Comparison of R1ρ Imaging Between Rapid Acquisition with Relaxation Enhancement (RARE) and Ultrashort TE (UTE) Sequence in the Assessment of Rat Liver Iron Overload at 11.7T

Abstract

Introduction: Since the most prominent effect of iron is increasing R2* and R2 relaxation rates, the iron-overload liver shows little signal with conventional T1ρ sequences like RARE. Whereas, with UTE MR imaging sequences, the signal can be detected from short T2/T2* relaxation components in tissues. This study aims to evaluate the difference in R1ρ profiles and compare the correlations between RARE-based and UTE-based sequences with LIC in the assessment of rat liver iron overload. Methods: Iron dextran (Sigma, 100 mg Fe/ml) was injected into thirty-five rats (25-100 mg/kg body weight), while the rats in the control group were injected with saline (n=5). The liver specimen was taken after one week. 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 RARE readout and 2D UTE readout pulses were developed to quantify R1ρ on a Bruker 11.7T MR system. Results: The mean R1ρ value of the rat liver with UTE-based R1ρ sequence was significantly higher compared to RARE-based R1ρ sequence (p<0.001). Spearman’s correlation analysis (two-tailed) indicated that the R1ρ values were significantly correlated with LIC for both UTE-R1ρ and RARE-R1ρ sequences (r = 0.727, P < 0.001, and r = 0.712, P < 0.001, respectively). Conclusion: The current study adds to evidence that there is a correlation between iron concentration and R1ρ. Moreover, UTE-based R1ρ sequence is more sensitive to the liver iron than the RARE-based R1ρ sequence. R1ρ might serve as a complementary imaging biomarker for liver iron overload quantification.