Revisiting signal intensity ratios for extending the dynamic range for iron overload estimation.

PURPOSE: The principles of signal-intensity-ratio (SIR) estimation were revisited to derive a set of generalizable equations to quantify liver iron concentration (LIC), extending the effective dynamic range of LIC-R2* estimation or detect when LIC- R2* estimation degenerates.

THEORY AND

METHODS

Equations for LIC were derived for two techniques: (1) relaxometry of multi-echo gradient echo and (2) SIR of liver and muscle tissue in a single echo. MRI was completed at 1.5T and 3T in human subjects with a range of liver iron burdens. LIC was estimated by both techniques and compared to assess concordance across the clinical range and high LIC performance.

RESULTS

Regression of Log(SIR) by LIC was linear for both 1.5T and 3T data with similar slopes. Comparison between SIR-LIC and R2*-LIC estimates demonstrated linear concordance until the echo time-based saturation of R2* estimates were reached, at which point SIR-LIC estimates continued to rise without a concomitant increase in R2*-LIC. Lack of spatial intensity correction caused systematic bias and increased variance in LIC estimates compared to intensity-corrected images.

CONCLUSIONS

LIC-SIR measurements can potentially be used to estimate liver iron concentration in examinations where R2* acquisitions were not performed or where the dynamic range of a multi-echo protocol is exceeded. Using spatial intensity correction where possible improves SIR-LIC estimates.