Final answer:
The pre-contrast image used in digital subtraction angiography is called the mask image.
Step-by-step explanation:
The pre-contrast image used to subtract pixel values from the post-contrast image showing contrast-filled blood vessels in digital subtraction angiography is called the mask image.
In digital subtraction angiography, a mask image is obtained before the injection of contrast agent. This initial image acts as a baseline and is then subtracted from the post-contrast image to specifically highlight the contrast-filled blood vessels.