title: Relaxivity-iron calibration in hepatic iron overload: Predictions of a Monte Carlo model.
abstract: OBJECTIVE
  R2* (1/T2*) and single echo R2 (1/T2) have been calibrated to liver iron concentration (LIC) in patients with thalassemia and transfusion-dependent sickle cell disease at 1.5T. The R2*-LIC relationship is linear, whereas that of R2 is curvilinear. However, the increasing popularity of high-field scanners requires generalizing these relationships to higher field strengths. In this study, we tested the hypothesis that numerical simulation can accurately determine the field dependence of iron-mediated transverse relaxation rates.
  
  
  METHODS
  We previously replicated the calibration curves between R2 and R2* and iron at 1.5T using Monte Carlo models incorporating realistic liver structure, iron deposit susceptibility, and proton mobility. In this paper, we extend our model to predict relaxivity-iron calibrations at higher field strengths. Predictions were validated by measuring R2 and R2* at 1.5T and 3T in six β-thalassemia major patients.
  
  
  RESULTS
  Predicted R2* increased twofold at 3T from 1.5T, whereas R2 increased by a factor of 1.47. Patient data exhibited a coefficient of variation of 3.6% and 7.2%, respectively, to the best-fit simulated data. Simulations over the range 0.25T-7T showed R2* increasing linearly with field strength, whereas R2 exhibited a concave-downward relationship.
  
  
  CONCLUSIONS
  A model-based approach predicts alterations in relaxivity-iron calibrations with field strength without repeating imaging studies. The model may generalize to alternative pulse sequences and tissue iron distribution.
authors.affiliation: Physical Sciences Platform, Sunnybrook Research Institute, Toronto, Canada.|Department of Medical Biophysics, University of Toronto, Toronto, Canada.
authors.forename: Nilesh R
authors.initials: NR
authors.lastname: Ghugre
authors.affiliation: Department of Biomedical Engineering, University of Southern California, Los Angeles, California, USA.
authors.forename: Eamon K
authors.initials: EK
authors.lastname: Doyle
authors.affiliation: Department of Radiology, New York University School of Medicine, New York, New York, USA.
authors.forename: Pippa
authors.initials: P
authors.lastname: Storey
authors.affiliation: Division of Cardiology and Radiology, Children's Hospital Los Angeles, Keck School of Medicine, University of Southern California, Los Angeles, California, USA.
authors.forename: John C
authors.initials: JC
authors.lastname: Wood
mesh_terms: D002138:Calibration; D006801:Humans; D007501:Iron / Q000032:analysis*; D019190:Iron Overload / Q000503:physiopathology*; D008099:Liver / Q000737:chemistry* / Q000502:physiology; D008279:Magnetic Resonance Imaging / Q000379:methods* / Q000706:statistics & numerical data; D008954:Models, Biological; D009010:Monte Carlo Method
chemical_list: D007501:Iron
keywords: 3T; Monte Carlo; high field; iron; liver; relaxation; relaxivity