JPS60251475A - Method and unit for optimizing gradation processing of subtraction picture - Google Patents

Abstract

PURPOSE:To obtain a subtraction picture excellent in diagnostic ability by finding out the maximum and minimum values of a differential signal obtained by subtraction and executing gradation processing in accordance with the previously determined maximum and minimum values of output picture density. CONSTITUTION:X-rays are irradiated to the same subjects on accumulative phospher sheets A, B with different energy, X-ray pictures are read out from these sheets A, B and digital picture signals logSA, logSB are obtained and stored in a storage means 17. The gradation processing maximizing method is applied and the signals logSA, logSB are read out from picture files 17A, 17A respective and inputted to a subtraction operation circuit 18. The circuit 18 weights both the l signals, subtracts in each corresponding picture element and inputs its digital differential signal Ssub to a picture processing circuit to execute gradation processing on the basis of a gradation conversion table 20b throug a signal converting circuit 20a. The processed differential signal Ssub' is inputted to a reproducing/recording device 21 and a subtraction picture is reproduced/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 of photoelectrically reading out two radiographic images taken under different conditions to obtain digital image signals, and then subtracting these digital image signals by matching each pixel of both images. This is a method of obtaining a difference signal to form an image of a specific structure in a radiographic image, and using the difference signal obtained in this way to reproduce a radiographic image in which only the specific structure has been extracted. I can do it.

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).
So-called temporal subtraction processing, which extracts specific structures by irradiating the same subject with radiation with different energy distributions, allows specific structures to have unique radiation energy absorption characteristics. Taking advantage of this fact, images with different specific structures are made to exist between two radiographic images, and then a subtract (subtracting) is performed in which the image signals of these two radiographic images are subtracted with appropriate weighting. This is a so-called energy subtraction process that extracts an image.

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 applicable to digital subtraction processing (usually Digital Ra
dioqraphy) and is 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 then when excitation light such as visible light is irradiated, the phosphor shines according to the amount of accumulated radiation energy. It exhibits exhaustion, has an extremely wide latitude (exposure range), and has extremely high resolution. Therefore, if the digital subtraction is performed using the radiographic image information stored on this phosphor sheet, sufficient image information can be obtained at all times even if the radiation dose fluctuates, and diagnostic performance can be improved. High quality radiographic images 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 is to form an output image, each output image has a different density range (or brightness range). and the contrast often varies. This is because the image signals of the two images subjected to subtraction vary due to the aforementioned variations in the sensitivity of the stimulable phosphor sheet, variations in the sensitivity of the image reader, and variations in the amount of radiographic radiation energy. It is something. Since the difference signal obtained by subtraction is minute, it is effective to perform gradation processing to amplify the difference signal, but if the image signal fluctuates as described above, the difference signal The variation component becomes considerably larger than the original signal component, and appears as large fluctuations in the 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 the ability to make a diagnosis by comparing multiple subtraction images. It has been pointed out that the density of the image deviates from the area.

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 subtraction image gradation processing optimization method of the present invention is based on the signal maximum value of the difference signal obtained by subtraction, the signal The minimum value is determined, and then the difference signal is subjected to gradation processing by making the signal maximum and minimum values correspond to predetermined output image density maximum and minimum values, respectively.

The signal maximum value and signal minimum value of the above-mentioned difference signal can be determined by sequentially comparing the difference signals, or by creating a histogram of the difference signal and determining it from this histogram. Compared to the method of creating and interlacing histograms, this method has the advantage that it can be easily configured using only hardware, and the calculation speed is fast.

In order to make the signal maximum value and minimum value of the difference signal obtained as described above correspond to the predetermined output image density maximum value and minimum value, the difference signal may be uniformly converted into signals before gradation processing. death,
Alternatively, the gradation conversion table may be corrected.

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. subtraction calculation means for obtaining a difference signal that forms an image of the specific structure; image processing means for performing gradation processing on this difference signal based on a gradation conversion table; a maximum/minimum calculation means for calculating a value, and the difference signal before being subjected to the gradation processing so that the maximum and minimum values of the signal correspond to predetermined maximum and minimum values of output image density, respectively. It consists of a signal conversion circuit that uniformly converts the signals.

In addition, instead of the signal conversion circuit, the gradation conversion table is corrected so that the signal maximum value and minimum value of the difference signal correspond to predetermined predetermined output image density maximum and minimum values, respectively. If you use the key conversion table correction circuit,
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. That is, the X-ray transmission 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 tube of the X-ray source 3 is By changing the voltage, X-ray images of the subject 1 with 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
It is assumed that the positional relationship of the 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,
An X-ray image is read from B by an image reading means as shown in FIG. 2, and a digital image signal representing the image is obtained. 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 according to the accumulated and recorded X-ray image. The stimulated luminescent light 13 enters the interior of the condensing plate 14 from one end surface of the condensing plate 14 made by molding a transparent acrylic plate, and reaches the photomultiplier 15 through repeated total reflection, where it is stimulated to emit light. The amount of light 13 emitted is output as an image signal @S. This output image signal S is converted into a logarithmic (1 ops) digital image signal 10QSA by a logarithmic converter 16 including an amplifier and an A/D converter. This digital image signal 10QsA 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 logsB is similarly stored in the storage medium 17.

Next, the digital image signal +0 obtained as described above is
Subtraction processing is performed using QsA and IoqSB. FIG. 3 shows the flow of subtraction 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 17,
The digital image signals 1ogs and 10g5B are read out and input to the subtraction calculation circuit 18. This subtraction calculation circuit 18 subjects the two digital image signals 1OGsA and 1oqsB to appropriate weighting and subtracts them for each corresponding pixel to obtain a digital difference signal 5sub=a @ 10QsA b ・10QsB+
c (a and b are weighting coefficients, and C is a bias component that makes the density approximately constant). This difference signal 5Sub
After being stored in the image file 19, it is input to the image processing circuit 20, and in the image processing circuit 20, it is converted to a tone conversion table 20b via a signal conversion circuit 20a, which will be described later.
Gradation processing is performed based on.

The difference signal 3sub' 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 5SUb'. FIG. 4 shows an image scanning and recording apparatus 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.
A visible image is formed on the photosensitive film 30 by modulating the image signal from the photosensitive film 30. If the difference signal 5sub' is used as the 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.

In the energy subtraction image recorded and reproduced on the photosensitive film 30 in this way, the difference signal 3sub fluctuates due to variations in sensitivity of the two stimulable phosphor sheets A and B, variations in imaging X-ray energy, etc. Therefore, 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 5sub is calculated.

Then, as shown in FIG. 5, the histogram calculation circuit 22 calculates, for example, the difference signal 3su from this histogram.
The maximum value Smax and minimum value Sm1n of b are determined and the information is input to the signal conversion circuit 20a. This signal conversion circuit 20a converts the difference signal 5sub so that the maximum value Smax and minimum value 51n always take constant values Cmax and Cm1n, respectively. That is, for example, 5sub-+p-8sub+q p-(Cmax-Cmin)/(SIIlaX
−8llin )q = (Cmax + Cm1
n) / 2- (Smax -+-8m1n)
/2 conversion is performed.

The converted signal p-3Sub+q is subjected to gradation processing based on the gradation conversion table 20b described above, where the constant values Cmax and Cm1n are as shown in FIG.
This signal is used to obtain the maximum density value DIIla× and the minimum density value Omin of the subtraction image. Therefore, the subtraction image based on the converted signal p-8sub+q always has a constant density range and contrast.

FIG. 6 shows the flow of energy subtraction processing performed by applying the gradation processing optimization method according to the second embodiment of the present invention. In this embodiment, instead of the histogram calculation circuit 220 used in the first embodiment, sequential comparison calculation circuits 122a and 122b are used as maximum and minimum calculation means, and gradation conversion circuits 122a and 122b are used as a replacement for the signal conversion circuit 20a. A table correction circuit 120 is used.

The sequential comparison calculation circuit 122a stores the first difference signal 5sub sent from the subtraction calculation circuit 18 as the maximum value, and compares the difference signal 3sub sent next with the stored difference signal 3sub. do. Then, the difference signal 3sub with the larger signal value is stored as the maximum value, and the same processing is sequentially performed on the subsequent difference signals 3sub sent. When this process is sequentially performed up to the last difference signal 5sub, the true maximum value Smax' in all five regions of the difference signal 3sub is found. In addition, the sequential comparison calculation circuit 122b similarly compares the difference signals 5sub and stores the smaller difference signal 5sub as the minimum value, so that the true minimum value 3 in the entire range of the difference signals 5sub
Enter m1n'.

Note that by using this sequential comparison method, the maximum of the difference signal 3sub! When setting the minimum value 3m1n + for aSIllax, in addition to setting the difference signal 5Sub sent first as the maximum value 3max' and minimum value 5m1n' as described above, An arbitrary signal value that is sufficiently small may be stored in advance as the maximum value, and an arbitrary signal value that is sufficiently larger than the assumed minimum value 3m1n' may be stored in advance as the minimum value.

The true maximum value 3max' and minimum value 3m1n' determined as described above are sent to the subtraction circuit 121a and the addition circuit 121b, respectively. In the subtraction circuit 121a, a predetermined signal level SΔa is subtracted from the true maximum value 3max,', and (3max'-8Δa) is output as the maximum value 3max for gradation processing optimization.

In addition, in the adder circuit 121b, the true minimum value 3m1 is
A predetermined signal level SΔb is added to n', (3min
' +3Δb〉 is the minimum value 3 m for gradation processing optimization
Output as in. Gradation conversion table correction circuit 12
0 receives the above maximum value smax and minimum value Smin, and as shown in FIG. Correct the gradation conversion table 20b as follows (indicated by a broken line in the figure).
. By correcting the gradation conversion table 20b in this way, the density range and contrast of the subtraction image can be kept constant, as in the first embodiment.

As shown in FIG. 7, the difference signal 3sub includes only noise components due to signal noise and scattered radiation during imaging on the maximum and minimum level sides. The signal level SΔa1 subtracted from the maximum value Smax' of the component and the signal level SΔb added to the true minimum value 31n' are set to values corresponding to the above-mentioned noise component. Therefore, the maximum value for gradation processing optimization is 3 maX, and the minimum value is 3 min.
as (Smax'-8Δa), (31Il
By using the value of in '+SΔb), it is possible to remove the effects of the signal noise and scattered radiation, and to make more effective use of the dynamic range of the recording medium of the subtraction image.

Another method is to use a value that is smaller than the true maximum value Smax' by a certain number of data and the true minimum value 3.
Values that are larger than m1n' by a certain number of data may be used as the maximum value Smax and minimum value Sm1n for gradation processing optimization, respectively.

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, axlogs + bxlog
gS C× 10Qs 8 B- +d etc. However, here, a, b, and C are weighting coefficients, and d is a bias component that makes the density approximately constant.

) It is also possible to obtain a subtraction image, and the present invention is also applicable to such a case. Further, the maximum value and minimum value of the difference signal may be determined only for a certain image portion (for example, a region of interest).

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 tube voltage of the radiation source as in the above embodiment. 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 stimulable 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. becomes possible.

And since the density and contrast can be maintained within the specified 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 drawings]

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 4 is a schematic diagram illustrating an overview of energy subtraction processing that performs gradation processing by applying the first embodiment of the method of the present invention; FIG. 4 is a schematic diagram illustrating an example of a subtraction image reproduction/recording system; FIG. 5 FIG. 6 is an explanatory diagram illustrating the gradation processing optimization method according to the first embodiment of the present invention, and FIG. FIG. 7 is an explanatory diagram illustrating the gradation processing optimization method according to the second embodiment of 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 122a, 12
2b...Sequential comparison calculation circuit A, 8...Storage phosphor sheet I o gSA, I o'asB...Digital image signal 5sub...Difference signal of digital image signal 5sub
'...Difference signal g after gradation processing + Figure 1!3 Figure 1!5 Figure

Claims (14)

[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 across these phosphor sheets to convert the radiation images into stimulated luminescence light, and the amount of luminescence of this stimulated luminescence light is converted into photoelectric radiation. This digital image signal is subtracted between corresponding pixels of each image to obtain a difference signal that forms an image of a specific structure in the radiographic image, and then this difference signal is converted into a digital image signal. In the tone processing of energy subtraction, which performs tone processing,
The signal maximum value and signal minimum value of the difference signal are determined, and then the difference signal is graded by making the signal maximum value and signal minimum value correspond to predetermined output image density maximum and minimum density values, respectively. A method for optimizing tone processing of a subtraction image, which is characterized by carrying out tone processing. (2) the signal maximum value is a value obtained by subtracting a predetermined signal level from the true maximum value in the entire region of the difference signal;
The subtraction image gradation processing optimization method according to claim 1, wherein the signal minimum value is a value obtained by adding a predetermined signal level to the true minimum value in the entire region of the difference signal. . (3) The signal maximum value is a value that is a certain number of data smaller than the true maximum value in the entire area of the difference signal, and the signal minimum value is the true minimum value in the entire area of the difference signal. 2. A method for optimizing gradation processing of a sub-1 motion image according to claim 1, wherein the number of data is calculated from . (4) The gradation of the subtraction image according to any one of claims 1 to 3, wherein the maximum signal value and the minimum signal value are determined by sequentially comparing the difference signals. Processing optimization method. (5) A patent characterized in that the maximum value and minimum value of the difference signal are made to correspond to the output image density maximum value and minimum density value by uniformly converting the difference signal before gradation processing. A subtraction image gradation processing optimization method according to any one of claims 1 to 4. (6) A patent characterized in that the maximum value and minimum value of the difference signal are made to correspond to the output image density maximum value and minimum density value by modifying the gradation conversion table used in the gradation processing. A subtraction image gradation processing optimization method according to any one of claims 1 to 4. (7) 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. a subtraction calculation means for obtaining a signal; an image processing means for performing gradation processing on the difference signal based on a gradation conversion table; a maximum-minimum calculation means for calculating a signal maximum value and a signal minimum value of the difference signal; a signal conversion circuit that uniformly converts the difference signal before undergoing the gradation processing so that the signal maximum value and minimum value correspond to predetermined output image density maximum and minimum values, respectively; A gradation processing optimization device for subtraction images. (8) The maximum/minimum calculation means sets the value obtained by subtracting a predetermined signal level from the true maximum value in the entire area of the difference signal as the signal maximum value, and sets it as the true minimum value in the entire area of the difference signal. 8. The subtraction image gradation processing optimization method according to claim 7, wherein the signal minimum value is a value obtained by adding a predetermined signal level. (9) The maximum/minimum calculation means calculates the number of data counted from the true maximum value in the entire area of the difference signal. 8. The method for optimizing gradation processing of a subtraction image according to claim 7, wherein the signal is configured to take a value that is larger than the minimum value by a certain number of data as the signal minimum value. (10) The maximum/minimum calculation means is a sequential comparison calculation means that sequentially compares the difference signals and obtains the signal maximum value and signal minimum value of the difference signal. The gradation processing optimization device for a subtraction image according to any one of Items 7 to 9 above. (11) 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. a subtraction calculation means for obtaining a signal; an image processing means for performing gradation processing on the difference signal based on a gradation conversion table; a maximum-minimum calculation means for calculating a signal maximum value and a signal minimum value of the difference signal; and a gradation conversion table correction circuit that corrects the gradation conversion table so that the maximum and minimum signal values correspond to predetermined output image density maximum and minimum values, respectively. Processing optimization device. (12) The maximum/minimum calculation means sets a value obtained by subtracting a predetermined signal level from the true maximum value in the entire region of the difference signal as the signal maximum value, and sets a predetermined signal to the true minimum value in the entire region of the difference signal. 12. The subtraction image gradation processing optimization method according to claim 11, wherein the signal minimum value is a value obtained by adding a level. (13) The maximum/minimum calculation means sets a value that is smaller than the true maximum value in the entire region of the difference signal by a certain number of data as the signal maximum value, and determines the true minimum value in the entire region of the difference signal. 12. The subtraction image gradation processing optimization method according to claim 11, wherein the signal minimum value is set to a value that is larger by a certain number of data counted from . (14) Claim 11, characterized in that the maximum/minimum calculation means is a sequential comparison calculation means that sequentially compares the difference signals and obtains the signal maximum value and signal minimum value of the difference signal. 13. A subtraction image gradation processing optimization device according to any one of paragraphs 1 to 13.