title: Investigating the dependence of the calibration parameter M on echo time.
abstract: OBJECTIVE
  The calculation of the calibration parameter M, which represents the maximum theoretically possible blood oxygen level dependent (BOLD) signal increase, is an essential intermediate step in any calibrated fMRI experiment. To better compare M values obtained across different studies, it is common to scale M values from their original BOLD echo time (TE) to a different echo time according to the theory that M is directly proportional to TE. To the best of our knowledge, this relationship has never been directly tested.
  
  
  METHODS
  A pseudocontinuous arterial spin labeling sequence with five readouts (TE ranging from 20 to 78 ms) was implemented to test the relationship between M and TE, both with and without the application of flow crushing gradients.
  
  
  RESULTS
  Both M and the BOLD signal were found to be linear functions of TE, but with a nonzero intercept. This intercept was reduced when crusher gradients were added, suggesting that the deviation from theory is a result of nonnegligible intravascular signal.
  
  
  CONCLUSIONS
  The linear scaling method introduces some error when comparing M values acquired with different BOLD echo times. However, this error is small compared with other considerations, and would generally not preclude the continued use of this scaling method.
authors.affiliation: FMRIB Centre, University of Oxford, Oxford, United Kingdom.
authors.forename: Hannah V
authors.initials: HV
authors.lastname: Hare
authors.affiliation: FMRIB Centre, University of Oxford, Oxford, United Kingdom.
authors.forename: Daniel P
authors.initials: DP
authors.lastname: Bulte
mesh_terms: D001921:Brain / Q000378:metabolism*; D002138:Calibration; D002560:Cerebrovascular Circulation / Q000502:physiology*; D005260:Female; D064368:Healthy Volunteers; D006801:Humans; D006935:Hypercapnia; D007091:Image Processing, Computer-Assisted / Q000379:methods; D008279:Magnetic Resonance Imaging / Q000295:instrumentation / Q000379:methods*; D008297:Male; D010100:Oxygen / Q000097:blood; D013113:Spin Labels; D055815:Young Adult
chemical_list: D013113:Spin Labels; D010100:Oxygen
keywords: BOLD; MRI; calibration; echo time; hypercapnia