Quantitative single-voxel spectroscopy: the reciprocity principle for receive-only head coils

To correct MR spectra for local changes in the coil sensitivity for a widely used coil setup, consisting of a transmitting body coil and a receive-only head coil.

The method relies on the reciprocity principle for the body coil and a correction factor for signal amplitudes between body coil and head coil. The correction is based either on the local flip angle dependence of the stimulated echo acquisition mode signal (TFC) or on the automatic RF calibration (RFC). Water phantoms of different volumes were used to simulate variable coil loads, and B1 field inhomogeneities were assessed by varying the voxel position. Furthermore, the correction was tested by longitudinal measurements in one volunteer.

The correction in vitro yields a reduction of the variation coefficient of the water signal by about 77% (TFC) and 66% (RFC) for different coil loads, as well as 55% (TFC) for variable voxel positions. Slightly lower reductions were assessed for the variation coefficients of the metabolite signals in vivo.

This approach adequately compensates for local changes in coil sensitivity, when acquiring MR spectra with a receive-only head coil.