New methods for robust continuous wave T1ρ relaxation preparation

• Published in: NMR in biomedicine 2023-02, Vol.36 (2), p.e4834-n/a
• Main Authors: Pala, Swetha, Hänninen, Nina E., Nykänen, Olli, Liimatainen, Timo, Nissi, Mikko J.
• Format: Article
• Language: English
• Subjects: Adiabatic; Adiabatic flow; Biological products; Bloch simulation; Continuous radiation; contrast; Deposition; field inhomogeneity; Inhomogeneity; Magnetic fields; Production methods; Relaxation time; Robustness; rotating frame of reference; Sensitivity analysis; Simulation; T1ρ relaxation; Theoretical analysis
• ISSN: 0952-3480; 1099-1492
• DOI: 10.1002/nbm.4834

Measurement of the longitudinal relaxation time in the rotating frame of reference (T1ρ) is sensitive to the fidelity of the main imaging magnetic field (B0) and that of the RF pulse (B1). The purpose of this study was to introduce methods for producing continuous wave (CW) T1ρ contrast with improved robustness against field inhomogeneities and to compare the sensitivities of several existing and the novel T1ρ contrast generation methods with the B0 and B1 field inhomogeneities. Four hard‐pulse and four adiabatic CW‐T1ρ magnetization preparations were investigated. Bloch simulations and experimental measurements at different spin‐lock amplitudes under ideal and non‐ideal conditions, as well as theoretical analysis of the hard‐pulse preparations, were conducted to assess the sensitivity of the methods to field inhomogeneities, at low (ω1 > ΔB0) spin‐locking field strengths. In simulations, previously reported single‐refocus and new triple‐refocus hard‐pulse and double‐refocus adiabatic preparation schemes were found to be the most robust. The mean normalized absolute deviation between the experimentally measured relaxation times under ideal and non‐ideal conditions was found to be smallest for the refocused preparation schemes and broadly in agreement with the sensitivities observed in simulations. Experimentally, all refocused preparations performed better than those that were non‐refocused. The findings promote the use of the previously reported hard‐pulse single‐refocus ΔB0 and B1 insensitive T1ρ as a robust method with minimal RF energy deposition. The double‐refocus adiabatic B1 insensitive rotation‐4 CW‐T1ρ preparation offers further improved insensitivity to field variations, but because of the extra RF deposition, may be preferred for ex vivo applications. New and existing state‐of‐the‐art CW‐T1ρ schemes were compared theoretically, numerically, and experimentally at 9.4 T. Of the CW‐T1ρ methods using hard pulses, the proposed triple‐focus and earlier single‐refocus methods were highly robust, while the scheme using adiabatic excitation and double refocusing with BIR‐4 was the most robust.