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Abstract

Purpose/Introduction: Magnetic resonance spectroscopic imaging (MRSI) enables the acquisition of quantitative metabolite distribution maps. However, the combined effect of B1-inhomogeneity across the volume of interest and slice profiles of the selective RF pulses used for localization create a localization profile that induces spatially varying metabolite intensities independent of physiological changes. This effect needs to be corrected when an external reference is used for quantification. Hence, in this work, a localization profile correction method for ERETIC [1] based 1H MRSI quantification is presented and compared with internal water referencing [2]. Subjects and Methods: PRESS localized 2D-MRSI data were obtained in vitro in a homogenous phantom and in vivo in a healthy volunteer. With ERETIC, the areas of the metabolite resonances are converted into concentrations by comparing them to an externally generated signal, that is inductively coupled into the receive coil. To correct for the localization profile, the metabolite/ERETIC ratios have to be divided by the expected values for the observable magnetization in every voxel. In this work these values are calculated by performing a Bloch simulation of the PRESS sequence with the relevant pulse and gradient waveforms. The actual flip angles were taken from measured AFI maps [3], reception sensitivity was considered following the approach by Wang [4]. Results: Fig.1: Obtained in vitro water/ERETIC ratios over the whole VOI (blue rectangle) with no correction (B) and with localization profile correction (C). Differences of the scaled Cho/water and corrected Cho/ERETIC ratios are plotted in (D). Discussion/Conclusion: Any quantification method using an external reference can only be reliable for MRSI, if an appropriate localization profile correction is applied. As demonstrated by a homogeneous phantom metabolite/ERETIC map the localization profile correction method shown here compensate the metabolite intensity variations. In vivo Cr/water maps and Cr/ERETIC maps with localization profile correction are highly similar and reflect the expected higher Cr concentration in the grey matter along the interhemispheric fissure.