Minimum acquisition methods for simultaneously imaging T(1), T(2), and proton density with B(1) correction and no spin-echoes

Abstract

The spin lattice (T(1)) and spin-spin (T(2)) relaxation times, along with the proton density (PD) contain almost all of the information that (1)H MRI routinely uses in clinical diagnosis and research, but are seldom imaged directly. Here, three methods for directly imaging T(1), T(2), and PD with the least possible number of acquisitions - three, are presented. All methods utilize long 0° self-refocusing adiabatic pre-pulses instead of spin-echoes to encode the T(2) information prior to a conventional gradient-echo MRI sequence. T(1) information is encoded by varying the flip-angle (FA) in the 'Dual-τ Dual-FA' and 'Four-FA' methods, or the sequence repetition period, TR, in the 'Dual-τ Dual-TR' method. Inhomogeneity in the FA distribution and slice-selection profile are recognized as the main error sources for T(1) measurements. The former is remedied by integrating an extra FA-dependent acquisition into the 'Four-FA' method to provide self-corrected T(1), T(2), PD, and FA in just four acquisitions - again, the minimum possible. Slice profile errors - which manifest as differences between 2D and 3D T(1) measurements, can be addressed by Bloch equation analysis and experimental calibration. All three methods are validated in phantom studies, and the 'Dual-τ Dual-FA' and 'Four-FA' methods are validated in human brain studies using standard partial saturation and spin-echo methods for reference. The new methods offer a minimum-acquisition option for imaging single-component T(1), T(2), and PD. 'Four-FA' performs best overall in accuracy, with high efficiency per unit accuracy vs. existing methods when B(1)-inhomogeneity is appropriately addressed.

Keywords: B(1) correction; MRI; Measurement; Proton density; Spin–latice relaxation; Spin–spin relaxation.