Real-time motion and B_0 correction for localized adiabatic selective refocusing (LASER) MRSI using echo planar imaging volumetric navigators

摘要

A method is presented to correct the effects of motion and motion-related B_0 perturbations on spectroscopic imaging in real time through the use of a volumetric navigator. It is demonstrated that, for an axial slice, lifting the chin significantly disrupts the B_0 homogeneity in the zero-order (frequency), first-order Y (coronal) axis and second-order ZY term. This volumetric navigator is able to measure and correct in real time both head pose and zero- to first-order B_0 inhomogeneities. The volumetric navigator was validated in six volunteers who deliberately lifted and then dropped their chin during the scan. These scans show that motion correction alone is not sufficient to recover the spectral quality. By applying real-time shim adjustments, spectral quality was fully recovered to linewidths below 0.08ppm and the signal-to-noise ratio to within acceptable limits in five of six subjects. In the sixth subject, 83% of the spectra within the volume of interest were recovered, compared with the worst case nonshim-corrected scan, where none of the voxels fell within these quality bounds. It is shown that the use of a volumetric navigator comes at no additional cost to the scan time or spectral signal-to-noise ratio.