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Zusammenfassung:
High magnetic fields increase the sensitivity and dispersion in MR spectroscopy. Due to stronger susceptibility-induced effects, strategies for optimal shimming are of increasing importance at higher field. The problem of shim requirements which exceeded the active shim capabilities of our system by far was overcome by combined passive and active shimming. Optimal shimming enabled 1H MR spectroscopy in the primary visual cortex V1 of the macaque monkey brain.
Methods: Measurements were done on a 7T/60cm Bruker Biospec system, temporarily equipped with a Siemens AC44 gradient insert. MR setup and anesthesia were described previously [1]. Based on the shim field requirements as determined via FASTMAP field mapping, small pieces of μ-metal sheet were mounted on a plastic tube (length 34cm, diameter 25cm). Accuracy and reproducibility was achieved by a well defined positioning of the passive shims with respect to the magnet iso-center and the monkey head (Fig.1/2). After crude adjustment of all second order shim terms by passive shimming, the remaining shim requirements were within the dynamic range of the active shim device for the regarded brain region. Optimal shimming of all first and second order terms was done with FASTMAP on a 1cm3 cube volume in visual cortex, before the linear shim terms were refined locally for the MR spectroscopy voxel (yellow box, Fig.3)[2].
Results: Depending on the monkey anatomy and head rotation, required shim fields of the two critical second order shims Z2 and X2-Y2 were found to (1000±200) and (600±300) of the available shim fields. By passive shimming it was possible to cancel the major field distortions and to reduce the shim requirements to the dynamic range of the active shims. Passive shimming was reproducible between experiment days and no further modifiactions were needed. No eddy current effects nor heating was observed in any of the MR spectroscopy or imaging experiments. With combined shimming, linewidths of 11.5-12.5Hz for water and 10-11Hz for the Cr+PCr resonance at 3ppm were reproducibly achieved in 5x1.6x5mm3 MR spectroscopy voxels in V1 (Fig.3/4). The spectral separation of creatine and phosphocreatine was possible and 10+ metabolites were quantified.
Discussion: Even strong limitations of the active shim power can be overcome by combined passive and active shimming. This might be of high practical relevance for high field MR setups of limited shim power and/or shimming of body regions which require high shim fields.
