Single Voxel Spectroscopy in Human Brain
PROBE is the standard GE sequence for single voxel spectroscopy. It is a basic PRESS sequence, selecting the voxel with three slice selective pulses. Each pulse defines the voxel thickness in one of three orthogonal directions. The default voxel size is 2 cm cubed but successful scans have been made with 7.5x7.5x8 mm in 5 minutes. PROBE has the option to use Very Sharp Saturation (VSS) pulses to allow a sharp cutoff of the selected voxel. This is an advantage if spectra are from a voxel right next to a lipid area.
The echo time can be set to a minimum of 30 ms. However, these short echo time spectra need advanced data processing to account for the variable baseline (background) from macromolecules and other metabolites. At the shorter echo time many more metabolites are visible and some of them can be quantified. The most popular of these is Myo-Inositol since it is relatively strong. Processing of the spectra is done with the program LCModel. This program makes a best fit to the spectrum using reference signals. These reference signals are made using the same sequence on a series of model solutions, one for each metabolite in the spectrum. All these signals summed together result in the fit (red line) of the sample spectrum shown in Fig. 2.
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