Quadruple-refocused spin-locking: A robust method for high-amplitude T1ρ imaging

Abstract

Purpose: Longitudinal relaxation time in the rotating frame (T1ρ) is a source of tissue-specific contrasts, with applications in detecting myocardial fibrosis, liver fibrosis, and early-stage osteoarthritis. However, T1ρ measurements are sensitive to static (B0) and radiofrequency (B1) magnetic field inhomogeneities. Improving the accuracy of T1ρ 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. Methods: 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. Results: 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. Conclusion: The QR-SL module has the potential to produce more accurate T1ρ maps, while minimizing artifacts. Consequently, the QR-SL module is more favorable for T1ρ quantification, especially for low-field and ultralow-field quantitative MRI.