title: Aqueous urea as a model system for bi-exponential relaxation.
abstract: OBJECTIVE
  To evaluate the utility of aqueous urea, doped inner- and outer-sphere relaxation agents, as an adjustable two-component model system.
  
  
  METHODS
  T2 was measured from 12 molal urea mixtures at pH 7.0 with varying amounts of the MnCl2 and FeO(1.44) (Feridex, Berlex Inc, Montville, NJ).
  
  
  RESULTS
  Bi-exponential relaxation was observed, with rates that were bilinearly related to [MnCl2] and [FeO(1.44)]. FeO(1.44) had comparable relaxivities on both urea and water, while MnCl2 relaxivity was > 15x larger for water than for urea.
  
  
  CONCLUSIONS
  Aqueous urea, doped with inner- and outer-sphere contrast agents, is a two-compartment model system, which can be exhibit a wide range of different T2s and signal fractions.
authors.affiliation: Department of Biomedical Engineering, Vanderbilt University School of Engineering, Nashville, TN 37232-2310, USA.
authors.forename: R Adam
authors.initials: RA
authors.lastname: Horch
authors.forename: Mark D
authors.initials: MD
authors.lastname: Does
mesh_terms: D008279:Magnetic Resonance Imaging; D009682:Magnetic Resonance Spectroscopy; D008956:Models, Chemical; D012996:Solutions; D014508:Urea / Q000737:chemistry*; D014867:Water / Q000737:chemistry
chemical_list: D012996:Solutions; D014867:Water; D014508:Urea