Spatial specificity in spatiotemporal encoding and Fourier imaging

Abstract

PurposeUltrafast imaging techniques based on spatiotemporal encoding (SPEN), such as RASER (rapid acquisition with sequential excitation and refocusing), is a promising new class of sequences since they are largely insensitive to magnetic field variations which cause signal loss and geometric distortion in EPI. So far, attempts to theoretically describe the point-spread-function (PSF) for the original SPEN-imaging techniques have yielded limited success. To fill this gap a novel definition for an apparent PSF is proposed. TheorySpatial resolution in SPEN-imaging is determined by the spatial phase dispersion imprinted on the acquired signal by a frequency-swept excitation or refocusing pulse. The resulting signal attenuation increases with larger distance from the vertex of the quadratic phase profile. MethodsBloch simulations and experiments were performed to validate theoretical derivations. ResultsThe apparent PSF quantifies the fractional contribution of magnetization to a voxel's signal as a function of distance to the voxel. In contrast, the conventional PSF represents the signal intensity at various locations. ConclusionThe definition of the conventional PSF fails for SPEN-imaging since only the phase of isochromats, but not the amplitude of the signal varies. The concept of the apparent PSF is shown to be generalizable to conventional Fourier-imaging techniques.