JPH0262076B2 - - Google Patents

Description

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

(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 two methods for this subtraction process: 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 Radiograpty).
It is abbreviated as DR.

More recently, as shown in JP-A-58-163340, for example, two stimulable phosphor sheets with an extremely wide radiation exposure area have been used, and these phosphor sheets are coated with a contrast agent as described above. Radiation that has passed through the same subject is irradiated under different conditions, with and without, 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 images are It has also been proposed to read out the information by scanning with excitation light 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 excited light such as visible light is irradiated, the phosphor emits photostimulated light according to the accumulated energy. It has an extremely wide latitude (exposure range) and extremely high resolution. Therefore, if the digital subtraction is performed using the radiation image information accumulated and recorded on this phosphor sheet, a radiation 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 above background The 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 a subtraction image of the present invention includes the following steps in the time subtraction processing performed using a stimulable phosphor sheet as described above.
Prior to main reading to obtain the digital image signal used for time subtraction processing, pre-reading is performed to read the accumulated image information of the phosphor sheet using excitation light of a lower level than the excitation light used in the main reading. , find the maximum transmitted radiation dose of each pre-read digital image obtained in the pre-reading, and then find the difference Δχ between these maximum values,
When performing gradation processing on the difference signal obtained by subtraction, (i) the difference Δχ is applied to the difference signal before gradation processing;
(ii) A correction that shifts the gradation conversion table by the difference Δχ to the higher density side of the input signal axis.

Regarding the above “main reading” and “pre-reading”,
For example, it is disclosed in detail in Japanese Unexamined Patent Publication No. 58-89245.

In the automatic density correction method for subtraction images of the present invention, which implements the above method, when performing the correction in (i) above, the accumulation an image reading means for obtaining a digital image signal of a radiographic image of the phosphor sheet; and an image reading means for reading the image (pre-reading) using excitation light at a lower level than the excitation light used for image reading (main reading), a pre-reading means for obtaining a pre-read digital image signal of a radiation image of a stimulable phosphor sheet; a digital image signal of a subject in which a contrast agent is injected into a specific structure read by the image reading means; and a digital image signal of a subject in which a contrast agent is injected into a specific structure, and a contrast agent is not injected. subtraction calculation means for obtaining a difference signal for forming an image of a specific structure by subtracting a digital image signal of a subject between corresponding pixels; the maximum transmitted radiation dose of the pre-read digital image signal of the subject whose specific structure has been injected with a contrast agent and the pre-read digital image signal of the subject to which no contrast agent has been injected, read by the pre-reading means; and a signal correction circuit that calculates the difference Δχ between the maximum values and adds this difference Δχ to the difference signal before undergoing gradation processing.

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 Δχ, the above-mentioned correction (ii) can be performed. be able to.

The maximum transmitted radiation dose of each pre-read digital image signal read out from each stimulable phosphor sheet is determined, for example, from the histogram of the pre-read digital image signals.

The histogram of the pre-read digital image signal read out from each stimulable phosphor sheet is as shown in Figure 1, for example, from the minimum values y _A and y _B of the minimum transmitted radiation dose to the maximum of the background density. The transmitted radiation dose corresponding to is distributed between the maximum values χ _A and χ _B . Although the minimum values y _A and y _B may change depending on the presence or absence of the contrast agent, the maximum values χ _A and χ _B corresponding to the maximum background density should originally be constant. Therefore, if the maximum values χ _A and χ _B differ for each 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 sheet. 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.

Note that the excitation light used for pre-reading is at a lower level than the excitation light used for main reading.
This means that the effective energy of the excitation light that the stimulable phosphor sheet receives per unit area during pre-reading is smaller than that during main reading. As a method to make the excitation light of the pre-reading at a lower level than the excitation light of the main reading, there is a method of reducing the output of the excitation light source such as a laser light source, and a method of passing the excitation light emitted from the light source to an ND filter, AOM, etc. in its optical path. A method of attenuating the excitation light, a method of providing a light source for pre-reading and a light source for main reading separately, and making the output of the former smaller than the output of the latter, and furthermore, a method of increasing the beam diameter of the excitation light. , a method of increasing the scanning speed of excitation light, and a method of increasing the transport speed of the stimulable phosphor sheet.

(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. That is, for example, in digital angiography, an X-ray transmission image of the subject 1 before the contrast agent of the blood vessel is injected is stored and recorded on the first stimulable phosphor sheet A in the state shown in FIG. A contrast medium is injected into the vein of the same subject 1, and in the case of the abdomen, for example, after about 10 seconds, an X-ray transmission 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, and the subject 1
The positional relationship between and the phosphor sheets A and B is also the same,
Two X-ray images with no difference other than the presence or absence of a contrast agent are stored and recorded in A and B.

In this way, two radiographic images with different image signals from 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, in the look-ahead section 100, move the stimulable phosphor sheet A along the arrow
While moving the laser beam 11 from the laser light source 10 for sub-scanning in a direction substantially orthogonal to It is emitted as stimulated luminescence light 13 according to the 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 light emission amount of 13 is output as the pre-read image signal SP.
This output pre-read image signal SP is converted into a pre-read digital image signal logSP _A of logarithmic value (logSP) by a logarithmic converter 16 including an amplifier and an A/D converter. This look-ahead digital image signal logSP _A
is stored in a storage medium 17 such as a magnetic tape. Next, in exactly the same manner, the recorded image on another stimulable phosphor sheet B is read out, and the pre-read digital image signal logSP _B is similarly sent to the storage medium 17.
is memorized.

Next, the stimulable phosphor sheet A is sent to a main reading section 200 which includes a laser light source 40, a scanning mirror 42, a light condensing plate 44, and a photoprinter 45, similar to the pre-reading section 100 described above. In this main reading section 200, the stimulated luminescence light 43 emitted from the stimulable phosphor sheet A by irradiation with the laser beam 41 is input to the photoprinter 45,
The image information stored and recorded in is read out photoelectrically (main reading). Note that the output of the laser light source 10 for pre-reading is smaller than the output of the laser light source 40 for main reading (preferably 10% or less,
More preferably, it is set to about 3% or less) so that a large amount of radiation energy accumulated in the stimulable phosphor sheet A is not dissipated before reading.

The book reading image signal S output from the book reading format 45 is amplified by the amplifier 46, and the A/D
It is converted into a digital signal by a converter 47 and then inputted to a logarithmic converter 48.
Logarithmic (logS) digital image signal by
Converted to logS _A. This digital image signal logS _A
is stored in a storage medium 49 such as a magnetic tape. Next, in exactly the same manner, the recorded image on another stimulable phosphor sheet B is read out, and the digital image signal logS _B of the main reading is similarly stored in the storage medium 49. Next, subtraction processing is performed using the digital image signals logS _A and logS _B of the main reading obtained as described above. FIG. 4 shows the flow of subtraction processing performed while performing automatic density correction according to one embodiment of the method of the present invention. First, the digital image signals logS _A and logS _B described above are read out from the image file 49A and the image file 49B in the storage medium 49, respectively, and input to the subtraction calculation circuit 50. This subtraction calculation circuit 50 subtracts the two digital image signals logS _A and logS _B for each corresponding pixel to obtain a digital difference signal Ssub. This difference signal Ssub is once transferred to the image file 5.
1 and then input to the image processing circuit 52, where the image is subjected to gradation processing based on the gradation conversion table 52a.

The difference signal Ssub′ that has undergone image processing is, for example, a CRT
The subtraction image is inputted to a display device such as, or a reproducing/recording device 53 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, which records an X-ray image before the contrast medium was injected into the abdomen, and 4B is an X-ray image of the same area after the contrast medium was injected. The image obtained from the second stimulable phosphor sheet B, 4C, is 4
This is a subtraction image obtained by subtracting the digital image signal representing the image 4A from the digital image signal representing the image B, 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, the density of this background BK portion varies due to variations in the intensity of the imaging radiation and variations in the sensitivity of the stimulable phosphor sheets A and B. Therefore, as shown in FIG. 4, image files 17a, 1 of the storage medium 17 are
Pre-read digital image signal stored in 7b
The logSP _A and logSP _B are input to a histogram calculation circuit 60, and the histograms of the respective signals logSP _A and logSP _B are determined in the histogram calculation circuit 60. As shown in FIG. 1, these histograms indicate the maximum values χ _A , χ _B of the maximum transmitted radiation dose, and the minimum values y _A , y B of the minimum transmitted radiation dose, which correspond to the maximum density of the background _BK , respectively. distributed between. As mentioned above, the maximum values χ _A and χ _B are supposed to be constant in both images, but as mentioned above, there are differences in radiation intensity during imaging and the sensitivity of the stimulable phosphor sheets A and B. Due to the difference, the background density changes from sheet to sheet, and the maximum values χ _A and χ _B differ. Therefore, the signals representing the maximum values χ _A and χ _B are sent to the signal correction circuit 61, and the difference Δχ between the two maximum values χ _A and χ _B is calculated.
This difference Δχ is uniformly added to the difference signal Ssub. By adding the difference Δχ to the difference signal Ssub in this way, the signal component (χ _A −χ _B , i.e. Δχ)
is corrected, and the background signal always has a 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 χ _A and χ _B are input from the histogram calculation circuit 60 to the gradation conversion table correction circuit 161 . The gradation conversion table 52a used in the image processing circuit 52 is as shown in FIG. 8, but the gradation conversion table correction circuit 161, like the signal correction circuit ₆₁ , The difference Δχ from _B is calculated, and the original gradation conversion table 52a (solid line in FIG.
(dashed line in FIG. 8). In this way, the gradation conversion table 5
2a 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 Δχ 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 by the original gradation conversion table 52a. Therefore, in the subtraction image obtained by the difference signal Ssub', the density of the background BK is always constant.

In the above embodiment, the output of the laser light source for pre-reading is made smaller than the output of the laser light source for main reading, so that the excitation light for pre-reading is at a lower level than the excitation light for main reading. The method of setting the excitation light for pre-reading and the excitation light for main reading at a lower level is not limited to this, and another method as described above may be adopted. Furthermore, the image reading means for performing main reading and the pre-reading means may be used together, and the excitation light level may be changed to perform "main reading" and "pre-reading".

Furthermore, in the present invention, the method of determining the maximum value of transmitted radiation dose of the digital image signal obtained by pre-reading is not limited to the method of calculating from the histogram. Methods such as selecting the following can be applied. Therefore, it goes without saying that in the first and second embodiments described above, the histogram calculation circuit 60 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 the time subtraction image obtained using two stimulable phosphor sheets can be always set constant, making it extremely easy to diagnose. This makes it possible to obtain subtraction images with excellent quality and high utility in the medical field.

[Brief explanation of the drawing]

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. Figure 5 is a schematic diagram showing an example of a subtraction image reproduction/recording system, and Figure 6 is a diagram showing radiographic images with contrast agent injection, radiographic images without contrast agent injection, and time subtraction images obtained from these radiographic images. A schematic diagram illustrating an example; FIG. 7 is a schematic diagram illustrating an overview of subtraction processing for automatic density correction according to a 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, 10,40...Laser light source, 11,4
1... Laser light, 14, 42... Scanning mirror,
13, 43... Stimulated luminescent light, 15, 45... Photomal, 16, 48... Logarithmic converter, 46... Amplifier, 47... A/D converter, 50... Subtraction calculation circuit, 60 ... Histogram calculation circuit, 61 ... Signal correction circuit, 100 ... Look-ahead section, 161 ... Gradation conversion table correction circuit, 20
0...Main reading section, A, B...Storage phosphor sheet, logSP _A , logSP _B ...Pre-read digital image signal, logS _A , logS _B ...Digital image signal by main reading, Ssub...Digital image signal difference signal.

Claims (1)

[Scope of Claims] 1. Two stimulable phosphor sheets are each irradiated with radiation that has passed through a subject whose specific structure is injected with a contrast agent and a subject where no contrast agent is injected. accumulating and recording radiation images with mutually different image information of the specific structure on a light 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 correspondence between the two images is determined. 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 a radiation image as described above, prior to the main reading to obtain the digital image signal used in the time subtraction processing, the excitation light having a lower level than the excitation light used in the main reading is used. Pre-reading is performed to read the accumulated image information of the two phosphor sheets, and the maximum transmitted radiation dose of each pre-read digital image signal obtained in the pre-reading is determined, and then the difference Δχ between these maximum values is determined, and then ( i) a correction that adds the difference Δχ to the difference signal before the gradation processing; (ii) a correction that shifts the gradation conversion table by the difference Δχ to the higher density side of the input signal axis. An automatic density correction method for subtraction images characterized by performing correction. 2. Scanning excitation light onto a stimulable phosphor sheet on which a radiation image has been accumulated and recorded, thereby photoelectrically reading out the stimulated luminescent light emitted from the stimulable phosphor sheet and converting it into a digital image signal. and scanning the stimulable phosphor sheet with excitation light of a lower level than the excitation light of the image reading means prior to actual reading by the image reading means, whereby the stimulable phosphor sheet is scanned with excitation light of a lower level than the excitation light of the image reading means a pre-reading means for photoelectrically reading the stimulated luminescence light emitted from the body sheet and converting it into a pre-read digital image signal; and a digital image signal of a subject having a specific structure injected with a contrast agent, which is read by the image reading means. and a digital image signal of the subject to which no contrast agent is injected, between corresponding pixels to obtain a difference signal to form an image of the specific structure; an image processing means that performs gradation processing based on a conversion table; a pre-read digital image signal read by the pre-reading means of a subject whose specific structure is injected with a contrast agent; and a pre-read digital image signal of the subject to which no contrast agent is injected. A sub-unit consisting of an arithmetic means for calculating the maximum value of each transmitted radiation dose of the image signal, and a signal correction circuit that calculates the difference Δχ between the respective maximum values and adds the difference Δχ to the difference signal before undergoing the gradation processing. Automatic density correction device for traction images. 3. Scanning excitation light onto a stimulable phosphor sheet on which a radiation image has been accumulated and recorded, thereby photoelectrically reading out the stimulated luminescent light emitted from the stimulable phosphor sheet and converting it into a digital image signal. and scanning the stimulable phosphor sheet with excitation light of a lower level than the excitation light of the image reading means prior to actual reading by the image reading means, whereby the stimulable phosphor sheet is scanned with excitation light of a lower level than the excitation light of the image reading means a pre-reading means for photoelectrically reading the stimulated luminescence light emitted from the body sheet and converting it into a pre-read digital image signal; and a digital image signal of a subject having a specific structure injected with a contrast agent, which is read by the image reading means. and a digital image signal of the subject to which no contrast agent is injected, between corresponding pixels to obtain a difference signal to form an image of the specific structure; an image processing means that performs gradation processing based on a conversion table; a pre-read digital image signal read by the pre-reading means of a subject whose specific structure has been injected with a contrast agent; and a pre-read digital image signal of the subject to which no contrast agent has been injected. a calculation means for calculating each maximum value of transmitted radiation dose of the image signal; and a gradation conversion table for calculating the difference Δχ between the respective maximum values and shifting the gradation conversion table by the difference Δχ toward the higher density side of the input signal axis. An automatic density correction device for subtraction images consisting of a correction circuit.