JPH0628411B2 - Digital subtraction system - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (a) Field of Industrial Application The present invention is an image of only a portion (for example, a blood vessel) where a contrast agent is present by digitally performing subtraction between images in an X-ray TV fluoroscopy system. It is related to the improvement of the digital subtraction system which extracts.

(B) Prior Art In the digital subtraction system, an X-ray TV fluoroscopy system is used to first obtain an image (mask image) before injection of the contrast agent, and then obtain an image (live image) after injection of the contrast agent.
Digitally subtract the mask image from the live image. Then, since the image other than the contrast agent portion is the same before and after the injection of the contrast agent, this is canceled, and only the image of the blood vessel or the like filled with the contrast agent is extracted. Therefore, it is extremely useful for diagnosing blood vessels and the like.

By the way, the canceling action of the image in the portion other than the contrast agent portion due to this subtraction does not work effectively unless the X-ray intensities are the same at each imaging time point before and after the injection of the contrast agent. That is,
In portions other than the contrast agent portion, if the X-ray intensities are not the same at both time points of imaging, the concentration value will not be 0 even by subtraction, and will be deviated to + or −. That is, it is premised that the X-ray intensities are the same before and after the injection of the contrast agent.

However, in reality, it is inevitable that the X-ray intensity fluctuates slightly over time, and if there is a slight variation in the X-ray intensity between when the mask image is taken and when the live image is taken, the contrast Where the subtraction result of the part that is not affected by the drug should be 0, it becomes + or-slightly. Since the subtraction image obtained as a result of the subtraction is usually contrast-enhanced, this slight deviation is emphasized, and the base density, which should be gray as an intermediate value on the screen of the monitor device, is set in each of the subtraction images. The irregular distribution of the black and white sides hinders the diagnosis.

(C) Objective The present invention aims to provide an improved digital subtraction system so as to automatically correct the variation in the base density of each subtraction image due to the variation of the X-ray intensity generated in a series of imaging. To do.

(D) Configuration In the digital subtraction system according to the present invention, an X-ray TV fluoroscopic system for obtaining a video signal of an X-ray fluoroscopic image, a live image after injection of the contrast agent obtained from the X-ray TV fluoroscopic system, and before injection of the contrast agent Using the obtained mask image, a subtracter that digitally subtracts the latter from the former to obtain a subtraction image, an enhancement circuit that adjusts the brightness and contrast of the subtraction image, and a subtraction image that passes through the enhancement circuit A frame memory that stores once and a D / A that converts the signal of the subtraction image stored in this frame memory into an analog signal
A converter, a density value detection circuit that detects a density value for each of a plurality of adjacent pixels from the signal of the subtraction image output from the subtractor, and outputs the average value as a density value; A circuit that creates a histogram of the distribution of density values, a circuit that reads the density value that is the peak in this histogram, and this density value is compared with a reference value,
It is characterized in that the enhancement circuit is provided so that the density value coincides with the reference value, and the control circuit adjusts the density value of each pixel of the original subtraction image by a constant value.

(E) Example In FIG. 1, X-ray tube 11 to X is applied to subject 10.
The X-rays that have been irradiated with the rays and transmitted through the subject 10 enter the image intensifier 12. In the image intensifier 12, the X-ray transmission image is converted into an optical image and its brightness is amplified, and the output image is converted into a video signal through the TV camera 13 and the camera control circuit 14. If this video signal is sent directly to the monitor device 15 without passing through the video processor 20 to display an X-ray transmission image or recorded by the recording device 16, a normal X-ray T
It is the same as the configuration of the V perspective system.

The video processor 20 converts the sent video signal into A /
The digital signal is converted by the D converter 21 and sent to the frame memories 24 and 25 via the logarithmic converter 22. First, the video signal obtained before injecting the contrast agent into the subject 10 is sent to the frame memory 24, and the mask image is stored in the frame memory 24. Next, after injection of contrast agent,
Video signals obtained one after another are sent to the frame memory 25 to store a live image. Here, the adder 23 is inserted in order to avoid the influence of noise by adding images of a plurality of frames when storing the mask image and the live image. In the subtractor 26, the live image read from the frame memory 25 is subtracted by the mask image read from the frame memory 24. The subtraction image thus obtained is temporarily stored in the frame memory 28 via the enhancement circuit 27, then converted into an analog signal via the D / A converter 29, and sent to the monitor device 15 and the recording device 16. The CPU 31 controls the entire system and the enhancement circuit 27. The value of the central gray level on the display screen set by the level setter 32 and the gray level on the display screen set by the display width setter 33 are set. The enhancement circuit 27 is controlled on the basis of the width of the contrast image, and the degree of enhancement of the brightness and contrast of the subtraction image displayed by the monitor device 15 is controlled. In this way, after the injection of the contrast agent, the subtraction image is operated every time a live image is obtained, and the subtraction image is sequentially displayed on the monitor device 15. The configuration so far is the same as that of a normal digital subtraction system.

According to the present invention, the enhancement circuit 27 is controlled. That is, the subtraction image output from the subtractor 26 is sent to the density value detection circuit 41. The density value detection circuit 41 does not simply output the density value of each pixel, but outputs the average value of the densities of eight pixels adjacent in the direction of the horizontal scanning line for every eight pixels. The reason why the density values of a plurality of pixels are averaged in this way is that if the density values of each pixel are adopted as they are, they will be affected by noise, and the effect of noise will be removed by averaging. 8 each
The density value obtained as the average value of the pixels is sent to the histogram creation circuit 42. The histogram creation circuit 42 is mainly composed of a random access memory, and uses a density value as an address, and when a certain density value is given, 1 is added to the address corresponding to the density value. Is stored as data. By executing this for one frame, a distribution histogram of density values in the subtraction image of one frame as shown in FIG. 2 is created. In this way, when one frame of the live image is written in the frame memory 25, at the same time, the histogram of the subtraction image is completed in the histogram creating circuit 42.

The histogram has a peak at a specific density value as shown by a solid line, a dotted line, and a chain line in FIG. 2, and the density value corresponding to this peak can be regarded as the base density. Because,
This is because it is generally considered that the number of pixels corresponding to the base density is the largest when the density distribution is viewed over all the pixels in the image of one subtraction image. When the X-ray intensity does not change between when the mask image is taken and when the live image is taken, the density of the base portion is the same for the mask image and the live image. The base density value of should be 0, that is, the reference value. That is, if a histogram like the solid line in FIG. 2 is obtained when there is no fluctuation in the X-ray intensity, the density value corresponding to the peak of this histogram is 0 (reference value), and if there is fluctuation, the dotted line or 1 The concentration value corresponding to the peak is deviated from 0 to the − side or the + side as indicated by the dotted line.

Therefore, in the subsequent one frame period, the peak density value reading circuit 43 detects the peak in the histogram and reads the address of this peak. And CPU3
The control circuit 1 controls the enhancement circuit 27 so that 1 compares this address, that is, the base density value with the reference value, and uniformly adds the value corresponding to the difference to all the pixels of the subtraction image. In this way, the base density of the subtraction image is made to match the reference value, and the base density is kept constant regardless of variations in the X-ray intensity. The above density correction operation is repeated every time a subtraction image is obtained to make the base density constant for all subtraction images.

By performing such density correction, the corrected subtraction image is output with a delay of one frame period as compared with the case without this correction operation, but there is no problem with the intermittent operation such as the serial mode. Does not occur. When continuous operation is performed as in the continuous mode, the frame memory 25 may be expanded by another frame to serve as a buffer memory, and the delay time may be adjusted.

The histogram may be created only in one field of the subtraction image. In this case, it is possible to read the density value corresponding to the peak and correct the density of the subtraction image during the period of the other field, so that the real-time property can be further improved.

(F) Effect According to the present invention, it is noted that when the densities are examined over all the pixels in one subtraction image, the number of pixels corresponding to the base density generally appears most. Therefore, for a series of subtraction images, When a histogram of the density distribution is taken on, the peak has a density value corresponding to the base density in any image, and the deviation of the density value corresponding to this peak from the reference value corresponds to the fluctuation of the X-ray intensity. Therefore, by adding or subtracting the density value corresponding to the deviation to each pixel of the subtraction image so as to eliminate this deviation, the base density can be kept constant for each of the series of subtraction images regardless of the fluctuation of the X-ray intensity. . This eliminates the variation in density between each of the series of subtraction images,
Easy to observe and diagnose.

Further, when creating the above histogram, the density value for each of a plurality of adjacent pixels is detected from the signal of the subtraction image output from the subtractor, the average value is calculated, and the histogram is created using this. Because 1
The inconvenience of being affected by various noises when a histogram is created using pixel-by-pixel density values can be solved by the above averaging, and the base density correction based on fluctuations in X-ray intensity can be accurately performed. Can be done.

[Brief description of drawings]

FIG. 1 is a block diagram of an embodiment of the present invention, and FIG. 2 is a graph showing a histogram. 10 ... Subject, 11 ... X-ray tube 12 ... Image intensifier 13 ... TV camera 14 ... Camera control circuit 15 ... Monitor device, 16 ... Recording device 20 ... Video processor, 21 ... A / D converter 22 ... Logarithmic converter, 23 ... Adder 24, 25, 28 ... Frame memory 26 ... Subtractor, 27 ... Enhance circuit 29 ... D / A converter, 31 ... CPU 32 ... Level setting device, 33 ... Display width setting device 41 ... Concentration value detection circuit 42 ... Histogram creation circuit 43 ... Peak concentration value reading circuit

Claims (1)

[Claims] 1. An X-ray TV for obtaining a video signal of an X-ray fluoroscopic image.
A subtraction image is obtained by digitally subtracting the latter from the former using a fluoroscopy system, a live image after injection of the contrast agent obtained from the X-ray TV fluoroscopy system, and a mask image obtained before injection of the contrast agent. A calculator and an enhancement circuit for adjusting the brightness and contrast of the subtraction image,
A frame memory that temporarily stores the subtraction image that has passed through the enhancement circuit, a D / A converter that converts the signal of the subtraction image stored in the frame memory into an analog signal, and a subtraction image of the subtraction image output from the subtractor. A density value detection circuit that detects the density values of a plurality of adjacent pixels from the signal and outputs the average value as the density value, a circuit that creates a histogram of the distribution of the detected density values, and this histogram The density value of each pixel of the original subtraction image is set to a constant value by comparing the density value with the reference value and the circuit that reads the density value that becomes the peak in A digital subtraction system having a control circuit for adjusting the amount.