MR-ANGIOGRAPHY OF THE CERVICAL BLOOD-VESS ELS WITH OPTIMIZED LINEARLYINCREASING FLIP ANGLES

Spin saturation effects in 3D time-of-flight (TOF) MR angiography (MRA ) can be reduced by using radio-frequency (RF) pulses with linearly in creasing flip angles (ramp pulses) in the main direction of flow. MR a ngiograms of the cervical arteries of five healthy volunteers and elev en patients suffering from stenosis of the internal carotic arteries w ere acquired using a standard flow-compensated 3D FISP sequence with a single volume technique. Compared with the conventional MR angiograms acquired with constant flip angles, the MR angiograms could be consid erably improved using the ramp pulses. Especially the distal parts of the vessels were depicted more clearly due to significantly reduced sa turation effects. Since the vessel signal largely depends on the shape of the ramp pulses, we developed a model for calculating the signal d istribution of the flowing protons for different ramp shapes and a giv en excitation volume (slab). The calculated signals were largely consi stent with the signals in the MR angiograms, thus permitting numerical optimization of the shape of the ramp pulses for the cervical arterie s. This optimized shape of the ramp pulse provided a high homogeneous vessel signal across the slab.