title: Limitations of using logarithmic transformation and linear fitting to estimate relaxation rates in iron-loaded liver.
abstract: BACKGROUND
  MRI is being increasingly used to evaluate tissue relaxation in the setting of iron overload. Diagnostic accuracy is strongly dependent upon the acquisition and analysis methods employed. Typically, a multi-echo train of relaxation data is acquired, the resulting curve is fit using a non-linear (exponential) function, and the derived relaxation time is converted to iron concentration by a calibration formula derived from paired MRI-biopsy samples. A theoretically valid processing alternative is to fit a straight line to the relaxation data after logarithmic transformation (log-linear). This log-linear method is more computationally efficient, allowing a full relaxation map to be generated in near real time. This method is present on all scanner platforms and has been published for use in assessing iron concentration. These factors imply methodological validity.
  
  
  OBJECTIVE
  To use in vivo and simulation data to show that log-linear fitting can generate highly erroneous relaxation results in iron-loaded tissues.
  
  
  METHODS
  After IRB approval, exponential and linear fitting were compared in a cohort of 20 patients being evaluated for hepatic iron overload. Simulation analyses were performed to characterize the main factors impacting derived results.
  
  
  RESULTS
  In human subjects, log-linear analyses demonstrated gross deviation from exponential results at a moderate relaxation shortening (T2* ~5 ms). Simulation analyses demonstrated that the discrepancy was caused by noise effects and additional signal components violating mono-exponential function shape.
  
  
  CONCLUSIONS
  Log-linear processing results in increasingly erroneous estimation of T2* with iron-loading. Therefore, this method should not be employed for measurement of relaxation behavior in clinical samples.
authors: Department of Radiology, Seattle Children's Hospital, 4800 Sandpoint Way, Room R4488, Seattle, WA 98105, USA.
authors: Randolph
authors: R
authors: Otto
authors: Mark R
authors: MR
authors: Ferguson
authors: Kenneth
authors: K
authors: Marro
authors: John W
authors: JW
authors: Grinstead
authors: Seth D
authors: SD
authors: Friedman
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