Inter-rater, intra-rater, and inter-machine reliability of quantitative ultrasound measurements of the patellar tendon

The use of ultrasound (US) to perform quantitative measurements of musculoskeletal tissues requires accurate and reliable measurements between investigators and ultrasound machines. The objective of this study was to evaluate inter-rater and intra-rater reliability of patellar tendon measurements between providers with different levels of US experience and inter-machine reliability of US machines. Sixteen subjects without a history of knee pain were evaluated with US examinations of the patellar tendon. Each tendon was scanned independently by two investigators using two different ultrasound machines. Tendon length and cross-sectional area (CSA) were obtained, and examiners were blinded to each other's results. Tendon length was measured using a validated system involving surface markers and calipers, and CSA was measured using each machine's measuring software. Intra-class correlation coefficients (ICCs) were used to determine reliability of measurements between observers, where ICC > 0.75 was considered good and ICC > 0.9 was considered excellent. Inter-rater reliability between sonographers was excellent and revealed an ICC of 0.90 to 0.92 for patellar tendon CSA and an ICC of 0.96 for tendon length. ICC for intra-rater reliability of tendon CSA was also generally excellent, with ICC between 0.87 and 0.96. Inter-machine reliability was excellent, with ICC of 0.91-0.98 for tendon CSA and 0.96-0.98 for tendon length. Bland-Altman plots were constructed to measure validity and demonstrated a mean difference between sonographers of 0.03 mm(2) for CSA measurements and 0.2 mm for tendon length. Using well-defined scanning protocols, a novice and an experienced musculoskeletal sonographer attained high levels of inter-rater agreement, with similarly excellent results for intra-rater and inter-machine reliability. To our knowledge, this study is the first to report inter-machine reliability in the setting of quantitative musculoskeletal ultrasound.