title: Ultrasound and magnetic resonance imaging are not interchangeable to assess the Achilles tendon cross-sectional-area.
abstract: OBJECTIVE
  The major aim of this study was to compare ultrasound (US) and magnetic resonance imaging (MRI) measurements of the Achilles tendon cross-sectional area (CSA). Further aims were to conduct reliability analyses and to assess the influence of transducer pressure on the tendon properties in US measurements.
  
  
  METHODS
  The Achilles tendon CSA of 15 participants was assessed at two positions with US and MRI by use of a standardized protocol. Method comparison was performed by two-way analysis of variance (ANOVA) and paired t test. Reliability was assessed by coefficients of variation (CV), intraclass correlation (ICC2,2), standard error of measurement (SEM), and minimal detectable change (MDC95). A paired t test was performed to investigate the effect of probe pressure on tendon CSA and thickness.
  
  
  RESULTS
  Mean US measurements provided a ~5.5% smaller CSA compared to MRI measurements. Intra-rater reliability analyses of US demonstrated CV values of 1.5-4.9%, ICC of 0.89-0.97, SEM and MDC95values of 0.22-0.77 mm2and 0.61-2.16 mm2for both raters, whereby CV values for intra-rater reliability of MRI ranged from 1.0 to 3.7%. Inter-rater reliability was lower for both modalities. Pressure applied on the transducer altered Achilles tendon CSA and thickness significantly (p < 0.05).
  
  
  CONCLUSIONS
  Our findings show that US and MRI cannot be used interchangeably for Achilles tendon CSA assessments, however, each imaging modality separately is reliable to assess this property. Pressure applied on the transducer during US measurements causes alterations of the tendon's morphology and should be avoided.
authors.affiliation: Institute of Sports Science, University of Graz, Mozartgasse 14, 8010, Graz, Austria.
authors.forename: Annika
authors.initials: A
authors.lastname: Kruse
authors.affiliation: Centre of Sport Science, University of Vienna, Auf der Schmelz 6a, 1150, Vienna, Austria. savvas.stafylidis@univie.ac.at.
authors.forename: Savvas
authors.identifier: http://orcid.org/0000-0003-3960-1551
authors.initials: S
authors.lastname: Stafilidis
authors.affiliation: Institute of Sports Science, University of Graz, Mozartgasse 14, 8010, Graz, Austria.
authors.forename: Markus
authors.initials: M
authors.lastname: Tilp
mesh_terms: D000125:Achilles Tendon / Q000000981:diagnostic imaging*; D000328:Adult; D005260:Female; D006801:Humans; D008279:Magnetic Resonance Imaging; D008297:Male; D012680:Sensitivity and Specificity; D014463:Ultrasonography
keywords: Achilles tendon; Cross-sectional area; Magnetic resonance imaging; Reproducibility; Ultrasonography