Time after excision and temperature alter ex vivo tissue relaxation time measurements

Previously unreported effects of tissue storage were recently observed in the authors' experimental magnetic resonance (MR) studies. To evaluate the effect of elapsed time after excision and storage temperature on tissue relaxation time measurements, tissue samples from the liver, pancreas, kidney, testis, spleen, and brain were obtained in rats. T1 and T2 were first measured within 5 minutes of excision, and between subsequent measurements, tubes were kept in a water bath at 40 degrees C, at room temperature (28 degrees C), or in an ice bath (4 degrees C). Cellular and organellar integrity was assessed with electron microscopy and correlated with the MR findings. At 40 degrees C (20-MHz spectrometer), the T1 of liver decreased from 280 msec +/- 8 to 212 msec +/- 10 during the first 60 minutes; the T1 of pancreas decreased from 276 msec +/- 3 to 208 msec +/- 2. Other tissues showed less than a 5% decrease in T1. T2 changes were smaller than T1 changes in all tissues. Electron microscopy of pancreatic acinar cells showed postmortem changes in mitochondria evolving over the first 60 minutes after death. Manganese loading experiments implicated mitochondrial manganese stores in the observed enhanced postmortem decrease in T1. This study calls into question reported relaxation time data for liver and pancreas. MR studies of excised tissues must account for time and temperature to prevent systematic experimental errors.