Sensitivity of fiber orientation dependent R2∗ to temperature and post mortem interval

Abstract

Purpose: $R_2^{\ast }$ imaging of brain white matter is well known for being sensitive to the orientation of nerve fibers with respect to the B₀ field of the MRI scanner. The goal of this study was to evaluate whether and to which extent fiber orientation dependent $R_2^{\ast }$ differs between in vivo and post mortem in situ examinations, and to investigate the influence of varying temperatures and post mortem intervals (PMI).

Methods: Post mortem in situ and in vivo MRI scans were conducted at 3T. $R_2^{\ast }$ was acquired with a multi-echo gradient-echo sequence, and the orientation of white matter fibers was computed using diffusion tensor imaging (DTI). Fitting of the measured fiber orientation dependent $R_2^{\ast }$ was performed using three different formulations of a previously proposed model.

Results: $R_2^{\ast }$ increased with increasing fiber angle for in vivo and post mortem in situ examinations, whereby the orientation dependency was lower post mortem. The different formulations of the fiber orientation model resulted in an identical fit, but showed large variations of the estimated parameters. The higher order orientation dependent $R_2^{\ast }$ components significantly decreased with decreasing temperature, while the orientation independent $R_2^{\ast }$ components showed no significant correlation with either temperature or PMI.

Conclusion: Although the mean diffusivity is strongly reduced post mortem, we could successfully estimate the fiber angle using DTI. Due to the strong correlation of the higher order orientation dependent $R_2^{\ast }$ components with temperature, the decreased $R_2^{\ast }$ fiber orientation dependency post mortem in situ might primarily be attributed to the lower brain temperature.

Keywords: ${\mathrm{R}}_2^{\ast }$ anisotropy; ${\mathrm{R}}_2^{\ast }$ fiber orientation; fiber orientation; in situ; post mortem; quantitative MRI; white matter.