title: Spatial specificity in spatiotemporal encoding and Fourier imaging.
abstract: OBJECTIVE
  Ultrafast 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.
  
  
  BACKGROUND
  Spatial 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.
  
  
  METHODS
  Bloch simulations and experiments were performed to validate theoretical derivations.
  
  
  RESULTS
  The 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.
  
  
  CONCLUSIONS
  The 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.
authors.affiliation: CMRR and Radiology, University of Minnesota, Minneapolis, MN, USA. Electronic address: goerk006@umn.edu.
authors.forename: Ute
authors.initials: U
authors.lastname: Goerke
mesh_terms: D003198:Computer Simulation; D017352:Echo-Planar Imaging / Q000379:methods*; D005583:Fourier Analysis; D008279:Magnetic Resonance Imaging / Q000379:methods*; D019047:Phantoms, Imaging
keywords: Frequency-swept pulse; Magnetic resonance imaging (MRI); Point-spread-function; RASER; Spatiotemporal encoding; Ultrahigh magnetic field