The Subtraction Process of DSA

The subtraction process in Digital Subtraction Angiography (DSA) involves four key steps: capturing a plain film, preparing a mask image, capturing an angiographic film, and registering the mask image with the angiographic film to create a subtraction image. Among these, the preparation of the mask image is crucial to the subtraction process.

A mask image is essentially a reference image taken without the use of a contrast agent. It is identical to the plain film image but exhibits inverse density. Broadly speaking, the mask serves as a baseline image that is subtracted from other images to highlight specific structures. Any frame captured during the subtraction process can be selected as the mask image.

Conversely, the image captured after the injection of a contrast agent is referred to as the angiographic or radiographic image. Similar to the mask, any frame during the subtraction imaging process can be used as the radiographic image.

The selection of the mask and angiographic images depends on the vascular phase being observed, such as the arterial, capillary, or venous phase. This careful selection ensures clarity and accuracy in the subtraction image, facilitating detailed visualization of vascular structures.