On quantification errors of R2*$$ {R}_2^{\ast } $$ and proton density fat fraction mapping in trabecularized bone marrow in the static dephasing regime

Abstract

Purpose: To study the effect of field inhomogeneity distributions in trabecularized bone regions on the gradient echo (GRE) signal with short TEs and to characterize quantification errors on $R_2^{*}$ and proton density fat fraction (PDFF) maps when using a water-fat model with an exponential $R_2^{*}$ decay model at short TEs.

Methods: Field distortions were simulated based on a trabecular bone micro CT dataset. Simulations were performed for different bone volume fractions (BV/TV) and for different bone-fat composition values. A multi-TE UTE acquisition was developed to acquire multiple UTEs with random order to minimize eddy currents. The acquisition was validated in phantoms and applied in vivo in a volunteer's ankle and knee. Chemical shift encoded MRI (CSE-MRI) based on a Cartesian multi-TE GRE scan was acquired in the spine of patients with metastatic bone disease.

Results: Simulations showed that signal deviations from the exponential signal decay at short TEs were more prominent for a higher BV/TV. UTE multi-TE measurements reproduced in vivo the simulation-based predicted behavior. In regions with high BV/TV, the presence of field inhomogeneities induced an $R_2^{*}$ underestimation in trabecularized bone marrow when using CSE-MRI at 3T with a short TE.

Conclusion: $R_2^{*}$ can be underestimated when using short TEs (<2 ms at 3 T) and a water-fat model with an exponential $R_2^{*}$ decay model in multi-echo GRE acquisitions of trabecularized bone marrow.

Keywords: $$$ {R}_2^{\ast } $$$ mapping; Gaussian decay; PDFF mapping; chemical shift encoding (CSE); magnetically inhomogeneous tissues; signal decay; static dephasing regime; trabecularized bone; ultra-short echo time (UTE).