Projection presaturation: A fast and accurate technique for multidimensional spatial localization

Abstract In recent years, several techniques of spatial presaturation have appeared in the literature for applications in in vivo volume-localized spectroscopy and imaging. All existing techniques of spatial localization by outer volume suppression utilize large-tip-angle RF saturation pulses with linear static-field gradients, applied in orthogonal directions. They are generally sensitive to errors in the tip angle and imperfections in the slice-selection profile, which cause inaccurate localization of the region of interest (ROI ). A fast spatial presaturation technique for multidimensional outer volume suppression has been developed, which uses a series of short, selective, small-tip-angle RF pulses during a single half cycle of a rotating gradient. Each individual RF pulse excites a one-dimensional region outside the projection of the ROI onto the instantaneous direction of the rotating gradient. Multiple excitation during the rotating gradient, combined with continuous dephasing of the transverse magnetization by the rotating gradient, then leads to spatial presaturation of a two-dimensional outer region, while the magnetization within the ROI remains virtually unaffected ( ∼ 1 % signal loss). The magnetization within the ROI is then interrogated with an excitation pulse, which can be made selective in the direction of the axis of rotation to acquire data from a three-dimensionally localized volume. This method virtually eliminates the tip-angle errors and greatly reduces the imperfections in the slice-selection profile, common to existing presaturation techniques, leading to more accurate spatial localization. It can be easily implemented on any existing MR scanner in conjunction with conventional imaging or spectroscopic techniques.