2.7. The physical basis of poor fat/muscle contrast observed at low temperatures in post-mortem MR (PMMR) imaging

Introduction To successfully apply MRI in the forensic setting, a better understanding of the temperature-dependence of MR image contrast is needed. For example, low fat signal and poor fat/muscle contrast has been observed in cold PM subjects imaged using typical clinical MR parameters. Because MR contrast is based on differences in the relaxation times T 1 and T 2 for different tissues, our goal is to explain MR contrast changes by measuring and analyzing the temperature-dependence of tissue T 1 and T 2 values. Methods Fresh, ex vivo mammalian tissues were imaged using a standard Spin Echo sequence by varying TR (with TE=10ms), or by varying TE (with TR=3200ms), over the temperature range 4−38 ° C. For each tissue type, signal intensity was plotted vs. time (TR or TE) and fit with the appropriate function to extract the relaxation time constant ( T 1 or T 2). Results In general, non-fatty body tissues exhibited a decreasing T 1 with decreasing temperature, while fat and liver T 1 values exhibited little temperature dependence. For nearly all tissues, the temperature-dependence of T 2 was very weak; however, the fat T 2 decreased substantially as temperature is reduced. Conclusions As subject temperature is decreased, the decreasing muscle T 1 (with the fat T 1 remaining approximately constant) results in a smaller difference in T 1 between muscle and fat, while the large drop in the fat T 2 increases T 2-weighting, even in nominally T 1-weighted sequences. Taken together, these findings explain poor fat/muscle contrast and low fat signal intensity previously observed at low temperatures in PMMR.