Blood vessels in MR and CT Angiography data sets are best viewed as Maximum Intensity Projection (MIP) images. This method of reformation takes the brightest pixels in the image volume and projects them into the final MIP image. Since blood usually has a higher pixel intensity than other structures in the background, the blood vessels are displayed brighter than the background.

MIP image of large aneurysm in the brain (MR Angiography)

Original or source image from MR Angiography data set showing the large aneurysm.