VARIABLE-ANGLE UNIFORM SIGNAL EXCITATION (VUSE) FOR 3-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 an giography [MRA] is presented. The pulse, called variable-angle uniform signal excitation [VUSE], was designed to provide uniform signal resp onse and improved contrast for blood flowing through a 3D imaging volu me during a FLASH sequence. The VUSE excitation profile was optimized on the basis of the number of pulses seen by the blood, which varied w ith the velocity of through-plane flow, repetition time, and slab thic kness with the maximum flip angle at the flow exit constrained at 90 d egrees. The theoretical results show that the optimal RF pulse gives m ore uniformity for flow signal than does a linear ramp excitation prof ile or a Gaussian pulse combined with a presaturation pulse. After tru ncation and filtering of the VUSE pulse in the time domain, the genera l shape of the VUSE RF excitation profile is maintained, but the maxim um flip angle is reduced. The arteries of the neck in a healthy volunt eer were imaged with the VUSE pulse, a constant flip angle [flat] puls e, and a linear ramp pulse in flow-compensated 3D MRA sequences. The V USE pulse produced the most uniform signal as evidenced by the lowest relative dispersion of signal along the left vertebral artery [18.0 ve rsus 26.4 to 23.6 for the other studies]. F-distribution tests also sh owed 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.