Contrast enhancement in TOF cerebral angiography at 7 T using saturation and MT pulses under SAR constraints: Impact of verse and sparse pulses

Cerebral three-dimensional time of flight (TOF) angiography significantly benefits from ultrahigh fields, mainly due to higher signal-to-noise ratio and to longer T₁ relaxation time of static brain tissues; however, specific absorption rate (SAR) significantly increases with B₀. Thus, additional radiofrequency pulses commonly used at lower field strengths to improve TOF contrast such as saturation of venous signal and improved background suppression by magnetization transfer typically cannot be used at higher fields. In this work, we aimed at reducing SAR for each radiofrequency pulse category in a TOF sequence. We use the variable-rate selective excitation principle for the slab selective TOF excitation as well as the venous saturation radiofrequency pulses. In addition, magnetization transfer pulses are implemented by sparsely applying the pulses only during acquisition of the central k-space lines to limit their SAR contribution. Image quality, angiographic contrast, and SAR reduction were investigated as a function of variable-rate selective excitation parameters and of the total number of magnetization transfer pulses applied. Based on these results, a TOF protocol was generated that increases the angiographic contrast by more than 50% and reduces subcutaneous fat signal while keeping the resulting SAR within regulatory limits.