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The chemical exchange (CE) process has been exploited as a novel and powerful contrast mechanism for MRI, which is primarily performed in the form of chemical exchange saturation transfer (CEST) imaging. A spin-lock (SL) technique can also be used for CE studies, although traditionally performed and interpreted quite differently from CEST. Chemical exchange imaging with spin-lock technique (CESL), theoretically based on the Bloch-McConnell equations common to CEST, has the potential to be used as an alternative to CEST and to better characterize CE processes from slow and intermediate to fast proton exchange rates through the tuning of spin-lock pulse parameters. In this study, the Z-spectrum and asymmetric magnetization transfer ratio (MTRasym) obtained by CESL are theoretically analyzed and numerically simulated using a general two-pool R1? relaxation model beyond the fastexchange limit. The influences of spin-lock parameters, static magnetic field strength B0 and physiological properties on the Z-spectrum and MTRasym are quantitatively revealed. Optimization of spin-lock frequency and spin-lock duration for the maximum CESL contrast enhancement is also investigated. Numerical simulation results in this study are compatible with the findings in the existing literature on CE imaging studies. [ABSTRACT FROM AUTHOR]

T1&rgr; relaxation is traditionally described as a mono-exponential signal decay with spin-lock time. However, T1&rgr; quantification by fitting to the monoexponential model can be substantially compromised in the presence of field inhomogeneities, especially for low spin-lock frequencies. The normal approach to address this issue involves the development of dedicated composite spin-lock pulses for artifact reduction while still using the mono-exponential model for T1&rgr; fitting. In this work, we propose an alternative approach for improved T1&rgr; quantification with the widely-used rotary echo spin-lock pulses in the presence of B0 inhomogeneities by fitting to a modified theoretical model which is derived to reveal the dependence of T1&rgr;-prepared magnetization on T1&rgr;, T2&rgr;, spin-lock time, spin-lock frequency and off-resonance, without involving complicated spin-lock pulse design. It has potentials for T1&rgr; quantification improvement at low spin-lock frequencies. Improved T1&rgr; mapping was demonstrated on phantom and in vivo rat spin-lock imaging at 3 T compared to the mapping using the mono-exponential model. [ABSTRACT FROM AUTHOR]

T1ρrelaxation has recently been found to be sensitive to liver fibrosis and has potential to be used for early detection of liver fibrosis and grading. Liver T1ρimaging and accurate mapping are challenging because of the long scan time, respiration motion and high specific absorption rate. Reduction and optimization of spin lock times (TSLs) are an efficient way to reduce scan time and radiofrequency energy deposition of T1ρ imaging, but maintain the near-optimal precision of T1ρ mapping. This work analyzes the precision in T1ρ estimation with limited, in particular two, spin lock times, and explores the feasibility of using two specific operator-selected TSLs for efficient and accurate liver T1ρ mapping. Two optimized TSLs were derived by theoretical analysis and numerical simulations first, and tested experimentally by in vivo rat liver T1ρ imaging at 3 T. The simulation showed that the TSLs of 1 and 50 ms gave optimal T1ρ estimation in a range of 10-100 ms. In the experiment, no significant statistical difference was found between the T1ρ maps generated using the optimized two-TSL combination and the maps generated using the six TSLs of [1, 10, 20, 30, 40, 50] ms according to one-way ANOVA analysis (p = 0.1364 for liver and p = 0.8708 for muscle). [ABSTRACT FROM AUTHOR]

The chemical exchange (CE) process has been exploited as a novel and powerful contrast mechanism for MRI, which is primarily performed in the form of chemical exchange saturation transfer (CEST) imaging. A spin-lock (SL) technique can also be used for CE studies, although traditionally performed and interpreted quite differently from CEST. Chemical exchange imaging with spin-lock technique (CESL), theoretically based on the Bloch-McConnell equations common to CEST, has the potential to be used as an alternative to CEST and to better characterize CE processes from slow and intermediate to fast proton exchange rates through the tuning of spin-lock pulse parameters. In this study, the Z-spectrum and asymmetric magnetization transfer ratio (MTR(asym)) obtained by CESL are theoretically analyzed and numerically simulated using a general two-pool R(1ρ) relaxation model beyond the fast-exchange limit. The influences of spin-lock parameters, static magnetic field strength B(0) and physiological properties on the Z-spectrum and MTR(asym) are quantitatively revealed. Optimization of spin-lock frequency and spin-lock duration for the maximum CESL contrast enhancement is also investigated. Numerical simulation results in this study are compatible with the findings in the existing literature on CE imaging studies.

T(1ρ) relaxation is traditionally described as a mono-exponential signal decay with spin-lock time. However, T(1ρ) quantification by fitting to the mono-exponential model can be substantially compromised in the presence of field inhomogeneities, especially for low spin-lock frequencies. The normal approach to address this issue involves the development of dedicated composite spin-lock pulses for artifact reduction while still using the mono-exponential model for T(1ρ) fitting. In this work, we propose an alternative approach for improved T(1ρ) quantification with the widely-used rotary echo spin-lock pulses in the presence of B(0) inhomogeneities by fitting to a modified theoretical model which is derived to reveal the dependence of T(1ρ)-prepared magnetization on T(1ρ), T(2ρ), spin-lock time, spin-lock frequency and off-resonance, without involving complicated spin-lock pulse design. It has potentials for T(1ρ) quantification improvement at low spin-lock frequencies. Improved T(1ρ) mapping was demonstrated on phantom and in vivo rat spin-lock imaging at 3 T compared to the mapping using the mono-exponential model.

T(1ρ) relaxation has recently been found to be sensitive to liver fibrosis and has potential to be used for early detection of liver fibrosis and grading. Liver T(1ρ) imaging and accurate mapping are challenging because of the long scan time, respiration motion and high specific absorption rate. Reduction and optimization of spin lock times (TSLs) are an efficient way to reduce scan time and radiofrequency energy deposition of T(1ρ) imaging, but maintain the near-optimal precision of T(1ρ) mapping. This work analyzes the precision in T(1ρ) estimation with limited, in particular two, spin lock times, and explores the feasibility of using two specific operator-selected TSLs for efficient and accurate liver T(1ρ) mapping. Two optimized TSLs were derived by theoretical analysis and numerical simulations first, and tested experimentally by in vivo rat liver T(1ρ) imaging at 3 T. The simulation showed that the TSLs of 1 and 50 ms gave optimal T(1ρ) estimation in a range of 10-100 ms. In the experiment, no significant statistical difference was found between the T(1ρ) maps generated using the optimized two-TSL combination and the maps generated using the six TSLs of [1, 10, 20, 30, 40, 50] ms according to one-way ANOVA analysis (p = 0.1364 for liver and p = 0.8708 for muscle)., (© 2012 Institute of Physics and Engineering in Medicine)