JPS5983487A - Subtraction processing method of radiant ray picture - Google Patents

Abstract

PURPOSE:To improve the picture quality, by subtracting digital data of two radiant ray pictures, extracting the image and applying a picture frame around the subtraction picture. CONSTITUTION:The two radiant ray pictures having at least a part differing in picture information are stored on separate storage fluorescent sheet, an stimulated light is scanned onto each storage fluorescent sheet, and the radiant ray pictures, 3A, 3B are read out from the sheet by accelerated illumination. The subtraction picture 3C is obtained by making the position of markers 8A, 8B, 8A' and 8B' coincident and applying subtraction between digital data corresponding to the two pictures. A picture frame 7 and spots 9A, 9B having the degree of gray corresponding to that of the average gray of the subtraction picture 3C are formed on the unnecessary part 6 and the marker images 8A'' and 8B'' at the circumference of the subtraction picture 3C so as to obtain the subtraction picture 3D.

Description

[Detailed description of the invention]
Then, a radiation image is recorded on this, and then excitation light is irradiated to cause the radiation image to be stimulated to emit light from the stimulable phosphor sheet, and this stimulated emitted light is photoelectrically read by a photodetector. The present invention relates to a method for creating an image frame during subtraction processing in a radiographic image recording and reproducing method for reproducing a captured image signal as a visible image.

When certain types of phosphors are irradiated with radiation (X-rays, alpha rays, beta rays, gamma rays, ultraviolet rays, etc.), some of this radiation energy is accumulated in the phosphors, causing them to emit visible light. It is known that when irradiated with excitation light such as, a phosphor exhibits stimulated luminescence depending on the accumulated energy, and a phosphor exhibiting this property is called a stimulable phosphor.

Using this stimulable phosphor, a radiation image of the human body, etc. is recorded on the stimulable phosphor provided on a sheet, and this phosphor sheet is scanned with excitation light such as a laser beam to make it shine. Radiation that generates stimulated emission light, photoelectrically reads out the obtained stimulated emission light to obtain an image signal, and outputs it as a visible image to a recording material such as a photographic light-sensitive material, CI'tT, etc. based on this image signal. An image record reading method has already been proposed by the applicant. (Unexamined Japanese Patent Publication No. 5-124.29, 56-1
No. 1395 etc. ) This method has the very practical advantage of being able to record images over a much wider radiation exposure latitude compared to conventional radiographic systems using silver halide photography. In other words, in stimulable phosphors, it is recognized that the amount of emitted light that is stimulated and emitted by excitation after accumulation is proportional to the amount of radiation exposure over an extremely wide range, and therefore, depending on various imaging conditions, radiation exposure Even if the amount fluctuates considerably, the amount of emitted light is read by the photoelectric conversion means by setting the read gain to an appropriate value and converted into an electrical signal, and this electrical signal is used to record on photographic light-sensitive materials, etc. A radiation image that is not affected by variations in radiation exposure can be obtained by outputting the radiation image as a visible image on a display device such as a material or a CRT.

In addition, according to this system, after converting the radiation image stored and recorded in the stimulable phosphor into an electrical signal, appropriate signal processing is performed, and this electrical signal is used to display a recording material such as a photographic light-sensitive material such as a CRT. By having the apparatus output as a visible image, a very large effect can be obtained in that a radiographic image with excellent suitability for observation and interpretation (diagnosis suitability) can be obtained.

In this way, in a radiation imaging system using a stimulable phosphor, the reading gain can be set to an appropriate value to photoelectrically convert the stimulated luminescence light and output it as a visible image. Fluctuations in radiation exposure amount due to fluctuations in tube voltage or MAS value, fluctuations in sensitivity of stimulable phosphors, fluctuations in sensitivity of photodetectors, changes in exposure amount due to subject conditions, or changes in radiation transmittance depending on the subject. Even if the energy stored in the stimulable phosphor differs due to various factors, or even if the amount of radiation exposure is reduced, it is possible to obtain radiographic images that are not affected by fluctuations in these factors. Furthermore, by first converting the luminescent light into an electrical signal, and then applying appropriate signal processing to this electrical signal, it is possible to produce radiation that is suitable for any diagnostic site required by the observer, such as the chest or heart. It is possible to obtain images and improve the aptitude for observation and interpretation.

By the way, two or more pieces of X-ray film can be used to image specific structures of the human body (e.g. organs, bones, etc.).
Conventionally, a subtraction processing method has been known in which images of blood vessels, etc.) are extracted, and a specific structure can be diagnosed more accurately using the extracted images. Here, subtraction processing methods can be roughly divided into the following two methods. In other words, X
The so-called temporal subtraction processing method extracts specific structures by subtracting (subtraction) an X-ray image in which no contrast agent has been injected from the X-ray image, and X-rays with different energy distributions for the same subject. irradiation, thereby causing an image of a specific structure to exist between the two X-ray images, which results from the specific structure having a specific X-ray energy absorption property, and then between the two X-ray images. The so-called Energy Zabu extracts an image of a specific structure by performing subtraction with appropriate weighting!・A traction processing method is known.

However, when trying to obtain a subtraction image using the X-ray photographic film, the X-ray photographic film becomes
Generally, it has a non-linear gradation and a narrow latitude, so it has the disadvantage that it is not possible to obtain a high-quality subtraction image. Moreover, in order to obtain a subtraction image using X-ray photographic film, one X-ray image is inverted, two X-ray photographic films are manually superimposed, and a third photographic film is created to show the difference. This is done by imprinting images. Therefore, it is difficult to accurately superimpose the X-ray images taken on two X-ray films and remove images other than the diagnostic structure, and the problem is that it takes a lot of time and effort to align them. . Therefore, although it cannot always be an effective diagnostic method, it is a diagnostic method that has been barely used only in special cases.Therefore, the above-mentioned subtraction treatment method using radiographic film is not commonly used. The current situation is that this has not been done.

In recent years, a so-called digital subtraction processing method (Digital Radiography) has been developed from the viewpoint that if the image data is a digital value, subtraction can be performed by linear computer processing without using the troublesome and non-linear photographic subtraction method. (hereinafter referred to as DR) has started to attract attention.

By the way, in the radiation imaging system using a stimulable phosphor sheet that has already been proposed by the applicant,
The radiation image stored and recorded on the stimulable phosphor sheet is read out using excitation light, the stimulated luminescence light obtained by this readout is detected by a photodetector, and the electrical signal obtained by this detection is converted into a digital signal. Conversion and further,
After various signal processing is performed, the final image can be outputted to various output devices, so that a radiation imaging system using such a stimulable phosphor sheet can also be subjected to various digital processing.

Therefore, in addition to having the advantage of the above-mentioned DR, that is, digital processing is possible, this radiation imaging system also has the advantage that the beam diameter of the excitation light (laser light) that scans the stimulable phosphor sheet. make it smaller,
The number of pixels per unit area is increased, and the final output of the image take after subtraction processing and various image processing can be directly recorded on the photosensitive material t1 such as silver salt, which is superior to conventional DTl. In principle, it is possible to obtain a clear subtraction image with a spatial resolution that is lower than the discrimination resolution of human vision. Since there is no technical problem in increasing the area of the sheet, it is possible to obtain subtraction images over a large area covering a wide range of parts of the human body at once.
It has a major feature that conventional I)R does not have.

However, it has been found that when an attempt is made to obtain a Zaff]-Lumpf image in a radiation system using a stimulable phosphor sheet as described above, the following problems occur.

That is, in a radiographic imaging system using stimulable phosphor sheets, two (or three or more) different stimulable phosphor sheets are used! - are inserted into the imaging table sequentially or simultaneously to capture radiographic images to be subtracted,
After that, in the process of individually inserting the stimulable phosphor sheet 1 into the reading device and reading out the radiation image to be subtracted, the mechanical precision of all devices involved in imaging and reading was improved. However, positional and rotational deviations occur between images that should be subtracted, and as a result, images that should be erased during subtraction may not be erased, or conversely, images that should be extracted may be erased, resulting in false images and inaccurate images. It was found that it was not possible to obtain a subtraction image, which caused serious problems in diagnosis.

When such a discrepancy occurs between the radiation image information stored and recorded on the stimulable phosphor sheet, the radiation image is stored and recorded in the stimulable phosphor as a latent image, so the X-ray image becomes a visible image. Unlike the case of X-ray photographic film, which can be taken as a photograph, it is not possible to match two X-ray photographs by visual inspection, making misalignment correction extremely difficult.

Furthermore, even if positional and rotational deviations that occur between two radiographic images can be detected by some means, the take of the read radiographic images must be corrected (if conventionally known arithmetic processing is performed, especially when correcting rotational deviations) It takes a lot of time and poses a huge problem in practice.

In order to solve this problem, the present applicant has filed a patent application filed in
In the specification of No. 45473, when recording a radiation image on a stimulable phosphor sheet, a marker providing a reference line or a reference point is simultaneously recorded, and the radiation image read out from the stimulable phosphor sheet is recorded at the same time. Detect the positional coordinates of the reference point or reference line from the digital image data, calculate the positional deviation and rotational deviation between the two radiographic images to be subtracted from the detected positional coordinates, and based on this positional deviation and rotational deviation. Rotate either one of the radiographic images to the digital image data,
We proposed a subtraction processing method for radiographic images in which the image data is moved and then image data is subtracted between each corresponding pixel of two radiographic images. (Positional misalignment refers to vertical and horizontal misalignment of the radiation image (subject image) with respect to the stimulable phosphor sheet.) According to this method, the difference between the radiation images accumulated and recorded on the stimulable phosphor sheet Since the positional and rotational deviations that occur can be automatically corrected, it is possible to obtain a subtraction image that has high contrast resolution and high spatial resolution, and is free from false images and excellent in suitability for observation and interpretation.

Furthermore, by using the approximate rotation operation described in the specification of Japanese Patent Application No. 57-454.73, this method can perform deviation correction in a significantly shorter time than conventionally known arithmetic processing methods. be able to.

Other misalignment correction methods include a mechanical position correction method when reading a stimulable phosphor, or a special This can also be achieved by the position correction method using a synchronization signal during reading, which is described in the specification of Japanese Patent Application No. 57-142494'1.

However, even if these deviations are corrected and subtraction processing is performed on the radiographic image, the non-overlapping parts of the radiographic image will be reproduced as unreadable unnecessary parts on the periphery of the subtraction reconstructed image. Therefore, unnecessary images palX appear around the subtraction image, making the image difficult to see and deteriorating the quality of the subtraction image.

In order to solve the problem described in ``-'', the present invention provides a subtraction method for radiographic images that can erase unnecessary parts that appear at the periphery of a subtraction image when subtraction processing is performed, and obtain a high-quality, easy-to-see subtraction image. The purpose is to provide a processing method.

The radiation image subtraction processing method of the present invention records two radiation images having different image information at least in part on separate stimulable phosphor sheets, and then exposes each of the stimulable phosphor sheets to excitation light. is scanned to cause the radiation image to be stimulated to emit light from the stimulable phosphor sheet, and this stimulated emitted light is read out photoelectrically by a photodetection means, and subtraction is performed between the digital data corresponding to each of the radiation images obtained. By performing this, an image of a specific structure in the radiographic image is extracted, and at this time, an arbitrary image is applied to an unnecessary part that occurs in the periphery of the subtraction image because the radiographic images do not overlap. This method is characterized in that a subtraction image is obtained by creating an image frame with a density of .

Hereinafter, the present invention will be explained in detail using the drawings.

FIG. 1 shows an example of recording markers on a stimulable phosphor sheet.

During radiographic imaging, two circular marker forming members 2A and 2B made of radiation shielding material are fixed to the two corners of the imaging table 1 along with the subject 3. The Xm emitted by the radiation source 4 is caused to accumulate and record on the stimulable phosphor sheet 5 below the imaging table 1.

The marker forming member 2A fixed to the imaging stand 1 in this way
. The relative positional relationship between the eyes and the subject image 3' does not change depending on the feeding object.

Therefore, it is necessary to obtain a subtraction image (mechanical precision is required when different stimulable phosphor sheets are sequentially or simultaneously conveyed to an imaging table in order to record different radiation images of the same subject on the stimulable phosphor sheet). Even if there is a deviation in the absolute positional relationship of the subject image 3' with respect to the stimulable phosphor sheet 5 and the absolute positional relationship of the scanning excitation light with respect to the subject image 3' that occurs during reading, the markers 2A', 2B The relative positional relationship between ' and the subject image 3' does not change.

That is, the position of the marker recorded on each stimulable phosphor sheet 5 is detected, and based on the detected position of the marker, the deviation between images is corrected on the read image data, and the Zabtraction process is performed. By performing this, a good subtraction image can be obtained.

Next, using FIG. 2, a description will be given of how an image frame is created in an unnecessary portion around the periphery of a subtraction image. 3A
shows an image obtained from the first phosphor sheet A, which records an X-ray image corresponding to a normal X-ray photograph in which the abdominal bones, blood vessels and kidneys with enhanced contrast due to injection of a contrast agent are visible; 3B is a transmission image obtained from the second phosphor sheet B in which only the bones can be seen in a transmitted image of the same area when the contrast medium has not passed through. 3C is a transmission image of 313 obtained from the digital image signal representing the image of 3A. When subtracting the digital image signals representing the image, the two images were corrected for the deviation and subtraction processing was performed to make the blood vessels and kidney thorns visible.As images 3A and 3B did not overlap, the peripheral areas were distorted. Unwanted parts (5, fi' have occurred. 3D shows unnecessary parts 6, fi' appearing at the periphery of the image.
This is a subtraction image in which an image frame 7 having a gray level (gradation level of the digitally converted image) matching the average chromaticity of the subtraction image is recorded at 6', and unnecessary parts 6 and 6' are deleted.

Note that the gray level in the present invention refers to the gray level of an image that has been digitally converted (including logarithmic compression), and is a quantity that is mainly handled within an image processing computer. - Actual density refers to the shading on the final film output, and is detected after being influenced by the image output device, the gradation characteristics of the film, etc.

By the way, when performing misalignment correction, marker 8 on 3A
A, 8B and 3BJ two Marsono-8N.

8. Set B' to overlap.

That is, the image 3A necessary to detect the reference position coordinates provided from the markers 8A, 8B on the image 3A and the markers 8A, ', 8B' on the image 3B and match the corresponding reference points. , 3B are calculated, and based on these values, images 3A and 3B are
The image data is subtracted between each corresponding pixel by correcting the deviation between the pixels.

Note that the markers 8A, 8B, 8A, ', 8B' themselves are also subjected to subtraction processing, but the marker images 8A, 8B, with the peripheral parts of the markers remaining on 3C within the range of image sampling error (for example, about 05 pixels) " may be reproduced. These marker images 8A and 8B also reduce diagnostic suitability, so after detecting the position of marker images 8A; 8B, It is erased by recording the spots 9A and 9B having the following values. In addition, image frame 7 and spots 9A, 9
B is created using digital data before being D/A converted in the image recording machine.

As mentioned above, image frame 7 and spora 1-9A.

The gray level of 9B is set to match the average chromaticity of the subtraction image. The average grayness measuring method used to determine this average method chromaticity will be explained below.

Ideally, once a subtraction image has been created, it would be preferable to sample the entire screen and obtain the average legal chromaticity. However, in reality, there are limitations on processing time and image storage memory capacity, especially for fine-grained images. This is a particular problem in radiographic image recording and reproducing systems using stimulable phosphor sheets that reproduce clear images with excellent spatial resolution through pixel sampling (5 to 10 pixels/mm).
Ru. Therefore, the following method can be used to determine the average chromaticity of an image in a short processing time.

(2) After carrying out subtraction calculation for one buffer memory, the subtraction image in the buffer memory is sampled to obtain the average law chromaticity, and the image frame 7 and spora) 9A and 9B are added in the buffer memory.

(2) Every time the subtraction calculation is performed for one scanning line, (2) the average law chromaticity is determined for the scanning line, and the image frame 7 and spora) 9A and 9B are added in units of one scanning line.

■ Before performing subtraction calculation, two images 3
Calculate and store the average values of the latter samples of A and 3B, and when it is necessary to create a subtraction image, immediately subtract the average grayness value of the two images calculated above once. Find the average chromaticity of the subtraction image by simply doing the following:

Next, a method for determining the thickness of the image frame 7 in this embodiment will be explained using FIG. 3. In Figure 3, the amount of misalignment correction between the two images on which subtraction is performed is Δθ(
The minimum required frame thicknesses in the X and Y directions are Fx (m+++) and Fy (mrn).
is expressed by the following equation.

Based on the above formula, the necessary thickness of the image frame is determined for each image, and frames are created one after another.

In the embodiment of the present invention described above, the image frame is created on digital data before being D/A converted in the image recording machine, and its density is adjusted to match the average normal chromaticity of the subtraction image. The size of the image frame is set according to the amount of misalignment correction between the two images to be subtracted as described in (1) above.

Although it is determined by equation (2), the subtraction processing method for radiographic images of the present invention is not limited to this.

For example, when recording a subtraction image with an image recording machine, the image frame may be recorded by providing a frame of a constant size and a constant density on a recording medium (for example, a photosensitive material). In addition, the density value of the image frame is a gray level, which is usually quantized to a digital level of 8 to 10 bits, and the maximum or minimum value of this digital level (255 for 8 bits).
Alternatively, it may be set to 0). Preferably, the average chromaticity of the subtraction image is made to match that of the subtraction image as in the above embodiment. By bringing the gray level of the image frame closer to the gray level of the subtraction image in this way, the edges of the image frame will be less emphasized when the subtraction image is subjected to blur mask processing (this will result in a subtraction image that is easier to see visually). .

As explained above, according to the present invention, an image frame is created to cover the unnecessary part that occurs at the periphery of the subtraction image because the images do not overlap when subtraction is performed. So,
A visually easy-to-see subtraction image can be obtained, which will have a positive impact on diagnosis.

Further, according to a preferred embodiment of the present invention, markers are used when recording radiographic images, and the deviation of the markers recorded in the two radiographic images to be subtracted on digital data is calculated, and the deviation of the subtraction images is calculated. Since the image frame is corrected and the minimum necessary image frame is created according to the amount of correction, it is possible to minimize the loss of correct information due to recording the image frame. Furthermore, since the density of the image frame is made to match the average chromaticity of the subtraction image, the contrast with the image within the frame is reduced, making it possible to obtain a subtraction image that is easier to see visually, further improving diagnostic suitability.

[Brief explanation of drawings]

FIG. 1 is a perspective view showing an example of a method for accumulating and recording a radiation image of a subject on a stimulable phosphor sheet, and FIG. 2 is a system showing a procedure for obtaining a subtraction image with a frame recorded in the present invention. FIG. 3 is a diagram showing an example of an image frame in the present invention. 1...Photography stand 3...Subject 5...Storage phosphor sheet

Claims (1)

[Scope of Claims] 1) Two radiation images differing in at least some image information are recorded on separate stimulable phosphor sheets, and then each of the stimulable phosphor sheets is scanned with excitation light to The radiation image is stimulated to emit light from the stimulable phosphor sheet, and the stimulated emitted light is read out photoelectrically by a photodetector, and subtraction is performed between the digital data corresponding to each of the radiation images obtained. Corresponding parts of the two radiographic images are superimposed to obtain a subtraction image in which an image of a specific structure in the radiographic image is extracted. 1. A method for subtraction treatment of radiographic images, which comprises creating an image frame of arbitrary density in an unnecessary part that occurs in the periphery of a subtraction image so as to cover the unnecessary part. 2) having a shape that provides a reference point or reference line to the stimulable phosphor sheet when the two radiographic images to be subtracted are stored on the separate stimulable phosphor sheet; A marker is simultaneously recorded at a fixed position with respect to the radiation image, and when subtraction is performed between digital data corresponding to the two radiation images to be subtracted, the reference of the marker is calculated from the digital data of the radiation image. The positional coordinates of the reference line are detected, and the rotational deviation and positional deviation between the two radiation images are calculated from the detected positional coordinates. Subtracting the radiation image according to claim 1, wherein the subtraction of the radiation image is performed by correcting the deviation of the two radiation images and subtracting the image data between corresponding pixels of the two radiation images to be subtracted. Traction treatment method. 3) The radiation image burrtraction processing method according to claim 1 or 2, characterized in that the image frame is created on the digital data. 4) The subtraction processing method for a radiation image according to claim 2 or 3, characterized in that the density of the image frame is changed according to the average density of the subtraction image. 5) The radiation according to any one of claims 2 to 4, characterized in that the size of the image frame is changed according to the amount of correction of the rotational deviation and positional deviation. Image subtraction processing method.