MR angiography using velocity‐selective preparation pulses and segmented gradient‐echo acquisition

We describe a cardiac‐gated MR angiographic imaging method that employs velocity‐selective preparation (VSP) pulses in conjunction with segmented gradient‐echo acquisitioin and subtraction to produce images that, ideally, contain no signal from stationary tissues and display vessels with a signal intensity that is dependent on the velocity of the blood in the vessels. The novel features of this method are a) it acquires several phase‐encoding valueslapplication of a single VSP pulse, b) it uses subtraction to eliminate signal that is not sufficiently suppressed by the VSP pulses, and c) it uses VSP pulses that are synchronized with the cardiac cycle so it can be used to produce ghost‐free images of pulsatile blood. An advantage of this sequence is that it detects a signal that, after preparation, is relatively unaffected by changes 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 gradient can be applied in a preparatory interval. Signal loss that results from dephasing during the longer TE preparation interval can be reduced or eliminated by allowing the dephased spins to flow out of the region of complex flow, and perhaps out of the field‐of‐view, by introducing a delay between the finish of the VSP pulse and the beginning of data acquisition.