title: Fast approximation to pixelwise relaxivity maps: validation in iron overloaded subjects.
abstract: OBJECTIVE
  Liver iron quantification by MRI has become routine. Pixelwise (PW) fitting to the iron-mediated signal decay has some advantages but is slower and more vulnerable to noise than region-based techniques. We present a fast, pseudo-pixelwise mapping (PPWM) algorithm.
  
  
  METHODS
  The PPWM algorithm divides the entire liver into non-contiguous groups of pixels sorted by rapid relative relaxivity estimates. Pixels within each group of like-relaxivity were binned and fit using a Levenberg-Marquadt algorithm.
  
  
  RESULTS
  The developed algorithm worked about 30 times faster than the traditional PW approach and generated R2* maps qualitatively and quantitatively similar. No systematic difference was observed in median R2* values with a coefficient of variability (CoV) of 2.4%. Intra-observer and inter-observer errors were also under 2.5%. Small systematic differences were observed in the right tail of the R2* distribution resulting in slightly lower mean R2* values (CoV of 4.2%) and moderately lower SD of R2* values for the PPWM algorithm. Moreover, the PPWM provided the best accuracy, giving a lower error of R2* estimates.
  
  
  CONCLUSIONS
  The PPWM yielded comparable reproducibility and higher accuracy than the TPWM. The method is suitable for relaxivity maps in other organs and applications.
authors.affiliation: CMR Unit, Fondazione G. Monasterio CNR-Regione Toscana and Institute of Clinical Physiology, Pisa, Italy.
authors.forename: Antonella
authors.initials: A
authors.lastname: Meloni
authors.forename: Heather
authors.initials: H
authors.lastname: Zmyewski
authors.forename: Hugh Young
authors.initials: HY
authors.lastname: Rienhoff
authors.forename: Amber
authors.initials: A
authors.lastname: Jones
authors.forename: Alessia
authors.initials: A
authors.lastname: Pepe
authors.forename: Massimo
authors.initials: M
authors.lastname: Lombardi
authors.forename: John C
authors.initials: JC
authors.lastname: Wood
mesh_terms: D000293:Adolescent; D000328:Adult; D000465:Algorithms; D005260:Female; D006801:Humans; D007091:Image Processing, Computer-Assisted / Q000379:methods; D007501:Iron / Q000737:chemistry*; D019190:Iron Overload / Q000175:diagnosis* / Q000473:pathology; D008099:Liver / Q000378:metabolism / Q000473:pathology*; D008279:Magnetic Resonance Imaging / Q000379:methods*; D008297:Male; D015588:Observer Variation; D015203:Reproducibility of Results; D055815:Young Adult
chemical_list: D007501:Iron
keywords: Fast R2* mapping; Iron overload assessment; Liver