Heterogeneous metabolic changes in the calf muscle of the rat during ischaemia-reperfusion: in vivo analysis by 31P nuclear magnetic resonance chemical shift imaging and 1H magnetic resonance imaging.

Non-localized and localized 31P nuclear magnetic resonance (NMR) spectra of rat calf muscle during arterial occlusion and after reperfusion were concurrently observed using chemical shift imaging and 1H magnetic resonance imaging (MRI). During ischaemia, the levels of high-energy phosphates (phosphocreatine and adenosine 5'-triphosphate) were depleted and that of inorganic phosphate was increased. In addition, the signal intensity on 1H MRI was increased. These changes were observed to be relatively homogeneous throughout the calf. In contrast, the changes during reperfusion were heterogeneous. In the central part of the calf, inorganic phosphate disappeared, phosphocreatine was restored immediately on reperfusion and the 1H signal was decreased. However, in the regions of the tibialis anterior muscle and the superficial part of the gastrocnemius muscle, inorganic phosphate persisted for several hours, phosphocreatine was not restored and the signal intensity on T2-weighted 1H MRI was increased further. The heterogeneous changes detected by 31P NMR spectroscopy and 1H MRI showed close agreement. The susceptibility of different calf muscles to ischaemia and reperfusion seems to depend on their predominant muscle fibre type (i.e. fast-twitch or slow-twitch fibres). Reversible and irreversible ischaemic changes could be non-invasively distinguished by in vivo 31P NMR spectroscopy and 1H MRI.