DIGITAL SUBTRACTION ANGIOGRAPHY

DIGITAL SUBTRACTION ANGIOGRAPHY

 Digital subtraction angiography (DSA) is a special method of fluoroscopy, which gives image of the vessels that are filled with contrast. Digital images are obtained before and after injection of contrast medium, to differentiate vascular pathology from surrounding anatomy. It is a noninvasive procedure, that provides improved image quality with lesser use of contrast medium. The principle of DSA imaging is as follows:

Step 1: Image of a particular anatomical region is recorded. This is called mask image (A), which shows normal anatomy of the region. It require 2 frames, the first is used to stabilize the system (technical factors) and the second frame is stored as mask image in the computer. 

Step 2: The patient is injected with contrast to fill the vessels and the image is taken. It is called contrast image (B), which shows contrast filled vessels, superimposed on the anatomy.

Step 3: The mask image is subtracted from the contrast image (B-A), by pixel by pixel basis and stored as contrast-mask image (C). This image reveals only vessels filled with contrast medium.

The final image is viewed in real time. There should not be any movement of the patient during the above procedure and the images are obtained rapidly. Movement between frames may lead to miss registration of images especially at the bone edges. This can be minimized by the movement of mask, which is called pixel shifting. Pixel shifting can be done manually of automatically.

To reduce the scatter, special algorithm is used. Taking successive frames reduce quantum mottle and increases signal to noise ratio. However, subtracted images reduce the signal to noise ratio and the image looks noisier. Hence, higher mA is used in DSA.
               

If the length of the anatomy is greater than the FOV (e.g. leg), then multiple images are to be taken to cover the entire anatomy. Each time the contrast is injected to the region. However, modern machine provides software that acquires several mask images and contrast images over the full length of the anatomy. This is facilitated by the table movement and its position for each image. Appropriate mask image for a given table position is subtracted from the contrast image. There is a chance of movement during the above procedure.

DSA Equipment

 Digital subtraction angiography equipment is basically a fluoroscopy X-ray machine with II tube or CCD camera or flat panel DR system. The signal as image frames are acquired, digitized with ADC and stored in random access memory (RAM). There are two memories, one for the mask image and the other for the contrast image. The image content is subtracted in a arithmetic unit, processed and converted back into a analog signal by DAC. Then, the signal is displayed in a high resolution monitor. Images can be stored on a magnetic hard disk and computer needs large memory of the order of 512 MB–1GB. The acquisition and processing is controlled by the CPU of the computer.