Contrast-enhanced MR angiography of the carotid bifurcation.

With contrast-enhanced MR angiographic techniques, a T1-shortening contrast agent is injected into the blood stream. Imaging during the first pass of the contrast agent permits acquisition of a high-contrast MR angiogram. Scan parameters such as flip angle, repetition time, echo time, and scan duration, and injection parameters, such as dose and rate, must be carefully chosen to achieve maximum contrast between blood vessels and stationary tissues. A critical parameter affecting image quality is the timing of the acquisition relative to the injection. If the collection of the center of k-space does not coincide with peak arterial concentration, artifacts, reduced SNR, and venous enhancement may result. Several techniques have been developed to address the timing issue. Post-processing techniques such as subtracting a pre-contrast image from a post-contrast angiogram can be used to improve image quality. Intravascular contrast agents that may also lead to improved image quality are currently being developed.