On‐resonance and off‐resonance continuous wave constant amplitude spin‐lock and T_1ρ quantification in the presence of B₁ and B₀ inhomogeneities

Abstract

Spin-lock MRI is a valuable diagnostic imaging technology, as it can be used to probe the macromolecule environment of tissues. Quantitative T imaging is one application of spin-lock MRI that is reported to be promising for a number of clinical applications. Spin-lock is often performed with a continuous RF wave at a constant RF amplitude either on resonance or off resonance. However, both on- and off-resonance spin-lock approaches are susceptible to B and B inhomogeneities, which results in image artifacts and quantification errors. In this work, we report a continuous wave constant amplitude spin-lock approach that can achieve negligible image artifacts in the presence of B and B inhomogeneities for both on- and off-resonance spin-lock. Under the adiabatic condition, by setting the maximum B amplitude of the adiabatic pulses equal to the B amplitude of spin-lock RF pulse, the spins are ensured to align along the effective field throughout the spin-lock process. We show that this results in simultaneous compensation of B and B inhomogeneities for both on- and off-resonance spin-lock. The relaxation effect during the entire adiabatic half passage (AHP) and reverse AHP, and the stationary solution of the Bloch-McConnell equation present at off-resonance frequency offset, are considered in the revised relaxation model. We demonstrate that these factors create a direct current component to the conventional relaxation model. In contrast to the previously reported dual-acquisition method, the revised relaxation model just requires one acquisition to perform quantification. The simulation, phantom, and in vivo experiments demonstrate that the proposed approach achieves superior image quality compared with the existing methods, and the revised relaxation model can perform T quantification with one acquisition instead of two.

Probing chemical exchange using quantitative spin‐lock R1ρ asymmetry imaging with adiabatic RF pulses

“…However, ACCSL can be used to mitigate this problem. When performing ACCSL, the spins are locked along the effective spin‐lock field after adiabatic half passage (AHP) at an angle θ from the longitudinal direction, which is determined by the nominal spin‐lock B 1 , nominal resonance FO, and B 1 RF and B 0 field inhomogeneities . Figure A shows the θ values with and without B 0 field inhomogeneity when ACCSL was used at a range of resonance FOs.…”

“…R 1ρ asymmetry can be calculated after using ACCSL to acquire the R 1ρ at each resonance FO. However, this approach and the corresponding relaxation model may require a long‐duration RF pulse at large FOs to accommodate the prolonged T 1ρ at increased FOs. Additionally, during the asymmetry analysis, any small quantification error because of noise can be amplified after the subtraction process.…”

“…where M ini1 and M ini2 are the initial magnetizations after the AHP and at the beginning of the spin‐lock multiplied by a scaling factor; and C is a term including the contributions from the steady‐state magnetization and relaxation effect . Note that the same C term is used in Equations and , as this term is calculated using the steady state magnetization, relaxation parameters, and adiabatic waveforms that remain constant regardless of the toggling RF pulse status. Note that M ini1 is equal to the negative M ini2 determined using the original HP‐iTIP when the toggling RF pulse is a perfect 180° pulse.…”

“…As the AC‐iTIP approach requires 4 acquisitions to measure R 1ρ values, 4 TSLs (0, 20, 40, and 60 ms) were used to simulate R 1ρ ‐fitting approach data and emulate an equal scan time between the 2 approaches. The R 1ρ ‐spectrum determined via the R 1ρ ‐fitting approach was obtained by fitting the data to our previously reported relaxation model . Three SNRs were used: 15, 25, and 50.…”

“…However, that approach ignored B 0 field inhomogeneity. Other reports suggest that by replacing hard pulses with adiabatic pulses with RF amplitudes matching that of the spin‐lock pulse, the spins would be locked along the effective field for both on‐ and off‐resonance spin‐lock, even in the presence of B 1 RF and B 0 field inhomogeneities . This approach should therefore provide artifact‐free spin‐lock images and T 1ρ quantification with simultaneous compensation of B 1 RF and B 0 field inhomogeneities for both on‐ and off‐resonance spin‐lock.…”