Digital subtraction angiography system

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Digital subtraction angiography (DSA) is a new medical imaging technology emerging in the 1980s S. It is a new method of examination combining computer and conventional X-ray angiogram, under the premise of establishing this technology, medical imaging gradually develops towards digitalization. This section describes the basic principle, system structure, imaging method, and processing technology of DSA.

I. Basic components and principles of DSA

The I .I-TV imaging system has been introduced in Section 3, which can be viewed as the application of this system plus Digital Subtraction Technique in the area of angiogram.

The basic content of the subtraction technique is to subtract two frames of images in the same part of the human body to obtain their difference. images without contrast agent are called mask images or masks, the image obtained after the contrast agent is injected is called the image creation or filling image. Broadly speaking, a mask is like an image to be subtracted from an image, while an image to be created is a subtracted image. Background images such as bones and soft tissue are eliminated in the image reduction, leaving only vascular images containing contrast agents. The process of Digital Subtraction is 5-23.

 

 

 

Figure 5-23 DSA processing Flowchart

 

Before performing subtraction, the X-ray image is often subjected to logarithm transformation. The logarithm transformation can be achieved by using a logarithm amplifier or a digital search table placed after the/D converter, so that the gray scale of the digital image is proportional to the attenuation coefficient of the human body tissue to the X-ray. Because the contrast of the vascular image is low, contrast enhancement must be performed on the reduced image, but the image signal and noise increase at the same time. Therefore, the original image must have a high signal-to-noise ratio to make the reduced image clearer.

 

 

Figure 5-24 digital image hardware Diagram

 

Figure 5-24 is the hardware structure diagram of the digital image in the DSA system. The search table in the figure is a real-time digital transformation function module. The input search table can be used for input image logarithm transformation, and the output search table for real-time image enhancement transformation and display transformation. Frame storage is used to store Mask Images, series of images, and reduced images. The data exchange between it and the computer determines the image post-processing speed. ALU is a real-time arithmetic logic generator, which is a key component of real-time subtraction. It provides fast computing speed, reduces mutual access with computers, and synchronizes processing speed with video signal refreshing speed.

Ii. How DSA works

In the DSA system, there are various methods for digital subtraction based on different purposes, such as time and energy subtraction, the difference mainly lies in the difference between the acquisition method of the masked image and the filling image of the contrast agent.

(1) subtraction of time

Time Subtraction is a common method of DSA. One or more frames of images are stored as mask images before the injected contrast agent enters the area of interest, and subtract from the image creation in chronological order. In this way, the same image part in the two frames is eliminated, and the high-density part formed when the contrast agent passes through the vascular is highlighted. This method is called time subtraction because the time obtained by the mask image and the image creation is different. The disadvantage is that due to the patient's autonomous or autonomous Motion During photography, the mask image and the image creation cannot be precisely matched, resulting in poor image registration artifacts or blurring. Because the number of frames and the collection time of the mask image used in the subtraction are different from those of the image creation, it can be divided into the following methods:

1. the pulse image (PI) method PI uses intermittent X-ray Pulse to form the mask image and image creation (as shown in 5-25), and consumes several frames per second, the pulse duration is generally greater than the time when the video signal is one frame. When the contrast agent does not flow into the interest vascular, the mask image is taken, and the X-ray image is collected and subtracted during the gradual diffusion of the contrast agent to obtain a series of continuous and separated image reduction series, there is a large interval (for example, 0.15 s) between frames ).

 

 

Figure 5-25 pulse image mode

 

M: mask image; s: subtraction image; T: pulse cycle because the pulse width of X-ray Pulse exposed in PI mode is large (for example, About ms), the dose is high, the obtained image has a high signal-to-noise ratio and is a common method. It is mainly used for cerebrovascular, cervical, liver, four and other parts of the artery with slow activity. When using the PI method to collect images, it is technically necessary to ensure that the X-ray dose received by the front and back frames is constant, this requires high voltage, Stable pulse timing, and reasonable and accurate sampling time of the X-ray machine. These requirements can be achieved for general large and medium-sized X-ray machines.

2. The super pulse image (SPI) mode is 6 ~ The rate of 30 frames is used for X-ray Pulse imaging, and then the image is reduced repeatedly at a frame-by-frame high speed, which features high frequency and narrow pulse width, as shown in Figure 5-26. The X-ray exposure pulse is consistent with the camera field synchronization. The effective time of the exposure signal should be within the Yin period, so the pulse frequency is up to 50 ~ 60Hz, pulse width is 3 ~ 4 ms. This method can continuously observe x-ray digital images or subtract images at the speed of real-time video, with a high dynamic definition.

 

 

Figure 5-26 ultra-pulse image mode

 

M: mask image; s: subtraction image; T: 1/30 seconds. This method can adapt to fast-moving organs such as lung, coronary artery, and heart, and the image motion is blurred. However, high requirements on the x-ray machine require a large capacity of X-ray tube and a low-latency control circuit. The X-ray machine that uses relays to control the exposure generally cannot meet the requirements and cannot achieve the pulse width stability less than Ms, pulse control methods such as thyristor are required.

3. The continuous image (CI) mode CI mode (as shown in 5-27) is the same as that of fluoroscopy, X-ray continuous illumination, and the synchronization frequency with the camera is 25-seconds ~ For 30 consecutive images, the X-ray can be continuous or impulsive. Because it is a continuous radiation for a long time, the load of the X-ray tube is quite large, so the use of X-ray tube with a large thermal capacity, such as the use of render tube current exposure, the resulting image reduction signal-to-noise ratio is very low, therefore, the CI method generally uses continuous exposure photography with a small focus and 15mA tubes of current.

 

 

Figure 5-27 continuous image modes

 

M: mask image; s: subtraction image; T: 1/30 s ci mode, high image acquisition frequency, can display fast moving parts such as the heart, large blood vessels, the number of image frames per unit of time is large, and the time resolution is high. However, the radiation dose of each frame is low, which reduces the contrast resolution. Therefore, image superposition, time filtering, and other processing methods are generally used to improve the signal-to-noise ratio.

4. in the Time Interval Difference (TID) mode, the image used as the mask image when the contrast agent was not injected into the vascular area, A sequence of X-ray images containing contrast media are used for image subtraction. The TID method is used to randomly determine an image (for example, 3rd frames, you can select an image taken when the contrast agent is just injected into the vascular, and perform subtraction (3-6) at a certain interval (for example, every 3 frames ), after frame-by-frame subtraction (4-7), (5-8 )......, Form the subtraction sequence, as shown in Figure 5-28.

 

 

Figure 5-28 TID Mode

 

In TID mode, the mask image is updated while performing subtraction. The two subtracted images are separated in a short time, which can enhance the changes of the high-frequency image, reduce the impact of patient activity on low frequency. For parts with periodic activity offset, such as the heart, appropriate selection of image interval frames and TID subtraction can eliminate the artifacts caused by phase deviation. The TID method can be used for subtraction and post-processing. The image acquisition frequency of the TID method is 25 or 30 frames per second, which can be checked in real time, but it is difficult to set an appropriate interval. If the image is obtained using the CI or SPI method, then the image in the frame is reduced at a certain interval, which is better than the direct real-time processing.

5. electrocardio cardiogram (ECG) trigger pulse mode because the heart movement is at different phases at each time point, in order to make the phase of the mask image and the image as close as possible, to reduce Motion Artifacts of images, the subtraction image is required to synchronize the motion of the heart. ECG is usually used to trigger the X-ray Pulse. External ECG signals trigger X-ray Image Acquisition in three ways.

(1) Continuous ECG marking: marks the pictures with ECG signals in a continuous way. The minimum frequency of this method is 5 frames/second.

(2) Pulse ECG Tag: The image is collected in a pulse manner and marked on the screen closest to the ECG signal. The minimum frequency is 5 frames/second.

(3) ECG gating trigger: The X-ray generator is triggered by the ECG signal to mark the image. The specific method is to store the output signal of the ECG machine after A/D conversion in the ECG memory and extract the R wave mark from the R wave signal as the reference of the ECG phase. As shown in Figure 5-29.

 

 

Figure 5-29 relationship between ECG waveform and R wave signal

 

During ECG-controlled acquisition, if the X-ray exposure is synchronized with the R wave mark, the image can be reduced regularly by the R wave. This method is mainly used for the DSA examination of the cardiac vascular, and the exposure matches the heart vascular pulsation rhythm to ensure that each frame of the image in the image series is in the same phase as the heart rhythm, eliminating Artifacts Caused by heart beats.

(2) subtraction of energy

Energy Subtraction is also called dual-energy subtraction. When performing angiogram in the interest area, two different Tube voltages (such as 70kv and 130kv) were used to obtain two frames of images and perform subtraction; because the two images use different energy X-ray, they are called energy subtraction.

This subtraction method utilizes the characteristic that the attenuation coefficient of iodine and surrounding soft tissue on X-ray varies significantly under different energy conditions (the attenuation curve changes sharply when the iodine is at the 3350nm energy level, and the attenuation coefficient suddenly increases, the soft tissue attenuation curve is continuous, and the greater the energy, the smaller the attenuation coefficient ). If an tissue containing bone, soft tissue, air, and trace iodine is exposed with an X-ray (70 and 130kv respectively) with slightly lower energy and slightly higher than 33kv, then, the latter image is about 80% less than the iodine signal of the previous image, the bone signal is about 40% less, the soft tissue signal is about 25% less, and the gas is almost no attenuation at two levels. If the two images are subtracted, the resulting images will effectively eliminate gas shadows and retain a small amount of soft tissue shadows and obvious bone shadows and iodine signals. If the image collected at 1.33 KV is weighted by a factor of about, the soft tissue and gas shadows can be effectively eliminated, leaving only a small amount of bone signals and obvious iodine signals.

The energy subtraction method can also separate the tissue with different attenuation coefficients, such as removing the bone or soft tissue from the X-ray image to obtain images with only soft or bone tissue. The specific method is to use two kinds of energy X-ray to obtain two images, one obtained under the low energy X-ray, the other obtained under the high energy X-ray, all images are weighted by logarithm transformation to eliminate bone or soft tissue.

In principle, the Energy Subtraction Method is a better method, but in implementation, the tube voltage must be able to switch between the two kinds of energy at high speed, increasing the complexity of the X-ray machine, this method is not applicable to general X-ray machines. This method is not easy to eliminate the skeleton shadow.

(3) Mixed Subtraction

Combining energy and time subtraction technology produces a hybrid subtraction technology. The basic principle is to perform a dual-energy subtraction before the contrast agent is injected to obtain images containing a small amount of bone tissue signals, the dual-energy subtraction image after this image is injected with contrast agent in the same vascular area is used for subtraction, and a simple vascular image is obtained. The hybrid subtraction requires a high load on both devices and X-ray tubes.

Iii. Special requirements and technical measures of DSA for devices

Unlike ordinary DF systems, DSA not only needs to digitize X-ray images but also to produce high-quality vascular reduction images. Therefore, the DSA system has a series of special requirements.

(1) X-ray occurrence and Imaging System

Includes X-ray tubes, High-Voltage Generators, image enhancement devices, optical systems, TV cameras and monitors.

1. The X-ray Generator requires that the X-ray tube be able to withstand the load of continuous pulse exposure. for medium and large DSA devices, the heat capacity of the X-ray tube should be above khu, and the tube voltage range is 40 ~ 150kv, usually 800 ~ 1250mA. The high-voltage generator is required to generate stable DC High Voltage. It adopts medium and high-frequency technology and is controlled by a microcomputer to generate almost pure DC voltage. The X-ray machine can be quickly exposed with multiple pulses, and the imaging speed can reach up to 150 frames/second.

2. the image booster usually uses the I. i, such as 31cm I. I can have 10, 16, 22, 31, and other CM fields, which can be flexibly selected based on the needs of the angiogram. The spatial resolution is inversely proportional to the screen size and field of view, which is generally 1.1 ~ 2.5lp/mm. In order to improve the sensitivity and resolution, the input screen is made of materials such as docus oxychloride. The new flat-panel enhancement device has hundreds of thousands of optical fibers between the light emitting body and the photoelectric layer on the input screen. The light of each pixel is coupled to the photoelectric layer, so that the image has a high brightness, improved I. I conversion efficiency, so it is very promising. Currently, the high-performance I. I quantum detection efficiency (DQE) reaches 85%. According to some materials, the resolution is up to 6.8lp/mm.

3. to adapt to the large X-ray dose range (that is, the variation range of input light), the optical system requires the use of large aperture, aperture can automatically adjust the lens, some lenses also contain electric neutral filter, to prevent glare.

4. video cameras require high sensitivity, high resolution, and low residual images. video channels must have various compensation circuits to ensure high signal-to-noise ratio and High Fidelity Video signals. X-ray exposure and image acquisition must be carried out simultaneously. However, due to the delay characteristics of the vacuum camera tube, the signal amplitude of each field varies with the pulse Imaging Mode and the interline scanning mode, sampling can be performed only after the signal amplitude is stable. Therefore, the exposure pulse width is increased, which wastes the dose. This situation can be improved by using CCD cameras and line-by-line scanning. With the improvement of CCD product quality, vacuum camera tubes will be replaced. High-performance CCD camera, with a resolution of 1249/1023 lines (50 ~ 60Hz), S/N is greater than 2500, and the frequency band is greater than 10.5 MHz.

5. The monitor must be equipped with a high-definition, large screen monitor, such as a line-by-line scan of more than 1024 lines, more than 51cm type. Nowadays, the monitor in the contrast room usually uses multi-screen, multi-split or picture-in-picture forms for easy comparison at any time. The high-performance monitor uses an ambient brightness sensor to automatically adjust the brightness. The flat-plane picture tube without blinking achieves non-blinking image display when the field frequency is higher than Hz.

6. automatic Control of the brightness of X-ray images in DSA due to the large changes in the tissue density of the object to be taken, it is necessary to ensure that images with sufficient diagnostic information can be obtained under various photography and photography conditions, eliminate blur and halo. DSA is a simulated image signal formed by the I .I-TV imaging system. The dynamic range of I. I is large, about 104, and images with good contrast can be output at different exposure doses. However, the illumination range of the target plane of the TV camera tube is 10-1 ~ At X, the output current changes between the dark and saturated current values, and the dynamic range is within several hundred. Some inspection sites (such as chest and abdomen) X-ray exposure dose range of up to 103 ~ 104, beyond the range where the camera can accurately copy signals, so a series of automatic control measures are required to ensure that the input light volume of the camera tube changes within its dynamic range.

"There are three main automatic control measures: ① controlling the output light volume of I. I. Controlling the exposure dose of X-ray means controlling I. the input light of I can automatically control the brightness of the X-ray image by automatically controlling the exposure time using the video signal output by the camera, or automatically adjusting the kV and Ma values of the X-ray tube;

② Control the output light volume of the optical system. The video signal is used to automatically control the aperture of the lens. The f1.4 aperture of the lens can be automatically adjusted to 6.6 × 104, in this way, the input illuminance of the camera tube is always within the normal range;

③ The dynamic range of X-ray information can be reduced by using the compensation filter so that it is consistent with the dynamic range of equipment components. The compensatory filtration apparatus is to place additional attenuation material between the X-ray tube and the patient, and select a specific attenuation area within the field of view to provide a more even dose distribution.

7. X-ray dose management is a task of dose management system, which consists of a series of modern technologies to minimize the amount of X-ray radiation received by patients while ensuring image quality.

(1) Gate Control Technology: at the interval of each pulse exposure, a negative potential is added to the gate to offset the activation and afterglow of the exposure pulse, so as to eliminate soft rays, improve the effective Ray quality, and 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 X-ray spectrum. The partition of the collimator has the shape of square, circle, and parallel four sides. The filter board and DSA compensation filter board located in the X-ray tube window also have various shapes, such as the polygon filter board for the head and the rectangle for the neck and limbs, the heart and lung are arc-shaped. The ideal filter board allows the image density within the display range to be basically the same, so as to avoid saturation artifacts. If there is no filter plate in the pulmonary DSA examination, the density of the lung and the heart is too different. When the X-ray dose is suitable for the heart, the small vessels in the lung are penetrated, and the dose is suitable for the lung, and the heart structure is unrecognizable. Various filters and separators can be automatically or manually controlled for easy adjustment. However, it is not recommended to use a filter board that is too thick. Otherwise, the load on the X-ray tube will be significantly increased, hardening the X-ray bundle and reducing the signal-to-noise ratio.

The filter grid placed in front of I. I is also used to eliminate the scattered lines when x-rays pass through the human body. It can be arranged in parallel, convergence, taper, and crossover modes. This technique reduces the X-ray radiation dose by about 20%.

(3) Pulse pivoting technology: this technology is implemented based on the digitization of perspective images. Therefore, it can enhance, smooth, and remove noise of pulse pivoting images to improve the definition of images. The device's pulse fluoroscopy frequency can be set to 25 frames/second, 12.5 frames/second, and 6 frames/second. The lower the frequency, the smaller the pulse width, and the smaller the radiation dose. However, when the pulse frequency is too low, the live image perspective will show an animated beating and dragging; when the pulse width is too narrow, the quality of the perspective image will decrease. Using this technique, it is estimated that the dose of conventional fluoroscopy radiation is reduced by about 40%.

(4) image freezing technology: the last frame of the image in each perspective is saved and displayed on the monitor. It is called the image freezing (lastimagehold, lih ). The full use of lih technology can reduce unnecessary fluoroscopy, significantly shorten the total fluoroscopy time, and reduce the radiation dose. The DSA filter board and partition can be adjusted in the lih state.

In addition, there are automatic display technology of radiation dose, fluoroscopy dose adjustment function next to the examination bed, and lead protective screen hanger.

(2) Mechanical System

It mainly includes racks and inspection beds, which require a large range of motion, fast speed, and all-round.

1. racks and bed racks have C, U, double C, and C-arm, and L + C-arm. There are two installation methods: landline or suspension, which can ensure that the angiogram is switched in multiple directions; comprehensive Selection and observation of projection angles can be achieved to reduce dead corners and avoid interfering with the operations of surgeons. To judge the performance of the rack, the rotation and longitudinal movement of the l arm are mainly determined. The angle of rotation of the C arm to the left front and right front and the range of axial movement of the head and foot are as follows, the speed and stability of motion, and the up and down motion of the image booster require the device to automatically display the arm position, angle, and other data. The longitudinal and lateral motion ranges of the inspection bed must be large and can be rotated left and right.

Modern angiogram machines use dual, single-C-arm three-axis (three motors drive the rotation axis, ensure that the C-arm is centered around the patient for the same center movement, flexible operation, accurate positioning) or L + C-arm three-axis system. The dual-C-arm product reduces the number of injection and X-ray exposures, and increases the motion angle. The inspection bed has two-way motion of 180 °, which increases the activity space and facilitates patient placement and rescue. The three-axis system is the basis for rotating angiogram and computer-assisted optimal angle positioning of vascular.

The modern angiogram machine is also equipped with an automatic security device, which can automatically warn and control the C-arm, I. i. motion speed: the sensor is used to perceive the distance between the surrounding objects to automatically slow down or stop (for example, to stop when the sensor is 10cm away from the object and to stop when the sensor is 1cm away from the object ).

2. the body position memory technology is designed for surgeons to store up to 100 individual positions. The positions can be pre-configured or stored at any time during angiogram, making the angiogram procedural, speed up angiogram.

3. automatic Tracking and Playback Technology when the C-arm is switched to the desired angle for perspective observation, the system can automatically search for and replay the existing images of the angle for reference when doctors diagnose or intervene in the treatment; you can also automatically turn the C-arm to this position based on the image for a new fluoroscopy. This technique is especially beneficial for angiogram of the heart and cerebral vessels, especially for coronary intervention.

(3) image data collection and storage system

The general structure of the system is shown in Figure 5-24. Because DSA requires real-time subtraction of more than 25 frames/Second, such high processing speed must be achieved through dedicated hardware. Some manufacturers add an image board on a general microcomputer to implement processing functions such as A/D conversion of video signals and real-time subtraction, the Board consists of A/D converter, input lookup table, high-speed recorder, frame storage, output lookup table, and D/A converter.

The sampling clock rate is determined based on the size of the collection matrix. For the 512x512 matrix, the sampling frequency must be greater than 10 MHz. For the 768X572 matrix and 1024x1024 matrix, the sampling frequencies are 15 MHz and 20 MHz respectively. Select the quantitative level of the/D converter according to the requirements for digital image gray level, that is, the number of BIT, generally 8bit or 10bit. The frame storage capacity generally needs to be able to store 16 digital images. When each pixel is 8 bits (1 byte, byte) data, the frame storage capacity is 4 MB or 16 Mb. Images of dynamic organs such as the heart and coronary artery must be continuously collected for 5S or 10 seconds at a rate of 25 frames/second. Larger image storage capacity (massive storage) is required ), some devices use 64 MB high-speed massive frame storage, which can save 512x512x8 bit images and 250 frames. If the size of the real-time frame storage is small, only the Cine angiogram can be used for the heart and coronary artery. A collection image generally does not exceed 10 s, but the image stored in the frame can be transferred to the CD or hard disk during the interval between the two collection images. Therefore, the frame storage capacity exceeds 64 MB, instead of film.

Large-capacity real-time image storage generally uses dynamic storage. Because the maximum real-time access speed is 50 frames per second, 512 × 512 × 8 bit images, they must be transmitted through the video bus, at the same time, a computer bus interface is required for read/write control and image transfer between frame storage and hard disk.

(4) Computer Systems

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

1. the system control process is shown in 5-30. The entire device is controlled by a computer. The signal to be connected according to the control process is as follows:

 

 

Figure 5-30 general control flowchart of the DSA system

 

(1) start switch signal: start switch 1 closed so that the X-ray machine is controlled by a computer, the computer sends an exposure preparation signal to the X-ray machine, and sends an aperture control signal to narrow the aperture. Enable switch 2 closed so that the angiogram starts, the computer starts the high-pressure injector, and the X-ray machine is exposed.

(2) contact signal: after the preparation of the X-ray machine is complete, the ready signal is sent to the computer, indicating that pulse exposure can be performed. After the exposure starts, a sampling start signal is sent to the/D conversion circuit. After the conversion ends, the computer is notified to read the digital signal, and the next frame of image is collected.

2. image post-processing this article mainly describes the processing of logarithm transformation, correction of Motion Artifacts, improvement of the Time-filtering processing and Automatic Parameter analysis functions of the image s/n.

(1) Logarithm transformation processing: The contrast difference between the angiogram image obtained at different time points will be generated due to the background changes. The logarithm transformation is performed before the subtraction, this difference can be eliminated. For example, a vascular with the same diameter exists at two points A and B with different thicknesses, the vascular reduction images obtained from different backgrounds and time ranges have different contrast values. If the subtraction is performed after the logarithm transformation, the image is displayed with the same contrast, which is irrelevant to the background of the vascular.

(2) Correction of Motion Artifacts: The mask image is well registered with the image creation, which is a prerequisite for ensuring the quality of DSA. The cause of poor image registration is the movement of the patient's body, the movement of Intestinal gases and the pulsation of the heart. The mask replacement method can be used to correct motion, intestinal gas, pixel Shift method can be used to correct motion, heart subtraction can be used to correct pulsation and other artifacts. The three methods are described below.

1) mask replacement (re-masked) 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 vascular to be checked, assume that the first exposure is the set mask image exposure, and then the image exposure. If a patient moves after the first image is taken and then a series of images are taken, the image will become blurred due to the moving artifacts. In this case, you can select a 2nd-frame image as the mask image minus the subsequent image creation to ensure that the registration between the subtraction objects is good. Because no initial mask is used, it is called a replacement mask.

When replacing the mask, the operator should carefully observe the series of images of the angiogram and use the try-out method to determine an ideal subtraction pair, generally, the instant image before the arrival of the contrast block is paired with the image at the peak of the contrast agent.

2) pixel Shift: a technology that eliminates moving artifacts through computer programs. If the human body moves between the two images, the subtraction of the two images will produce poorly registered artifacts. To improve the registration of subtraction, you can move the partial or all pixels of the mask to a certain distance in the opposite direction, so that the corresponding pixels can be better registered. Because the patient's movement is in three-dimensional space, and the pixel movement is only in 2D space, the pixel Shift has limited ability to improve artifacts. Figure 5-31 is a pixel Shift, where figure (1) is the superposition of two images, the dotted line represents the first image, only contains one bone structure, and the solid line represents the second image, in addition to the bone structure, arterial vessels that are filled with Iodine contrast agents are also included, and the bone structure is moved during the acquisition of two images. Figure (2) indicates that one of the two images moves to the left, so that the bone signal is aligned with each other, and only the artery is left after the subtraction.

 

 

Figure 5-31 pixel Shift

 

3) cardiac Subtraction Method: When DSA is used to check the heart, it is necessary to use ECG-controlled acquisition method because of the pulsation Artifacts Caused by the inconsistent cardiac phase between the mask image and the image. However, the acquisition speed of this method is low, and only one or two frames of images can be captured in a heart cycle. This method is not suitable for heart checks, the number of image frames in the heart cycle must be supplemented (the average speed is 30 ~ 30 when the image collection speed is 30 frames/second ~ 32 frames ). A heartbeat phase mask image is collected, and ECG signals are collected at the same time. The relationship between the images of each frame and the heartbeat phase is compared frame by frame starting from the R wave, find a frame in the same phase as the R wave as the first frame mask image, and collect images in the next several heart cycles. After the examination, in order to correct the pulsation artifacts, continuous subtraction can be performed for the mask image and the image making with the same heart phase. This method is called the heart Subtraction Method. The schematic of cardiac Subtraction is 5-32.

 

 

Figure 5-32 schematic diagram of cardiac Subtraction

 

(3) Time Filtering: The image sequence used for Subtraction is taken during the time when the contrast agent passes through the vascular of interest, and each frame creation interval changes with time. The purpose of subtraction is to extract time-dependent vascular images from the image of the entire anatomical structure, that is, filter out. Therefore, the subtraction process can be considered as a filtering process, called time filtering. The simplest Time Filtering Method is mask subtraction, which uses two frames of image subtraction. In addition, there are also integral masks, matched filtering, and Recursive Filtering. They use two or more images for subtraction, in order to reduce noise and increase s/n.

(4) image reduction processing: In the DSA system, some common image processing methods are basically used, such as black/white inversion, image filtering, shift and rotation, edge enhancement and detection, dynamic window position and window width adjustment, histogram balancing, image filtering, etc. The following describes several processing and measurement analysis methods.

1) interpolation and local amplification: select a local area from the entire storage image to zoom in and display it. The magnification can be selected, but more than 4 times is meaningless. As the Pixel Distribution of the enlarged image becomes thinner, the interpolation method can be used to supplement the pixel. The simplest interpolation method is to take the average value of the data of adjacent sampling points as the interpolation value. For example, if the data of two adjacent sampling points is a and B, the interpolation value is c = (a + B)/2. This can be seen clearly, but the resolution will not be improved without increasing the amount of information. The above is also called playback amplification, which is used to zoom in on the acquired images.

If the local zoom-in image is implemented by changing the size of the sampling area, it is the real local zoom-in. For example, if the input field of the image enhancement device is reduced and the sampling frequency remains the same, the pixels in the unit area increase and the spatial resolution is improved, which is called acquisition amplification.

2) Demarcation MARK: this technique provides an anatomical marker for the DSA subtraction image to precisely locate the lesions or vessels. Because only the vascular images containing contrast agents are displayed, the anatomical positioning is not obvious, and the original mask is overlapped with the DSA subtraction with an enhanced brightness frame, in this way, the vascular and reference structures can be displayed simultaneously, that is, the border images, structures such as bones or soft tissue as markers.

(5) Automatic Analysis: after ventricular and angiogram, the computer uses the analysis software to extract functional information related to quantitative diagnosis in real time and add it to the morphological image. The following describes several analysis functions.

1) left ventricular volume calculation and analysis function: The left ventricular volume is calculated by using the left ventricular extension final image and the left ventricular contraction final image obtained from the DSA image; based on the above results, the function parameters such as ejection fraction, ventricular wall movement, cardiac displacement, cardiac weight, and myocardial blood reserve are calculated.

2) coronary artery or vascular analysis software: the computer uses geometric and density methods to measure the vascular diameter, maximum stenosis coefficient, narrow or plaque area, lesions and blood flow conditions.

3) Functional Imaging: this is an image that uses a video density meter to plot a Time Video density curve for a series of captured images and then is formed based on the parameters obtained from the curve. This image reflects functional information, which is different from the traditional image that reflects morphological information. From the curve, we can extract the Time-Dependent Parameters of contrast agent flow in the vascular, the parameters of Local vascular volume or depth (thickness), and parameters of local organ vascular perfusion, these parameters are indispensable for the diagnosis and treatment of cardiovascular diseases and can be found in the early stage.

(5) New technologies of DSA Processing

DSA not only serves diagnosis, but also provides advanced means for disease treatment. DSA is often used in interventional therapy. The method of drawing the path graph can guide the surgeon to perform quick and correct operations. The image acquisition method of ECG-triggered pulse has unique features for clear imaging of the motion part; the peak value holding acquisition method can improve the signal-to-noise ratio of the reduced image. For motion position DSA imaging, dynamic DSA Technology (that is, during the image acquisition process, x-ray tubes, checklists, and detectors for regular motion) can greatly reduce artifacts, most common problems include film subtraction, rotating angiography, contrast-based contrast-tracking angiography, step-by-step angiography, and automatic optimal angle positioning.

1. pathmap technology is created for the convenience of intubation in complex locations and the need for interventional therapy. The specific method is to inject a little contrast agent before photography, real-time and dynamic subtraction of the image obtained from the first fluoroscopic and later fluoroscopic images, overlapping the vascular shadow and intubation process, and displaying at the same time. This clearly shows the direction of the catheter and the specific position of the tip, allowing the surgeon 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 vascular filling contrast agent is the most, and the contrast is the highest, the Auxiliary Mask is replaced by the filling image; ③ When the contrast agent in the vascular is emptied, the fluoroscopy image is subtracted from the filling Image Mask, and the vascular is displayed with the maximum contrast, so that the catheter can be operated along the track accurately.

In short, the path graph technology uses the natural image of the fluoroscopy as the auxiliary mask, and then replaces the auxiliary mask with the filling image to become the actual mask, which is subtract from the perspective image without contrast agent, the vascular images containing only contrast agents can be obtained as a route map for intubation, which can clearly observe the dynamic movement of vascular catheter, which is helpful for the comparison and safety of interventional therapy.

2. Digital Film subtraction uses digital fast Short Pulse for image acquisition. Real-Time Imaging: 25 to 25 seconds ~ 50 frames. Generally, 50 frames per second and 25 frames per second in two directions can be recorded on the film. This acquisition method is used for the movement of the heart, coronary artery and other locations, so that the Motion Artifacts after subtraction are almost zero. This method is often supplemented by ECG trigger.

3. when the DSA system starts to collect images, the C-arm stent rotates around the patient, and the parameters of a certain vascular and its branches are collected 180 °, so that the human body remains static, the X-ray tube and booster are synchronized to obtain three-dimensional images. This technique significantly increases the angle of observation and provides more diagnostic information, especially for cerebrovascular, cardiac and coronary angiogram.

4. step-by-Step angiogram uses rapid pulse exposure to collect images. During the exposure, the X-ray tube and booster remain static, and the catheter bed carries the human body automatically and uniformly forward to obtain the whole vascular image reduction process, it is mainly used for artery examination and interventional therapy of the four limbs.

5. remote Control contrast agent tracking technology after injecting contrast agent, during the collection of images, hand-controlled or programmable bed surface movement speed, tracking contrast agent collection, it is particularly suitable for peripheral arterial and thoracic aortic angiogram that requires multiple visual fields and multiple injections.

6. automatic angle positioning system automatic angle positioning refers to the computer to analyze and determine the optimal display angle of the lesions Based on the display of vessels in the left and right anterior oblique positions, the C-arm is automatically transferred to this position for angiogram. If the operator gives two angles (at least 30 ° interval) to the common vascular and presses the function key (marked as COMPAS), the computer automatically finds the optimal projection angle and displays the image of the vascular, this function is especially suited for coronary and cerebrovascular angiogram until Optimal imaging is obtained.

7. The peak value holding sampling technique sets the maximum and minimum brightness units in the frame memory. before sampling, initialize the two units to the lowest and brightest values respectively. During the sampling process, when the current image becomes brighter, the current value is written to the maximum unit. Similarly, when the current image is darker, the current value is written to the minimum unit, the above process repeats until the sampling is complete. The maximum unit is always the data of the memory mask image, while the memory process of the minimum unit is from the mask image to the partial filling image, and then to the full filling image. Subtract the maximum value from the minimum frame storage unit to get a series of fully-filled image reduction. This process is to maintain the peak value sampling. Its advantage is that it can improve the quality of image reduction, or get the image effect of ordinary DSA acquisition with a small dose of radiation.

8. two-plane angiogram, One Direction of X-ray angiography is likely to affect the observation because of the overlapping vascular, the dual-C-arm X-ray machine (DSA) system can implement synchronous control of two identical DSA instances by using software, and obtain the positive and side image building at a rate of 25 frames/second in real time. In one direction, blood vessels may not overlap. Doctors can use their clinical experience to obtain hidden three-dimensional information from two images in different directions. For example, two images in different directions are displayed on two monitors respectively, and images with a real stereoscopic effect can be seen through a special observation mirror. As long as we know the spatial coordinates of the X-ray source in two directions, we can use the probe software to accurately calculate the three-dimensional spatial location of the lesion. This method of Dual-plane angiography through software connection can avoid multiple injection of contrast agent and multi-faceted projection, thus shortening the examination time and reducing the amount of contrast agent.

To sum up, with the continuous development of DSA technology, the continuous improvement of equipment performance and imaging methods, the shortcomings of DSA have been improved. For example, the post-processing of the image increases the value of S/N. Due to the small field of view, the large part requires multiple exposures. the input field of I is solved by using remote control contrast agent tracking technology and step-by-step exposure. Motion part imaging and Motion Artifacts can be improved by improving the high-voltage generator and using ultra-short pulse rapid exposure; digital Pulse fluoroscopy can reduce the dose of X-ray radiation by nearly half.