Reduced field-of-view MRI with two-dimensional spatially-selective RF excitation and UNFOLD.

When the region of interest (ROI) is smaller than the object, one can increase MRI speed by reducing the imaging field of view (FOV). However, when such an approach is used, features outside the reduced FOV will alias into the reduced-FOV image along the phase-encoding direction. Reduced-FOV methods are designed to correct this aliasing problem. In the present study, we propose a combination of two different approaches to reduce the acquired FOV: 1) two-dimensional (2D) spatially-selective RF excitation, and 2) the unaliasing by Fourier-encoding the overlaps using the temporal dimension (UNFOLD) technique. While 2D spatially-selective RF excitation can restrict the spins excited within a reduced FOV, the UNFOLD technique can help to eliminate any residual aliased signals and thus relaxes the requirement for a long RF excitation pulse. This hybrid method was implemented for MR-based temperature mapping, and resulted in artifact-free images with a fourfold improvement in temporal resolution.