Measuring T₂ and T₁, and imaging T₂ without spin echoes

Abstract

During adiabatic excitation, the nuclear magnetization in the transverse plane is subject to T(2) (spin-spin) relaxation, depending on the pulse length τ. Here, this property is exploited in a method of measuring T(2) using the ratio of NMR signals acquired with short and long-duration self-refocusing adiabatic pulses, without spin-echoes. This Dual-τ method is implemented with B(1)-insensitive rotation (BIR-4) pulses. It is validated theoretically with Bloch equation simulations independent of flip-angle, and experimentally in phantoms. Dual-τT(2) measurements are most accurate at short T(2) where results agree with standard spin-echo measures to within 10% for T(2) ≤ 100 ms. Dual-τ MRI performed with a long 0° BIR-4 pre-pulse provides quantitative T(2) imaging of phantoms and the human foot while preserving desired contrast and functional properties of the rest of the MRI sequence. A single 0° BIR-4 pre-pulse can provide T(2) contrast-weighted MRI and serve as a "T(2)-prep" sequence with a lower B(1) requirement than prior approaches. Finally, a Tri-τ experiment is introduced in which both τ and flip-angle are varied, enabling measurement of T(2), T(1) and signal intensity in just three acquisitions if flip-angles are well-characterized. These new methods can potentially save time and simplify relaxation measurements and/or contrast-weighted NMR and MRI.