DSA

DIGITAL SUBTRACTION ANGIOGRAPHY
(DSA)


. Angiograhpy is an X-ray examination with radio-opaque contrast medium in the vascular system to image the configuration of vascular circulation. In conventional angiography procedure, images are acquired by exposing an area of interest with time-controlled x-rays while injecting contrast medium into the blood vessels. The image obtained would also include all overlying structure besides the blood vessels in this area like bone and soft tissue.

Digital Subtraction Angiography (DSA) is a digital vascular imaging used in interventional radiology to clearly visualize blood vessels without the image result also include all overlying structure in this area like bone and soft tissue by subtracting a 'pre-contrast image' or the mask from later images, once the contrast medium has been introduced into a structure. So DSA combines the digitization of an image with subtraction technique. The most common use of DSA is with flouroscopic angiography as a subtitute for static serial angiographic films produced by a rapid film changer..

DSA was developed during the 1970s by groups at the University of Wisconsin, The University of Arizona, and the Kinderklinik at the University of Kiel. This work led to the development of commercial system that were introduced in 1980. Within the next few years many manufacturers of x-ray equipment introduced DSA product. After several years of rapid change , the system evolved to those available today. The primary changes since the introduction of DSA in 1980 include improved image quality, larger pixel matrices (up to 1024 x 1024), and fully digital system. Image Quality has improved for two reason : (1) the component parts (e.g., the image intensifier, television camera) have been improved, and (2)the component parts have been more effectively integrated into the system, since the early system were built using component part selected “off the shelf’ and they may or may not have been properly matched.


Equipment and Apparatus
An Image intensifier-television system (fluoroscopy) can be used to form images with little electrical interference, provide moderate resolution, and yield diagnostic quality images when combined with a high-speed image processor in DSA system. The television camera is focused onto the image-intensifier output phosphor and converts the light intensity into an electrical signal.
The image processor consist of a computer and image processing hardware. The computer control various components (e.g., memories, image processing hardware, and x-ray generator), and the image processing hardware gives the system the speed to do many images processing operation in real time.

DSA depends on the mating of high-resolution image-intensifier and television technology with computerized information manipulation and storage
X-ray Image Intensifiers
An image intensifier is a device that intensifies low light-level images to light levels that can be seen with the human eye or can be detected by a video camera. An image intensifier is a vacuum tube, having an input window on which inside surface a light sensitive layer called the photocathode has been deposited. Photons are absorbed in the photocathode and give rise to emission of electrons into the vacuum. These electrons are accelerated by an electric field to increase their energy and focus them. After multiplication by an MCP (multi channel plate) these electrons will finally be accelerated towards the anode screen. The anode screen contains a layer of phosphorescent material that is covered by a thin aluminium film. When striking the anode the energy of the electrons is converted into photons again. Because of the multiplication and increased energy of the electrons the output brightness is higher as compared to the original input light intensity.
Modern image intensifiers no longer use a separate fluorescent screen. Instead, a caesium iodide phosphor is deposited directly on the photocathode of the intensifier tube. On a typical general purpose system, the output image is approximately 105 times brighter than the input image. This brightness gain comprises a flux gain (amplification of photon number) and minification gain (concentration of photons from a large input screen onto a small output screen) each of approximately 100. This level of gain is sufficient that quantum noise, due to the limited number of x-ray photons, is a significant factor limiting image quality.
Image intensifiers are available with input diameters of up to 45 cm, and a resolution of approximately 2-3 line pairs mm-1.
Flat-panel detectors
The introduction of flat-panel detectors allows for the replacement of the image intensifier in fluoroscope design. Flat panel detectors offer increased sensitivity to X-rays, and therefore have the potential to reduce patient radiation dose. Temporal resolution is also improved over image intensifiers, reducing motion blurring. Contrast ratio is also improved over image intensifiers: flat-panel detectors are linear over a very wide latitude, whereas image intensifiers have a maximum contrast ratio of about 35:1. Spatial resolution is approximately equal, although an image intensifier operating in 'magnification' mode may be slightly better than a flat panel.
Flat panel detectors are considerably more expensive to purchase and repair than image intensifiers, so their uptake is primarily in specialties that require high-speed imaging, e.g., vascular imaging and cardiac catheterization.

Fluoroscopy
Fluoroscopy is a dinamic radiographic examination, compared to diagnostic radiography, which is static in character. Fluoroscopy is an imaging technique to obtain real-time images of the internal structures of a patient through the use of a fluoroscope In its simplest form, a fluoroscope consists of an x-ray source and fluorescent screen between which a patient is placed. However, modern fluoroscopes couple the screen to an x-ray image intensifier and CCD video camera allowing the images to be played and recorded on a monitor. The use of x-rays, a form of ionizing radiation, requires that the potential risks from a procedure be carefully balanced with the benefits of the procedure to the patient. While physicians always try to use low dose rates during fluoroscopy procedures, the length of a typical procedure often results in a relatively high absorbed dose to the patient. Recent advances include the digitization of the images captured and flat-panel detector systems which reduce the radiation dose to the patient still further.
The Basic Principles of DSA

Under the flouroskopy control the patient is injected with contrast medium direcly to blood vessel or through a catheter and the blood vessels in the anatomical region of interest are then highlighted on a sequence of radiographical images.
In order to clearly visualize blood vessels in a bony or dense soft tissue environment., first a mask image is acquired. The mask image is simply an image of the same area before the contrast is administered. The radiological equipment used to capture this is usuallly an image intensifier, which will then keep producing images of the same area at a set rate (1 - 6 frames per second), taking all subsequent images away from the original 'mask' image. The radiologist controls how much contrast media is injected and for how long. Smaller structures require less contrast to fill the vessel than others. Images produced appear with a very pale grey background, which produces a high contrast to the blood vessels, which appear a very dark grey.The images are all produced in real time by the computer, as the contrast is injected into the blood vessels.
Radiation Exposure
Radiation exposure from X-ray angiography procedures are relatively high when compared with conventional radiographic procedures. Angiography procedures can generate highly localized doses to the skin of patients, which may be above the threshold for deterministic injuries as well as carrying an increased risk of cancer induction. Staff doses are linked to patient doses because they result from secondary scattered radiation arising mainly from the patient. Staff may also be exposed to primary leakage radiation that is generated at the X-ray target and which has penetrated the leaded X-ray tube housing. Without due care and understanding, multiple procedures could lead to serious injury. This highlights the need to optimize the imaging equipment used during angiography and to properly use any dose saving techniques. The training of staff working in the vicinity of X-ray equipment is also of paramount importance. Radiation exposure to patients and laboratory staff has been recognized as a necessary hazard in angiography.
Procedures that utilize ionizing radiation should be performed in accordance with the As Low As Reasonably Achievable (ALARA) philosophy. Thus, personels ordering and performing angiography should be very familiar with the dosage of radiation from angiography procedures and ways in which dose can be minimized.
The Ionising Radiations Regulations 1999 require that measures are taken to minimize the radiation dose received by those working in a radiation environment. This is normally achieved by ensuring that those persons working within "Controlled Areas" are adequately trained in matters relating to radiation protection. For some of these groups (e.g. radiologist, cardiologists and radiographers), training in such matters forms a significant part of their basic training.
Specific points to impart are:
1. Digital acquisitions lead to much higher doses that fluoroscopy.
2. When imaging oblique angles, the scatter on the X-ray tube side is greater than that on the intensifier side.
3. Lead protection must be carefully placed to ensure continuity of protection.
4. Distance from the patient is an effective method of dose reduction.