Variable-angle uniform signal excitation (VUSE) for three-dimensional time-of-flight MR angiography.

A spatially asymmetric RF pulse that improves the uniformity of blood signal intensity and vascular contrast in three-dimensional (3D) MR angiography (MRA) is presented. The pulse, called variable-angle uniform signal excitation (VUSE), was designed to provide uniform signal response and improved contrast for blood flowing through a 3D imaging volume during a FLASH sequence. The VUSE excitation profile was optimized on the basis of the number of pulses seen by the blood, which varied with the velocity of through-plane flow, repetition time, and slab thickness with the maximum flip angle at the flow exit constrained at 90 degrees. The theoretical results show that the optimal RF pulse gives more uniformity for flow signal than does a linear ramp excitation profile or a Gaussian pulse combined with a presaturation pulse. After truncation and filtering of the VUSE pulse in the time domain, the general shape of the VUSE RF excitation profile is maintained, but the maximum flip angle is reduced. The arteries of the neck in a healthy volunteer were imaged with the VUSE pulse, a constant flip angle (flat) pulse, and a linear ramp pulse in flow-compensated 3D MRA sequences. The VUSE pulse produced the most uniform signal as evidenced by the lowest relative dispersion of signal along the left vertebral artery (18.0 versus 26.4 to 23.6 for the other studies). F-distribution tests also showed that the signal distribution obtained with the VUSE pulse in a 3D FLASH sequence was statistically different from that for the flat and the linear ramp pulses.