JPH0544871B2 - - Google Patents

Description

[Detailed Description of the Invention] (a) Industrial Application Field The present invention relates to a digital subtraction system (hereinafter simply referred to as ``digital subtraction system'') which obtains a difference image by digitally subtracting a video signal of a fluoroscopic image obtained by an X-ray TV system.
(abbreviated as DSS).

(b) Prior art DSS first stores the fluoroscopic image before contrast agent injection as a mask image, and then digitally subtracts the mask image from the fluoroscopic image obtained after contrast agent injection (this is called a live image). , which obtains an image (difference image) by subtracting the mask image from the live image, and is mainly used for angiography. When this DSS is used for angiography, it is extremely useful for diagnosis because it extracts only images of the blood vessels in the process of the flow of the contrast medium.

By the way, conventional DSS has been improved by storing the maximum and minimum values of many pixels of live images of many frames obtained from before the contrast agent arrives until it flows away. This is a peak-hold type DSS that obtains peak-hold images for two screens (the maximum value is called the positive peak-hold image, and the minimum value is called the negative peak-hold image), and subtracts between these images. A system that can be called a system has been proposed (Patent Application No. 58-247498).
This system can reduce the amount of X-rays and reduce radiation exposure for patients, there is no need to consider the timing of taking microphone images, and the entire process (that is, trajectory) of the flow of the contrast agent can be displayed on a single screen. It has advantages such as:

However, this system also has some points to improve. In other words, the peak hold method can eliminate the influence of stationary noise such as quantum noise, but on the other hand, the peak value is stored as is due to the principle of the peak hold method, so fluctuations in the video signal (for example, X-ray dose The problem is that the image quality is significantly degraded when these things occur.

(c) Purpose This invention provides peak-hold type DSS that
The purpose is to suppress the effects of fluctuations in video signals, noise, etc., and to improve the ability to obtain high-quality differential images.

(d) Configuration This invention provides a peak-hold type DSS that includes:
It is characterized in that a recursive filter is used to obtain positive and negative peak hold images from the filtered video signal.

(E) Embodiment In FIG. 1, X-rays are continuously emitted from an X-ray tube 1 toward a subject 2. In this case, the X-ray dose may be low, and the tube current may be about several mA to several tens of mA. The transmitted X-ray image of the X-rays transmitted through the subject 2 is converted into an optical image whose brightness is amplified by the image intensifier 3, and this optical image is converted into a video signal via the image pickup tube 4 and camera control unit 5. Ru. These configurations are normal X-ray TV
It is similar to the system.

The video signal is digitally processed by an image processing device 6. The video signal is first converted into a digital signal by the A/D converter 7 and then sent to the logarithmic converter 8. The digital signal is also sent to the switching circuit 14, and the switching circuit 14 selects this digital signal until just before starting the peak hold operation. Therefore, the digital signal from the A/D converter 7 is directly converted into an analog signal by the D/A converter 15, and a video signal that has not undergone any processing is output from the image processing device 6. It turns out. The output of this image processing device 6 is sent to an image display device, an image recording device, etc., and even after the contrast agent is injected, a mere X-ray fluoroscopic image is observed on the image display device before the contrast agent reaches the imaging site. be done.

On the other hand, the signal that has passed through the logarithmic converter 8 passes through a recursive filter 9 and undergoes recursive filtering for each pixel.
Sent to 1. These peak hold image storage devices 10 and 11 each consist of a frame memory and a comparison circuit, and when a video signal is sent one frame at a time, each pixel is compared with the signal stored in the frame memory. An operation is performed in which the signal that has just been sent is compared and the larger (brighter) or smaller (darker) signal is written into the frame memory. Therefore, when several frames of signals are sent, the signal with the maximum value or minimum value among those frames is held for each pixel. It is assumed that the peak hold image storage device 10 stores the maximum value, that is, the positive peak hold image, and the peak hold image storage device 11 stores the minimum value, that is, the negative peak hold image. The signals of the images held by the storage devices 10 and 11 are always output and subtracted by the subtracter 12, and the difference image obtained by subtracting these peak held images is always output from the subtracter 12. be done. This difference image is contrast-enhanced by an image intensifier 13 and sent to a switching circuit 14.

After the contrast medium is injected, the switching circuit 14 is switched at an appropriate timing before the contrast medium reaches the imaging site. Then, a difference image that is the result of subtracting both peak hold images is output from the image processing device 6, and this difference image is displayed or recorded. A positive peak hold image represents an image (corresponding to a mask image) of a bright area, that is, an area unaffected by the contrast medium (where the contrast medium does not flow), and a negative peak hold image represents an image of a dark area. It represents an image of the trajectory of a certain contrast agent and an image of the background where the contrast agent does not flow (corresponding to a live image). Therefore, by subtracting these peak hold images, only the image of the trajectory of the contrast agent is extracted.
Therefore, using this difference image, it is possible to observe how the trajectory of the contrast medium is added one after another as the contrast medium flows, and the surgeon is free to stop imaging at a point that satisfies him/her. . In other words,
For example, it is easy to select the object to be imaged, such as only the arterial layer, peripheral blood vessels, or venous layer. When a satisfactory image is obtained, the peak hold operation by the peak hold image storage devices 10 and 11 is stopped, and at the same time, the X-ray exposure is also stopped. At this time, the difference image at the time when the peak hold is stopped is displayed on the image display device.

Here, the recursive filter 9 includes a frame memory 91 and multipliers 92 and 93, as shown in FIG.
and an adder 94, and its operation is as follows. When a new frame of video signal is sent, a multiplier 92 calculates a constant K for each pixel.
(0≦K≦1) is multiplied. At the same time, signals are read out for each pixel from the frame memory 91,
A multiplier 93 multiplies each pixel by a constant (1-K). Then, the output of the multiplier 92 and the output of the multiplier 93 are added for each pixel by an adder 94, and the addition result is written to the frame memory 91 for each pixel. Therefore, the signal for each pixel stored in the frame memory 91 contains the signal components of the previous frame in a suppressed state as the distance from the current frame increases, which is the same as afterimages being superimposed. It's becoming like that. If K is made smaller, the afterimage component will increase, and the signal will be smoothed accordingly, making it unaffected by fluctuations in the video signal and noise, but on the other hand, the responsiveness to changes in the image will deteriorate.
On the other hand, if K is increased, afterimage components will be reduced and responsiveness will be improved, but the influence of noise will become more apparent. Therefore, here, the value of K in the multipliers 92 and 93 is made variable, and K is increased when the imaged area is a stationary part with relatively little movement, such as the head, and when the area to be imaged is a stationary part with relatively little movement, such as the area near the heart. It is possible to reduce K in the case of parts that move quickly.

(F) Effect The peak-hold type DSS according to the present invention uses a recursive filter to obtain positive and negative peak-hold images from the filtered video signal, which reduces noise inherent to the peak-hold type. In addition to the effects, deterioration in image quality due to fluctuations in video signals and external noise can be reduced, further improving image quality, and subtraction images can be obtained at lower doses.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of an embodiment of the present invention, and FIG. 2 is a block diagram of a recursive filter. 1... X-ray tube, 2... Subject, 3... Image intensifier, 4... Image pickup tube, 5... Camera control unit, 6... Image processing device,
7... A/D converter, 8... Logarithmic converter, 9...
Recursive filter, 10, 11...Peak hold image storage device, 12...Subtractor, 13...Image enhancer, 14...Switching circuit, 15...D/A converter, 91...Frame memory, 92, 93...
Multiplier, 94...Adder.

Claims (1)

[Claims] 1. A first image storage means for storing the maximum value and a minimum value for each pixel from video signals of a large number of fluoroscopic images obtained from before the contrast agent arrives to when it flows away in the X-ray TV system. and arithmetic means for performing pixel-by-pixel subtraction between the image stored in the first image storage means and the image stored in the second storage means. The traction system includes recursive filter means for recursively filtering video signals of the plurality of fluoroscopic images, and sends the recursively filtered video signals to the first and second image storage means. A digital subtraction system characterized by: