Adiabatic Pulse Shape Influence on the Orientation Dependence of T_1ρ Relaxation

To determine how adiabatic pulse shape influences the balance between magic angle effect (MAE) suppression and T₁ contamination in adiabatic T_1ρ (AdT_1ρ) mapping of ordered tissues. We systematically compared continuous-wave (CW), hyperbolic-secant 4 (HS4), and tan-tanh (TAN) preparations ex vivo, and evaluated the feasibility and repeatability of UTE-AdT_1ρ mapping of the knee in vivo.

UTE-AdT_1ρ imaging was performed at 3 T. A bovine tendon was imaged at 0°, 55°, and 90° using CW, HS4 (3/6 ms), and TAN (3/6 ms) preparations at spin-lock amplitudes of 500-1000 Hz. Five healthy volunteers and five patients with knee osteoarthritis were scanned. Data were fit with mono-exponential (ME) and stretched-exponential (SE) models. Intrasubject repeatability was assessed in healthy volunteers.

All preparations exhibited strong MAE (anisotropy ratios > 40%). TAN pulses consistently produced lower T_1ρ values than HS4, indicating reduced T₁ contamination (ex vivo). ME-T_1ρ remained orientation-dependent across all pulses, whereas the SE parameter α was largely orientation-independent (anisotropy ratio ≈2%-20%). TAN-6 ms yielded T_1ρ values most consistent with CW, while HS4-6 ms produced substantially higher values in the knee joint. TAN-6 ms (CV = 3.47%) and HS4-6 ms (CV = 4.24%) demonstrated better repeatability than CW (CV = 7.24%).

Adiabatic pulse shape strongly influences T₁ contamination, but none of the tested preparations eliminated MAE. TAN-6 ms offered the best balance of reduced T₁ contamination and high repeatability. The near orientation-independence of the SE parameter α suggests that it may serve as a more robust biomarker for highly ordered tissues.