JP2631032B2 - Radiation image energy subtraction method and apparatus - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method and an apparatus for energy subtraction of radiographic images.

2. Description of the Related Art In various fields, reading a recorded radiation image to obtain an image signal, performing appropriate image processing on the image signal, and reproducing and recording the image have been performed. For example, an X-ray image is recorded using an X-ray film having a low gamma value designed to be compatible with the subsequent image processing, and the X-ray image is read from the film on which the X-ray image is recorded, and is converted into an electric signal. After converting and subjecting the electric signal (image signal) to image processing and reproducing it as a visible image in a copy photograph or the like, a reproduced image having good image quality performance such as contrast, sharpness, and graininess can be obtained. A system has been developed (see Japanese Patent Publication No. 61-5193).

In addition, the applicant has determined that radiation (X-ray, α-ray, β-ray,
Ray, electron beam, ultraviolet ray, etc.), a part of this radiation energy is accumulated, and then, when irradiated with excitation light such as visible light, a stimulable fluorescent light that emits a quantity of photostimulated light corresponding to the accumulated energy. Utilizing light bodies (stimulable phosphors)
A radiation image of a subject such as a human body was once photographed and recorded on a sheet-shaped stimulable phosphor, and the stimulable phosphor sheet was scanned with excitation light such as a laser beam to generate stimulated emission light. A radiation recording / reproducing system has already been proposed that photoelectrically reads the stimulated emission light to obtain an image signal, and based on this image signal, outputs a radiation image of the subject as a visible image to a recording material such as a photographic material or a CRT. (Japanese Patent Laid-Open No. 55-12429)
Nos. 56-11395, 55-0163472, 56-164645
No. 55-116340, etc.).

This system has the practical advantage of being able to record images over a very large radiation exposure area compared to conventional radiographic systems using silver halide photography. That is, it has been recognized that the amount of stimulating light emitted by excitation after accumulation with respect to the amount of radiation exposure is proportional over a very wide range, and thus the amount of radiation exposure varies considerably depending on various imaging conditions. Also, the stimulable light emitted from the stimulable phosphor sheet is read, the gain is set to an appropriate value, read by photoelectric conversion means, converted into an electric signal (image signal), and this image signal is used. By outputting a radiation image as a visible image to a display device such as a photographic material or a CRT, a radiation image that is not affected by a change in radiation exposure can be obtained.

In a system using a recording sheet such as an X-ray film or a stimulable phosphor sheet as described above, a plurality of radiation images recorded on the recording sheet are read, and a plurality of image signals are obtained. The subtraction processing of the radiation image may be performed based on the subtraction processing.

Here, the subtraction processing of the radiation image refers to a processing of obtaining an image corresponding to a difference between a plurality of radiation images captured under different conditions, and specifically, the plurality of radiation images are obtained at a predetermined sampling interval. By reading, a plurality of digital image signals corresponding to each radiation image are obtained, and a subtraction process is performed for each corresponding sampling point of the plurality of digital image signals to emphasize only a specific subject portion in the radiation image. Or, a process of obtaining an extracted radiation image.

This subtraction processing basically includes the following two. That is, the subtraction (subtract) of the radiation image in which the contrast agent is not injected is performed by subtracting the radiation image in which the contrast agent is not injected from the radiation image in which a specific portion of the subject (for example, a blood vessel when a human body is a subject) is enhanced by the injection of the contrast agent. So-called time difference subtraction for extracting a specific portion (for example, a blood vessel), and radiation having different energies in soft tissue and bone tissue in a specific portion of the subject, for example, when the chest of a human body is used as the subject. Irradiating radiation having different energies to the same subject (for example, the chest of a human body) by utilizing different radiation absorptances with respect to a plurality of radiations by each radiation having these different energies Obtain images and calculate the difference by appropriately weighting these multiple radiation images Particular part of the subject by the (e.g. soft tissue in breast only or bone tissue only)
There is a so-called energy subtraction that extracts The present applicant has also proposed energy subtraction using a stimulable phosphor sheet (JP-A-59-83).
486, JP-A-60-225541).

Here, in the method described in Japanese Patent Application Laid-Open No. Sho 60-225541, two radiographs using radiations having different energies are performed, and two radiographic images obtained as a result are read to obtain two radiographs. A digital image signal is obtained, and subtraction is performed based on these image signals. Also, Japanese Patent Application Laid-Open No. 59-83486 discloses an energy subtraction method in which one recording is performed by irradiating two recording sheets sandwiching filters having different absorptances by radiation energy with radiation transmitted through a subject. Have been proposed.

(Problems to be Solved by the Invention) For example, when an X-ray image is obtained by irradiating an X-ray to a subject including a soft part and a bone part, such as a human chest, the X-ray has a distribution spread over a predetermined energy range. When the X-rays pass through the subject, the X-rays on the low energy side are attenuated first, so that the X-rays transmitted through the subject are X-rays on the higher energy side than the X-rays incident on the subject. A so-called hardening phenomenon in which the components are increased occurs. Since this hardening differs depending on each part of the subject, for example, when a soft part image in which only the soft part shadow is displayed by performing energy subtraction processing so that the shadow of the bone does not disturb the observation, Part of the image, for example, in a chest X-ray image, a shadow of a bone portion mainly remains at a portion of the clavicle, and the shadow of the portion is not completely erased from the entire image. There is a problem in that the shadow of the bone that is being made gives an impression as if it is something abnormal in soft tissue.

The present invention has been made in view of the above circumstances, and has as its object to provide an energy subtraction method and apparatus for forming an image in which a shadow of a bone does not obstruct observation and is not mistaken for a shadow of a soft part. Things.

(Means for Solving the Problems) In order to achieve the above object, an energy subtraction method of the present invention is directed to a method of transmitting a plurality of radiations having different energy distributions transmitted through the same subject composed of a bone part and a soft part. Are respectively recorded on a plurality of recording sheets, and the plurality of radiation images are read from the plurality of recording sheets to obtain a plurality of image signals representing each of the plurality of radiation images, based on the plurality of image signals. Performing a subtraction process to obtain a processed image signal representing a radiation image in which the density of the bone is inverted and the density of the bone is higher than the density of the soft part. It is characterized by outputting a reproduced image based on a signal.

Here, it is preferable that the subtraction process is performed by changing a parameter of the subtraction process for each portion of the radiation image.

In addition, the energy subtraction apparatus of the present invention can simultaneously generate a radiation source that emits radiation, a subject placement unit where a subject including a bone part and a soft part is placed, and a plurality of radiation images of radiation having different energy distributions from each other. A recording unit comprising a plurality of recording sheets for sequentially recording, and a sheet holding portion provided at a position facing the radiation source with the subject arrangement portion interposed therebetween; A reading unit that obtains a plurality of image signals representing each of the plurality of radiation images of the subject from the plurality of recording sheets on which the recording of the radiation image has been performed, based on the plurality of image signals obtained by the reading unit; By performing the subtraction processing, the density of the bone part is inverted, and the shadow of the bone part is A processing unit for obtaining a processed image signal representing a radiographic image expressed at a density higher than that of a soft part, and an output unit for outputting a reproduced image based on the processed image signal. .

Here, the "recording sheet" refers to a sheet-like material on which a radiation image is recorded by irradiating radiation thereon, such as the above-described stimulable phosphor sheet or X-ray film.

The “density” includes the luminance when an image is displayed on, for example, a CRT display or the like. In this case, “low luminance” corresponds to “high density”.

(Effect) The above-described energy subtraction method and apparatus of the present invention stop forcibly removing the shadow of the bone, and instead invert the density of the bone to make the shadow of the bone higher in density than the shadow of the soft part. Thus, the shadow of the skeleton is made less noticeable, thereby facilitating the observation of the shadow of the soft tissue.

Also, although the shadow of the bone is not noticeable, it appears on the image, so compared to the case where the soft image is the observation target,
There is also an advantage that it is easy to grasp the arrangement relationship between the shadow of the soft tissue and the shadow of the bone.

(Example) Hereinafter, an example of the present invention will be described with reference to the drawings. Here, an example in which the above-described stimulable phosphor sheet is used will be described.

FIG. 1 is a schematic view of an X-ray imaging apparatus which is an embodiment of a recording unit of the energy subtraction apparatus of the present invention.

X-rays 3 emitted from an X-ray tube 2 of the X-ray imaging apparatus 1
Illuminates the subject 4. X-ray transmitted through subject 4
3a is irradiated to the first stimulable phosphor sheet 5, and a part of the energy of the X-ray 3a is accumulated in the first stimulable phosphor sheet 5, whereby the X-ray of the subject 4 is applied to the sheet 5. Line images are stored and recorded. The X-rays 3b transmitted through the sheet 5 further pass through the filter 6, and the X-rays 3c transmitted through the filter 6 are irradiated on the second stimulable phosphor sheet 7. Thus, the X-ray image of the subject 4 is also stored and recorded on the sheet 7.
The subject 4 is provided with two marks 8 for performing a subtraction process.

Note that the X-ray imaging apparatus stores and records X-ray images on two stimulable phosphor sheets 5 and 7 in one photographing operation. Different X-rays may be used to capture one image each.

FIG. 2 is a perspective view of an X-ray image reading apparatus which is an embodiment of the reading unit of the energy subtraction apparatus of the present invention, and an image processing display apparatus which is an embodiment of an arithmetic unit and an output unit.

After imaging is performed by the X-ray imaging apparatus 1 shown in FIG.
The first and second stimulable phosphor sheets 5, 7 are each X
The line image reading device 10 is set at a predetermined position. Here, the case of reading the first X-ray image stored and recorded on the first stimulable phosphor sheet 5 will be described.

The stimulable phosphor sheet 5 which is set at a predetermined position and on which the first X-ray image is stored is recorded by a sheet conveying means 15 such as an endless belt driven by a driving means (not shown).
Is conveyed (sub-scan) in the direction of arrow Y. On the other hand, a light beam 17 emitted from a laser light source 16 is reflected and deflected by a rotating polygon mirror 19 driven by a motor 18 and rotated at a high speed in the direction of the arrow, passes through a focusing lens 20 such as an fθ lens, and then passes through an optical path by a mirror 21. Instead, it is incident on the sheet 14 and performs main scanning in an arrow X direction substantially perpendicular to the sub-scanning direction (arrow Y direction). From the portion of the stimulable phosphor sheet 14 irradiated with the light beam 17, a stimulable luminescent light 22 is emitted in an amount corresponding to the accumulated and recorded X-ray image information.
Is guided by a light guide 23 and is photoelectrically detected by a photomultiplier (photomultiplier tube) 24. The light guide 23 is formed by molding a light-guiding material such as an acrylic plate, and is arranged so that the linear incident end face 23a extends along the main scanning line on the stimulable phosphor sheet 14. The photomultiplier 2 is placed on the injection end face 23b formed in an annular shape.
4 light receiving surfaces are coupled. Light guide from entrance end face 23a
The photostimulated luminescent light 22 entering the light guide 23 travels through the interior of the light guide 23 by repeating total reflection, exits from the exit end face 23b, is received by the photomultiplier 24, and is represented by a photostimulable light 22 representing a radiation image. Is converted into an electric signal by the photomultiplier 24.

The analog signal S output from the photomultiplier 24 is logarithmically amplified by a log amplifier 25, and then input to an A / D converter 26, where it is sampled to obtain a digital image signal S0. The image signal S0 represents the first X-ray image formed by the X-rays on the relatively low energy side, which is stored and recorded on the first stimulable phosphor sheet 5, and is represented here by the first X-ray image. referred to as one image signal S0 _1. The first image signal S0 ₁ is temporarily stored in the internal memory of the image processing and displaying apparatus 30.

The image processing and display apparatus 30 includes a keyboard 31 for inputting various instructions, a CRT display 32 for displaying a visible image based on auxiliary information and image signals for the instructions, and a floppy disk as an auxiliary recording medium loaded and driven. A floppy disk drive unit 33 and a main unit 34 having a CPU and an internal memory are provided.

Next, in the same manner as described above, the second stimulable phosphor sheet 7
Image signal SO ₂ representing the second X-ray image formed by the relatively higher energy X-rays stored and recorded in
Is obtained, and this second image signal SO ₂ is
It is temporarily stored in the internal memory in 0.

FIG. 3 shows two image signals representing first and second X-ray images stored in an internal memory in the image processing and display device.
Based on S0 _1, S0 _2, is a diagram showing the flow of processing performed by the image processing and displaying apparatus.

Stored in the internal memory of the image processing display device, first and second X-ray image signal S0 _1, S0 ₂ is first X-ray image 41, respectively shown in FIG. 3, the second X-ray image The signal carrying 42. The first X-ray image 41 has a relatively low energy X
Although the second X-ray image 42 is an image based on relatively high energy X-rays, the density of the soft part and the bone part is different from each other, but both of them are original images in which both the soft part and the bone part are recorded. It is an image.

These first and the second X-ray image signal S0 _1, S0 ₂ is
Read from the internal memory of the image processing and displaying apparatus 30 shown in FIG. 2, first, these two image signals S0 _1, S0 ₂ the relative alignment of the respective X-ray images 41 and 42 carrying respective image signals (See JP-A-58-163338). This alignment is performed by the two marks 8 shown in FIG.
Are performed by relatively linearly moving and rotating the two X-ray images so that.

After that, in the present embodiment, in order to reduce the effect of hardening of each part on the X-ray image, the parameters of the subtraction processing are changed for each part of the X-ray image as follows (Japanese Patent Application No. Hei 10-26139). 2-89367).

Here, a smoothed image in which the low spatial frequency of the X-ray image is emphasized is first obtained. This smoothed image is two X-ray images
Although it may be obtained based on either of the X-ray images 41 and 42, in the present embodiment, in order to further reduce noise components included in these X-ray images 41 and 42, the X-ray images 41 and 42 correspond to each other. For each pixel, a superimposed image is obtained by performing an operation according to the equation S0 = S0 ₁ + S0 ₂ (1), and a smoothed image is obtained by smoothing the superimposed image. Here, for each pixel of the superimposed image, the average value of a large number of image signals S0 respectively corresponding to a large number of pixels around each pixel is obtained, and a smoothed image signal representing a smoothed image by this average value is obtained. S _SM is required.

After the smoothed image signal S _SM is determined, subtraction processing according to the equation _{S1 = Ka · S0 2 -Kb ·} S0 1 + Kc ...... (2) for each pixel is performed, the shadow of the bone portion of the object 4 A soft part image 43 (see FIG. 3) which is erased and in which only the soft part shadow is extracted is obtained. Here, Ka, Kb are parameters parameters, Kc is defining a bias component defining the two image signals S0 _2, S0 ₁ weighting, where each parameter Ka, Kb, Kc and smoothed image signal is also
S is a function of _SM .

FIG. 4 is a diagram showing an example of a function of the parameters Ka, Kb, and Kc.

Parameters Ka, as shown in FIG. 4 (a), Kb is a monotonically increasing function according to the value of the smoothed image signal S _SM becomes large and has always been a Ka> Kb. The fourth
The parameter Kc is also a monotonically increasing function as shown in FIG.

The parameters Ka, Kb, and Kc as functions with the smoothed image signal S _SM as a variable are stored in advance in a memory (parameter storage means according to the present invention) in the image processing and display device 30, and according to the above equation (2). It is referred to when performing the subtraction processing.

Also similar to the above (2), subtraction processing according to the equation _{S2 = Ka '· S0 2 -Kb} ' · S0 1 + Kc '...... (3) for each pixel is performed, erasure shadow soft object 4 Then, a bone image 44 (see FIG. 3) from which only the shadow of the bone is extracted is obtained. Here, Ka 'and Kb' are two image signals S, like Ka and Kb.
Kc 'is a parameter that determines the weighting of 0 ₂ and S0 ₁
Are parameters that determine the bias in the same way as
b ′, Kc ′ are smoothed image signals S _SM similarly to the above Ka, Kb, Kc.
The function form is Ka, Kb, Kc shown in FIG.
The function form of Ka ′, Kb ′, and Kc ′ is omitted here.

When the bone image is obtained in this way, the image is inverted between black and white, and an inverted bone image 45 is obtained. In the inverted bone image 45, when a superimposed image is output as a visible image as described later, the shadow of the bone is expressed with an extremely high density so that the shadow of the bone is not conspicuous on the visible image. Image.

Next, an image signal representing the superimposed image 46 is generated by superimposing the inverted bone image 45 and the soft part image 43 obtained according to the equation (2), and a visible image based on the image signal is displayed on the CRT display 32. (See FIG. 2). The image displayed on this CRT display 32 is
Since the shadow of the bone is expressed in high density (low brightness), it is inconspicuous. Therefore, the observer can observe the shadow of the tumor, which appears in the soft tissue, without being bothered by the shadow of the bone. Can do it.

Although the superimposed image 46 is displayed on the CRT display 32 in the above embodiment, a hard copy of the superimposed image 46 may be obtained instead or together with this.

In the above example, (2), the subtraction processing based on the equation (3), Paremeta Ka of the subtraction process, Kb, Kc; Ka ', Kb', the Kc 'to the value of the smoothed image signal S _SM However, since the present invention does not eliminate the shadows of the bones but rather displays them in an aggressive (inconspicuous) manner, the effect of hardening is small, and thus the parameters Ka, Kb, Kc; Ka ′, K
b ', Kc' is regardless of the value of the smoothed image signal S _SM, those may be fixed.

Further, in the above embodiment, the soft part image 43 is calculated according to the equation (2).
Is obtained, and the bone image 44 is obtained according to the equation (3).
The inverted bone image 45 is obtained by inverting 44, and the inverted bone image 45 is obtained.
Although the procedure of obtaining the superimposed image 46 by superimposing the soft part image 45 and the soft part image 43 was performed, without performing this procedure, mathematically equivalent to this procedure, Ka / Kb> Ka ″ / Kb ″ > Ka '/ Kb' becomes Ka ", Kb" using, by performing _{S3 = Ka "· S0 2 -Kb} " · S0 1 + Kc "...... (4) becomes operational, the image was suddenly superimposed
An image signal S3 representing 46 can be obtained, and therefore, instead of performing the above procedure, an operation based on the equation (5) may be performed.

Further, the present invention is not limited to an X-ray image of the chest of a human body, but can be applied to, for example, an X-ray image of a limb of a human body. It can be widely applied when obtaining a line image.

Further, although the above embodiment is an example using a stimulable phosphor sheet, the present invention is not limited to a stimulable phosphor sheet, but an X-ray film. And the like) can also be applied.

(Effects of the Invention) As described in detail above, in the method and apparatus for energy subtraction of a radiographic image of the present invention, the density of a bone is inverted so that a shadow of a bone is expressed at a higher density than a shadow of a soft part. Obtaining a processed image signal representing the radiation image;
Since the reproduced image based on the processed image signal is output, the shadow of the bone does not hinder the observation and the mistake of the shadow of the soft part is prevented when observing the reproduced image. Since the shadow of the bone is also displayed, the positional relationship of the soft tissue can be accurately grasped.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of an X-ray imaging apparatus which is an embodiment of a recording unit of the energy subtraction apparatus of the present invention, and FIG. 2 is a view of one of a reading unit, an arithmetic unit and an output unit of the energy subtraction apparatus of the present invention. FIG. 3 is a perspective view of an X-ray image reading device and an image processing display device according to an embodiment; FIG. 3 is a diagram showing a flow of processing performed in the image processing display device; It is a figure showing an example. DESCRIPTION OF SYMBOLS 1 ... X-ray imaging apparatus, 2 ... X-ray tube 3, 3a, 3b, 3c ... X-ray, 4 ... Subject 5 ... First stimulable phosphor sheet 6 ... Filter 7 ... Second stimulable phosphor sheet 8 Mark 16 Laser light source 19 Rotating polygon mirror 22 Stimulated emission light 23 Light guide 24 Photomultiplier 30 Image processing and display device , 41, 42 ... Original image 43 ... Soft part image, 44 ... Bone part image 45 ... Inverted bone part image, 46 ... Overlay image

Claims (3)

(57) [Claims] 1. A plurality of radiation images of radiation having different energy distributions transmitted through the same subject composed of a bone part and a soft part are recorded on a plurality of recording sheets, respectively. The plurality of radiation images are read to obtain a plurality of image signals representing each of the plurality of radiation images, and a subtraction process is performed based on the plurality of image signals, whereby the density of the bone portion is inverted, and Obtaining a processed image signal representing a radiation image in which the shading of the part is expressed at a higher density than the shading of the soft part, and outputting a reproduced image based on the processed image signal. . 2. The method according to claim 1, wherein said subtraction processing is performed by changing parameters of said subtraction processing for each part of said radiation image. 3. A radiation source for emitting radiation, a subject arrangement section on which an object composed of a bone and a soft part is arranged, and a plurality of radiation images for recording a plurality of radiation images by radiation having different energy distributions from each other. A recording unit comprising: a sheet holding unit provided at a position facing the radiation source with the object placement part interposed therebetween, wherein recording sheets are simultaneously or sequentially disposed; and a recording unit for recording a radiation image of the object in the recording unit. A reading unit that obtains a plurality of image signals representing the plurality of radiation images of the subject from the performed recording sheet, by performing a subtraction process based on the plurality of image signals obtained by the reading unit, Radiation in which the density of the bone is inverted so that the shadow of the bone is expressed at a higher density than the shadow of the soft part. Energy subtraction device of the radiation image comprising the output unit for outputting computing Yunitsuto obtaining a processed image signal representing an image, and a reproduction image based on the processed image signal.