JPS60250792A - Method and device for gradation processing optimizing of subtraction picture - Google Patents

Abstract

PURPOSE:To obtain an energy subtraction picture of high diagnosing ability by finding out two signal values from a histogram of a differential signal and gradation processing the differential signal corresponding to specified output picture density. CONSTITUTION:Digital picture signals logSA, logSB are read out respectively from picture files 17A and 17B in a memory medium 17 and inputted to a subtraction arithmetic circuit 18. The circuit 18 weights signals logSA and logSB properly, and subtracts for each corresponding picture element and finds digital differential signal Ssub as shown in the expression. The signal Ssub is stored once in a picture file 19 and inputted to a picture processing circuit 20, and gradation processed in the circuit 20 through a signal converting circuit 20a basing on a gradation conversion table 20b. Gradation processed differential signal Ssub' is inputted to display unit such as CRT etc. and a reproduction recording device 21 such as a scanning recording device etc. and a subtraction picture is reproduced and recorded by the differential signal Ssub'.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of the Invention) The present invention provides subtraction processing of radiographic images, specifically, digital subtraction processing of radiographic images performed using a stimulable phosphor sheet, in which the density range and contrast are always constant. The present invention relates to a method and apparatus for performing gradation processing on a subtraction image so as to achieve the following.

(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 formed in a spiral shape under different conditions are photoelectrically read out 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 two methods for this subtraction process: In other words, the image signal of a radiographic image in which no contrast agent has been injected is subtracted from the image signal of a radiographic image in which a specific structure has been emphasized by contrast agent injection (subtraction).
We use so-called time subtraction processing to extract specific structures by irradiating the same subject with radiation having different energy distributions, and thereby detect that specific structures have unique radiation energy absorption characteristics. This method is used to create an image in which a specific structure differs between two radiographic images, and then subtraction (subtraction) is performed after applying appropriate weighting between the image signals of these two radiographic images to find the specific structure. This is a so-called energy subtraction process for extracting images.

This subtraction processing is an extremely effective method for diagnosis, especially in image processing of medical X-ray photographs.
It has received a lot of attention in recent years, and its research and development is actively progressing by making full use of electronic engineering technology. This technique is particularly suitable for digital subtraction processing (usually [digital R
abbreviated as DR.

More recently, as shown in Japanese Patent Laid-Open No. 58-163340, for example, stimulable phosphor sheets with an extremely wide radiation exposure area have been used, and these phosphor sheets have been exposed to different conditions as described above. Radiation that has passed through the same subject is irradiated, and radiation images with different image information of specific structures are accumulated and recorded on these phosphor sheets, and these accumulated images are read out by scanning with excitation light and converted into digital signals. However, it has also been proposed to perform the digital subtraction using these digital signals. The above-mentioned stimulable phosphor sheet is, for example, JP-A-55-12429
As disclosed in the publication, radiation (X-rays, alpha rays, beta
When irradiated with radiation (rays, gamma rays, ultraviolet rays, etc.), a portion of the radiation energy is accumulated in the phosphor, and when excitation light such as visible light is irradiated, the phosphor emits light according to the amount of accumulated radiation energy. The body exhibits stimulated luminescence, has an extremely wide latitude code (exposure area), and has extremely high resolution. Therefore, if the digital subtraction is performed using the radiographic image information accumulated and recorded on this phosphor sheet, sufficient image information can always be obtained even if the radiation dose fluctuates, and radiographic images with high diagnostic ability can be obtained. can be obtained.

However, when performing energy subtraction using a stimulable phosphor sheet as described above, if the difference signal obtained by subtraction is used as it is to form an output image, each output image will have different density ranges (or brightness ranges). Contrast often varies. This is due to fluctuations in the image signals of the two images to be subjected to subtraction due to the aforementioned variations in the sensitivity of the stimulable phosphor sheet, variations in the sensitivity of the image reader, variations in the imaging radiation energy, etc. It is. Since the difference signal obtained by subtraction is minute, it is effective to perform gradation processing that amplifies the difference signal, but if the image signal fluctuates as described above, the variation component in the difference signal will be It becomes considerably larger than the original signal component, and appears as large fluctuations in density range and contrast. As mentioned above, if the density (or brightness) and contrast of subtraction images vary, it may impede proper diagnosis when comparing multiple subtraction images, or it may hinder human visual characteristics. It has been pointed out that the density of the image deviates from certain areas.

In energy subtraction using a stimulable phosphor sheet, in order to obtain a radiation image for subtraction (that is, a radiation image with different image information of a specific structure), the radiation energy is increased by changing the tube voltage of the radiation source. In addition to changing the energy distribution, other known methods may be used, such as inserting and removing a filter that changes the radiation energy distribution between the radiation source and the subject. -193765
As disclosed in the above publication, a laminate of a stimulable phosphor sheet and a filter or the like may be used to obtain the image by one radiography.

(Objective of the Invention) The present invention has been made in view of the above circumstances, and is an optimization of gradation processing of subtraction images that can obtain energy subtraction images with constant density range and contrast and good diagnostic performance. It is an object of the present invention to provide methods and apparatus.

(Structure of the Invention) The method for optimizing gradation processing of a subtracted image of the present invention is to calculate the histogram of a difference signal obtained by subtraction in energy subtraction processing performed using a stimulable phosphor sheet as described above. At least two signal values different from each other are determined from the histogram, and then the difference signal is subjected to gradation processing by making the signal values correspond to at least two predetermined output image densities.
・It is designed to make sense.

By using the maximum and minimum signal values as at least two signal values in the above histogram and making these correspond to the predetermined maximum and minimum output image density values, the density range and contrast can be kept constant over the widest range. Can be set.

As mentioned above, in order to make a certain signal value of the difference signal histogram correspond to a predetermined output image density, it is possible to uniformly convert the difference signal before gradation processing, or to correct the gradation conversion daple. Good too.

The gradation processing optimization device for a subtraction image of the present invention that implements the above method uses the above-mentioned excitation light scanning and photoelectric readout of stimulated luminescence light to perform accumulation processing when uniformly converting difference signals. An image reading means for obtaining a digital signal of a radiographic image of a fluorescent phosphor sheet, and a digital image signal of a subject having different image information of a specific structure read by this image reading means are subtracted between corresponding pixels. ii of the specific structure! (ii) a subtraction calculation means for obtaining a difference signal forming an image; an image processing means for performing gradation processing on the difference signal based on a gradation conversion table; histogram calculation means for calculating at least two signal values; and uniformly calculating the difference signal before undergoing the gradation processing so that the at least two signal values correspond to at least two predetermined output image densities. It consists of a signal conversion circuit that converts into

Furthermore, instead of the signal conversion circuit, a tone conversion table correction circuit may be used that corrects the tone conversion table so that the at least two signal values correspond to at least two predetermined output image densities. , the difference signal can be used as is to optimize gradation processing.

(Embodiments) Hereinafter, the present invention will be described in detail based on embodiments shown in the drawings.

FIG. 1 shows a state in which two stimulable phosphor sheets A and B are irradiated with X-rays 2 that have passed through the same subject 1, each with different energies. Specifically, the X-ray transmitted image of the subject 1 is stored and recorded on the first stimulable phosphor sheet A, and then the stimulable phosphor sheets A and B are quickly replaced within a short period of time, and at the same time, the X-ray source is By changing the tube voltage of No. 3, X-ray images of the subject 1 having different energies of transmitted X-rays are accumulated and recorded on the stimulable phosphor sheet B. At this time, stimulable phosphor sheets A and B
The positional relationship of subject 1 is the same in both.

In this way, at least some of the image information differs between the two images.
Two radiation images are accumulated and recorded on two stimulable phosphor sheets A and B. Next, these two stimulable phosphor sheets A,
[From 3, X by an image reading means as shown in FIG.
The line image is read to obtain a digital image signal representing the image. First, while moving the stimulable phosphor sheet Δ for sub-scanning in the direction of the arrow Y, the laser beam 11 from the laser light source 10 is caused to main-scan in the X direction by the scanning mirror 12, and from the phosphor sheet A, The accumulated X-ray energy is emitted as stimulated luminescence light 13 according to the accumulated and recorded X-ray image. The stimulated luminescent light 13 is produced by molding a transparent acrylic plate, and is transmitted from one end surface of the light collecting plate 14 to the light collecting plate 14.
The photomultiplier enters the inside of the photomultiple 1 while repeating total reflection
5, the amount of light emitted by the stimulated luminescent light 13 is output as an image signal S. This output image signal S is sent to an amplifier and an A/
A logarithmic converter 16 including a D converter converts the logarithmic value (10 gS
) is converted into a digital image signal 10g5A. This digital image signal 10CISA is stored in a storage medium 17 such as a magnetic disk. Next, in exactly the same manner, the recorded image on another stimulable phosphor sheet B is read out, and its digital image signal 1 ops8 is similarly stored in the storage medium 17.

Next, the digital image signal 10 obtained as described above is
g5A, l0g5. ．． 3 to perform subtraction processing. FIG. 3 shows the flow of sub-1 to luxion processing performed by applying the gradation processing optimization method according to the first embodiment of the method of the present invention. First, from the image file 17A and the image file 17B in the storage medium 1'', the digital image signals IQ and IsA and IogS are respectively input.
BI! The data is read out and manually inputted to the subtraction calculation circuit 18. This subtraction calculation circuit 18 subjects the two digital image signals 1og3Δ and lOGSB to appropriate weighting and subtracts them for each corresponding pixel, resulting in a digital difference signal 5sub=a−10g5A bIIIOQsB +C(
a, b are weighting coefficients, and C is a bias component that keeps the concentration approximately constant). This difference signal 3sub is stored in the -tan image file 19 and then subjected to image processing times j! 820, and in the image processing circuit 2o, it is converted to a tone conversion table 2 via a signal conversion circuit 20'a, which will be described later.
Gradation processing is performed based on °b.

The difference signal 5sub' that has undergone gradation processing is input to a display device such as a CRT, or a reproducing/recording device 21 such as a scanning recorder, and a subtraction image is reproduced and recorded using the difference signal 3sub'. FIG. 4 shows li! as an example of a subtraximine image reproducing and recording system. ii shows an image scanning and recording device. The photosensitive film 30 is moved in the sub-scanning direction of arrow Y, and the laser beam 3 is caused to main-scan on the photosensitive film 30 in the X direction.
A visible image is formed on the photosensitive film 30 by modulating the laser beam 31 with an image signal from an image signal provider 33 using an A10 modulator 32 . If the difference signal 5sub' is used as this modulation image signal,
An image of only a desired specific structure can be reproduced and recorded on the photosensitive film 30 by digital subtraction processing.

The energy subtraction image recorded and reproduced on the photosensitive film 30 in this way is caused by the differences in the difference signal 3sub due to variations in the two stimulable phosphor sheets A, 8 m degree variations, and variations in the imaging X-ray energy. Because of this variation, the density range and contrast often vary from image to image. Therefore, as shown in FIG. 3, the difference signal 3SUb is input to the histogram calculation circuit 22, and a histogram of the difference signal 3SUb is calculated.

Then, as shown in FIG. 5, the histogram calculation circuit 22 calculates the difference signal 5su from this histogram, for example.
The maximum value Smax and minimum value 51n of b are determined and the information is input to the signal conversion circuit 20a. This signal conversion circuit 20a converts the difference signal 3sub so that the maximum value 5Illax1 and the minimum value S1n always take constant values Cmax and cmin, respectively. That is, for example, 5Sub→pISSub+q p= (Cmax - Cmin) / (Smax
-8m1n)a -(Crnax 1Cm1n>
/ 2 - (Smax + Sm1n) / 2 conversion is performed.

The converted signal p-3sub+q is subjected to gradation processing based on the above-mentioned gradation conversion table 20b, where the constant values Cmax and Cm1n are as shown in FIG.
Maximum density value of subtraction image □max, fi
There are some signals that have the minimum IDm1n. Therefore, the subtraction image based on the converted signal p''5sub1q always has a constant density range and contrast.

As described above, instead of converting the difference signal 5sub by the signal conversion circuit 20a, the gradation conversion table 2 is converted by the gradation conversion table correction circuit 120 as shown in FIG.
A similar effect can be obtained by modifying 0b as shown in FIG.

Further, the histogram of the difference signal may be created only for a certain image portion (for example, a region of interest).

The embodiment using two sheets A and B has been described above, but the other three sheets are radiographed with different energies, and the image signals obtained from these sheets are subjected to subtraction processing (for example, ax I 0QSA 10 b
It is also possible to obtain a subtraction image by adding X I OgSB to X I 0QS (: +d, etc., where a, b, and C are weighting coefficients, and d is a bias component that makes the density approximately constant.) The present invention is also applicable to such cases.

As mentioned above, in order to obtain a radiation image to be subjected to subtraction (that is, a radiation image in which the image information of a specific structure is different from each other), the radiation energy is increased by changing the voltage of the radiation source as in the embodiment described above. In addition to changing the radiation energy distribution, other known methods may be used, such as inserting and removing a filter that changes the radiation energy distribution between the radiation source and the subject. As disclosed in Japanese Patent Application No. 193765, a laminate of a 1F4 phosphor sheet and a filter may be used to obtain the image by one radiography.

(Effects of the Invention) As explained in detail above, according to the present invention, the overall density and contrast of energy subtraction images can be kept common among each image, so subtraction images with extremely excellent diagnostic ability can be obtained. It becomes possible to obtain.

And because the m-pupil and contrast can be kept within a predetermined range,
It becomes possible to make the most effective use of the dynamic range of the recording medium that outputs the subtraction image. Furthermore, since the method of the present invention can be automatically implemented by an electric circuit, in particular, subtraction calculation processing is performed continuously,
This is particularly effective when observing subtraction images in real time using a CRT.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram showing the step of accumulating and recording radiation images in the method of the present invention, FIG. 2 is a schematic diagram illustrating reading of radiation image information from the stimulable phosphor sheet on which the above-mentioned accumulation and recording has been performed, and FIG. 3 is obtained by applying the method of the first embodiment of the method of the present invention.
A schematic diagram illustrating an overview of energy subtraction processing that performs gradation processing, FIG. 4 is a schematic diagram illustrating an example of a subtraction image reproduction/recording system, and FIG. 5 is a gradation processing optimization method according to the first embodiment of the present invention. FIG. 6 is a schematic diagram illustrating an overview of energy subtraction processing that performs gradation processing by applying the second embodiment of the method of the present invention, and FIG. 7 is a diagram illustrating the second embodiment of the present invention. FIG. 2 is an explanatory diagram illustrating a gradation processing optimization method according to the present invention. 1... Subject 2... X-ray 3... X-ray source 10... Laser light source 11... Laser light 12... Scanning mirror 13... Stimulated luminescence light 15... Photomultiple 16... Logarithmic converter 18... Subtraction calculation circuit 20... Image processing circuit 20a... Signal conversion circuit 2
0b... Gradation conversion table 22... Histogram calculation circuit 120... Gradation conversion table correction circuit Δ, B... Stimulative phosphor sheet 10g5. 10g513...Digital image signal 3s
ub...Difference signal of digital image signal 3sub'...
Gradation processed difference signal 1￥1 Figure 3 Figure 5

Claims (6)

[Claims] (1) Two or more stimulable phosphor sheets are irradiated with radiation that has passed through an object that includes a specific structure with different radiation energy absorption characteristics, and these phosphor sheets are used to identify the object. Radiation images with different image information of the structure are accumulated and recorded, and excitation light is scanned over these phosphor sheets to convert the radiation images into stimulated luminescence light, and the amount of luminescence of this stimulated luminescence light is calculated. is photoelectrically read out and converted into a digital image signal, this digital image signal is subtracted between corresponding pixels of each image to obtain a difference signal forming an image of a specific structure in the radiographic image, and then this difference signal is In the gradation processing of energy subtraction in which gradation processing is performed on the histogram, the histogram of the difference signal is calculated to obtain at least two signal values that are different from each other, and then the difference signal is calculated from the at least two signal values. A method for optimizing gradation processing of a subtraction image, characterized in that gradation processing is performed in correspondence with each of at least two predetermined output image densities. (2) The at least two signal values are a maximum signal value and a minimum signal value, and these maximum signal values and minimum signal values are made to correspond to a predetermined maximum output image density value and minimum density value, respectively. Ill! of the subtraction image recited in claim 1, which is i-key processing optimization method. (3) The at least two signal values of the difference signal are made to correspond to the at least two predetermined output image densities by uniformly converting the difference signal before gradation processing. A method for optimizing subtraction image N-tone processing according to scope 1 or 2. (4) The at least two signal values of the difference signal are made to correspond to the at least two predetermined output image densities by modifying the tone conversion table used for the tone processing. A method for optimizing gradation processing of a subtraction image according to item 1 or 2 of the range. (5) Scanning excitation light onto a stimulable phosphor sheet on which a radiation image has been stored and recorded, and thereby photoelectrically reading out the stimulated luminescent light emitted from the stimulable phosphor sheet to convert it into a digital image signal. An image reading means to be converted and a digital image signal of a subject having different image information of the specific structure read by the image reading means are subtracted between corresponding pixels to form an image of the specific structure. subtraction calculation means for obtaining a signal; image processing means for performing gradation processing on the difference signal based on a gradation conversion table; and a histogram of the difference signal to obtain at least two mutually different signal values of the histogram. a histogram calculation means; and a signal conversion circuit that uniformly converts the difference signal before undergoing the gradation processing so that the at least two signal values correspond to at least two predetermined output image densities. A subtraction image gradation processing optimization device. (6) Scanning excitation light onto a stimulable phosphor sheet on which a radiation image has been accumulated and recorded, and thereby photoelectrically reading out the stimulated luminescent light emitted from the stimulable phosphor sheet to convert it into a digital image signal. An image reading means to be converted and a digital image signal of a subject having different image information of the specific structure read by the image reading means are subtracted between corresponding pixels to form an image of the specific structure. subtraction calculation means for obtaining a signal; image processing means for performing gradation processing on the difference signal based on a gradation conversion table; and a histogram of the difference signal to obtain at least two mutually different signal values of the histogram. a histogram calculation means, such that the at least two signal values correspond to at least two predetermined output images;
A subtraction image gradation processing optimization device comprising a gradation conversion table correction circuit that corrects the gradation conversion table.