JPS60227735A - Automatic density correcting method and apparatus of subtraction image - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of the Invention) The present invention relates to subtraction processing of radiographic images, particularly digital subtraction processing of radiographic images using a stimulable phosphor sheet, in which a constant and appropriate background density is always maintained. The present invention relates to a method and apparatus for automatically correcting the density of a subtracted image to obtain a subtracted image.

(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 medium has been injected is subtracted from the image signal of a radiographic image in which specific structures have been emphasized by contrast medium injection (Zabtract).
So-called temporal subtraction processing that extracts specific structures by irradiating the same subject with radiation with different energy distributions, and then applying appropriate weighting between the image signals of these two radiation images. This is a so-called energy subtraction process in which an image of a specific structure is extracted by performing a subtraction W (subtract).

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
diography) and 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 are 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 the contrast agent, 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 means that when irradiated with radiation (X-rays, α-rays, β-rays, γ-rays, ultraviolet rays, etc.), the radiation energy is released as disclosed in, for example, Japanese Patent Application Laid-Open No. 55-12429. When a portion of the phosphor is accumulated in the phosphor and then irradiated with excitation light such as visible light, the phosphor exhibits stimulated luminescence depending on the accumulated energy, and has an extremely wide latitude (exposure range). It has an extremely high resolving power. therefore,
By performing the digital subtraction using the radiation image information stored 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 time subtraction as explained above, the background, except for the specific structure into which the contrast agent has been injected, will always have a constant density. It should be. However, during radiography, even if the radiation intensity is set constant, the intensity of the actually irradiated radiation varies widely, and there is also variation in the sensitivity of the stimulable phosphor sheet. The m degree of the background often varies depending on each subtraction image. It has been pointed out that if the degree of delinquency varies, proper diagnosis may be 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 always has a constant background I! It is an object of the present invention to provide a method for automatic 8-degree correction of a temporal subtraction image and an apparatus for carrying out the method, which can obtain a subtraction image of Maro.

(Structure of the Invention) The automatic density correction method for subtraction images of the present invention includes:
As mentioned above, in the time subtraction processing performed using a stimulable phosphor sheet, prior to the main reading to obtain the digital image signal used in the time subtraction processing, excitation light of a lower level than the excitation light used in the similar book reading is used. perform pre-reading to read the accumulated image information of the phosphor sheet using the phosphor sheet, and determine the maximum transmitted radiation dose of each pre-read digital image obtained in the pre-reading,
After that, when calculating the difference Δχ between these maximum values and performing gradation processing on the difference signal obtained by subtraction, 1) Correction by adding the difference Δχ to the difference signal before gradation processing, ++>gradation Either one of the corrections is performed by shifting the key conversion table by the difference Δχ to the larger S degree side of the input signal axis.

Note that the above-mentioned "main reading" and "pre-reading" are disclosed in detail in, for example, Japanese Patent Application Laid-open No. 89245/1983.

The automatic density correction device for a subtraction image of the present invention that implements the above method, when performing the correction in 1) above,
An image reading means for obtaining a digital image signal of a radiation image of a stimulable phosphor sheet by scanning the excitation light and charging readout of the stimulated luminescent light as described above, and this image reading means is used for image reading (actual reading). a pre-reading means for performing image reading (pre-reading) with excitation light at a lower level than excitation light of a stimulable phosphor sheet to obtain a pre-read digital image signal of a radiation image of a stimulable phosphor sheet; and a specific structure read by the image reading means. subtraction calculation means for obtaining a difference signal for forming an image of a specific structure by subtracting between corresponding pixels a digital image signal of a subject to which a contrast medium has been injected and a digital image signal of a subject to which no contrast medium has been injected; an image processing means that performs gradation processing on this difference signal based on a gradation conversion table; a pre-read digital image signal of a subject in which a contrast agent has been injected into a specific structure read by the pre-reading means; and a contrast agent. calculation means for calculating the maximum value of each transmitted radiation dose of the pre-read digital image signal of the subject to which no radiation is injected; calculating the difference Δχ between the maximum values; and adding this difference Δχ to the difference signal before being subjected to gradation processing. It consists of a signal correction circuit.

Further, in place of the signal correction circuit, a tone conversion table correction circuit for shifting the tone conversion table to the higher density side of the input signal axis by the difference Δχ may be provided.
) can be corrected.

The maximum transmitted radiation dose of each pre-read digital image signal read out from each disposable 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, for example, as shown in Figure 1, and the minimum values V and VB, where the amount of transmitted radiation is minimum, correspond to the maximum background density. The transmitted radiation dose is distributed between the maximum values χ4 and χ8. The minimum values VA and yB vary depending on the presence or absence of the contrast agent, but the maximum value χ7 corresponding to the maximum background density,
χ etc. should originally be constant. Therefore, if the maximum values χΔ and χB differ from image to image, this is considered to be due to differences in radiation intensity at the time of imaging or differences in sensitivity of the stimulable phosphor sheet. Therefore, if the above-mentioned correction is performed to eliminate the difference in the maximum image signal value between each image, the 8I trace signal in the background part will always have an O (zero) value when both digital image signals are subtracted, and the subtraction image will be The back Jff1 degree is always constant.

Furthermore, the fact that the excitation light used for pre-reading is at a lower level than the excitation light used for main reading means that the effective energy of the excitation light that the stimulable phosphor sheet receives per unit area during pre-reading is lower than the excitation light used for main reading. It means smaller than the actual value. As a method of making the pre-reading excitation light lower than the main reading excitation light, there is a method of adjusting the output of an excitation light source such as a laser light source, or using an ND filter, AOM, etc. in the optical path of the excitation light emitted from the light source. There are two methods: 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 a method of increasing the beam diameter of the excitation light, Examples include a method of increasing the scanning speed of excitation light and a method of increasing the transport speed of the stimulable phosphor sheet.

(Embodiment) Embodiments of the present invention will be described below 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, that is, subject 1.
This shows the state in which a specific structure is irradiated with or without contrast agent injection. That is, for example, in angiography, the first stimulable phosphor sheet A is coated in the state shown in FIG.
The X-ray transmission images of the subject 1 before the contrast medium is injected into the blood vessels are accumulated and recorded, and then the contrast medium is injected into the veins of the same subject 1. For example, in the case of the abdomen, after about 10 seconds have elapsed, this subject is similarly imaged. The X-ray transmission images of No. 1 are accumulated and recorded. At this time, the tube voltage of the X-ray source 3 is the same, and the subject 1, the phosphor sheet A,
The positional relationship with AlB is also the same, and two X-ray images with no difference other than the presence or absence of a contrast agent are stored and recorded in AlB.

In this way, two radiographic images having different image information of the contrast agent injection part are accumulated and recorded on the two stimulable phosphor sheets t-A and t-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. 3, and a digital image signal representing the image is obtained. First, in the pre-reading section 100, the stimulable phosphor sheet A is
While moving the laser beam in the Y direction for sub-scanning, the laser beam 11 from the laser light source 10 is scanned in the X direction by the scanning mirror 12, and the accumulated X-ray energy is accumulated and recorded from the phosphor sheet 8. Stimulated luminescent light 13 is emitted according to the 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 a pre-read image signal QSP. The output pre-read image signal SP is converted to a logarithmic value (IO
QSP) is converted into a pre-read digital image signal 1oasP,a. This look-ahead digital image signal 1 o gS
The PA is stored in a storage medium 17 such as a magnetic tape.

Next, in exactly the same way, add another stimulable phosphor sheet B.
The recorded image is read out, and its pre-read digital image signal 1ooSPB is similarly stored in the storage medium 17.

Next, the stimulable phosphor sheet A includes a laser light source 40, a scanning mirror 42, a light condensing plate 4, and a laser light source 40, a scanning mirror 42,
4, it is sent to the book reading section 200 consisting of a photomultiplier 45. 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 photomultiple 45, and is stored and recorded on the stimulable phosphor sheet A. Image information is read out photoelectrically (book reading). In addition, the laser beam for pre-reading ii
The output of o is set to be smaller than the output of the main reading laser light source 40 (preferably about 10% or more, more preferably about 3% or less), and the radiation energy accumulated in the stimulable phosphor sheet 8 is set to It is designed so that the information is not lost in a lot of information before reading the book.

The main reading image signal S output from the main reading photomultiple 45 is amplified by an amplifier 46, converted into a digital signal by an A/D converter 47, and then input to a logarithmic converter 48. Numerical value (I ogs
) is converted into a digital image signal of 10g58. This digital image signal logS is stored in a storage medium 49 such as a magnetic tape. Next, in exactly the same way - C1, the recorded image on another stimulable phosphor sheet B is read out,
The digital image signal logSB of the book reading is similarly stored in the storage medium 49. Next, subtraction processing is performed on the digital image signal logS of the book reading obtained as described above using logSB. FIG. 4 shows row 6 while performing automatic density correction according to one embodiment of the method of the present invention.
This figure shows the flow of subtraction processing. First, the above-mentioned digital image signal 10 is obtained from the image file 49A and the image file 49B in the storage medium 49, respectively.
gSA 110QSB is read out and input to subtraction calculation circuit 50. This subtraction calculation circuit 50 processes the above two digital image signals IogS4.
and l0QsB are subtracted for each corresponding pixel to obtain a digital difference signal 3SUb.

This difference signal 3Sub is stored in the -tan image file 51 and then input to the image processing circuit 52, where the difference signal 3Sub is
Receive AI processing.

The difference signal 3Sub' that has undergone image processing is input to a display device such as a CRT, or a reproducing/recording device 53 such as a scanning recorder, and a subtraction image is reproduced and recorded using the difference signal 3sub'. FIG. 5 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 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.

Figure 6 shows that by subtraction as explained above,
This shows how to obtain an image of a desired specific structure.

In the figure, 4A is an image obtained from the first stimulable phosphor sheet A, which records an X-ray image before the contrast agent is injected into the abdomen.
B is an image obtained from the second stimulable phosphor sheet B, which records an X-ray image of the same area after injection of a contrast agent, 4C
is a subtraction image obtained by subtracting the digital image signal representing the 4A image from the digital image signal representing the 4B image, so that only blood vessels are visible.

In the subtraction image 4C, the background BK portion around the extracted specific structure (blood vessel) should originally have the difference signal 5sub of 0 (zero) and always have a constant density on the reproduced image. However, as mentioned above, there are variations in the imaging radiation intensity, and the stimulable phosphor sheet A
,,B, 8171 of this back 11BK portion will vary due to variations in sensitivity. Therefore, as shown in FIG. 4, the image file 17a of the storage medium 17 is
Pre-read digital image signal 1 stored in 117b
o Q S P and I OQ S P B are input to the histogram calculation circuit 60, and the histogram calculation circuit 6
0 at each signal 10 (JSPA, l0QS
Find the histogram of P. As shown in FIG. 1, these histograms indicate the maximum values χ and χB of the maximum transmitted radiation dose corresponding to the maximum density of the background BK, respectively;
The transmitted radiation dose is distributed between the minimum value yp, , '/B. As mentioned above, the maximum values χ and χ8 are supposed to be constant in both images, but as mentioned above, due to the difference in radiation intensity at the time of imaging and the difference in sensitivity of the stimulable phosphor sheet A1B, the background Ir! I changes from sheet to sheet, and each maximum value χa, χ,
are different. Therefore, a signal representing χB to the maximum value is sent to the signal correction circuit 61, the difference Δχ between the two maximum values χA and χ is calculated, and this difference Δχ is uniformly added to the difference signal 5Sub. By adding the difference Δχ to the difference signal 5sub in this way, the difference in imaging radiation intensity, the stimulable phosphor sheet A
, B, the signal component (χA−χB, that is, Δχ) included in the difference signal 3sub is corrected, and the signal in the background portion always has a 0 (zero) value. Therefore, in the reproduced and recorded subtraction image, the I! of background BK! Maro is always constant.

FIG. 7 shows the flow of subtraction processing for performing automatic density correction according to the second embodiment of the method of the present invention. In this case, a signal indicating the maximum value, χ, is sent from the histogram stem circuit 60 to the gradation conversion table correction circuit 161.
is input. The gradation conversion table 52a used in the image processing circuit 52GC is as shown in FIG. 8, but the gradation conversion table correction circuit 161, like the signal correction circuit 61, .

The original gradation conversion table 52a (solid line in FIG. 8) of the image processing circuit 52 is shifted to the higher density side in the input signal axis direction by the difference 61 (dotted line in FIG. 8). ). When the gradation conversion table 52a is shifted in this way, the difference signal 5sub' after gradation processing always contains signal components due to variations in the imaging radiation intensity and variations in sensitivity of the stimulable phosphor sheets A and B. The difference signal 5sub that does not include Δχ takes the same value as when it is subjected to gradation processing using the original gradation conversion table 52a.

Therefore, in the subtraction image obtained by the difference signal 5sub', the density of the background BK is always constant.

In addition, in the above-mentioned actual mode, 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 making the excitation light level lower than that of the excitation light of the main reading is not limited to this, and another method as described above may be adopted. Further, the image reading means for performing the main reading may be used as the pre-reading means, and the excitation light level may be changed to perform the "main reading" and the "pre-reading J".

Furthermore, in the present invention, the method of calculating the maximum value of the transmitted radiation dose of the digital image signal obtained by pre-reading is not limited to the method of calculating from the histogram. A selection method 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 calculating the maximum value.

(Effects of the Invention) As described in detail above, according to the present invention, the background density of the time subtraction image obtained using two stimulable phosphor sheets can be set to a constant value, which greatly improves diagnostic performance. It becomes possible to obtain subtraction images that are highly useful 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 step of accumulating and recording radiographic images in the method of the present invention, and Fig. 3 is a graph showing the steps of accumulating and recording the above-mentioned accumulation and recording. FIG. 4 is a schematic diagram illustrating the reading of radiation image information from a stimulable phosphor sheet; FIG. 4 is a schematic diagram illustrating the outline of subtraction processing that performs automatic 81-degree correction according to the first embodiment of the method of the present invention; FIG. 6 is a schematic diagram showing an example of a subtraction image reproduction/recording system; FIG. 6 is a schematic diagram showing an example of 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. 7 is a schematic diagram illustrating the outline of subtraction processing for performing automatic density correction according to the second embodiment of the method of the present invention, and FIG. 8 is a diagram illustrating the gradation conversion in the second embodiment method.
It is a graph explaining sol correction. 1... Subject 2... X-ray 3... X-ray dose 4A, 4B... X-ray image 4C...
Subtraction image 10.40... Laser light source 11.41... Laser light 12.42... Scanning mirror 13.43... Stimulated luminescent light 15.45... Photomultiple 16.48... Logarithmic converter 46...Amplifier 47...A/D converter 50...
Subtraction calculation circuit 60... Histogram calculation circuit 61... Signal correction circuit 100... Pre-read section 161... Gradation conversion table correction circuit 200... Main reading sections A, B... Accumulative phosphor Sheet 10QsP, 10QsPB...Pre-read digital image signal 1oss 10QsB 1...Digital image signal 3SUb by main reading...Difference signal of digital image signal 1!4 Fig. 116 Fig. 4A 4[3 aaY Fig. 8 Fig. 8 bsu. (Voluntary) Procedural Amendment Letter Patent Application No. 59-85723 September 17, 1980 Title of the invention Automatic density correction method for subtraction images and relationship with the person performing the subtraction correction Patent applicant Address Kanagawa Address: 210 Nakanuma, Minamiashigara City, Prefecture Name: Fuji Photo Film Co., Ltd. Agent 5-2-1 Rokigi, Minato-ku, Tokyo Date of amendment order None Number of inventions to be increased by amendment None Subject of amendment Description of the description Contents of amendments to the “Detailed Description of the Invention” column

Claims (1)

[Scope of Claims] 1) Two stimulable phosphor sheets are each irradiated with radiation that has passed through a subject whose specific structure has been injected with a contrast agent and a subject where no contrast agent has been injected. Accumulating and recording different radiation images of the specific structure on a phosphor sheet, scanning the phosphor sheets with excitation light to convert the radiation images into stimulated luminescence light, The amount of this stimulated luminescence light is read out photoelectrically and converted into a digital image signal, and this ÷digital image signal is subtracted between corresponding pixels of both images to form an image of a specific structure in the radiation image. In the temporal subtraction processing of a radiographic image in which a difference signal is obtained and the difference signal is then subjected to gradation processing based on a predetermined gradation conversion table, this method is used to obtain the digital image signal used in the temporal subtraction processing. First, pre-reading is performed to read the accumulated image information of the two phosphor sheets using excitation light of a lower level than the excitation light used in the similar book reading, and each pre-read digital image signal obtained in the pre-reading is read. Determine the maximum value of the transmitted radiation dose, then determine the difference Δχ between these maximum values, and then i) correct by adding the difference Δχ to the difference signal before performing the gradation processing; An automatic density correction method for a subtraction image, characterized in that one of the corrections is performed to shift an input signal axis to the higher density side by a difference Δχ. 2) 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 and converting it into a digital image signal. This image reading means and this image I! ! Prior to actual reading by the reading means, the stimulable phosphor sheet is scanned with excitation light of a lower level than the excitation light of the image reading means, thereby stimulating the stimulated luminescence emitted from the stimulable phosphor sheet. a pre-reading means for photoelectrically reading light and converting it into a pre-read digital image signal; a digital image signal of a subject to which a contrast agent has been injected into a specific structure read by the image reading means; and the subject to which no contrast medium has been injected. is subtracted between the corresponding pixels and the digital image signal of
subtraction calculation means for obtaining a difference signal forming an image i of the specific structure; image processing means for performing gradation processing on the difference signal based on a gradation conversion table; and a subtraction calculation means for obtaining a difference signal forming the image i of the specific structure; calculating means for calculating the maximum transmitted copper dose of the pre-read digital image signal of a subject to which a contrast agent has been injected and the pre-read digital image signal of the subject to which no contrast medium has been injected; and calculating the difference Δχ between the respective maximum values. and a signal correction circuit which adds the difference Δχ to the difference signal before undergoing the gradation processing. 33) Scanning excitation light onto a stimulable phosphor sheet on which a radiation image has been stored and recorded, and photoelectrically reading the stimulated luminescent light emitted from the stimulable phosphor sheet to generate a digital image signal. an image reading means for converting the stimulable phosphor into an image; and prior to actual reading by the image reading means, 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 out stimulated luminescence light emitted from a light sheet and converting it into a pre-read digital image signal; and a digital image of a subject in which a contrast agent has been injected into a specific structure read by the image reading means. subtraction calculation means for obtaining a difference signal for forming an image of the specific structure by subtracting the signal and a digital image signal of the subject to which no contrast agent is injected between corresponding pixels; and gradation conversion for this difference signal. an image processing means that performs gradation processing based on a table; a pre-read digital image signal of a subject whose specific structure has been injected with a contrast agent, which is read by the pre-read means; and a pre-read digital image of the subject to which no contrast agent has been injected; calculation means for calculating the maximum value of each transmitted radiation IIA amount of the signal; and gradation conversion for determining the difference Δχ between the respective maximum values and shifting the gradation conversion table by the difference Δχ to the higher density side of the input signal axis. An automatic temperature correction device for subtraction images consisting of a table correction circuit.