Nonaqueous magnetization following adiabatic and selective pulses in brain: T1 and cross‐relaxation dynamics

Inversion pulses are commonly employed in MRI for T1‐weighted contrast and relaxation measurements. In the brain, it is often assumed that adiabatic pulses saturate the nonaqueous magnetization. We investigated this assumption using solid‐state NMR to monitor the nonaqueous signal directly following adiabatic inversion and compared this with 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 using 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 biexponential decay models. Further fitting analysis was performed using only two inversion delay times. Adiabatic and sinc inversion left much of the nonaqueous magnetization along B0 and resulted in biexponential relaxation. Saturation of both aqueous and nonaqueous magnetization components led to effectively monoexponential T1 relaxation. Typical adiabatic inversion pulses do not, as has been widely assumed, saturate the nonaqueous proton magnetization in white matter. Unequal magnetization states in aqueous and nonaqueous 1H reservoirs prepared by soft and adiabatic pulses result in biexponential T1 relaxation. Both pools must be prepared in the same magnetization state (e.g., saturated or inverted) in order to observe consistent monoexponential relaxation. We use solid‐state NMR to monitor both aqueous and nonaqueous signals directly during inversion recovery using multiple preparation pulses in vivo and ex vivo. In contradiction to the common assumption, we show that applying typical adiabatic inversion pulses to white matter leaves substantial nonaqueous proton signal aligned along B0. This leads to biexponential T1 relaxation during inversion recovery, resembling that following a selective inversion pulse.