Optimization of RF excitation to maximize signal and T2 contrast of tissues with rapid transverse relaxation

Abstract

Ultrashort echo time MRI requires specialized pulse sequences to overcome the short T(2) of the MR signal encountered in tissues such as ligaments, tendon, or cortical bone. Theoretical work is presented, supported by simulations and experimental data on optimizing the radiofrequency excitation to maximize signal-to-noise ratio and contrast-to-noise ratio. The theoretical calculations and simulations are based on the classic Bloch equations and lead to a closed form expression for the optimal radiofrequency pulse parameters to maximize the MR signal in the presence of rapid T(2) decay. In the steady state, the spoiled gradient recalled echo signal amplitude in response to the radiofrequency excitation pulses is not maximized by the classic Ernst angle but by a more general criterion we call "generalized Ernst angle." Finally, it is shown that T(2) contrast is maximized by flipping the magnetization at the Ernst angle with a radiofrequency pulse duration proportional to the targeted T(2). Experimental studies on short T(2) phantoms confirm these optimization criteria for both signal-to-noise ratio and contrast-to-noise ratio.