title: Correction of susceptibility-induced GRE phase shift for accurate PRFS thermometry proximal to cryoablation iceball.
abstract: BACKGROUND
  The susceptibility contrast between frozen and unfrozen tissue disturbs the local magnetic field in the proximity of the ice-ball during cryotherapy. This effect should be corrected for in real time to allow PRFS-based monitoring of near-zero temperatures during intervention.
  
  
  METHODS
  Susceptibility artifacts were corrected post-processing, using a rapid numerical algorithm. The difference in bulk magnetic susceptibility between frozen and non-frozen tissue was approximated to be uniform over the ice-ball volume and was determined from the isothermal principle applied to the phase-transition frontier of compartments. Subsequently, the magnetic perturbation field was calculated rapidly in 3D using a Fourier-convolution. Experimental studies were performed for two scenarios: tissue defrosting in a water bath and induction of an ice-ball by a MR-compatible cryogenic probe.
  
  
  RESULTS
  The susceptibility artifacts yielded PRFS temperature errors as high as 10-12°C proximal to the ice-ball, positive or negative depending on the relative orientation of the position vector from the B(o) direction. These effects were fully corrected for to within the noise range. The susceptibility-corrected PRFS temperature values were consistent with the phase-transition isothermal condition, irrespective of the local orientation of the position vector.
  
  
  CONCLUSIONS
  By implementing on-line the post processing algorithm, PRFS MRT may be used as a safety tool for non-invasive and accurate monitoring of near-zero temperatures during MR-guided clinical cryotherapy.
authors: Eberhard-Karls-University Tübingen, Tübingen, Badem-Württenberg, Germany. antje.kickhefel@siemens.com
authors: Antje
authors: A
authors: Kickhefel
authors: Clifford
authors: C
authors: Weiss
authors: Joerg
authors: J
authors: Roland
authors: Patrick
authors: P
authors: Gross
authors: Fritz
authors: F
authors: Schick
authors: Rares
authors: R
authors: Salomir
mesh_terms: D000465:Algorithms; D016477:Artifacts; D003452:Cryosurgery; D017679:Cryotherapy / Q000379:methods*; D005583:Fourier Analysis; D005615:Freezing; D006801:Humans; D007091:Image Processing, Computer-Assisted; D021621:Imaging, Three-Dimensional / Q000379:methods; D060526:Magnetic Fields; D008279:Magnetic Resonance Imaging / Q000379:methods; D008280:Magnetics; D015233:Models, Statistical; D008962:Models, Theoretical; D009132:Muscles / Q000473:pathology; D015203:Reproducibility of Results; D013696:Temperature