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

Objective: 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.

Material and Methods: 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.

Results: 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.

Conclusion: 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.