title: Two-dimensional imaging in a lightweight portable MRI scanner without gradient coils.
abstract: OBJECTIVE
  As the premiere modality for brain imaging, MRI could find wider applicability if lightweight, portable systems were available for siting in unconventional locations such as intensive care units, physician offices, surgical suites, ambulances, emergency rooms, sports facilities, or rural healthcare sites.
  
  
  METHODS
  We construct and validate a truly portable (<100 kg) and silent proof-of-concept MRI scanner which replaces conventional gradient encoding with a rotating lightweight cryogen-free, low-field magnet. When rotated about the object, the inhomogeneous field pattern is used as a rotating spatial encoding magnetic field (rSEM) to create generalized projections which encode the iteratively reconstructed two-dimensional (2D) image. Multiple receive channels are used to disambiguate the nonbijective encoding field.
  
  
  RESULTS
  The system is validated with experimental images of 2D test phantoms. Similar to other nonlinear field encoding schemes, the spatial resolution is position dependent with blurring in the center, but is shown to be likely sufficient for many medical applications.
  
  
  CONCLUSIONS
  The presented MRI scanner demonstrates the potential for portability by simultaneously relaxing the magnet homogeneity criteria and eliminating the gradient coil. This new architecture and encoding scheme shows convincing proof of concept images that are expected to be further improved with refinement of the calibration and methodology.
authors.affiliation: A.A. Martinos Center for Biomedical Imaging, Department of Radiology, Massachusetts General Hospital, Charlestown, Massachusetts, USA; Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, Massachusetts, USA.
authors.forename: Clarissa Zimmerman
authors.initials: CZ
authors.lastname: Cooley
authors.forename: Jason P
authors.initials: JP
authors.lastname: Stockmann
authors.forename: Brandon D
authors.initials: BD
authors.lastname: Armstrong
authors.forename: Mathieu
authors.initials: M
authors.lastname: Sarracanie
authors.forename: Michael H
authors.initials: MH
authors.lastname: Lev
authors.forename: Matthew S
authors.initials: MS
authors.lastname: Rosen
authors.forename: Lawrence L
authors.initials: LL
authors.lastname: Wald
mesh_terms: D004867:Equipment Design; D019544:Equipment Failure Analysis; D005240:Feasibility Studies; D007089:Image Enhancement / Q000295:instrumentation*; D008279:Magnetic Resonance Imaging / Q000295:instrumentation*; D008904:Miniaturization; D019047:Phantoms, Imaging; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D014159:Transducers
keywords: Halbach magnet; low-field MRI; nonlinear SEMs; parallel imaging; portable MRI