title: A method for unwrapping highly wrapped multi-echo phase images at very high field: UMPIRE.
abstract: OBJECTIVE
  To develop a method of unwrapping phase images from multi-echo scans that works even where there are several wraps between echoes, and which generates unwrapped phase images in addition to phase difference (PD) images.
  
  
  BACKGROUND
  The difference between the echo spacings in an acquisition with three unevenly spaced echoes (an imposed delay) can be selected such that the phase evolution in that time is in the range -π to +π in all voxels of interest. Under this condition, an image of the difference between the phase evolutions in the two inter-echo periods, an estimate of ΔB0 , is free of wraps. This ΔB0 estimate can be used to identify and remove receiver phase offsets and wraps in phase images.
  
  
  METHODS
  The approach was tested on simulated data and high-resolution in vivo brain data acquired from six subjects at 7 Tesla.
  
  
  RESULTS
  The method generated wrap-free phase images. It was able to remove more wraps than is possible with PD imaging and was faster and more reliable than spatial unwrapping.
  
  
  CONCLUSIONS
  Unwrapping Multi-echo Phase Images with iRregular Echo spacings (UMPIRE) is conceptually simple, fast, reliable, and requires no fitting, thresholds, or operator intervention.
authors.affiliation: High Field MR Center of Excellence, Department of Radiology, Medical University of Vienna, Austria.
authors.forename: Simon
authors.initials: S
authors.lastname: Robinson
authors.forename: Horst
authors.initials: H
authors.lastname: Schödl
authors.forename: Siegfried
authors.initials: S
authors.lastname: Trattnig
mesh_terms: D000328:Adult; D016477:Artifacts; D017352:Echo-Planar Imaging / Q000379:methods*; D005260:Female; D006257:Head / Q000033:anatomy & histology*; D006801:Humans; D007089:Image Enhancement / Q000379:methods*; D007091:Image Processing, Computer-Assisted / Q000379:methods; D021621:Imaging, Three-Dimensional / Q000379:methods; D008297:Male
keywords: UMPIRE; high field; multi-echo; phase imaging; phase wraps; unwrapping
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