Post mortem brain temperature and its influence on quantitative MRI of the brain

Abstract

Objective MRI temperature sensitivity presents a major issue in in situ post mortem MRI (PMMRI), as the tissue temperatures differ from living persons due to passive cooling of the deceased. This study aims at computing brain temperature effects on the MRI parameters to correct for temperature in PMMRI, laying the foundation for future projects on post mortem validation of in vivo MRI techniques. Materials and methods Brain MRI parameters were assessed in vivo and in situ post mortem using a 3 T MRI scanner. Post mortem brain temperature was measured in situ transethmoidally. The temperature effect was computed by fitting a linear model to the MRI parameters and the corresponding brain temperature. Results Linear positive temperature correlations were observed for T1, T2* and mean diffusivity in all tissue types. A significant negative correlation was observed for T2 in white matter. Fractional anisotropy revealed significant correlations in all gray matter regions except for the thalamus. Discussion The linear models will allow to correct for temperature in post mortem MRI. Comparing in vivo to post mortem conditions, the mean diffusivity, in contrast to T1 and T2, revealed additional effects besides temperature, such as cessation of perfusion and active diffusion.

The Douglas Bell Canada Brain Bank Post-mortem Brain Imaging Protocol

“…The process of fixation also significantly impacts the diffusion-weighted images (DWI), where T1, T2, and T2* relaxation times as well as diffusivity decrease in formalin-fixed tissues (Birkl et al, 2016; Dawe et al, 2009; Miller et al, 2011; Schmierer et al, 2008). Temperature is another factor that significantly impacts relaxation times and diffusivity in postmortem tissues, with mean diffusivity (MD) values reportedly decreasing by ∼30% when the brain temperature is decreased from body temperature (∼37 C) to room temperatures (∼20 C) (Berger et al, 2021). As such, the impact of temperature and fixation on diffusion metrics must be modeled and adjusted for, to enable comparison with in vivo metrics.…”

“…The 3D meGSE sequence acquires 32 TEs in 7:16 minutes and allows for fitting the multi-exponential model to generate MWF maps (Duval et al, 2018). As mentioned previously, since the process of fixation and to a lesser extent temperature impact T1, T2, and T2* relaxation times (Berger et al, 2021; Birkl et al, 2016; Dawe et al, 2009), the impact of temperature and fixation needs to be modeled and adjusted for, to enable comparison with in vivo quantitative metrics. Figure 6 shows the T1, T2, T2*, and MWF maps for one specimen.…”

“…between 0 days to 22 years), allowing for future development of models that can be employed to adjust for the impact of fixation. (Miller et al, 2011) Ex vivo DWI acquisition is particularly challenging due to a number of factors, including a decrease of mean diffusivity with a decrease in temperature (Berger et al, 2021), shortening of T2, and the consequent decrease in SNR (Miller et al, 2011). As such, ex vivo DWI acquisitions generally have poorer image quality and diffusion contrast compared to in vivo settings (Miller et al, 2011;Pfefferbaum et al, 2004).…”

A temperature‐controlled cooling system for accurate quantitative post‐mortem MRI

“…The impact of temperature on quantitative post‐mortem imaging has been studied before, albeit with a focus on how temperature differences introduce discrepancy of tissue parameter estimates between in vivo and post‐mortem scans 14–17 . These studies used short scan times and assumed a stable temperature during the acquisition of each quantitative map.…”

“…The impact of temperature on quantitative post‐mortem imaging has been studied before, albeit with a focus on how temperature differences introduce discrepancy of tissue parameter estimates between in vivo and post‐mortem scans. 14 , 15 , 16 , 17 These studies used short scan times and assumed a stable temperature during the acquisition of each quantitative map. However, under certain conditions, such as long scan durations, low initial sample temperatures, and the use of high SAR pulses, our results demonstrate that this assumption may not be valid and quantitative measures from the post‐mortem data become biased.…”