Adiabatically prepared spin-lock could reduce the R(1ρ) dispersion

BACKGROUND: R(1ρ) (or spin-lock) imaging is prone to artifacts arising from field inhomogeneities that may impact the R(1ρ) quantification. Previous research has proposed two types of method to manage the artifacts in continuous-wave constant amplitude spin-lock, one is based on the composite block pulses to compensate for the field imperfections, another category uses adiabatic pulses in the R(1ρ) pre-pulse to excite and reverse the magnetization (named adiabatic prepared approach). Although both methods have proved their efficiency in alleviating artifacts, we observed that the adiabatic pulse approach could produce much lower R(1ρ) dispersion in human knee cartilage than the block pulse method (characterized by the R(1ρ) difference ∆R(1ρ) =11.4 Hz (from spin-lock field 50 to 500 Hz) for the block pulse method vs. ∆R(1ρ) =4.5 Hz for the adiabatic pulse approach). Prompted by this observation, the purpose of this study was to investigate the underlying factors that may affect the R(1ρ) dispersion through numerical simulations based on the two-pool exchanging Bloch-McConnell equations. METHODS: The effects of free water pool size P(a) (from 0.80 to 0.95), chemical exchange rate k(b) (from the bound to free water pool, ranged from 500 to 3,000 Hz), adiabatic pulse duration T(p) (from 5.0 to 25 ms), and the chemical shift of the bound pool ppm(b) (from 1.0 to 5.0 ppm) were examined on the degree of the R(1ρ) dispersion for the two R(1ρ) imaging methods. RESULTS: In general, the greater the ppm(b), k(b), T(p), and the smaller P(a), the more significant difference in R(1ρ) dispersion between the block and adiabatic approaches, with the dispersion curve of the adiabatic method becoming flatter. CONCLUSIONS: The adiabatic prepared approach may compromise the R(1ρ) dispersion, the effect is determined by the combination of the tissue and radiofrequency (RF) pulse properties. It is suggested that care should be taken when using the adiabatically prepared approach to study R(1ρ) dispersion.