Abstract
PURPOSE: CEST is commonly used to probe the effects of chemical exchange. Although R(1ρ) asymmetry quantification has also been described as a promising option for detecting the effects of chemical exchanges, the existing acquisition approaches are highly susceptible to B(1) RF and B(0) field inhomogeneities. To address this problem, we report a new R(1ρ) asymmetry imaging approach, AC-iTIP, which is based on the previously reported techniques of irradiation with toggling inversion preparation (iTIP) and adiabatic continuous wave constant amplitude spin-lock RF pulses (ACCSL). We also derived the optimal spin-lock RF pulse B(1) amplitude that yielded the greatest R(1ρ) asymmetry. METHODS: Bloch-McConnell simulations were used to verify the analytical formula derived for the optimal spin-lock RF pulse B(1) amplitude. The performance of the AC-iTIP approach was compared to that of the iTIP approach based on hard RF pulses and the R(1ρ) -spectrum acquired using adiabatic RF pulses with the conventional fitting method. Comparisons were performed using Bloch-McConnell simulations, phantom, and in vivo experiments at 3.0T. RESULTS: The analytical prediction of the optimal B(1) was validated. Compared to the other 2 approaches, the AC-iTIP approach was more robust under the influences of B(1) RF and B(0) field inhomogeneities. A linear relationship was observed between the measured R(1ρ) asymmetry and the metabolite concentration. CONCLUSION: The AC-iTIP approach could probe the chemical exchange effect more robustly than the existing R(1ρ) asymmetry acquisition approaches. Therefore, AC-iTIP is a promising technique for metabolite imaging based on the chemical exchange effect.