title: Effects of air susceptibility on proton resonance frequency MR thermometry.
abstract: OBJECTIVE
  The temperature dependence of the proton resonance frequency (PRF) is often used in MR thermometry. However, this method is prone to even very small changes in local magnetic field strength. Here, we report on the effects of susceptibility changes of surrounding air on the magnetic field inside an object and their inferred effect on the measured MR temperature.
  
  
  METHODS
  MR phase thermometry was performed on spherical agar phantoms enclosed in cylindrical containers at 7 T. The air susceptibility inside the cylindrical container was changed by both heating the air and changing the gas composition.
  
  
  RESULTS
  Changing the temperature of surrounding air from 23 to 69°C caused an apparent MR temperature error of 2 K. When ambient air was displaced by 100% oxygen, the MR temperature error increased to 40 K. The magnetic field shift and therefore error in inferred MR temperature scales linearly with volume susceptibility change and has a strong and nontrivial dependence on the experimental configuration.
  
  
  CONCLUSIONS
  Air susceptibility changes associated with oxygen concentration changes greatly affect PRF MR thermometry measurements. Air temperature changes can also affect these measurements, but to a smaller degree. For uncalibrated MR thermometry, air susceptibility changes may be a significant source of error.
authors.affiliation: Department of Neurophysics, Max Planck Institute for Human Cognitive and Brain Sciences, Stephanstr. 1a, 04103, Leipzig, Germany. streicher@cbs.mpg.de
authors.forename: Markus N
authors.initials: MN
authors.lastname: Streicher
authors.forename: Andreas
authors.initials: A
authors.lastname: Schäfer
authors.forename: Enrico
authors.initials: E
authors.lastname: Reimer
authors.forename: Bibek
authors.initials: B
authors.lastname: Dhital
authors.forename: Robert
authors.initials: R
authors.lastname: Trampel
authors.forename: Dimo
authors.initials: D
authors.lastname: Ivanov
authors.forename: Robert
authors.initials: R
authors.lastname: Turner
mesh_terms: D000362:Agar / Q000737:chemistry; D000388:Air; D001831:Body Temperature; D002138:Calibration; D004867:Equipment Design; D005740:Gases; D005782:Gels; D006358:Hot Temperature; D006801:Humans; D060526:Magnetic Fields; D009682:Magnetic Resonance Spectroscopy / Q000379:methods*; D010100:Oxygen / Q000737:chemistry; D019047:Phantoms, Imaging; D015203:Reproducibility of Results; D013696:Temperature
chemical_list: D005740:Gases; D005782:Gels; D000362:Agar; D010100:Oxygen