Parallel transmission: Physics background, pulse design, and applications in neuro MRI at ultra-high field

This chapter describes the challenges posed by radiofrequency (RF) field inhomogeneity at ultra-high magnetic field (UHF) (7 Tesla and above) and possible solutions for UHF neuro MRI. RF field inhomogeneity can alter image quality, limiting the ability of UHF MRI to deliver good-quality images over the whole brain. After introducing the physics of the problem, standard mitigation approaches such as composite and adiabatic pulses are briefly presented. The rest of the chapter is devoted to parallel transmission (pTx), one of the most promising technologies that can be used to solve the problem for neuro MRI at UHF. Emphasis is put on the concepts which cover the mathematical formulation of the pulse engineering problem with transmit array architectures, milestone developments, and in vivo demonstrations of the added value of pTx for neuro MRI at UHF. This chapter concludes by providing perspectives of how pTx may continue to evolve moving forward.