JPH0262074B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of the Invention) The present invention relates to subtraction processing of radiographic images;
In particular, it relates to a method and apparatus for automatically correcting the density of a subtraction image so that a constant and appropriate background density is always obtained in digital subtraction processing of a radiation image using a stimulable phosphor sheet. It is something.

(Technical Background of the Invention and Prior Art) Digital subtraction of radiographic images has been known for some time. Digital subtraction of radiographic images is a process in which two radiographic images taken under different conditions are read out photoelectrically to obtain digital image signals, and then these digital image signals are made to correspond to each pixel of both images. This method performs subtraction processing to obtain a difference signal to form an image of a specific structure in a radiographic image, and the difference signal obtained in this way is used to reproduce a radiographic image in which only the specific structure is extracted. can do.

There are basically the following two methods for this subtraction processing. That is, a specific structure is extracted by subtracting the image signal of a radiographic image in which no contrast agent has been injected from the image signal of a radiographic image in which a specific structure has been emphasized by contrast agent injection. So-called time subtraction processing involves irradiating the same subject with radiation having different energy distributions, and then performing subtraction (subtraction) after appropriately weighting the image signals of the two radiation images. This is a so-called energy subtraction process that extracts images of specific structures.

This subtraction processing is an extremely effective method for diagnosis, especially in image processing of medical X-ray photographs, so it has attracted much attention in recent years, and its research and development is actively progressing by making full use of electronic engineering technology. . This technique is specifically called digital subtraction processing (usually Digital Rediography).
It is abbreviated as DR.

More recently, as shown in Japanese Patent Application Laid-Open No. 58-163340, two stimulable phosphor sheets with an extremely wide radiation exposure area are used, and these phosphor sheets contain a contrast agent as described above. By irradiating the same object with radiation that has passed through it under different conditions, we accumulate and record radiation images with different image information of the areas where the contrast agent has been injected into these phosphor sheets, and then use these accumulated images with excitation light. It has also been proposed to read the data by scanning and convert it into digital signals, and to perform the digital subtraction using these digital signals. The above-mentioned stimulable phosphor sheet refers to radiation (X-rays, α-rays, β-rays,
When irradiated with radiation (rays, gamma rays, ultraviolet rays, etc.), a portion of the radiation energy is accumulated in the phosphor, and then when it is irradiated with excitation light such as visible light, the phosphor exhibits stimulated luminescence depending on the accumulated energy. It has an extremely wide latitude (exposure range) and extremely high resolution. Therefore,
By performing the digital subtraction using the radiographic image information stored on the phosphor sheet, a radiographic image with high diagnostic performance can be obtained.

When a subtraction image is formed on, for example, a photographic material using the difference signal obtained by the time subtraction as explained above, the area other than the specific structure into which the contrast agent has been injected, that is, the background, is Originally, the concentration should always be constant. However, during radiography, even if the radiation intensity is set constant, there are slight variations in the intensity of the actually irradiated radiation, and there are also variations in the sensitivity of the stimulable phosphor sheet, so the background density often varies depending on each subtraction image. It has been pointed out that when this background density varies, proper diagnosis is hindered when a diagnosis is made by comparing a plurality of subtraction images.

(Object of the Invention) The present invention has been made in view of the above-mentioned circumstances, and provides an automatic density correction method for a temporal subtraction image that can always obtain a subtraction image with a constant background density, and the same. The object is to provide an apparatus for carrying out the method.

(Structure of the Invention) The automatic density correction method for subtraction images of the present invention includes, in the time subtraction processing performed using a stimulable phosphor sheet as described above,
When performing gradation processing on the difference signal obtained by subtraction, the maximum transmitted radiation dose of each digital image signal read out from each phosphor sheet is determined, and then the difference Δx between these maximum values is calculated.
Then, (i) the difference Δx is added to the difference signal before gradation processing.
(ii) A correction that shifts the gradation conversion table to the higher density side of the input signal axis by the difference Δx.

The automatic density correction device for subtraction images of the present invention, which implements the above-mentioned method, performs the correction described in (i) above by scanning the excitation light and photoelectrically reading out the stimulated luminescence light as described above. an image reading means for obtaining a digital signal of a radiographic image of a fluorescent phosphor sheet, a digital image signal of a subject whose specific structure has been injected with a contrast agent, and a digital image signal of an object to which no contrast agent has been injected, read by the image reading means; subtraction calculation means for obtaining a difference signal by subtracting the image signal between corresponding pixels to form an image of a specific structure; and image processing for performing gradation processing on this difference signal based on a gradation conversion table. means, a calculation means for determining the maximum transmitted radiation dose of each of the digital image signals, and a signal correction circuit that determines the difference Δx between the maximum values and adds this difference Δx to the difference signal before undergoing gradation processing. Consists of.

Furthermore, if a tone conversion table correction circuit is provided in place of the signal correction circuit that shifts the tone conversion table to the higher density side of the input signal axis by the difference Δx, the above-mentioned correction (ii) can be performed. be able to. The maximum transmitted radiation dose of each digital image signal read out from each stimulable phosphor sheet is
For example, it can be determined from the histogram of these digital image signals.

The histogram of the digital image signal read out from each stimulable phosphor sheet is, for example, as shown in Figure 1.
From y _A and y _B , the transmitted radiation dose corresponding to the maximum background density is distributed between the maximum values x _A and x _B. The minimum values y _A and y _B may vary depending on the presence or absence of the contrast agent, but the maximum values x A and y _B corresponding to the maximum background density are
x _B should originally be constant. Therefore, if the maximum values x _A and x _B differ from image to image, this is considered to be due to differences in radiation intensity at the time of imaging or differences in sensitivity of the stimulable phosphor sheets. Therefore, if the above-mentioned correction is performed to eliminate the deviation in the maximum value of the image signal between each image, the density signal of the background part will always have a value of 0 (zero) when the two digital image signals are subtracted, and the subtraction image The background density of is always constant.

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

Figure 2 shows X-rays 2 transmitted through the same subject 1 on two stimulable phosphor sheets A and B under different conditions.
In other words, a contrast agent is injected into a specific structure of subject 1,
Alternatively, it shows the state of irradiation without injection. For example, angiography (Digital
angiography), the first stage is
The X-ray transmission image of the subject 1 before the contrast medium of the blood vessel is injected is stored and recorded on the stimulable phosphor sheet A, and then the contrast medium is injected into the vein of the same subject 1. After about a second has elapsed, the X-ray transmitted image of the subject 1 is similarly accumulated and recorded. At this time, the tube voltage of the X-ray source 3 is the same, the positional relationship between the subject 1 and the phosphor sheets A and B is also the same, and two X-ray images with no difference other than the presence or absence of contrast agent are A. ,B.

In this way, two radiation images with different image information of the contrast agent injection part are accumulated and recorded on the two stimulable phosphor sheets A and B. Next, an X-ray image is read from these two stimulable phosphor sheets A and B by an image reading means as shown in FIG. 3 to obtain a digital image signal representing the image. First, while moving the stimulable phosphor sheet A in the direction of the arrow Y for sub-scanning, the laser beam 11 from the laser light source 10 is caused to main scan in the X direction by the scanning mirror 12. The accumulated X-ray energy is emitted as stimulated luminescence light 13 in accordance with the accumulated and recorded X-ray image. The stimulated luminescent light 13 enters the interior of the condenser plate 14 from one end surface of the condensing plate 14 made by molding a transparent acrylic plate, and reaches the photomal 15 through repeated total reflection, where it becomes the stimulated luminescent light. The amount of light emission of 13 is output as an image signal S. This output image signal S is sent to an amplifier and an A/D
A logarithmic converter 16 including a converter converts the signal into a digital image signal logS _A having a logarithmic value (logS).
This digital image signal _logSA is stored in a storage medium 17 such as a magnetic tape. Next, in exactly the same way, the recorded image of another stimulable phosphor sheet B is read out, and its digital image signal logS _B
is similarly stored in the storage medium 17.

Next, subtraction processing is performed using the digital image signals logS _A and logS _B obtained as described above. FIG. 4 shows the flow of subtraction processing performed while performing automatic density correction according to the first embodiment of the method of the present invention. First, the digital image signals are extracted from the image file 17A and the image file 17B in the storage medium 17, respectively.
logS _A and logS _B are read out and input to the subtraction calculation circuit 18 . This subtraction calculation circuit 18 processes the above two digital image signals.
LogS _A and logS _B are subtracted for each corresponding pixel to obtain a digital difference signal Ssub. This difference signal Ssub is temporarily stored in the image file 19 and then input to the image processing circuit 20, where it is subjected to gradation processing based on the gradation conversion table 20a.

The difference signal Ssub′ that has undergone gradation processing is, for example, a CRT
The subtraction image is inputted to a display device such as, or a reproducing/recording device 21 such as a scanning recorder, and a subtraction image is reproduced and recorded using the difference signal Ssub'. Fifth
The figure shows an image scanning and recording device as an example of a subtraction image reproduction and recording system.
The photosensitive film 30 is moved in the sub-scanning direction of the arrow Y, and the laser beam 31 is main-scanned on the photosensitive film 30 in the X direction, and the laser beam 31 is sent to the A/O modulator 32 to generate an image from the image signal supply device 33. A visible image is formed on the photosensitive film 30 by modulating the signal. If the difference signal Ssub' is used as the image signal for modulation, an image of only a desired specific structure can be transferred to the photosensitive film 30 by digital subtraction processing.
Can be recorded and played on.

FIG. 6 shows how an image of a desired specific structure is obtained by the subtraction as described above. In the figure, 4A is an image obtained from the first stimulable phosphor sheet A that records an X-ray image before contrast medium was injected into the abdomen, and 4B records an X-ray image of the same area after contrast medium was injected. The image obtained from the second stimulable phosphor sheet B, 4C is 4B
This is a subtraction image obtained by subtracting the digital image signal representing the image 4A from the digital image signal representing the image 4A, so that only the blood vessels are visible.

In the subtraction image 4C above, the background BK part around the extracted specific structure (blood vessel) should originally have the difference signal Ssub of 0 (zero) and always have a constant density on the reproduced image. be. However, as described above, due to variations in the intensity of the imaging radiation and variations in the sensitivity of the stimulable phosphor sheets A and B, the density of this background BK portion varies. Therefore, as shown in FIG. 4, the digital image signals logS _A and logS _B are input to the histogram calculation circuit 22, and the respective signals logS _A and logS _B are inputted to the histogram calculation circuit 22.
Find the histogram of . These histograms are as shown in FIG. 1, respectively.
The maximum value of the transmitted radiation maximum corresponding to the concentration of x _A ,
It is distributed between x _B and the minimum values y _A and y _B of the minimum transmitted radiation dose. As mentioned above, the above maximum values x _A and x _B should originally be constant in both images, but as mentioned above, there are differences in radiation intensity during imaging and differences in sensitivity between stimulable phosphor sheets A and B. The background density changes from sheet to sheet, and the maximum values x _A and x _B are different.
Therefore, a signal representing the maximum values x _A and x _B is sent to the signal correction circuit 24, a difference Δx between the two maximum values x _A and x _B is determined, and this difference Δx is uniformly added to the difference signal Ssub. By adding the difference Δx to the difference signal Ssub in this way, the difference signal Ssub is
(x _A −x _B , i.e. Δx
) is corrected, and the background signal is always 0.
(zero) value. Therefore, the density of the background BK in the reproduced and recorded subtraction image is always constant.

FIG. 7 shows the flow of subtraction processing for automatic density correction according to the second embodiment of the method of the present invention. In this case, signals indicating the maximum values x _A and x _B are input from the histogram calculation circuit 22 to the gradation conversion table correction circuit 124 . The gradation conversion table 20a used in the image processing circuit 20 is as shown in FIG. 8, and the gradation conversion table correction circuit 124, like the signal correction circuit ₂₄ , The difference Δx from _B is calculated, and the original tone conversion table 20a (solid line in FIG. 8) of the image processing circuit 20 is converted to
(dashed line in FIG. 8). In this way, gradation conversion table 2
When 0a is shifted, the difference signal after gradation processing
In any case, Ssub′ is always a difference signal Ssub that does not include the signal component Δx due to variations in imaging radiation intensity and variations in sensitivity of stimulable phosphor sheets A and B.
takes the same value as when gradation processing is performed using the original gradation conversion table 20a. Therefore, in the subtraction image obtained by the difference signal Ssub', the density of the background BK is always constant.

In the present invention, the method for determining the maximum transmitted radiation dose of a digital image signal is not limited to the method of calculating from a histogram, but may also include, for example, a method of sequentially comparing image data and selecting a larger value. is applicable. Therefore, it goes without saying that in the first and second embodiments described above, the histogram calculation circuit 22 may be replaced with another calculation means for determining the maximum value.

(Effects of the Invention) As explained in detail above, according to the present invention, the background density of a time subtraction image obtained using two sheets can be always set constant.
Subtraction images with extremely excellent diagnostic properties and high utility in the medical field can now be obtained.

[Brief explanation of drawings]

FIG. 1 is a graph showing an example of a histogram of a digital image signal used for time subtraction, FIG. 2 is an explanatory diagram showing the steps of accumulating and recording radiation images in the method of the present invention, and FIG. FIG. 4 is a schematic diagram illustrating the reading of radiation image information from a stimulable phosphor sheet; FIG. The figure is a schematic diagram showing an example of a subtraction image reproduction/recording system. Figure 6 is an example of a radiographic image with contrast agent injection, a radiographic image without contrast agent injection, and a time subtraction image obtained from these radiographic images. FIG. 7 is a schematic diagram illustrating the outline of subtraction processing for automatic density correction according to the second embodiment of the method of the present invention;
FIG. 8 is a graph illustrating correction of the gradation conversion table in the method of the second embodiment. 1...Subject, 2...X-ray, 3...X-ray source, 4
A, 4B... X-ray image, 4C... Subtraction image, 10... Laser light source, 11... Laser light, 12... Scanning mirror, 13... Stimulated luminescence light, 15... Photomal, 16... ...logarithmic converter,
18... Subtraction calculation circuit, 20... Image processing circuit, 20a... Gradation conversion table, 22
... Histogram calculation circuit, 24 ... Signal correction circuit, 124 ... Gradation conversion table correction circuit, A,
B...Stormable phosphor sheet, logS _A , logS _B ...Digital image signal, Ssub...Difference signal of digital image signal, Ssub'...Difference signal subjected to gradation processing.

Claims (1)

[Scope of Claims] 1. Two stimulable phosphor sheets are each irradiated with radiation that has passed through a specific structure in which a contrast agent has been injected and the object in which no contrast agent has been injected. accumulating and recording radiation images with mutually different image information of the specific structure on a sheet;
These phosphor sheets are scanned with excitation light to convert the radiation image into stimulated luminescence light, and the amount of the stimulated luminescence light is read out photoelectrically and converted into a digital image signal, and the corresponding of both images is converted into a digital image signal. This digital image signal is subtracted between pixels to obtain a difference signal that forms an image of a specific structure in a radiation image, and then this difference signal is subjected to gradation processing based on a predetermined gradation conversion table. In the time subtraction processing of the radiographic image, the maximum transmitted radiation dose of each of the digital image signals is determined, and the difference Δx between these maximum values is determined, and then (i) A subtraction characterized by performing either one of the following corrections: (ii) a correction that adds the difference Δx to the difference signal; and (ii) a correction that shifts the gradation conversion table by the difference Δx toward the higher density side of the input signal axis. Automatic density correction method for images. 2. An image in which excitation light is scanned across a stimulable phosphor sheet on which a radiation image has been accumulated and recorded, thereby photoelectrically reading out the stimulated luminescence light emitted from the stimulable phosphor sheet and converting it into a digital image signal. a reading means, and a digital image signal of a subject whose specific structure has been injected with a contrast medium, read by the image reading means, and a digital image signal of the subject to which no contrast medium has been injected, are subtracted between corresponding pixels. subtraction calculation means for obtaining a difference signal to form an image of the specific structure; image processing means for performing gradation processing on the difference signal based on a gradation conversion table; automatic density of a subtraction image, comprising a calculation means for calculating a maximum value, and a signal correction circuit that calculates a difference Δx between the respective maximum values and adds the difference Δx to the difference signal before undergoing the gradation processing. correction device. 3. An image in which excitation light is scanned across a stimulable phosphor sheet on which a radiation image has been accumulated and recorded, thereby photoelectrically reading out the stimulated luminescence light emitted from the stimulable phosphor sheet and converting it into a digital image signal. a reading means, and a digital image signal of a subject whose specific structure has been injected with a contrast agent, read by the image reading means, and a digital image signal of the subject to which no contrast agent has been injected, are subtracted between corresponding pixels. subtraction calculation means for obtaining a difference signal to form an image of the specific structure; image processing means for performing gradation processing on the difference signal based on a gradation conversion table; and a gradation conversion table correction circuit that calculates the difference Δx between the respective maximum values and shifts the gradation conversion table to the higher density side of the input signal axis by the difference Δx. Automatic density correction device for traction images.