by reading a radiation image at a predetermined sampling interval, the image data does not carry aliasing only at a specific spatial frequency, and therefore this image data has high image quality with no noticeable aliasing noise. It carries image information of .

[Brief explanation of the drawing]

Figure 1 is a diagram showing an outline of an example of a radiation image capturing device. Figure 2 is a diagram showing an outline of an example of a radiographic image capturing apparatus. FIG. 3 is a graph showing the spatial frequency characteristics of the radiation image shown in FIG. 2 in the direction perpendicular to the striped pattern (left-right direction in FIG. 2). FIG. 4 is a perspective view of an example of a radiation image reading device using an embodiment of the radiation image data generation method of the present invention, and FIG. 5A is a diagram showing a radiation image (including a striped pattern). FIG. 5B is a graph showing the spatial frequency characteristics in the direction perpendicular to the striped pattern.
m) Diagram showing aliasing of T-sampled image data, Figure 5C with the same sampling interval 0.2 as in Figure 5B.
FIG. 3 is a diagram showing the spatial frequency characteristics of a radiation image carried by image data sampled in mm. 1...Radiation source 3...Subject 4...Grid 5...Subject image 6...Striped pattern
1■...Stormative phosphor sheet 17...Light beam 23...Photomultiplier 26...Log amplifier 27...A/D converter 2
9...Storage unit 5o...Image processing device first
Figure 2 Figure 3 Figure 4 Figure 5A Figure 5B Figure 5C

Claims (1)

[Claims] (1) Radiation shielding objects extending in a predetermined direction and radiation transmitting objects extending in the predetermined direction are repeatedly and alternately arranged between a subject and a recording sheet on which a radiographic image of the subject is recorded during radiography. A grid arranged to eliminate or reduce scattered radiation directed toward the recording sheet, characterized in that the radiation shields are arranged at irregularly varying intervals. (2) The maximum interval among the irregularly fluctuating intervals is 1/2 or less of the sampling interval at which the radiation image recorded on the recording sheet is read at a predetermined sampling interval to obtain an image signal. A grid according to claim 1, characterized in that: