Digital subtraction angiography (DSA) is a medical imaging technique used to visualize blood vessels in various parts of the body, particularly in the brain, spinal cord, and limbs. It's particularly useful in diagnosing conditions such as aneurysms, arterial stenosis, and arteriovenous malformations.

Here's how it works:

Contrast Injection: A contrast agent, typically iodine-based, is injected into the bloodstream. This contrast material absorbs X-rays and makes blood vessels visible on X-ray images.

Image Acquisition: X-ray images are taken before and after the contrast injection. For the 'before' images, the X-ray source and detector are positioned so that they are directly opposite each other, capturing a baseline image of the area of interest.

Subtraction: The 'after' images, taken after the contrast injection, are digitally subtracted from the 'before' images. This process removes the bone and tissue structures that obscure the blood vessels, leaving only the contrast-filled vessels visible.

Image Enhancement: The resulting subtracted image enhances the visibility of blood vessels, making it easier for the radiologist to identify any abnormalities or blockages.

Analysis: Radiologists analyze the subtracted images to assess the condition of the blood vessels, identify any abnormalities, and determine the appropriate course of treatment.

DSA offers several advantages over conventional angiography, including reduced radiation exposure, improved visualization of blood vessels, and the ability to precisely locate and measure abnormalities. However, it requires specialized equipment and expertise to perform and interpret effectively.