Time-of-flight magnetic resonance angiography at 7 T using venous saturation pulses with reduced flip angles

Abstract

Objectives: The visibility of the vasculature in time-of-flight (TOF) magnetic resonance angiography (MRA) highly profits from increased magnetic field strengths. However, the application of additional saturation pulses for suppression of the venous system is often not possible at 7 T; to remain within the regulatory specific absorption rate (SAR) limits, the repetition time (TR) needs to be prolonged, preventing the acquisition of high-resolution MRA data sets within clinically acceptable acquisition times. In this work, saturation pulses were modified regarding flip angle and duration to meet SAR constraints and minimize total measurement time.

Materials and methods: To ameliorate SAR restrictions, the variable-rate selective excitation (VERSE) algorithm was used for both excitation and saturation radio frequency pulses. In this way, saturation pulses (executed every TR) become applicable in high-resolution TOF MRA protocols but still lengthen total measurement time notably. In this work, saturation pulses were further modified in terms of flip angle and duration to meet SAR constraints and minimize total measurement time.

Results: In the considered parameter range for excitation flip angle α of 15° to 35° and TR of 20 ms to 35 ms, sufficient saturation flip angles (αSAT) were 30° to 50°.

Conclusions: This work shows that by lowering the flip angle αSAT, saturation pulses can be applied in high-resolution clinical TOF protocols using a TR as short as 20 ms. An αSAT of α + 15° is sufficient for suppression of the venous system in TOF MRA protocols in the parameter range normally used at 7 T. Instead of the standard 90° saturation pulse, only half the flip angle (or even less) is necessary, substantially ameliorating SAR constraints and enabling acquisition of high resolution in acceptable imaging time.