MR-ANGIOGRAPHY USING VELOCITY-SELECTIVE PREPARATION PULSES AND SEGMENTED GRADIENT-ECHO ACQUISITION

We describe a cardiac-gated MR angiographic imaging method that employ s velocity-selective preparation (VSP) pulses in conjunction with segm ented gradient-echo acquisition and subtraction to produce images that , ideally, contain no signal from stationary tissues and display vesse ls with a signal Intensity that is dependent on the velocity of the bl ood in the vessels. The novel features of this method are a) it acquir es several phase-encoding values/application of a single VSP pulse, b) it uses subtraction to eliminate signal that is not sufficiently supp ressed by the VSP pulses, and c) it uses VSP pulses that are synchroni zed with the cardiac cycle so it can be used to produce ghost-free ima ges of pulsatile blood. An advantage of this sequence is that it detec ts a signal that, after preparation, is relatively unaffected by chang es in blood velocity. This leads to a large signal-to-noise ratio for all the phase-encoding values, a reduction of ghosting artifacts, and the ability to visualize blood that is in motion for only a short time during the cardiac cycle. Because the signal is prepared during peak flow, venous signal can be suppressed by making the sequence sensitive to high velocities. An additional advantage of this sequence is that it permits sampling with a short TE because the velocity-encoding grad ient can be applied in a preparatory interval. Signal loss that result s from dephasing during the longer TE preparation interval can be redu ced or eliminated by allowing the dephased spins to flow out of the re gion of complex flow, and perhaps out of the field-of-view, by introdu cing a delay between the finish of the VSP pulse and the beginning of data acquisition.