RF encoding using a multielement parallel transmit system

Abstract

Radiofrequency (RF) encoding using spatially variant RF transmission fields represents an alternative to the conventional signal-encoding techniques applied in MRI, which are based on main field gradients. Thus, RF encoding might allow omitting the use of all main field gradients, alleviating acoustic noise and other main field gradient-related problems. This study investigates the potential of RF transmit encoding using spatially nonlinear RF fields generated by an eight-channel parallel transmit system equipped with the corresponding transmit array. Appropriate spatial encoding functions are determined iteratively by randomized superposition of the array element sensitivities and judging their performance. Besides RF amplitude-based signal encoding, the possibility of RF phase-dominated encoding is investigated to allow future fast imaging applications. The theoretical background is given and experimental phantom results are compared with predictions from corresponding simulations to prove the basic feasibility of the approach for low-resolution MRI. Deviations of roughly 10-15% between main field and RF encoded images were found for an in-plane resolution of 5 x 5 mm(2). The approach of RF encode MRI does not seem to be able to replace standard main field gradient encoding completely, but a suitable combination of both concepts could find promising applications.