title: Fully refocused multi-shot spatiotemporally encoded MRI: robust imaging in the presence of metallic implants.
abstract: OBJECTIVE
  An approach has been recently introduced for acquiring arbitrary 2D NMR spectra or images in a single scan, based on the use of frequency-swept RF pulses for the sequential excitation and acquisition of the spins response. This spatiotemporal-encoding (SPEN) approach enables a unique, voxel-by-voxel refocusing of all frequency shifts in the sample, for all instants throughout the data acquisition. The present study investigates the use of this full-refocusing aspect of SPEN-based imaging in the multi-shot MRI of objects, subject to sizable field inhomogeneities that complicate conventional imaging approaches.
  
  
  METHODS
  2D MRI experiments were performed at 7 T on phantoms and on mice in vivo, focusing on imaging in proximity to metallic objects. Fully refocused SPEN-based spin echo imaging sequences were implemented, using both Cartesian and back-projection trajectories, and compared with k-space encoded spin echo imaging schemes collected on identical samples under equal bandwidths and acquisition timing conditions.
  
  
  RESULTS
  In all cases assayed, the fully refocused spatiotemporally encoded experiments evidenced a ca. 50 % reduction in signal dephasing in the proximity of the metal, as compared to analogous results stemming from the k-space encoded spin echo counterparts.
  
  
  CONCLUSIONS
  The results in this study suggest that SPEN-based acquisition schemes carry the potential to overcome strong field inhomogeneities, of the kind that currently preclude high-field, high-resolution tissue characterizations in the neighborhood of metallic implants.
authors: Chemical Physics Department, Weizmann Institute of Science, 76100 Rehovot, Israel.
authors: Noam
authors: N
authors: Ben-Eliezer
authors: Eddy
authors: E
authors: Solomon
authors: Elad
authors: E
authors: Harel
authors: Nava
authors: N
authors: Nevo
authors: Lucio
authors: L
authors: Frydman
mesh_terms: D000465:Algorithms; D000818:Animals; D002138:Calibration; D017352:Echo-Planar Imaging / Q000379:methods; D007091:Image Processing, Computer-Assisted / Q000379:methods; D008279:Magnetic Resonance Imaging / Q000379:methods*; D009682:Magnetic Resonance Spectroscopy / Q000379:methods*; D008670:Metals / Q000737:chemistry*; D051379:Mice; D016513:Mice, SCID; D015233:Models, Statistical; D019047:Phantoms, Imaging; D019736:Prostheses and Implants; D015203:Reproducibility of Results; D012984:Software
chemical_list: D008670:Metals