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Improving high-field MRI using parallel excitation

William A Grissom, Laura Sacolick and Mika W Vogel

MRI at high magnetic field strengths promises to deliver clearer images of the body’s structure and function. However, high-field MRI currently faces several substantial engineering challenges before it can be considered robust and safe enough for routine clinical use. Challenges include increased radiofrequency (RF) energy deposition that can create unsafe patient heating, large main field inhomogeneities that prevent functional neuroimaging in the lower brain and inhomogeneous RFs transmit fields that cause spatially varying image contrast. Patient-tailored multidimensional RF excitation pulses have been proposed as a means to address many of these challenges. While these pulses show significant promise, their effectiveness has been severely limited by their long durations. Parallel excitation using multiple RF transmit coils driven simultaneously is a new technology that has been proposed to reduce multidimensional pulse durations. Parallel excitation provides an instantaneous spatial encoding mechanism that can be traded for conventional time-consuming gradient field encoding, and permits multidimensional RF pulses to be accelerated by several-fold. In general, parallel excitation gives the MRI applications developer degrees of freedom that can be used for many purposes to enhance the performance of high-field MRI. This article reviews the hardware and theoretical foundations of parallel excitation, its applications in high-field MRI and open research problems.