To evaluate the ultrasound (US) appearance of the normal Achilles tendon at increasing frequency and establish an anatomical correlation for US findings, 30 normal tendons were examined in vivo and three in vitro with 10 MHz and 15 MHz mechanical sector probes. Side-by-side comparison was performed in vitro between the sonograms and the corresponding anatomical sections. Two tendinous portions were detected by presence of an internal acoustic interface which had different appearances: one (type I) or two (type II) continuous lines of increased thickness and greater reflectivity than adjacent fibrils; or displacement (type III) of the distal portion of the well insonated sector of the tendon body. When, on coronal scans of the tendon, no intratendinous linear echoes of increased reflectivity were visible, the two portions of the tendon were identified through the converging course of their bundles (type 0 pattern). Different echogenicity allowed the detection of two tendinous portions, also on axial images. Scanning of isolated tendons allowed precise location of these interfaces at the boundary between anatomically distinct tendinous portions arising from the soleus and gastrocnemius muscles. Although the normal Achilles tendon is commonly regarded as a uniform structure by US, the use of high resolution probes allows identification of its constituent portions. Their identification may be useful to avoid misdiagnoses of pathological findings
