New methods for robust continuous wave T_1ρ relaxation preparation

Abstract

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) and high (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.

Whole knee joint mapping using a phase modulated UTE adiabatic T1ρ (PM‐UTE‐AdiabT1ρ) sequence

“…To further investigate the contrast mechanism of both AdiabT 1ρ and CW‐T 1ρ , we plan to perform a comparison study (i.e., PM‐UTE‐AdiabT 1ρ vs. PM‐UTE‐CWT 1ρ ) on a newly installed small‐bore Bruker 3 T scanner. In addition to the state‐of‐art preparation scheme for CW‐T 1ρ , a few new CW‐T 1ρ developments will also be incorporated with the PM‐UTE sequence structure in this comparison study 45 . Typically, the maximum spin‐lock frequency of CW‐T 1ρ is limited by the peak power of the RF amplifier and SAR restriction on a clinical 3 T scanner.…”

“…In addition to the state-of-art preparation scheme for CW-T 1ρ , a few new CW-T 1ρ developments will also be incorporated with the PM-UTE sequence structure in this comparison study. 45 Typically, the maximum spin-lock frequency of CW-T 1ρ is limited by the peak power of the RF amplifier and SAR restriction on a clinical 3 T scanner. The Bruker scanner offers more flexibility in adjusting the spin-lock frequencies than the clinical 3 T scanner used in this study, thus providing more useful information in studying the mechanisms of CW-T 1ρ and AdiabT 1ρ .…”

An adiabatic RF pulse in spin‐lock preparations: A double‐edged sword for quantitative R1ρ imaging in clinical applications

“…3,4 Recently, some research effort has focused on developing robust spin-lock (SL) preparation methods, many of which depend considerably on lengthy adiabatic RF pulses. [5][6][7] Although intrinsically less susceptible to ΔB 0 and ΔB 1 , 8 adiabatic pulses have a certain limitation for an accurate R 1ρ quantification. Unfortunately, this limitation has been largely overlooked in the literature.…”

“…4,20 This less than ideal performance may be somewhat remedied by the latest developments on refining an optimal SL preparation for quantitative R 1ρ imaging in clinical applications. 5,6…”