Normalisation of metabolite images in 1H NMR spectroscopic imaging

Single slice proton magnetic resonance spectroscopic images were taken from a volume of interest (VOI), which was localised using the point resolved spectroscopy (PRESS) sequence. Non-uniformities in the excitation profile of the VOI within the image plane were accounted for using a uniformity correction image. Metabolite images were divided on a voxel-by-voxel basis with a water image obtained from a VOI of the same spatial dimensions from a phantom acquisition. Normalisation procedures carried out on phantom metabolite images were needed to account for the spatial shift in the excitation profile of the VOI, which varies linearly with the chemical shift of the metabolite. Patient metabolite images were then normalised using a VOI water image from a phantom acquisition, which was spatially shifted according to the metabolite chemical shift. In a study of 14 stroke patients, metabolite maps of the distribution of choline, creatine and N-acetyl aspartate (NAA) were constructed. The spectral signal intensities from these metabolites in the infarcted and in the contralateral region were calculated using the raw maps and secondly with the same maps after the B1 normalisation process. In the initial analysis only the metabolite NAA showed a significant decrease, whilst all three metabolites showed a significant reduction in the normalised analysis. These results confirm that normalisation of PRESS localised images does have a significant bearing upon any quantitative measurements made using such images.