PURPOSE

Besides the conventional chemical exchange saturation transfer (CEST) sequence, an off-resonance spin-lock (CESL) sequence can also be applied to evaluate chemical exchange effect^1,2. The advantage of CESL sequence over CEST sequence is the reduction of saturation time. However, with an inhomogeneous B₀ field, the angle of the rotation pulses in CESL sequence would not be ideal, which may probably affect the accuracy of CESL signal. Till now, few studies focus on whether the CESL signal is still accurate with B₀ inhomogeneity. Using perturbation of longitudinal relaxation rate in rotating frame (PLRF) analysis, we compared the signals from short-saturation-time CESL sequence and long-saturation-time CEST sequence to evaluate the accuracy of the CESL signal.

THEORY

Based on analytical solution of Z-value, the process of chemical saturation transfer with saturation pulse amplitude B₁ and frequency offset Δω could be treated as T_1ρ relaxation along the direction of $$$\overrightarrow{\omega_{eff}}=(\gamma B_{1}, 0, \triangle\omega)$$$ tilted by an angle $$$\theta=\arctan(\gamma B_{1}/\triangle\omega)$$$ off the Z axis, combined with T_2ρ relaxation perpendicular to the direction of $$$\overrightarrow{\omega_{eff}}$$$ ^2,3. If the magnetization along T_2ρ direction ($$$M_{T_{2\rho}}$$$) has a nonzero contribution to the final magnetization along Z axis before signal acquisition, the signal would be contaminated, which places constrains on repeatable quantifications in practice. To avoid the contamination from $$$M_{T_{2\rho}}$$$ in CESL sequence, the flip angle of the rotation pulses β is frequency offsets variedly set to θ. It is theoretically ideal, but would be problematic with an inhomogeneous B₀ field. As a result, the acquired signal at any offset may be significantly biased. To overcome this problem, a fixed flip angle (FA) β could be applied. Thus, the contamination from $$$M_{T_{2\rho}}$$$ would probably be neglected or near a given frequency offset which satisfies $$$\beta\approx\theta$$$. It should be noticed that a change in saturation time (t_sat) would result in an alteration of ΔZ, therefore, to compare the signal from CESL sequence (short t_sat) with that from conventional CEST sequence (long t_sat) PLRF analysis is used for quantification, specifically, for converting traditional to ΔR_1ρ and/or SPACER³.

EXPERIMENTS

Phantoms with glucose and glutamate (GL phantom) or bovine serum albumin (BSA phantom) were prepared and used to test the effect of CESL. MnCl₂ was added into phantoms for adjusting relaxation times. Continuous wave rectangular saturation pulse followed by a SE-EPI sequence was used for CEST imaging, in which B₁=1μT, TR=2s and t_sat=1.5s. were applied. CESL sequences with or without a fixed flip angle (β=14.93°) were tested. For both kinds of CESL sequences, two B₀ inhomogeneity (ΔB₀=30Hz and 50Hz) and three t_sat (516ms, 258ms, and 129ms) as well as B1=1μT and TR=2s were applied for imaging. The corresponding maps of B₁, T₁ and T₂ were also obtained for quantification.

RESULT

Signal from the CEST sequence is treated as the gold standard to measure chemical transfer effect. If a fixed flip angle is not applied, the accuracy of CESL signal will strongly depend on B₀ inhomogeneity (ΔB₀). When facing relative small ΔB₀ (e.g. 30Hz), after quantification, the spectra (ΔR_1ρ and SPACER) from CESL sequence fits fairly well to the corresponding spectra from CEST sequence in the range of 1ppm to 4ppm, although with a decreased t_sat there is a little difference between each pair of spectra (Fig. 1). However, when a relative larger ΔB₀ comes (e.g. 50Hz), the spectra (ΔR_1ρ and SPACER) from CESL sequence have poor performance. Specifically, in the range from 1ppm to 2ppm (Fig. 2), significant difference exists between these spectra and corresponding spectra from CEST sequence. On the contrary, if a fixed flip angle is applied, in the offset range near the frequency offset satisfying β=θ, specifically Δω round 160Hz for β=14.93° and B₁=1μT, the spectra (ΔR_1ρ and SPACER) from CESL sequence are nearly identical to the corresponding spectra from CEST sequence, despite the values of ΔB₀ and t_sat (Fig. 3, 4).

DISCUSSION and CONCLUSION

Theoretically, CESL sequence is feasible in measuring chemical exchange effect with a shorter saturation time than that required by the CEST sequence. This study reveals that when there is great difference between θ and the flip angle β, CESL sequence cannot provide reliable signal. To ensure the CESL signal is of high quality, the B₀ inhomogeneity should be relative small and t_sat should be relative large; or a fixed flip angle β could be applied to maintain the high quality of CESL signal at the offset satisfying $$$\beta\approx\theta$$$.

Acknowledgements

No acknowledgement found.