NMR spectroscopic measurements of diffusion in heart
A decrease in the apparent diffusion coefficient (ADC) of water is becoming an important tool for the detection of acute and chronic brain disorders, yet it is not known whether changes in myocardial ADCs hold similar potential. Consequently, this work determined whether the ADCs of water or intracellular metabolites could be used to show ischaemia or cell swelling in the isolated rat heart. A modified STEAM pulse sequence was designed to measure the ADCs of the 1H-NMR detectable metabolites, taurine and creatine, with 4 min time resolution in a 3 mm myocardial slice. Experiments included the measurement of: a) metabolite diffusion coefficients and diffusion tensors in solutions, and ADCs and diffusion tensors in the isolated, KCl-arrested rat heart; b) taurine and creatine content during 32 min total, global ischaemia in the isolated rat heart; c) metabolite and water ADCs before, during and after ischaemia; and d) changes in average cardiomyocyte diameter during perfusion and ischaemia using the taurine ADC measurements. At a diffusion time of 50 ms, the myocardial ADCs were 1.06 x 10-3 mm2/s for water, 0.29 x 10-3 mm2/s for taurine and 0.26 x 10"3 mm 2 /s for creatine. Neither taurine nor creatine was lost from the heart during ischaemia, making either suitable for ischaemic diffusion measurements. Contrary to changes in the brain, myocardial water and taurine ADCs remained constant during ischaemia; however, the total creatine ADC increased by 35% which was shown to result from hydrolysis of PCr to creatine. Using the taurine ADC measurements at diffusion times between 50 ms and 1510 ms, the average myocyte diameter was calculated to be 40 μm during perfusion and 27 μm by the end of ischaemia. The decrease in myocyte diameter indicates that the buffer perfused heart is highly oedematous. This is the first time that: 1) metabolite ADCs have been measured in isolated heart, and 2) NMR spectroscopy has been used to determine the myocyte diameter. Thus ADC changes may not have potential for detecting ischaemia in the heart, although the measurement of myocyte diameter using taurine ADCs could indicate myocardial oedema.