Non-aqueous magnetization following adiabatic and selective pulses in brain: T1 and cross-relaxation dynamics.

Abstract

Inversion pulses are commonly employed in MRI for T1 -weighted contrast and relaxation measurements. In brain, it is often assumed that adiabatic pulses saturate the non-aqueous magnetization. We investigated this assumption using solid-stateNMRto directly monitor the non-aqueous signal following adiabatic inversion and compared this to signals following hard and soft inversion pulses. The effects of the different preparations on relaxation dynamics were explored. Inversion recovery experiments were performed on ex vivo bovine and porcine brains in 360 MHz (8.4 T) and 200 MHz (4.7 T) NMR spectrometers, respectively, using broadband rectangular, adiabatic, and sinc inversion pulses as well as a long rectangular saturation pulse. Analogous human brain MRI experiments were performed at 3 T using single-slice echo-planar imaging. Relaxation data were fitted by mono- and bi-exponential decay models. Further fitting analysiswas performed using only two inversion delay times. Adiabatic and sinc inversion left much of the non-aqueous magnetization along B0 and resulted in bi-exponential relaxation. Saturation of both aqueous and non-aqueous magnetization components led to effectively mono-exponential T1 . relaxation. Typical adiabatic inversion pulses do not, as has been widely assumed, saturate the non-aqueous proton magnetization in white matter. Unequal magnetization states in aqueous and non-aqueous .H reservoirs prepared by soft and adiabatic pulses result in bi-exponential T1 . relaxation. Both pools must be prepared in the same magnetization state (e.g. saturated or inverted) in order to observe consisten mono-exponential relaxation.