Tilt optimized flip uniformity (TOFU) RF pulse for uniform image contrast at low specific absorption rate levels in combination with a surface breast coil at 7 Tesla

Abstract

Purpose Going to ultrahigh field MRI (e.g., 7 Tesla [T]), the nonuniformity of the B1+ field and the increased radiofrequency (RF) power deposition become challenging. While surface coils improve the power efficiency in B1+, its field remains nonuniform. In this work, an RF pulse was designed that uses the slab selection to compensate the inhomogeneous B1+ field of a surface coil without a substantial increase in specific absorption rate (SAR). Theory and Methods A breast surface coil was used with a decaying B1+ field in the anterior-posterior direction of the human breast. Slab selective RF pulses were designed and compared with adiabatic and spokes RF pulses. Proof of principle was demonstrated with FFE and B1+ maps of the human breast. Results In vivo measurements obtained with the breast surface coil show that the tilt optimized flip uniformity (TOFU) RF pulses can improve the flip angle homogeneity by 31%, while the SAR will be lower compared with BIR-4 and spokes RF pulses. Conclusion By applying TOFU RF pulses to the breast surface coil, we are able to compensate the inhomogeneous B1+ field, while keeping the SAR low. Therefore stronger T1-weighting in FFE sequences can be obtained, while pulse durations can remain short, as shown in the human breast at 7T. Magn Reson Med 74:482-488, 2015.