Working mode of subtraction angiography

In DSA system, according to different purposes, there are many digital subtraction methods, such as time subtraction and energy subtraction. The main difference is that the acquisition methods of two subtraction images, namely mask and contrast agent filled image, are different.

(1) time subtraction

Time subtraction is a common way of DSA. Before the injected contrast agent enters the region of interest, one or more frames of images are stored as mask images, which are subtracted from the created images that appear in time sequence. In this way, the same image parts in the two frames are eliminated, and the high-density parts formed when the contrast agent passes through the blood vessels are highlighted. This working mode is called time subtraction, because the time to obtain the mask image and the image is different. Its disadvantage is that the mask image and the created image cannot be accurately matched due to the patient's intentional or unintentional movement in the process of photography, resulting in poor or blurred image registration artifacts. In view of the difference in the number of frames and acquisition time between the mask image and the created image used in subtraction, it can be divided into the following ways:

1. Pulse image (PI) mode PI mode uses intermittent X-ray pulses to form mask images and create images (as shown in Figure 5-25), and captures several frames of images every second, and the pulse duration is generally longer than that of video signals. When the contrast agent does not flow into the blood vessel of interest, a mask image is taken, and X-ray images are collected and subtracted during the gradual diffusion of the contrast agent, and a series of continuous and spaced subtraction images are obtained, with a large interval between each frame of subtraction images.

2. Super pulse imaging (SPI) mode SPI mode takes X-ray pulse photography at the rate of 6-30 frames per second, and then repeats high-speed subtraction frame by frame, which has the characteristics of high frequency and narrow pulse width, as shown in Figure 5-26. The X-ray exposure pulse is synchronous with the camera field, and the effective time of the exposure signal should be within the field blanking period, so the maximum pulse frequency is 50 ~ 60 Hz, and the pulse width is 3 ~ 4 ms This method can continuously observe the X-ray digital image or subtraction image at the speed of real-time video, with high dynamic clarity.

3. Continuous Image (CI) Mode CI Mode (as shown in Figure 5-27) Like fluoroscopy, X-rays are continuously irradiated, and continuous images synchronized with the camera are obtained at a frequency of 25-30 frames per second. X-rays used can be continuous or pulsed. Because it is continuous irradiation for a long time, the load of X-ray tube is quite large, so X-ray tube with large heat capacity should be used. For example, the signal-to-noise ratio of subtraction image is very low in X-ray tube current exposure, so CI method generally adopts small focal length. Current continuous exposure photography of 15mA tube.

4. Time interval difference (TID) method The above subtraction methods all use the image when the contrast agent is not injected into the blood vessels of the contrast site as the mask image, and the sequence X-ray image containing the contrast agent as the subtraction image. The TID method does not fix the mask image, but randomly determines a frame image (for example, the third frame, the image taken just after the contrast agent is injected into the blood vessel). Then, subtraction processing (3-6) is performed on the image (frame 6) at certain intervals (for example, every three frames), and then subtraction (4-7) and (5-8) are performed frame by frame to form a subtraction image sequence.

5. Electrocardiogram (ECG) trigger pulse mode Because the heart movement is in a different phase at every moment, in order to make the phase of the mask image and the created image as close as possible and reduce the motion artifacts of the subtraction image, ECG is usually used to trigger the X-ray pulse mode. External ECG signals trigger X-ray image acquisition in three ways.

(1) Continuous ECG marking: Take images in a continuous way and mark them on the pictures with ECG signals. The minimum frequency of this method is 5 frames per second.

(2) Pulse ECG marking: the image is shot in a pulse mode, and the picture closest to the ECG signal is marked, and the lowest frequency is also 5 frames per second.

(3) ECG gating trigger: ECG signal starts the X-ray generator to shoot gated images and mark the images. The specific method is to store the output signal of ECG machine into ECG memory after A/D conversion, and at the same time extract the R wave sign from the R wave signal as the reference of ECG phase. In ECG gated acquisition, if the X-ray exposure is synchronized with the R-wave marker, the subtraction image timed by the R-wave can be obtained. This method is mainly used for DSA examination of cardiac vessels, and the exposure matches the rhythm of cardiac vessels, so as to ensure that every frame in the image sequence is in phase with the rhythm and eliminate the artifacts caused by cardiac pulsation.

2) Energy subtraction

Energy subtraction is also called double energy subtraction. When performing angiography in the region of interest, two images are obtained with two different tube voltages (such as 70kV and 130kV) almost at the same time, and they are subtracted. Because the two images were taken by X-rays with different energies, it is called energy subtraction.

This subtraction method takes advantage of the fact that the attenuation coefficients of iodine and surrounding soft tissues are obviously different at different energies (the attenuation curve of iodine jumps at the energy level of 33keV, and the attenuation coefficient suddenly increases, while the attenuation curve of soft tissues is continuous, and the attenuation coefficient decreases with the increase of energy). If a tissue containing bone, soft tissue, air and trace iodine is irradiated with X-rays with energy slightly lower than 33 kV and slightly higher than 33 kV (70 kV and 130 kV, respectively), the iodine signal of the latter image is weakened by about 80%, the bone signal is weakened by about 40%, the soft tissue signal is weakened by about 25%, and the gas is hardly attenuated at the two energy levels. If these two images are subtracted, the gas shadow will be effectively eliminated, and a small amount of soft tissue shadow, obvious bone shadow and iodine signal will be preserved. If the images collected at 130kV are weighted with a coefficient of about 1.33 and then subtracted, soft tissue and gas shadows can be well eliminated, leaving only less bone signals and obvious iodine signals.

Energy subtraction can also separate tissues with different attenuation coefficients, such as removing bone tissue or soft tissue from X-ray images, so as to obtain images with only soft tissue or bone tissue. The specific method is to obtain two images with two kinds of energy X-ray beams, one under low-energy X-ray and the other under high-energy X-ray. The images are weighted and subtracted by logarithmic transformation, and bones or soft tissues are eliminated.

In principle, energy subtraction is a good subtraction method, but in practical application, it is required that the tube voltage can be switched between two kinds of energy at high speed, which increases the complexity of X-ray machine, and the general X-ray machine cannot adopt this method. This method is not easy to eliminate the residual image of bones.

(3) Mixed subtraction

Combining energy and time subtraction techniques, a hybrid subtraction technique is produced. The basic principle is to do a dual-energy subtraction before injecting contrast agent to get an image containing a small amount of bone tissue signals. After injecting contrast agent into blood vessels, subtract this image from the dual-energy subtraction image to get a simple blood vessel image. Hybrid subtraction requires high equipment and X-ray tube load.

Third, DSA's special requirements and technical measures for equipment

DSA is different from the common DF system. It not only digitizes X-ray images, but also obtains better quality subtraction images. Therefore, DSA system has a series of special requirements.

(1) X-ray generation and imaging system

Including X-ray tube, high-voltage generator, image intensifier, optical system, TV camera and monitor.

The 1.X-ray generator requires that the X-ray tube can withstand the load of continuous pulse exposure. For medium and large DSA equipment, the heat capacity of X-ray tube should be above 200kHU, the tube voltage range is 40 ~ 150 kV, and the tube current is usually 800 ~ 1250 Ma. It is required that the high voltage generator can generate stable DC high voltage, adopt medium and high frequency technology and be controlled by microcomputer to generate almost pure DC voltage. X-ray machine can quickly expose in multi-pulse mode, and the highest imaging speed is 150 frames/second.

2. Image intensifiers usually use I.i. with variable field of view, such as 775px I.i., which can have four fields of view: 10, 16, 22, 3 1, and can be selected flexibly according to the needs of photographing. Spatial resolution is inversely proportional to screen size and field of view, generally 1. 1 ~ 2.5lp/mm ... In order to improve sensitivity and resolution, the input screen is made of cesium iodide and other materials. The newly developed flat panel intensifier has hundreds of thousands of optical fibers between the emitter and the photoelectric layer of the input screen, which couples the light of each pixel to the photoelectric layer, thus making the image have higher brightness and improving the I/I conversion efficiency, so it has a broad prospect. At present, the quantum detection efficiency (DQE) of high performance I.i. is 85%. Some data show that the highest resolution is 6.8 LP/mm.

3. In order to adapt to a larger X-ray dose range (that is, the range of input light), the optical system requires the use of large-aperture lenses with automatic adjustable aperture, and some lenses also contain electric neutral density filters to prevent the ingestion of strong light.

4. TV cameras require the camera tube to have the characteristics of high sensitivity, high resolution and low afterimage, and the video channel should have various compensation circuits to ensure the output of high signal-to-noise ratio and high fidelity video signals. X-ray exposure and image acquisition must be carried out at the same time. However, due to the hysteresis characteristics of the vacuum camera tube, in the pulse image mode and interlaced scanning mode, the image signal amplitude of each field is different, and sampling can only be carried out after the signal amplitude is stable, thus increasing the exposure pulse width and wasting the dose. Using CCD camera and progressive scanning system can improve this situation. With the improvement of the quality of CCD products, it will further replace the vacuum camera tube. High-performance CCD camera adopts high-definition system with resolution of 1249/ 1023 line (50 ~ 60hz), signal-to-noise ratio greater than 2500, and frequency band greater than10.5mhz..

5. The monitor should be equipped with a high-definition large-screen monitor, for example, the types that scan more than 1024 lines and more than 1275px line by line. At present, the display of the contrast room often adopts the form of multi-screen, multi-segmentation or picture-in-picture, which is convenient for comparison at any time. High-performance displays use ambient brightness sensors to automatically adjust brightness; Flashless flat-panel kinescope realizes flicker-free image display when the field frequency is higher than 100Hz.

6. Automatic control of X-ray image brightness in 6.DSA, because the tissue density of the subject changes greatly, it is necessary to ensure that images with sufficient diagnostic information can be obtained under various photographic subjects and conditions to eliminate blur and halo. DSA is an analog image signal formed by I.I-TV imaging system. I.I has a large dynamic range of about 10, and can output images with good contrast under different exposure doses. But the illumination range of the target surface of the TV camera tube is 10? When ~ ~ 10x, the output current varies between dark current value and saturated current value, and the dynamic range is within several hundred. The X-ray exposure dose range of some examination parts (such as chest and abdomen) is 10 ~ 10, which is beyond the range that the camera can accurately copy signals. Therefore, a series of automatic control measures are needed to ensure that the input light quantity of the camera tube changes within its dynamic range.

"There are three main self-control measures: ① Controlling the output light of i.i. Controlling the exposure dose of X-rays means controlling the input light of I.i, so as to automatically control the exposure time by using the video signal output by the camera, or automatically adjust the kV and mA values of the X-ray tube, so as to automatically control the brightness of X-ray images.

② Control the output light quantity of the optical system. The aperture of the lens is automatically controlled by the video signal. With the help of computer-controlled optical filter, the lens with F 1.4 aperture can automatically adjust the light quantity to 6.6× 10, thus ensuring that the input illumination of the camera tube is always within the normal range.

③ The compensation filter can also reduce the dynamic range of X-ray information to make it consistent with the dynamic range of equipment parts. The compensation filter is to place additional attenuation material between the X-ray tube and the patient, and select a specific attenuation area in the field of view to provide a more uniform dose distribution.

7.X-ray dose management The task of the dose management system is to minimize the X-ray radiation dose to patients while ensuring the image quality. It consists of a series of modern technologies.

(1) grid control technology: add a negative potential to the grid at the interval of each pulse exposure to offset the start and afterglow of the exposure pulse, thus eliminating soft rays, improving the quality of effective rays and shortening the pulse width.

(2) spectral filtering technology: aluminum filter plate is placed in the window of I.I or X-ray tube to eliminate soft rays, reduce secondary radiation and optimize X-ray spectrum. The shape of the collimator partition is square, round and parallelogram; The filter located in the window of X-ray tube and DSA compensation filter also have various shapes, such as polygonal filter in the head, rectangular filter in the neck and limbs, double arc filter in the heart and lungs, etc. The ideal filter plate can make the image density in the display screen basically the same, thus avoiding saturation artifacts. If there is no filter plate in DSA examination of lung, the density difference between lung and heart is too large. When the X-ray dose is suitable for the heart, the small blood vessels in the lungs are penetrated, and when the dose is suitable for the lungs, the structures in the heart cannot be recognized. All kinds of filter plates and partitions can be controlled automatically or manually, and the adjustment is very convenient. However, it should be noted that the filter plate should not be too thick, otherwise it will obviously increase the load of the X-ray tube, harden the X-ray harness and reduce the signal-to-noise ratio.

The grid placed in front of i.i. is also used to eliminate the scattered lines of X-rays passing through the human body, which are parallel, convergent, conical and cross-arranged. After adopting this technology, the X-ray radiation dose can be reduced by about 20%.

(3) Pulse fluoroscopy technology: It is realized on the basis of digitization of fluoroscopy images, so it can enhance, smooth and denoise the pulse fluoroscopy images to improve their clarity. There are 25 frames per second, 12.5 frames per second, 6 frames per second, etc. The lower the frequency, the narrower the pulse width and the smaller the radiation dose. But when the pulse frequency is too low, the moving image perspective will jump and drag like animation; When the pulse width is too narrow, the quality of fluorescence image decreases. Using this technique, the estimated radiation dose is reduced by about 40% compared with conventional fluoroscopy.

(4) Image freezing technology: the last frame of each view is temporarily stored and displayed on the monitor, which is called image freezing (LIH). Making full use of LIH technology can reduce unnecessary fluoroscopy, obviously shorten the total fluoroscopy time and achieve the purpose of reducing radiation dose. In LIH state, DSA filter plates and partitions can also be adjusted.

In addition, there are radiation dose automatic display technology, bedside perspective dose adjustment function, lead protective screen hanger and so on.

(2) Mechanical system

It mainly includes a frame and an examination table, which requires a large range of motion, high speed and omnibearing.

1. Frames and bedsteads are C-shaped, U-shaped, double C-shaped arms, L+C-shaped arms, etc. There are two installation methods: sitting on the ground or hanging, which can ensure that the photos can be cut in from multiple directions; It can select and observe the projection angles in all directions to reduce the dead angle and try not to hinder the operation of the surgeon. Judging the performance of the rack mainly depends on the rotation and longitudinal movement of the L arm, the rotation angles of the C arm inclined to the left and right, the axial movement range of the head and feet, the speed and stability of the movement, and the up-and-down movement of the image intensifier. The device is required to automatically display the position and angle of the arm. The sickbed has a wide range of longitudinal and lateral movements and can rotate left and right.

Modern angiography machines mostly adopt double or single C-arm triaxial system (three motors drive the rotating shaft to ensure the concentric movement of the C-arm around the patient, flexible operation and accurate positioning) or L+C-arm triaxial system. Double C-arm products reduce the times of drug injection and X-ray exposure, and increase the angle of motion. The movement of the examination table is bidirectional 180, which increases the activity space and facilitates the positioning and rescue of patients. Triaxial system is the basis of rotational angiography and computer-aided optimal angle positioning of blood vessels.

Modern angiography machines are also equipped with automatic safety protection devices. The computer can automatically warn and control the moving speed of the C-arm and I-I according to the position of the frame and the bed, and use the sensor to feel the distance of the surrounding objects, so as to automatically slow down or stop (for example, when the distance is 250px, it will stop when it is 25px).

2. Posture memory technology A projection posture memory device specially designed for surgeons can store up to 65,438+000 postures. Various postures can be set in advance or stored at any time when taking pictures, so that the pictures can be programmed and the imaging speed can be accelerated.

3. Automatic tracking and playback technology When the C-arm rotates to the angle required for perspective observation, the system can automatically search and play back the existing images at this angle for doctors' reference in diagnosis or interventional therapy; You can also automatically turn the C-arm to this position according to the perspective image. This technique is especially beneficial for angiography of heart and cerebral vessels, especially for coronary intervention.

(3) Image data acquisition and storage system

The overall structure of the system is shown in Figure 5-24. Because DSA needs real-time subtraction of more than 25 frames per second, such a high processing speed must be realized by special hardware. Some manufacturers add image boards to general-purpose microcomputers to realize video signal processing functions, such as A/D conversion and real-time subtraction. The board consists of A/D converter, input look-up table, high-speed arithmetic unit, frame memory, output look-up table and D/A converter.

The sampling clock rate is determined according to the size of the acquisition matrix. For the 5 12×5 12 matrix, the sampling frequency should be greater than 10 MHz. For 768×572 matrix and 1024× 1024 matrix, the sampling frequency is 15MHz and 20MHz respectively. According to the requirements of digital image gray scale, that is, the number of bits, the quantization level of A/D converter is selected, which is generally 8 bits or 10 bits. Generally, the capacity of the frame memory should be able to store 16 frames of digital images. When each pixel is 8 bits (that is, 1 byte), the frame storage capacity is 4MB or 16MB. For the angiography of dynamic organs such as heart and coronary artery, it is necessary to continuously collect 5s or 10s images at the rate of 25 frames per second, which requires a larger image memory (mass memory). Some devices use 64MB high-speed massive frame storage, which can store 250 frames of 5 12x 5 12 x8-bit images. If the real-time frame storage capacity is small, the heart and coronary artery can only be imaged by movie. Generally, one image capture does not exceed 10s. During the interval between two image captures, images stored in frames can be transferred to optical disks or hard disks. Therefore, if the frame storage capacity exceeds 64MB, the film can be replaced.

Large-capacity real-time image memory generally adopts dynamic memory. Because the maximum real-time access speed should reach 50 frames of 5 12×5 12×8bit images per second, it must be transmitted through the video bus, and there should be a computer bus interface to control reading and writing, so as to realize the image transmission between the frame memory and the hard disk.

4) Computer system

In DSA system, computers are mainly used for system control and image post-processing.

1. The control flow of the system is as shown in Figure 5-30, with the computer as the main body to control the whole equipment. According to the control flow, the signals to be connected are as follows:

1) Start switch signal: Turn off the start switch 1 so that the X-ray machine is controlled by the computer, and the computer sends an exposure preparation signal to the X-ray machine; An aperture control signal is sent to reduce the aperture of the aperture. Turn off the start switch 2 and start the imaging process. The computer starts the high-pressure injector and sends the exposure signal to the X-ray machine.

(2) Contact signal: After the X-ray machine is ready, send a ready signal to the computer, indicating that pulse exposure can be carried out. Sending a sampling start signal to the A/D conversion circuit after the exposure starts; After the conversion is completed, inform the computer to read the digital signal, perform pulse exposure again, and collect the next frame of image.

2. Post-processing of image This paper mainly explains logarithmic transformation processing, correction processing of moving artifacts, time filtering processing to improve image signal-to-noise ratio and automatic parameter analysis function.

(1) Logarithmic transformation processing: Due to the change of background, the contrast of subtraction images of contrast vessels obtained at different times will be different, and this difference can be eliminated by logarithmic transformation before subtraction. For example, there are blood vessels with the same diameter at points A and B with different thicknesses. If subtraction is carried out without logarithmic transformation, the subtraction images of blood vessels obtained in different backgrounds and times will have different contrast. If subtraction is performed after logarithmic transformation, it will be displayed with the same contrast regardless of the blood vessel background.

(2) Motion artifact correction: Good registration of mask image and created image is the premise to ensure the quality of DSA inspection. The reasons for poor image registration are the patient's body movement, intestinal gas movement and heart beat. Mask replacement method can correct artifacts caused by body motion and intestinal gas, and pixel shift method can correct body motion and cardiac subtraction, and can correct pulsation. These three methods are introduced below.

1) mask replacement (re-masking) method: It is the most important image registration method in DSA. Its principle is to generate an exposure pulse sequence when the contrast agent flows through the blood vessel to be examined. It is assumed that the first exposure is the set mask image exposure, and then the image exposure is performed. If the patient moves after taking the first image and then takes a series of images, the subtraction image will become blurred due to moving artifacts. In this case, the second frame image can be selected as the mask image to subtract the subsequent images to ensure good registration between subtraction pairs. Because the initial mask is not used, it is called mask replacement.

When changing the mask, the operator should carefully observe the series of contrast images and determine the ideal subtraction pair by trial and error. Generally, the image immediately before the arrival of the contrast agent is paired with the image of the contrast agent peak.

2) Pixel shifting: It is a technology to eliminate motion artifacts through computer programs. If the human body moves between two images, the subtraction of the two images will produce poor registration artifacts. In order to improve the registration of subtraction pairs, some or all pixels of the mask can be moved in the opposite direction for a certain distance, so that the corresponding pixels can be better registered. Because the patient's motion is carried out in three-dimensional space, and the pixel's motion is only carried out in two-dimensional image, the ability of pixel shifting to improve artifacts is limited.

3) Cardiac subtraction: When DSA examines the heart, pulsation artifacts are generated due to the mismatch between the cardiac time of the mask image and the imaging image, so ECG gated acquisition method is needed. However, the acquisition speed of this method is low, and only 1 or 2 frames can be acquired in a cardiac cycle, which is not suitable for cardiac examination, and the number of image frames in the cardiac cycle must be supplemented (when the acquisition speed is 30 frames per second, the average is 30 ~ 32 frames). Collect a mask image of a cardiac cycle, collect ECG signals at the same time, compare the relationship between each frame image and cardiac phase with R wave as the starting point, find a frame in phase with R wave as the first frame mask image, and collect and create images of the next few cardiac cycles. After examination, in order to correct pulsation artifacts, mask images and imaging images with consistent cardiac phases can be extracted for continuous subtraction, which is called cardiac subtraction.

(3) Temporal filtering: The image sequence used for subtraction is taken during the passage of the contrast agent through the blood vessel of interest, and the imaging of each frame changes with time. The purpose of subtraction is to extract the time-related vascular image from the image of the whole anatomical structure, that is, to filter it out. Therefore, the subtraction process can be considered as a filtering process, which is called time filtering. The simplest temporal filtering is mask subtraction, which subtracts two images. In addition, there are integral masks, matched filtering, recursive filtering, and subtraction with more than two frames of images in order to reduce noise and improve signal-to-noise ratio.

(4) Subtraction image processing: In DSA system, some common image processing methods are basically adopted, such as black-and-white inversion, image filtering, translation and rotation, edge enhancement and detection, dynamic window level and width adjustment, histogram equalization, image filtering, etc. The following briefly introduces several processing and measurement analysis methods.

1) interpolation and local enlargement: select a local area from the whole stored image for enlargement. Magnification can be selected, but it is meaningless to exceed 4 times. Because the pixel distribution of the enlarged image becomes finer, interpolation method can be used to supplement the pixels. The simplest interpolation method is to take the average value of adjacent sampling points as the interpolation value. For example, the data of two adjacent sampling points are A and B, and the interpolation value is C=(A+B)/2. This can be seen clearly, but it does not increase the amount of information, so it will not improve the resolution. The above is also called playback magnification, which enlarges and displays the captured image.

If the local enlargement of the image is realized by changing the size of the sampling area, it is a real local enlargement. For example, when the input field of an image intensifier decreases and the sampling frequency remains unchanged, the number of pixels per unit area increases and the spatial resolution is improved, which is called acquisition amplification.

2) Landmark: Landmark technology is mainly to provide an anatomical mark for DSA subtraction images and accurately locate lesions or blood vessels. Because the subtraction image only shows the image of blood vessels containing contrast agent, the anatomical position is not obvious, so a frame of DSA subtraction with enhanced brightness is used to overlap the original mask, so that the blood vessels and reference structures are displayed at the same time, that is, landmark images, bones or soft tissues are used as markers.

(5) Automatic analysis function: After ventriculography and angiography, the computer uses analysis software to extract the functional information related to quantitative diagnosis in real time and add it to the morphological image. Several analysis functions are described below.

1) left ventricular volume calculation and analysis function: it is to calculate left ventricular volume by using left ventricular end-diastolic image and end-systolic image obtained from DSA image; According to this result, functional parameters such as ejection fraction, wall motion, cardiac output, heart weight and myocardial blood flow reserve can be calculated.

2) Coronary artery or blood vessel analysis software: it is a computer for measuring blood vessel diameter, maximum stenosis coefficient, stenosis or plaque area, lesion range and blood flow state by geometric and density methods.

3) Functional image: It is an image formed by drawing a time video density curve for a series of shot images with a video densitometer and then according to the parameters obtained from the curve. This kind of image reflects functional information, which is different from the traditional image that reflects morphological category information. From this curve, we can extract the time-related parameters of contrast agent flowing in blood vessels, the parameters of local blood vessel volume or depth (thickness) and the parameters of local organ parenchymal blood perfusion. These parameters are indispensable for the diagnosis and treatment of cardiovascular diseases, and the lesions can be found at an early stage.

New technology of DSA processing

DSA not only serves for diagnosis, but also provides advanced means for disease treatment. DSA is a common means of interventional therapy, and the method of drawing path map can guide the operator to operate quickly and correctly. The image acquisition method of ECG trigger pulse is unique, and the imaging of moving parts is clear; Peak-hold acquisition method can improve the signal-to-noise ratio of subtraction image; For DSA imaging of moving parts, dynamic DSA technology (that is, X-ray tube, examination table and detector move regularly during image acquisition) can greatly reduce artifacts, such as cine subtraction, rotational angiography, contrast agent tracking angiography, step-by-step angiography and automatic optimal angle positioning.

1. Roadmap technology is produced to facilitate intubation and interventional therapy in complex parts. The specific method is to inject a little contrast agent first, then take pictures, and make real-time dynamic subtraction between the first fluoroscopic image and the subsequent fluoroscopic image, so that the vascular shadow overlaps with the intubation process and is displayed at the same time. This clearly shows the direction of the catheter and the specific position of the tip, so that the operator can smoothly insert the catheter into the destination. This method is divided into three stages: ① active digital perspective to form an auxiliary mask image; ② When the contrast agent in the blood vessel is the most filled and the contrast is the highest, the filling image is used to replace the auxiliary mask; ③ When the contrast agent in the blood vessel is emptied, the fluoroscopic image is subtracted from the filled image mask to display the blood vessel with maximum contrast, which can make the catheter operate accurately along the trajectory.

In a word, the road map technology is to use the perspective natural image as an auxiliary mask, and then use the filling image instead of the auxiliary mask to become the actual mask, and subtract it from the perspective image without contrast agent to get the blood vessel image containing only contrast agent, which can be used as a road map for intubation, and can clearly observe the dynamic movement of the intravascular catheter, which is very helpful for the comparison and safety of interventional therapy.

2. Digital film subtraction uses digital fast short pulses for image acquisition. Real-time imaging is 25 ~ 50 frames per second, generally up to 50 frames per second in one direction and 25 frames per second in two directions. Images can be recorded on film. This acquisition method is used in heart, coronary artery and other moving parts, which makes the motion artifacts after subtraction almost zero. This method is often supplemented by ECG trigger.

3. At the same time when the rotating DSA system starts to collect images, the C-arm bracket rotates around the patient and collects the parameters of a blood vessel and its branches at 180. The human body remains still, and the X-ray tube and intensifier move synchronously, thus obtaining a three-dimensional image. This technique obviously increases the observation angle and obtains more diagnostic information, which is especially suitable for angiography of cerebral vessels, cardiac chambers and coronary arteries.

4. Step-by-step angiography uses rapid pulse exposure to collect images. During the exposure, the X-ray tube and intensifier remain stationary, and the catheter bed moves forward automatically and evenly with the human body, thus obtaining the whole subtraction image of blood vessels, which is mainly used for arterial examination and interventional therapy of limbs.

5. Remote contrast tracking technology: After the contrast agent is injected, the moving speed of the bed surface is manually controlled or programmed during the imaging period to track the contrast agent imaging, which is especially suitable for peripheral artery and thoracic and abdominal aorta angiography that needs multi-field and multiple injections.

6. Automatic angle positioning system Automatic angle positioning means that the computer analyzes and determines the best display angle of the lesion according to the display of the left and right oblique diseased blood vessels, and the C-arm automatically turns to this position for angiography. As long as the operator gives the general blood vessel two angles at will (at least 30 apart), and then presses the function key (marked COMPAS), the computer will automatically find the best projection angle and display the blood vessel image until the best image is obtained. This function is especially suitable for coronary angiography and cerebral angiography.

7. Peak-hold sampling technology sets the maximum brightness unit and the minimum brightness unit in the frame memory, and initializes these two units to the darkest and brightest values respectively before starting sampling. In the sampling process, the current value is written into the maximum value unit only when the current image becomes bright; Similarly, only when the current image is dark, the current value is written into the minimum unit, and the above process is repeated until the sampling ends. The largest unit always remembers the mask image data, while the smallest unit's memory process is from the mask image to the partially filled image to the completely filled image. By subtracting the maximum and minimum frame storage units, a series of subtraction images from partial filling to complete filling are obtained. This process is peak-hold sampling. Its advantage is that the quality of subtraction image can be improved, or the image effect of common DSA acquisition mode can be obtained with a small irradiation dose.

8. Double-plane angiography One direction of X-ray angiography is likely to affect observation because of blood vessel overlap. The DSA system of double C-arm X-ray machine can realize the synchronous control of two identical DSA through software, and obtain real-time imaging in two directions at the speed of 25 frames per second. The blood vessels in one direction may not overlap, and doctors can obtain implicit three-dimensional information from two images in different directions by virtue of their own clinical experience. For example, if two images in different directions are displayed on two monitors, images with real stereoscopic effect can be seen through special observation glasses. As long as we know the spatial coordinates of the X-ray source in two directions, we can use sounding software to accurately calculate the three-dimensional spatial position of the lesion. This method of realizing biplane angiography through software connection can avoid multiple injections of contrast media and multi-directional projection, thus shortening the examination time and reducing the amount of contrast media.

To sum up, with the continuous development of DSA technology and the continuous improvement of equipment performance and imaging methods, the shortcomings of DSA have been improved. For example, the post-processing of the image improves the signal-to-noise ratio; Because of the small field of view, large parts need multiple exposures, which can be solved by improving the input field of view of I.i., adopting remote control contrast tracking technology and stepping exposure. The imaging of moving parts and motion artifacts can be improved by improving the high voltage generator and using ultra-short pulse rapid exposure. Using digital pulse fluoroscopy can reduce the X-ray radiation dose by nearly half.