R1ρ at high spin-lock frequency could be a complementary imaging biomarker for liver iron overload quantification

Purpose: To compare the correlations among the R1 rho, R2, and R2(star) relaxation rates with liver iron concentration (LIC) in the assessment of rat liver iron content and explore the application potential of R1 rho in assessing liver iron content.Methods: Iron dextran (dosage of 0, 25, 50, 100, and 200 mg/kg body weight) was injected into 35 male rats to increase the amount of iron storage in the liver. After one week, all rats were euthanized with isoflurane. A portion of the largest hepatic lobe was extracted to quantify the LIC by inductively coupled plasma, and the remaining liver tissue was stored in 4% buffered paraformaldehyde for 24 h before MRI. Spin-lock preparation with a RARE (rapid acquisition with relaxation enhancement) readout (9 different spin-lock times and 7 different spin-lock frequencies (FSLs)) and multi-echo UTE (ultrashort TE) pulses were developed to quantify R1 rho and R2(star) on a Bruker 11.7 T MR system. For comparisons with R1 rho and R2(star), R2 was acquired using the CPMG sequence.Results: Mean R1 rho values displayed dispersion, with decrease in R1 rho at higher FSLs. Spearman's correlation analysis (two-tailed) indicated that the R1 rho values were significantly associated with LIC at FSL = 2000, 2500, and 3000 Hz (r = 0.365 and P = 0.031, r = 0.608 and P < 0.001, and r = 0.764 and P 0.001, respectively), and were not significantly associated with LIC at FSL = 500, 1000, 1250, and 1500 Hz (all P 0.05). R2 and R2(star) showed significant linear correlations with LIC (r = 0.787 and P < 0.001, and r = 0.859 and P < 0.001, respectively). Correlation analysis across R1 rho, R2, and R-star also suggested that the correlation strength between R1 rho and R2 and between R1 rho and R-star showed an increasing trend with increase in FSL.Conclusion: In this study, a strong association was observed between R1 rho and LIC at high FSLs further confirming previous findings. The results demonstrated that R1 rho at high FSL might serve as a complementary imaging biomarker for liver iron overload quantification.