In the DSA system, according to different purposes, there are various methods of digital subtraction, such as time and energy subtraction, etc. The difference mainly lies in the acquisition of the two images of phase subtraction, i.e., the mask and the contrast-filled image.
(I) Temporal subtraction
Temporal subtraction is commonly used in DSA, where one or more frames are stored as a mask before the injected contrast agent enters the region of interest and is subtracted one by one from the chronologically appearing contrast image. In this way, portions of the image that are the same in both frames are eliminated, and the high-density portions formed as the contrast agent passes through the vessel are highlighted. This mode of operation is called temporal subtraction because of the different time sequence in which the mask image and the contrast image are acquired. Its shortcoming is that the mask image and the contrast image cannot be precisely matched due to the patient's voluntary or involuntary movements during the photographic process, resulting in poorly aligned artifacts or blurring of the image. In view of the subtraction of the mask image and contrast image used in the number of frames, acquisition time is different, and can be divided into the following ways:
1. Pulse image (pulseimage, PI) mode PI mode using intermittent X-ray pulses to form the mask image and contrast image (as shown in Figure 5-25), several frames of the image per second, the duration of the pulse is generally greater than the time of a frame of the video signal. The mask image is taken when the contrast agent is not flowing into the vessel of interest, and the X-ray image is acquired and subtracted during the gradual diffusion of the contrast agent, resulting in a series of consecutive and spaced subtracted image series, with large intervals between each subtracted image frame.
2. superpulseimage (SPI) mode SPI mode at a rate of 6 to 30 frames per second X-ray pulse camera, and then frame by frame high-speed repeated subtraction, with a high frequency, narrow pulse width characteristics, see Figure 5-26. X-ray exposure pulse and the camera field synchronization to maintain consistency, the effective time of the exposure signal should be in the field of the fading period, so the pulse frequency up to 50 to 60 Hz. The pulse frequency is up to 50-60Hz, pulse width are 3-4ms. this way can be real-time video speed continuous observation of X-ray digital image or subtracted image, has a high dynamic clarity.
3. Continuous image (continuousimage, CI) mode CI mode (shown in Figure 5-27) and fluoroscopy, X-ray continuous irradiation, synchronized with the camera, the frequency of 25 to 30 frames per second continuous image, the X-ray can be continuous or pulsed. Because it is a long period of continuous irradiation, the load of the X-ray tube is quite large, so it is necessary to use a large thermal capacity of the X-ray tube, such as exposure with fluoroscopic tube current, the resulting subtracted image of the signal-to-noise ratio is very low, so the CI mode is generally used with a small focal point ?15mA tube current of the condition of the continuous exposure of the photographic.
4. Time interval difference (timeintervaldifference, TID) mode The previous several subtraction modes use the image when the contrast agent is not injected into the angiographic site of the blood vessels as a mask image, and subtract the image with the contrast agent containing the sequence of X-ray images as a contrast image, the TID mode is not a fixed mask image, but to randomly determine the frame of the image (for example, frame 3, can be). Instead, an image is randomly determined (e.g., frame 3, which can be taken when the contrast agent is just injected into the vessel), and then subtracted from the contrast image (frame 6) at a certain interval (e.g., every 3 frames) after it (3-6), and then subtracted frame by frame (4-7) and (5-8) ...... to form a subtracted image sequence.
5. Electrocardiogram (ECG) trigger pulse method due to each moment of cardiac motion in different phases, in order to make the mask image and contrast image phase as close as possible, in order to reduce the subtraction of image motion artifacts, the requirement of the subtraction of the image of the synchronization of the cardiac motion, usually use the ECG trigger X-ray pulse method. The external ECG signal triggers the X-ray acquisition image in three ways.
(1) Continuous ECG marking: the image is acquired in a continuous manner, marking the frame on which the ECG signal occurs, with a minimum frequency of 5 frames/second for this modality.
(2) Pulsed electrocardiogram marking: the image is taken in a pulsed fashion, marking the frame closest to where the ECG signal occurs, with a minimum frequency of 5 frames/second.
(3) ECG gated triggering: the ECG signal initiates the gated acquisition of the X-ray generator to mark the image. The specific method is to memorize the output signal of the ECG machine in the ECG memory after A/D conversion, and at the same time to draw the R-wave marker from the R-wave signal as the reference of ECG phase. During ECG gated acquisition, if the X-ray exposure is synchronized with the R-wave marker, a subtracted image timed by the R-wave is obtained. This modality is mainly used for DSA examination of the large vessels of the heart, where the exposure is matched to the rhythm of the cardiac vascular beat to ensure that each frame in the image series is in phase with the cardiac rhythm and to eliminate artifacts due to cardiac beat.
II) Energy subtraction
Energy subtraction is also known as dual-energy subtraction. In angiography of the region of interest, two frames are obtained almost simultaneously with two different tube voltages (e.g., 70 kV and 130 kV), which are subtracted; the two frames are called energy subtraction because they are taken with X-rays of different energies.
This subtraction method makes use of the fact that the attenuation coefficients of iodine and the surrounding soft tissues for X-rays are significantly different at different energies (iodine jumps in the attenuation curve at the 33-keV energy level, with a sudden increase in attenuation coefficients, whereas the attenuation curve of the soft tissues is continuous and the higher the energy, the smaller the attenuation coefficients). If a piece of tissue containing bone, soft tissue, air, and trace amounts of iodine is exposed to X-rays with energies slightly below and slightly above 33 keV (70 and 130 kV, respectively), the latter image shows approximately 80% reduction in iodine, 40% reduction in bone, and approximately 25% reduction in soft tissue signals compared to the former image, while the gas shows virtually no attenuation at both energy levels. If these two frames are subtracted, the resulting image will effectively eliminate the gas shadow, retaining a small amount of soft tissue shadow and significant bone shadow and iodine signal. If the image acquired at 130kV is weighted by a factor of about 1.33 and then subtracted, the soft tissue and gas shadows can be eliminated very well, leaving only a small amount of bone signal and obvious iodine signal.
Energy subtraction can also separate tissues with different attenuation coefficients, such as removing bone or soft tissue from the X-ray image to obtain an image of only soft or bone tissue. The specific method is to use two energy X-ray beams to obtain two images, one in the low-energy X-ray obtained, the other in the high-energy X-ray obtained, the image are logarithmically transformed by weighted subtraction, the elimination of bone or soft tissue.
From the principle, energy subtraction is a better subtraction method, but in the implementation of the tube voltage required to be able to switch between the two energies at high speed, increasing the complexity of the X-ray machine, the general X-ray machine can not be used this method. This method is also less likely to eliminate residual images of bones.
(C) Hybrid subtraction method
The combination of energy and time subtraction techniques produces the hybrid subtraction technique. The basic principle is that before the contrast agent is injected, a double-energy subtraction is done first to obtain an image containing a small portion of the bone tissue signal, and then this image is subtracted from the double-energy subtraction image of the blood vessels injected with the contrast agent, and then a pure vascular image is obtained. Hybrid subtraction is more demanding in terms of equipment and X-ray tube loading.
Three, DSA on the special requirements of the equipment and technical measures
DSA and ordinary DF system is different, not only to digitize the X-ray image, but also to achieve a better quality of vascular subtraction image, therefore, the DSA system has a series of special requirements.
(A) X-ray generation and imaging system
Including X-ray tube, high voltage generator, image intensifier, optical system, TV camera and monitor.
1. X-ray generator requires X-ray tube can withstand the load of continuous pulse exposure, for medium and large DSA equipment, the general thermal capacity of the X-ray tube should be more than 200kHU, the tube voltage range of 40 ~ 150kV, tube current is usually 800 ~ 1250mA. high voltage generator is required to produce a stable DC high voltage, the use of medium and high-frequency technology, by the micro-computer control, to produce almost pure DC voltage. The X-ray machine is capable of fast exposure with multiple pulses and imaging speed up to 150 frames/second.
2. The image intensifier usually adopts variable field of view I.I, such as 775px I.I can have 10, 16, 22, 31cm four kinds of field of view, according to the needs of imaging flexible choice. The spatial resolution is inversely proportional to the screen size and field of view, generally 1.1~2.5LP/mm. In order to improve the sensitivity and resolution, the input screen is made of cesium iodide and other materials. The newly developed flat-plate type intensifier has hundreds of thousands of optical fibers between the luminous body of the input screen and the photovoltaic layer, coupling the light of each pixel to the photovoltaic layer, thus making the image have a high luminance and improving the conversion efficiency of I.I. Therefore, it is promising. Currently, high-performance I.I has a quantum detection efficiency (DQE) of 85%. Some sources say that the resolution of up to 6.8LP/mm.
3. Optical system in order to adapt to the use of X-ray dose range (i.e., the range of changes in the amount of input light) is large, requires the use of a large aperture, aperture can be automatically adjusted lens, and some lenses also contain motorized neutral filters to prevent the intake of bright light.
4. TV cameras require the camera tube with high sensitivity, high resolution and low residual image characteristics, the video channel to have a variety of compensation circuits to ensure that the output of high signal-to-noise ratio, high-fidelity video signals. x-ray exposure and image acquisition must be synchronized, but due to the hysteresis characteristics of the vacuum camera tube, in the pulsed image and interlaced scanning system, the image of the image of the signal amplitude is not equal to each one, the sampling Sampling needs to wait until the signal amplitude is stabilized, thus increasing the exposure pulse width and wasting the dose. This situation can be improved by using CCD cameras and progressive scanning. With the improvement of CCD product quality, will further replace the vacuum camera tube. High-performance CCD camera, using high-definition system, the resolution of 1249/1023 lines (50 ~ 60Hz), S/N is greater than 2500, the band is greater than 10.5MHz.
5. Monitor requirements with high-definition, large-screen monitor, such as progressive scanning of more than 1024 lines, more than 1275px type. Now the monitor in the imaging room is often used in the form of multi-screen, multi-split or picture-in-picture, easy to compare at any time. High-performance monitors use ambient brightness sensors to automatically adjust brightness; flicker-free planar CRTs realize flicker-free image display at field frequencies higher than 100Hz.
6. Automatic control of X-ray image brightness in the DSA due to the subject's tissue density varies greatly, should ensure that in a variety of different photographic objects and photographic conditions can be obtained with sufficient diagnostic information image, to eliminate the blurring and halo. DSA is the formation of analog image signals by the I.I-TV imaging system, I.I.'s dynamic range is large, about 10, in the different exposure dose It can output images with good contrast at different exposure doses. However, when the target surface illumination of the TV camera tube ranges from 10?~10x, the output current varies between dark and saturated current values, and the dynamic range is within a few hundred. Some parts of the examination (such as chest, abdomen) X-ray exposure dose variation range of 10 ~ 10, more than the camera can accurately replicate the signal range, so there is a need for a series of automatic control measures to ensure that the camera tube's light input varies within its dynamic range.
"There are three main types of self-control measures: ① Control of I.I. output light volume. Control of the exposure dose of X-ray is to control the input light volume of I.I to use the video signal output of the camera to automatically control the exposure time, or automatically adjust the kV, mA value of the X-ray tube, you can automatically control the brightness of the X-ray image;
② control of the output light volume of the optical system. With the video signal to automatically control the size of the lens aperture, F1.4 aperture lens in the controlled computer with the assistance of the filter, automatic adjustment of the amount of light can reach 6.6 × 10, thus ensuring that the input illumination of the camera tube is always in the normal range;
③ the use of compensating filters can also be reduced in the dynamic range of the X-ray information, so that it and the dynamic range of the equipment components coincide with the dynamic range. A compensating filter is an additional attenuating material placed between the x-ray tube and the patient, which selects specific areas of attenuation within the field of view to provide a more uniform dose distribution.
7. X-ray dose management Minimizing the dose of X-ray exposure received by the patient while maintaining image quality is the task of the dose management system, which consists of a range of modern technologies.
(1) Grid control technology: in each pulse exposure interval to the gate to add a negative potential to offset the exposure pulse of the start and afterglow, thus eliminating soft rays, improve the quality of the effective ray, shorten the pulse width.
(2) Spectral Filtering Technology: An aluminum filter plate is placed in the window of the I.I or X-ray tube to eliminate soft rays, reduce secondary radiation, and optimize the spectrum of X-rays. The spacer of the collimator has square, round and parallelogram shapes; the filter plate located in the window of the X-ray tube and the DSA compensatory filter plate also have various shapes, such as polygonal filter plate for the head, rectangular for the neck and limbs, and double arc for the heart and lungs. The ideal filter plate can make the image density within the range of the display is basically the same, so as not to produce saturation artifacts. If there is no filter plate for DSA examination of the lungs, the density difference between the lungs and the heart is so large that the small blood vessels in the lungs are penetrated when the X-ray dose is suitable for the heart, and the intracardiac structures cannot be recognized when the dose is suitable for the lungs. Various filter plates and spacers can be automatically or manually controlled and are easily adjusted. However, it should be noted that it is not advisable to use too thick a filter plate, otherwise it will significantly increase the X-ray tube load, but also harden the X-ray beam and reduce the signal-to-noise ratio, etc..
Filter grids placed in front of the I.I are also used to eliminate scattered rays when X-rays pass through the body, and are arranged in parallel, convergent, conical and crossed. X-ray radiation dose can be reduced by about 20% with this technique.
(3) pulse fluoroscopy technology: is realized on the basis of fluoroscopic image digitization, so the pulse fluoroscopy image can be enhanced, smoothed, denoising and other filtering processes to improve the clarity of the image. The pulse fluoroscopy frequency of the equipment has 25 frames/second, 12.5 frames/second, 6 frames/second and other types of options, the lower the frequency, and the narrower the pulse width, the smaller the radiation dose. However, when the pulse frequency is too low, the moving image fluoroscopy will appear animated jumping and dragging; when the pulse width is too narrow, the quality of the fluoroscopic image will be reduced. With this technique, it is estimated that the radiation dose is reduced by about 40% compared with conventional fluoroscopy.
(4) Image Freezing Technology: The last frame of each fluoroscopy is stored temporarily and retained for display on the monitor, called lastimagehold (LIH). Making full use of LIH technology can reduce unnecessary fluoroscopy, significantly shorten the total fluoroscopy time, and achieve the purpose of reducing the radiation dose. It is also possible to adjust the DSA filter and septum in the LIH state.
In addition, there are automatic display technology of radiation dose, fluoroscopic dose adjustment function beside the examination bed, and lead protection screen hanger.
(2) mechanical system
Mainly including the rack and examination bed, they are required to have a large range of motion, fast speed and all-round.
1. Rack and bed rack C, U, double C isomorphic arm, L + C arm, etc.; installation mode has a seat on the ground or suspension of two kinds of contrast can be guaranteed from a number of directions into the cut; can do a full range of selection and observation of the projection angle in order to reduce the dead angle, as far as possible 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 angle of rotation of the C-arm to the left front oblique, to the right front oblique and the range of axial movement to the head and to the foot, the speed and stability of the movement, the up and down movement of the image intensifier, and the requirement that the equipment can automatically display the position of the arm, angle and other data. The longitudinal and transverse movement range of the examination bed should be large and can be rotated left and right.
Modern angiography machines mostly use double, single C-arm three-axis (three motor-driven rotary axis to ensure that the C-arm around the patient for the same center of motion, flexible operation, accurate positioning) or L + C-arm three-axis system. Double C-arm products reduce the number of drug injection and X-ray exposure, and increase the angle of movement. The examination bed moves 180° in both directions, which increases the space for movement and facilitates patient positioning and resuscitation. The three-axis system is the basis for rotational imaging and computer-assisted vascular positioning at the optimal angle.
Modern angiography machine is also equipped with automatic safety protection device, the computer can automatically warn and control the speed of C-arm and I.I. movement according to the position of the frame and bed, and use the sensor to feel the distance of the surrounding objects to automatically realize the deceleration or stopping (e.g. deceleration when it is 250px away from the object, stopping when it is 25px away from the object).
2. Body position memory technology specially designed for surgeons to project the body position memory device, which can store up to 100 body positions, and all kinds of positions can be preset in advance or stored at any time in the imaging, so as to make the imaging programmed and speed up the imaging speed.
3. Automatic tracking and playback technology: When the C-arm is turned to the desired angle for fluoroscopic observation, the system automatically searches for and replays the existing imaging images at that angle for the doctor's reference in diagnosis or interventional therapy; the C-arm can also be automatically turned to that position to perform fluoroscopic imaging again according to the images. This technology is especially beneficial for cardiac and cerebrovascular angiography, especially for coronary intervention procedures.
(C) Image data acquisition and storage system
The general structure of this system has been shown in Figure 5-24. Since DSA requires real-time subtraction of more than 25 frames/second, such a high processing speed must be realized by dedicated hardware. Some manufacturers in the general-purpose microcomputer to add a video board to realize the video signal A/D conversion and real-time subtraction and other processing functions, the board consists of A/D converter, input lookup table, high-speed operator, frame memory, output lookup table, D / A converter and other components.
According to the size of the acquisition matrix to determine the rate of the sampling clock, the 512 × 512 matrix, the sampling frequency needs to be greater than 10MHz; 768 × 572 matrix and 1024 × 1024 matrix, the need for the sampling frequency of 15MHz and 20MHz, respectively. according to the requirements of the digital image gray level to select the quantization level of the A/D converter, i.e., the number of bits (bit), generally 8bit or 10bit. The capacity of the frame memory should be able to save 16 frames of digital images in general, and when each pixel is 8bit (i.e., 1 byte, byte) of data, the capacity of the frame memory is 4MB or 16MB. For the imaging of the heart and the coronary arteries and other dynamic organ parts, the need for real-time continuous acquisition of 5s or 10s of images at a rate of 25 frames per second requires the use of a higher-capacity image Some devices have adopted 64MB high-speed massive frame memory, which can save 250 frames of 512×512×8bit images. If the capacity of real-time frame memory is small, the heart and coronary artery can only be imaged by movie. An acquisition of the image is generally not more than 10s, and in the interval between two acquisitions of the image of the frame can be stored in the image of the transfer to the CD-ROM or hard disk, so the frame memory capacity of more than 64MB, can replace the movie film.
Large-capacity real-time image memory is generally used dynamic memory, due to the highest real-time access speed to reach 50 frames per second 512 × 512 × 8bit image, so it must be transmitted through the video bus, but also to have a computer bus interface, in order to carry out the read-write control and the realization of the frame and hard disk between the image of the memory transfer.
D) computer system
In the DSA system, the computer is mainly used for system control and image post-processing.
1. System control control process shown in Figure 5-30, the computer as the main body to control the entire device. According to the control process needs to be connected to the signal is as follows:
1) start switch signal: start switch 1 closed so that the X-ray machine to accept the computer control, by the computer to the X-ray machine to send exposure preparation signal; send diaphragm control signal, so that the aperture of the aperture is reduced. The start switch 2 is closed so that the contrast process begins, the computer starts the high-pressure injector, and an exposure signal is sent to the X-ray machine.
(2) Contact Signal: When the X-ray machine is ready, a ready signal is sent to the computer to indicate that pulse exposure can be performed. After the start of exposure, send a sampling start signal to the A/D conversion circuit; after the end of the conversion, notify the computer to read the digital signal, and then carry out pulse exposure again to collect the next frame of image.
2. Image post-processing here mainly explains the logarithmic transformation processing, correction processing of mobility artifacts, improve the image S/N time filtering processing and automatic parameter analysis function.
(1) Logarithmic transformation processing: the contrast difference of angiographic subtraction images obtained at different moments will be produced by the change of background, and this difference can be eliminated by logarithmic transformation before subtraction. For example, at two points A and B with different thicknesses, there are blood vessels of the same diameter. If the subtraction is performed without logarithmic transformation, the subtracted images of blood vessels obtained at different moments will have different contrast due to the different backgrounds. If logarithmically transformed and then subtracted, they will be displayed with the same contrast, independent of the background of the vessels.
(2) Mobility artifact correction processing: mask image and contrast image alignment is good, is to ensure the quality of DSA examination prerequisite. Poor image alignment is due to patient body movement, intestinal gas movement and heart beat. The mask replacement method corrects for artifacts such as body movement, gas-induced bowel movements, pixel shift method corrects for body movement, and cardiac subtraction corrects for artifacts such as pulsatility. These three methods are described below.
1) Replacement of the mask (re-masking) method: the most important image alignment method in DSA, the principle is to generate a sequence of exposure pulses when the contrast agent flows through the blood vessels to be examined, assuming that the first exposure is the mask image exposure that is set, and the subsequent exposure is the contrast image exposure. If patient movement occurs after the first image frame is taken, followed by a series of images, the subtracted image will be blurred by movement artifacts. In this case, frame 2 can be selected as the mask image to subtract the subsequent contrast image to ensure good alignment between subtraction pairs. Since the initial mask is not used, this is called changing the mask.
When replacing the mask, the operator should carefully observe the series of contrast images and decide on a better subtraction pair by trial-and-error method, generally selecting the image of the instant before the arrival of the contrast mass to match with the image of the peak of the contrast agent.
2) Pixel shift: a technique to eliminate movement artifacts by a computer program. If the body moves between two image acquisitions, the subtraction of the two images produces poorly aligned artifacts. To improve the alignment of the subtracted pairs, some or all of the pixels of the mask can be shifted by a certain distance in the opposite direction so that the corresponding pixels are better aligned. Because patient movement takes place in three dimensions and pixel shifting takes place only on images in two dimensions, pixel shifting has limited ability to improve artifacts.
3) Cardiac subtraction method: When DSA is used to examine the heart, it is necessary to use ECG-gated acquisition because of the beating artifacts caused by the incompatibility of the cardiac phases of the mask image and the contrast image. However, the acquisition speed of this method is low, and only 1 or 2 frames can be acquired in one cardiac cycle, which is unsuitable for cardiac examination, and the number of image frames in the cardiac cycle must be supplemented (the average is 30-32 frames when the acquisition speed is 30 frames/second). A mask image is acquired for one cardiac cycle, and the ECG signal is acquired at the same time. The R wave is used as the starting point to compare the relationship between each frame and the cardiac phase frame by frame, and the frame that is in phase with the R wave is found to be the first mask image, and the contrast image is acquired in the following several cardiac cycles. At the end of the examination, in order to correct the beating artifacts, the mask image with the same cardiac phase and the contrast image can be withdrawn 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 vessel of interest, and each frame of the contrast image changes over time. The purpose of subtraction is to extract, i.e. filter out, the vascular images with time-dependent characteristics from the images of the entire anatomical structure. Therefore, the subtraction process can be thought of as a process of filtering, called temporal filtering. The simplest temporal filtering is mask mode subtraction, which utilizes two frames of image subtraction. In addition, there are integral mask, matched filtering and recursive filtering, etc., they use more than two frames of images for subtraction, the purpose is to reduce the noise and improve the S/N.
(4) Subtracting Image Processing: In the DSA system, some general image processing methods are basically used, such as black and white inversion, image filtering, shift and rotate, edge enhancement and detection, dynamic window position and window width adjustment, histogram equalization, image filtering, and so on. The following is a brief introduction to several processing and measurement analysis methods.
1) interpolation and local magnification: from the entire stored image to select the local area to be enlarged to display, the magnification can be selected, but more than 4 times the loss of meaning. As the pixel distribution of the enlarged image becomes thinner, interpolation can be used to supplement the pixels. The simplest interpolation method is to take the average of neighboring sample point data as the interpolation value, for example, two neighboring sample point data for A and B, then the interpolation value of C = (A + B) / 2. This can be seen more clearly, but does not increase the amount of information, it will not improve the resolution. The above is also known as playback zoom, zooming in to show the captured image.
True local zoom is achieved if the local zoom image is realized by transforming the size of the sampling area. For example, after the input field of the image intensifier is reduced and the sampling frequency is unchanged, the pixels per unit area are increased and the spatial resolution is improved, which is known as acquisition magnification.
2) Boundary markers: Boundary marker technology mainly provides an anatomical landmark for the subtraction image of DSA to make a precise localization of the lesion area or vessel. Since the subtracted image only shows the image of the vessels containing the contrast agent and the anatomical localization is not obvious, a frame of DSA subtraction whose brightness has been enhanced is overlaid with the original montage so that both the vessels and the reference structures can be shown at the same time, i.e., for the borderline image, and the structures, such as bones or soft tissues, are used as markers.
(5) Automatic analysis function: After ventricular and angiography, the computer uses analysis software to extract functional information related to quantitative diagnosis in real time and add it to the morphology image. Several analysis functions are described below.
1) Left ventricular volume calculation and analysis function: It is to calculate the volume of the left ventricle by using the end-dilated image and end-systolic image of the left ventricle obtained from the DSA image; and based on this result, functional parameters such as the ejection fraction, ventricular wall motion, cardiac output, cardiac weight, and myocardial flow reserve are then calculated.
2) Coronary artery or vascular analysis software: It is a computer that uses geometric and densitometric methods of processing to measure vessel diameter, maximum stenosis coefficient, stenosis or plaque area, extent of the lesion, and blood flow status.
3) Functional imaging: is the use of video densitometer on the ingested series of images plotted on the time video density curve, and then according to the parameters obtained from the curve to form a kind of image. This type of image reflects functional information, which is different from the traditional image that reflects information in the morphological category. From the curves, it is possible to extract time-dependent parameters of the flow of the contrast medium through the vessels, parameters of the volume or depth (thickness) of the local vessels, and parameters of the parenchymal perfusion of the local organs, which are indispensable for the diagnosis and treatment of cardiovascular diseases, and for the detection of lesions at an early stage.
(E) New technology of DSA processing
DSA not only serves for diagnosis, but also provides advanced means for disease treatment. DSA is often used in interventional therapy, and adopts the method of drawing path diagrams, which can guide the operator to operate quickly and correctly; the image acquisition method of ECG triggering pulse is unique for clear imaging of the moving parts; the peak-holding acquisition method can increase the signal-to-noise ratio of the image; for the image acquisition method of motion, the peak-holding acquisition method can increase the signal-to-noise ratio of the image; and the image acquisition method can increase the signal-to-noise ratio of the image. The signal-to-noise ratio of the image; for DSA imaging of moving parts, the use of dynamic DSA technology (i.e., regular movement of the X-ray tube, examination bed, and detector during image acquisition) can greatly reduce artifacts, and the common ones are cine subtraction, rotational angiography, contrast tracking angiography, step-by-step angiography, and automatic optimal angular positioning.
1. Path map technology created for the convenience of intubation of complex sites and the needs of interventional therapy, the specific method is to inject a small amount of contrast agent and then photography, the image obtained from the first fluoroscopy and later fluoroscopy of the image of real-time dynamic subtraction, so that the vascular shadow and the process of insertion of the overlap of the catheterization process, and display at the same time. This clearly shows the course of the catheter and the exact location of the tip, allowing the operator to smoothly insert the catheter into the destination. This method is divided into three phases: (1) active digital fluoroscopy forms an auxiliary mask image; (2) when the vessel is filled with the most contrast and the highest contrast is exposed, the auxiliary mask is replaced by the filling image; and (3) when the vessel is emptied of contrast, the fluoroscopic image is subtracted from the filling image mask, and the vessel is displayed with the maximum contrast, enabling the catheter to be accurately maneuvered along the trajectory.
In summary, the path diagram technique is to use the natural image of fluoroscopy as an auxiliary mask, and then replace the auxiliary mask with the filling image to become the actual mask, which is subtracted from the contrast-free fluoroscopic image to obtain the contrast-only image of the blood vessel, which is used as the path diagram for catheterization to clearly observe the dynamic motion of the catheter within the vessel, which is very helpful for the comparison and safety of interventional therapy.
2. Digital cine subtraction performs image acquisition with digital fast short pulses. Real-time imaging is 25 to 50 frames per second, generally up to 50 frames per second in one direction and 25 frames per second in both directions, allowing images to be recorded on movie film. This type of acquisition is used for motion sites such as the heart and coronary arteries, resulting in almost zero motion artifacts after subtraction. The use of this method is often supplemented with ECG triggering.
3. The rotational angiography DSA system begins to acquire images at the same time, the C-arm stent around the patient for rotational movement, a blood vessel and its branches for 180 ° parameter acquisition, the human body to remain stationary, the X-ray tube and the intensifier for synchronized movement, so as to obtain three-dimensional images. The use of this technique significantly increases the angle of observation, obtaining more diagnostic information, especially for cerebrovascular, cardiac cavity and coronary angiography.
4. Step-by-step angiography uses rapid pulse exposure to collect images, the X-ray tube and intensifier remain stationary during exposure, and the catheter bed moves forward automatically and uniformly with the human body, thus obtaining a full subtractive image of the blood vessels, which is mainly used for arterial examination of the extremities and interventional therapy.
5. Remote control contrast agent tracking technology after injection of contrast agent, in the image during the hand-controlled or programmed control of the speed of bed movement, tracking the contrast agent image, especially for the peripheral arteries and thoracic and abdominal aortograms that require multiple horizons and multiple injections to complete.
6. Automatic Angle Positioning SystemAutomatic Angle Positioning System means that the computer analyzes and determines the optimal display angle of the lesion according to the display of the lesion vessels in the left and right anterior oblique positions, and the C-arm is automatically turned to that position for imaging. The operator just needs to press the function key (labeled COMPAS) after giving any 2 angles (at least 30° interval) to the general blood vessels, 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 artery and cerebral angiography.
7. Peak hold sampling technology in the frame memory to set the brightness of the maximum value of the unit and the minimum value of the unit, before the start of the sampling, the two units were initialized to the darkest and brightest values. Sampling process, in the current image becomes brighter, before the current value is written into the maximum value unit; similarly, in the current image is darker before the current value is written into the minimum value unit, the above process is repeated until the end of the sampling. The maximum value unit always memorizes the mask image data, while the minimum value unit memorizes the process from the mask image to the partially filled image and then to the fully filled image. The maximum value and the minimum value of the frame memory unit is reduced to obtain a series of partial to fully filled subtracted image, this process is the peak hold sampling. Its advantage is that it can improve the quality of the subtracted image, or with a smaller dose of irradiation to obtain the image effect of ordinary DSA acquisition.
8. Dual-plane angiography X-ray angiography in one direction is likely to be affected by the overlap of the blood vessels and the observation, dual C-arm X-ray machine DSA system can be realized through the software is identical to the synchronous control of the two DSAs, at a rate of 25 frames / second real-time acquisition of the front and side of the two directions of the contrast image. Vessels may not overlap in one of the directions, and physicians can combine their clinical experience to obtain implied three-dimensional information from the contrast images in two different directions. For example, if the contrast images in two different directions are displayed on two separate monitors, the images can be viewed in true three-dimensionality through a specialized viewer. As long as you know the spatial coordinates of the X-ray sources in both directions, you can also accurately calculate the three-dimensional spatial position of the lesion by using probing software. This method of biplane angiography through software linkage avoids multiple injections of contrast and multi-directional projections, thus shortening the examination time and reducing the amount of contrast.
In summary, the shortcomings of DSA have been improved with the continuous development of DSA technology and the continuous improvement of equipment performance and imaging methods. For example, the post-processing of the image to improve the S/N; due to the small field of view, large parts need multiple exposures, which can be solved by improving the input field of the I.I, using remote contrast tracking technology, step-by-step exposure; imaging of moving parts and kinematic artifacts, which can be improved by improving the high-voltage generator, the use of ultrashort pulses for rapid exposure to be improved; and the use of digital pulsed fluoroscopy can reduce the dose of X-ray radiation by nearly half.