Projection Presaturation. III. Accurate Selective Excitation or Presaturation of the Regions of Tailored Shape in the Presence of Short-T1 Species

A new method is described which further increases the accuracy of localization by projection presaturation (PP), a technique for multidimensional spatial localization of contoured regions of interest (ROIs), whose shape, size, position, and number can be chosen arbitrarily (J. Magn. Reson.90, 313, 1990). By applying a nonselective radiofrequency inversion pulse after the PP saturation cycle and nulling the residual longitudinal magnetization of the region outside the ROI, the outer-volume signal is suppressed to the "noise" level, thus improving the accuracy of the basic PP localization, even when saturating a large exterior volume of short-T₁ species. The ability of the method to perform spatial and spectral localization simultaneously is also described. Alternatively, the single-shot methods for selective saturation (instead of selective excitation) of single or multiple contoured regions at arbitrary positions in an extended volume are presented. In particular, the results of selective excitation and saturation of a region of conformal geometric shape in two and three dimensions in a phantom as well as in vivo (healthy volunteers) are presented to demonstrate the efficacy of the method. Applications of these methods include flow- and motion-artifact suppression in body MRI, accurate conformal localization for in vivo spectroscopy in the presence of chemical shift, outer-volume suppression for resolution or speed enhancement in ultrafast imaging (Magn. Reson. Med.13, 77, 1990; J. Phys. C10, L55, 1977), and spin tagging by selective excitation/saturation of flowing spins for flow studies and angiography.