title: Assessment of limb muscle and adipose tissue by dual-energy X-ray absorptiometry using magnetic resonance imaging for comparison.
abstract: OBJECTIVE
  To use magnetic resonance imaging (MRI) to validate estimates of muscle and adipose tissue (AT) in lower limb sections obtained by dual-energy X-ray absorptiometry (DXA) modelling.
  
  
  METHODS
  MRI measurements were used as reference for validating limb muscle and AT estimates obtained by DXA models that assume fat-free soft tissue (FFST) comprised mainly muscle: model A accounted for bone hydration only; model B also applied constants for FFST in bone and skin and fat in muscle and AT; model C was as model B but allowing for variable fat in muscle and AT.
  
  
  METHODS
  Healthy men (n = 8) and women (n = 8), ages 41-62y; mean (s.d.) body mass indices (BMIs) of 28.6 (5.4) kg/m2 and 25.1 (5.4) kg/m2, respectively.
  
  
  METHODS
  MRI scans of the legs and whole body DXA scans were analysed for muscle and AT content of thigh (20 cm) and lower leg (10 cm) sections; 24h creatinine excretion was measured.
  
  
  RESULTS
  Model A overestimated thigh muscle volume (MRI mean, 2.3 l) substantially (bias 0.36 l), whereas model B underestimated it by only 2% (bias 0.045 l). Lower leg muscle (MRI mean, 0.6 l) was better predicted using model A (bias 0.04 l, 7% overestimate) than model B (bias 0.1 l, 17% underestimate). The 95% limits of agreement were high for these models (thigh, +/-20%; lower leg, +/-47%). Model C predictions were more discrepant than those of model B. There was generally less agreement between MRI and all DXA models for AT. Measurement variability was generally less for DXA measurements of FFST (coefficient of variation 0.7-1.8%) and fat (0.8-3.3%) than model B estimates of muscle (0.5-2.6%) and AT (3.3-6.8%), respectively. Despite strong relationships between them, muscle mass was overestimated by creatinine excretion with highly variable predictability.
  
  
  CONCLUSIONS
  This study has shown the value of DXA models for assessment of muscle and AT in leg sections, but suggests the need to re-evaluate some of the assumptions upon which they are based.
authors.affiliation: MRC Dunn Clinical Nutrition Centre, Cambridge, UK.
authors.forename: N J
authors.initials: NJ
authors.lastname: Fuller
authors.forename: C R
authors.initials: CR
authors.lastname: Hardingham
authors.forename: M
authors.initials: M
authors.lastname: Graves
authors.forename: N
authors.initials: N
authors.lastname: Screaton
authors.forename: A K
authors.initials: AK
authors.lastname: Dixon
authors.forename: L C
authors.initials: LC
authors.lastname: Ward
authors.forename: M
authors.initials: M
authors.lastname: Elia
mesh_terms: D015502:Absorptiometry, Photon; D000273:Adipose Tissue / Q000033:anatomy & histology / Q000000981:diagnostic imaging*; D000328:Adult; D015992:Body Mass Index; D005260:Female; D006801:Humans; D007866:Leg / Q000033:anatomy & histology / Q000000981:diagnostic imaging; D008279:Magnetic Resonance Imaging; D008297:Male; D008875:Middle Aged; D018482:Muscle, Skeletal / Q000033:anatomy & histology / Q000000981:diagnostic imaging*; D012016:Reference Values; D012680:Sensitivity and Specificity