title: Comparison of R2' measurement methods in the normal brain at 3 Tesla.
abstract: OBJECTIVE
  R2', the reversible component of transverse relaxation, is an important susceptibility measurement for studies of brain physiology and pathologies. In existing literature, different R2' measurement methods are used with assumption of equivalency. This study explores the choice of measurement method in healthy, young subjects at 3T.
  
  
  METHODS
  In this study, a modified gradient-echo sampling of free induction decay and echo (GESFIDE) sequence was used to compare four standard R2' measurement methods: asymmetric spin echo (ASE), standard GESFIDE, gradient echo sampling of the spin echo (GESSE), and separate R2 and R2* mapping.
  
  
  RESULTS
  GESSE returned lower R2' measurements than other methods (P < 0.05). Intersubject mean R2' in gray matter was found to be 2.7 s(-1) using standard GESFIDE and GESSE, versus 3.4-3.8 s(-1) using other methods. In white matter, mean R2' from GESSE was 2.3 s(-1) while other methods produced 3.7-4.3 s(-1) . R2 correction was applied to partially reduce the discrepancies between the methods, but significant differences remained, likely due to violation of the fundamental assumption of a single-compartmental tissue model, and hence monoexponential decay.
  
  
  CONCLUSIONS
  R2' measurements are influenced significantly by the choice of method. Awareness of this issue is important when designing and interpreting studies that involve R2' measurements.
authors.affiliation: Department of Radiology, Stanford University, Stanford, California, USA; Department of Electrical Engineering, Stanford University, Stanford, California, USA.
authors.forename: Wendy
authors.initials: W
authors.lastname: Ni
authors.forename: Thomas
authors.initials: T
authors.lastname: Christen
authors.forename: Zungho
authors.initials: Z
authors.lastname: Zun
authors.forename: Greg
authors.initials: G
authors.lastname: Zaharchuk
mesh_terms: D000328:Adult; D000465:Algorithms; D001921:Brain / Q000033:anatomy & histology* / Q000502:physiology*; D017097:Electric Impedance; D005260:Female; D006801:Humans; D007090:Image Interpretation, Computer-Assisted / Q000379:methods*; D008279:Magnetic Resonance Imaging / Q000379:methods*; D008297:Male; D012016:Reference Values; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity
keywords: GESFIDE; GESSE; R2′; susceptibility; transverse relaxation
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