JPH0363877B2 - - 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. This 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 Radiography).
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. Radiation that has passed through the same subject is irradiated under different conditions, and radiographic images with different image information of the areas where the contrast agent is injected into these phosphor sheets are accumulated and recorded, and these accumulated signals are used 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, it should be possible to always output at a constant density (or brightness). However, during radiography, even if the radiation intensity is set constant, there are slight variations in the intensity of the actually irradiated radiation, and furthermore, there are variations in the sensitivity of the stimulable phosphor sheet.
The density of the background 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 it is possible to always obtain a subtraction image with a constant background density. It is an object of the present invention to provide an automatic density correction method for a subtraction image and an apparatus for implementing the method.

(Structure of the Invention) The automatic density correction method for a subtraction image of the present invention is based on a difference signal obtained by subtraction in subtraction processing performed using a stimulable phosphor sheet as described above. When performing gradation processing, a histogram of the difference signal is obtained to find its maximum frequency point signal, and the maximum frequency point signal is calculated from a standard background density signal that carries the standard background density of the subtraction image by the difference signal. The difference Δχ is calculated by subtracting the difference Δχ, and then (i) the difference Δχ
(ii) the gradation conversion table used for the gradation processing,
A value −Δχ whose absolute value is equal to and opposite in sign to the difference Δχ
In this embodiment, one of the corrections to shift the input signal in the input signal axis direction is performed.

The automatic density correction device for subtraction images of the present invention, which implements the above method, performs the above-mentioned correction (i) 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 for forming an image of a specific structure by subtracting the image signal between corresponding images; and image processing for performing gradation processing on this difference signal based on a gradation conversion table. means for calculating a histogram of the difference signal to obtain its maximum frequency point signal; storage means for storing a standard background density of a subtraction image based on the difference signal; and reading from the storage means. and a signal correction circuit that subtracts the maximum frequency point signal from the standard background density signal carrying the standard background density that has been obtained, and adds the difference Δχ to the difference signal before being subjected to gradation processing.

Furthermore, in place of the signal correction circuit, a tone conversion table correction circuit may be provided which shifts the tone conversion table by -Δχ, which is equal in absolute value to the difference Δχ and has an opposite sign, in the direction of the input signal axis. , as mentioned above
Amendment (ii) may be made.

The maximum frequency point density in the histogram of the difference signal is usually the background density of the subtraction image, so if the above-mentioned correction (i) or (ii) is performed, the background density will always be the same in the subtraction image. It will now be set to a constant standard background density.

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

Figure 1 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 image of the subject 1 after passing through the X-rays is similarly stored 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. 2 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. 3 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'. Fourth
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. 5 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 above subtraction image 4C, the background BK part around the extracted specific structure (blood vessel) originally has the difference signal Ssub of 0 (zero), and always has a constant density (brightness of 5) on the reproduced image. However, as mentioned 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 will vary. The difference signal Ssub is input to the histogram calculation circuit 22 to obtain a histogram of the difference signal Ssub.As shown in FIG. 6, this histogram usually consists of a large mountain M representing the background BK area. , and a small peak m representing the specific structure portion into which the contrast agent has been injected.Therefore, if the position of the large peak M is always the same, the background
BK always has a constant concentration. However, for the reasons mentioned above, the histogram changes as shown by the broken line in FIG. Therefore, if we assume that the solid line histogram in _FIG . The maximum frequency point signal χ of the actual histogram is subtracted from the background density signal (which is the background density signal), and the difference Δχ (=χ ₀ −χ) is obtained. As shown in FIG. 3, this calculation is performed using the maximum frequency point signal
and a storage means 23 storing the standard background density.
The standard background density signal χ ₀ read from the signal is input to the signal correction circuit 24, and the signal correction circuit 24 performs the correction. The signal correction circuit 24 then processes the difference signal Ssub extracted from the image file 19.
The above difference Δχ is uniformly added to . Therefore, the difference signal Ssub is always corrected so that the maximum frequency point signal of its histogram becomes the above-mentioned X ₀ , and in the subtraction image obtained based on the difference signal Ssub after this correction, the background density is also has the above standard background density.

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, the maximum frequency point signal χ from the histogram calculation circuit 22 and the standard background density signal χ ₀ from the storage means 23 are input to the gradation conversion table correction circuit 124. The image processing circuit 20
The gradation conversion table 20a used in the gradation conversion table 20a is as shown in FIG.
After finding Δχ=χ ₀ −χ in the same way as in 4, the correction term −
ΔX is sent to the image processing circuit 20, and the original gradation conversion table 20a (solid line in FIG. 8) is shifted by -ΔX in the direction of the input signal axis (broken line in FIG. 8).
When the gradation conversion table 20a is shifted in this way, the difference signal Ssub' after gradation processing is always the difference signal Ssub in which the maximum frequency point signal of the histogram is the standard background density signal χ ₀ . 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 the standard background density.

As mentioned above, the standard background density signal χ ₀ can be obtained by finding the maximum frequency point signal of the histogram of the difference signal Ssub that forms a subtraction image with a preferable standard background density. However, the background density signal can be extracted by specifying the image coordinate points from the difference signal Ssub that forms a standard background density subtraction image.
Alternatively, a signal representing a preferred standard background density may be generated and used.

(Effects of the Invention) As explained in detail above, according to the present invention, the background density of the time subtraction image obtained using two stimulable phosphor sheets can be always set to a constant value, resulting in extremely high diagnostic performance. This makes it possible to obtain subtraction images that are highly useful in the medical field.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram showing the step of accumulating and recording radiation images in the method of the present invention, FIG. A schematic diagram illustrating an overview of subtraction processing for performing automatic density correction according to the first embodiment of the method of the present invention;
Fig. 4 is a schematic diagram showing an example of a subtraction image reproduction/recording system, and Fig. 5 is a radiographic image with contrast medium injection, a radiographic image without contrast medium injection, and a time subtraction image obtained from these radiographic images. FIG. 6 is a graph showing an example of a histogram of a difference signal forming a subtraction image; FIG. 7 is a schematic diagram showing an example of a histogram of a difference signal forming a subtraction image; FIG. FIG. 8 is a graph explaining the 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, 23 ... storage means,
24... Signal correction circuit, 124... Gradation conversion table correction circuit, A, B... Stimulative 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 radiological image obtained by the difference signal, a histogram of the difference signal is obtained, and then its maximum frequency point signal is obtained from the standard background density signal carrying the standard background density of the subtraction image by the difference signal. A difference Δχ is obtained by subtracting the maximum frequency point signal, and then (i) correction is performed by adding the difference Δχ to the difference signal before the gradation processing, (ii) a gradation conversion table used for the gradation processing is calculated. ,
An automatic density correction method for a subtraction image, characterized in that either one of the corrections is performed by shifting the input signal in the axial direction by a value -Δχ whose absolute value is equal to and opposite to the difference Δχ. 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 forming an image of the specific structure; image processing means for performing gradation processing on the difference signal based on a gradation conversion table; and obtaining a histogram of the difference signal. a histogram calculation means for calculating the maximum frequency point signal;
a storage means for storing a standard background density of a subtraction image based on the difference signal; subtracting the maximum frequency point signal from a standard background density signal carrying the standard background density read from the storage means; An automatic density correction device for a subtraction image, comprising a signal correction circuit that adds Δχ to the difference signal before undergoing the gradation processing. 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 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 forming an image of the specific structure; image processing means for performing gradation processing on the difference signal based on a gradation conversion table; and obtaining a histogram of the difference signal. a histogram calculation means for calculating the maximum frequency point signal;
a storage means for storing a standard background density of a subtraction image based on the difference signal; and a storage means for subtracting the maximum frequency point signal from a standard background density signal carrying the standard background density read from the storage means, An automatic density correction device for a subtraction image, comprising a tone conversion table correction circuit that shifts a tone conversion table by -Δχ having an absolute value equal to the difference Δχ and having an opposite sign in an input signal axis direction.