Variable tip angle slab selection pulses for carotid and cerebral time-of-flight MR angiography. Theory and...

To determine the optimal parameters of variable tip angle slab selection (tilted optimized nonsaturating excitation, TONE) pulses in time-of-flight MR angiography, in order to produce the best visualization of vessels distal to the entry partition.

The influences of the mean flip angle (MFA) and the profile tilt of the TONE pulse in three-dimensional (3D) time-of-flight MR angiography were evaluated with a mathematical modeling of the flow signal amplitude. We compared the calculated flow signals with the signals from flow-phantom models and with those from 6 normal volunteers. 3D gradient echo images (TR/TE 30/7) were acquired with a single variable of the MFA and the profile tilt. The flow velocity of the phantom was maintained at 15 cm/s. Regions of interest were measured on the source images. We also evaluated the visualization of the cerebral vessels on maximum intensity projection images obtained with and without TONE pulses in 5 other healthy volunteers.

The changes of flow signals in the phantom and in the volunteers were in good accord with those of the signals mathematically predicted and plotted by a computer, as each parameter was varied. The peripheral saturation was weaker at smaller MFA than at larger MFA. A greater profile tilt produced a more pronounced TONE effect than a lower one. The visualization of the peripheral cerebral vessels was markedly improved by the addition of the TONE pulse.

Computer simulation is useful for examining the optimal TONE parameters. The TONE pulse markedly improves the visualization of the distal branches of the cerebral vessels.