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Abstract

Introduction: The information content of the upfield (0-4.5ppm) and down- field (4.7-8ppm) spectral range in 1 H MRS is complementary [1-3], with specific diagnostic relevance for phenyl ketonuria [4] of the downfield range. Hence it is desirable to acquire metabolic information from the upfield and downfield resonances simultaneously with high regional specificity. In this work, a novel implementation of inner volume saturated (IVS) [5], magnetization transfer compensated SPECIAL [6, 7] at 7T is presented that allows for regional specific, reproducible acquisition and quantification of up- downfield metabolites in the human brain simultaneously. Materials Methods: A 1 H quadrature surface coil (RAPID Biomedical) was used to acquire data from a voxel in the occipital cortex of 4 healthy volunteers on a Philips 7T Achieva Scanner (Philips Medical Systems, Cleveland). Volume based power optimization [8] and second order shimming were performed. SPECIAL with a minimum echo time of 11.8ms and a repetition time of 7000ms was used for localization. Due to the small downfield signals 160 fids were acquired and IVS (Fig 1) was used to obtain an effective voxel size of 21.3x13.2x24.6mm. The outer volume saturation pulses were interleaved with VAPOR water suppression [11]. The SPECIAL inversion pulse was adjusted to prevent artifacts from magnetization transfer and residual NOE effects [7]. Four consistent downfield peaks were fitted in JMRUI [10] while the upfield part was fitted using LCModel [9]. Results Discussion: It could be shown that SPECIAL together with IVS can be used to acquire spatially selective full range spectra with a very small chemical shift displacement. Fig. 2 shows the excellent spectral quality and reproducibility over all volunteers in the up- and down field part of the spectrum. In the downfield 4 major peaks could clearly be identified and fitted. Quantification results in Figure 3a 3b also show good reproducibility even for small downfield peaks.