title: Median nerve cross-sectional area and MRI diffusion characteristics: normative values at the carpal tunnel.
abstract: OBJECTIVE
  Enlargement of the median nerve is an objective potential imaging sign of carpal tunnel syndrome. Diffusion tensor MRI (DTI) may provide additional structural information that may prove useful in characterizing median neuropathy. This study further examines normal values for median nerve cross-sectional area (CSA), apparent diffusion coefficient (ADC), and fractional anisotropy (FA).
  
  
  METHODS
  Twenty-three wrists in 17 healthy volunteers underwent MRI of the wrist at 3 T. In 13 subjects, DTI was performed at a B value of 600 mm(2)/s. Median nerve CSA, ADC, and FA were analyzed at standardized anatomic levels.
  
  
  RESULTS
  Mean (SD) median nerve CSA within the proximal carpal tunnel was 10.0 (3.4) mm(2). The mean (SD) FA of the median nerve was 0.71 (0.06) and 0.70 (0.13) proximal to and within the carpal tunnel, respectively. There was a significant difference between nerve CSA and ADC, but not FA, at the distal forearm and proximal carpal tunnel. Nerve CSA, ADC, and FA did not differ between men and women or between dominant and non-dominant wrists. Nerve CSA at the proximal carpal tunnel was positively correlated with subject age and body mass index.
  
  
  CONCLUSIONS
  Our results suggest a 90% upper confidence limit for normal median nerve CSA of 14.4 mm(2) at the proximal carpal tunnel, higher than normal limits reported by many ultrasound studies. We observed a difference between the CSA and ADC, but not the FA, of the median nerve at the distal forearm and proximal carpal tunnel levels.
authors.affiliation: Clinical Center, Radiology and Imaging Sciences, 10 Center Drive, Bethesda, MD 20892, USA. lyao@cc.nih.gov
authors.forename: Lawrence
authors.initials: L
authors.lastname: Yao
authors.forename: Neville
authors.initials: N
authors.lastname: Gai
mesh_terms: D000328:Adult; D019412:Anatomy, Cross-Sectional / Q000379:methods*; D002348:Carpal Bones / Q000033:anatomy & histology*; D038524:Diffusion Magnetic Resonance Imaging / Q000379:methods*; D005260:Female; D006801:Humans; D008297:Male; D008475:Median Nerve / Q000033:anatomy & histology*; D008875:Middle Aged; D012016:Reference Values; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D014955:Wrist Joint / Q000033:anatomy & histology*; D055815:Young Adult