11 1 General Introduction recognizable tenderness upon Achilles tendon palpation. Despite the challenges inherent in diagnosing Achilles tendinopathy, experts concur that the above-mentioned criteria are reliable.2,3 The use of imaging to diagnose Achilles tendinopathy is a topic of debate.2 In cases where not all clinical diagnostic criteria are present, imaging can play an important role to confirm the diagnosis.2,25 When imaging is used, ultrasound is the preferred modality as it is a cheap and accessible method.25 Conventional X-rays are typically only used to exclude bony abnormalities and MRI may be considered if ultrasound is unavailable, prior to potential surgery or when the findings on ultrasound are not consistent with the clinical picture.25 The Ultrasound Tissue Characterization (UTC) procedure is a reliable technique that allows for accurate depiction of Achilles tendon geometry and structure.26-29 The UTC is a customized tracking and ultrasonographic data-collection device that provides objective, standardized measurements, which in practice are often translated to conventional ultrasound.26 The UTC procedure is carried out by positioning participants prone on an examination table with a maximal dorsiflexion angle of the ankle. A multi-frequency 5-16 MHz linear-array transducer is used, which is placed in a transverse position to the Achilles tendon (Figure 1). The transducer in the UTC tracking and data-collection device moves automatically from proximal to distal over a distance of 12 cm, collecting digital transverse images at regular intervals of 0.2 mm which result in a three-dimensional data block (Figure 2). Figure 2. The Ultrasound Tissue Characterization procedure. The typical appearance of the Achilles tendon on ultrasound is a pattern of parallel fibrillar lines in the longitudinal plane and a round-to-oval shape in the transverse plane.25 The
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