Velocity-selective excitation for MR angiography

A pulse sequence has been implemented on a Philips MR imaging system that enables angiography without subtraction. The velocity-selective excitation of flowing spins only is achieved by a 900-1800-900 pulse combination, in which the last 900 pulse resets stationary spins magnetization to the Z-axis. Flowing spins are dephased by two equal field gradient blocks, positioned symmetrically around the 1800 pulse. Consequently, moving spins are not fully reset and produce an MR imaging signal. A gradient echo scheme is used to acquire the signal. Different versions of the sequence were investigated and used for projective imaging, both in phantom and in vivo; results are shown and compared with those of other MR angiographic methods