Chemical shift imaging with phase-encoding RF pulses

An inherent problem of conventional chemical shift imaging is signal contamination into adjacent voxels. This is especially severe in proton spectroscopy of the central nervous system, where the lipid signal from the skull is several orders of magnitude higher than the metabolite resonances from the brain. The reason for the contamination lies in the fact that the distribution of spins does not coincide with the discrete sampling matrix of the discrete Fourier transformation if a continuous phase-encoding scheme using magnetic field gradients is being used. This problem can be overcome if phase encoding is performed by appropriately phase-modulated RF pulses. It is demonstrated that contamination free spectra can be acquired from voxels immediately adjacent to the skull. Using small voxels of 1 x 1 x 2-ml regional variations of N-acetyl aspartate in the cerebellum of patients with oligopontine cerebellar atrophy could be observed.