Quadruple-refocused spin-locking: A robust method for high-amplitude T_1ρ imaging

Longitudinal relaxation time in the rotating frame (T_1ρ) is a source of tissue-specific contrasts, with applications in detecting myocardial fibrosis, liver fibrosis, and early-stage osteoarthritis. However, T_1ρ measurements are sensitive to static (B₀) and radiofrequency (B₁) magnetic field inhomogeneities. Improving the accuracy of T_1ρ quantification can enable earlier and more reliable detection of pathological changes, providing a basis for early intervention. Therefore, we have developed an improved, quadruple-refocused spin-locking (QR-SL) technique based on existing preparation schemes to obtain a more robust compensation for B₀ and B₁ field inhomogeneities.

The QR-SL module consists of four 180° refocusing pulses with opposite phases and five spin-locking (SL) pulses with phase cycling according to the rotary-echo principle. The performance of the proposed QR-SL module is evaluated through numerical simulations and experimental validation in comparison to composite-SL (C-SL), balanced-SL (B-SL), and triple-refocused-SL (TR-SL) preparation modules.

Numerical simulations indicate that the QR-SL module demonstrates improved tolerance to a range of B₀ and B₁ field inhomogeneities compared to the other three modules. In scenarios involving inhomogeneities of both fields, the experimental results show that the residual sum of squares of the QR-SL module was decreased by 24.3%, 68.9%, and 12.5% for in vivo knee cartilage, respectively, compared to the composite SL, balanced SL, and triple-refocused SL preparation modules.

The QR-SL module has the potential to produce more accurate T_1ρ maps, while minimizing artifacts. Consequently, the QR-SL module is more favorable for T_1ρ quantification, especially for low-field and ultralow-field quantitative MRI.