title: Slice offset frequency and shim adjustment for interactive steady-state free-precession (SSFP) imaging.
abstract: OBJECTIVE
  To devise a method allowing real-time optimization of center frequency (CF) and shim for an interactive steady-state free-precession (SSFP) sequence by reformatting a previously acquired field map in the same orientation as the interactive acquisition.
  
  
  METHODS
  Field maps were acquired in a rectangular parallel-piped phantom and a normal volunteer. An SSFP sequence was modified to communicate the current slice offset and rotation to an external program that reformatted the field map into the same plane, calculated the CF and shim offsets, and passed them back to the sequence. CF offsets as a function of position for the phantom were compared with the scanner prescan-determined offset.
  
  
  RESULTS
  In the phantom, the CF measurements agreed with the scanner-determined offsets. Bland-Altman analysis showed a bias of -14 Hz (field map - prescan) and limits of agreement of -28 to 0 Hz. In the volunteer there was a qualitative improvement in image quality when using the optimized center frequencies and shims.
  
  
  CONCLUSIONS
  The proposed method demonstrates how CF and shim can be optimized for any interactively positioned slice, resulting in reduced off-resonance artifacts.
authors.affiliation: Department of Radiology, Cambridge University Hospitals NHS Foundation Trust, Cambridge, UK. mjg40@radiol.cam.ac.uk
authors.forename: Martin J
authors.initials: MJ
authors.lastname: Graves
authors.forename: Pauline
authors.initials: P
authors.lastname: Wong
authors.forename: Andrew N
authors.initials: AN
authors.lastname: Priest
authors.forename: Richard T
authors.initials: RT
authors.lastname: Black
authors.forename: David J
authors.initials: DJ
authors.lastname: Lomas
mesh_terms: D000465:Algorithms; D016477:Artifacts; D006801:Humans; D007089:Image Enhancement / Q000379:methods; D007090:Image Interpretation, Computer-Assisted / Q000379:methods*; D008279:Magnetic Resonance Imaging / Q000379:methods*; D019047:Phantoms, Imaging; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity