A QUANTITATIVE STUDY OF RAMPED RADIO-FREQUENCY, MAGNETIZATION-TRANSFER, AND SLAB THICKNESS IN 3-DIMENSIONAL TIME-OF-FLIGHT MAGNETIC-RESONANCE ANGIOGRAPHY IN A PATIENT POPULATION

RATIONALE AND OBJECTIVES. The authors compare the effectiveness of var ious magnetic resonance (MR) angiography acquisition strategies in enh ancing the visibility of small intracranial vessels. METHODS. Blood ve ssel contrast-to-noise ratio (MR) in time-of-flight MR angiography was studied as a function of vessel size and several selectable imaging p arameters, Contrast-to-noise measurements were made on 257 vessel segm ents ranging in size from 0.3 mm to 4.2 mm in patients who recently ha d undergone intraarterial cerebral angiography. Imaging parameters stu died included magnetization transfer, spatially variable radio frequen cy (RF) pulse profile (ramped RF), and imaging slab thickness. RESULTS . The combination of thin slabs (16 slices/slab), ramped RF, and magne tization transfer resulted in the highest CNR for all but the smallest vessel sizes, The smallest vessels (< 0.5 mm) had the highest CNR, us ing the thick slab (64 slices/slab) with ramped RF and magnetization t ransfer, Magnetization transfer always improved vessel CNR, but the im provement diminished as the slab thickness was reduced, The CNR increa sed with a decrease in slab thickness for all but the smallest vessel sizes. CONCLUSIONS. Overall, the results provide a quantitative demons tration that inflow enhancement of blood is reduced for small vessels, Thus, whereas magnetization transfer is important at all vessel sizes , it becomes the primary factor in improving the, visibility of the sm allest vessels.