Image findings

Functional radiography could successfully reveal the kinetics of the Achilles tendon during ankle dorsiflexion. Sequential images demonstrated the condition of the Achilles tendon under the spontaneous ankle movement, which could not be provided by MRI approach. The large FOV allowed us to evaluate Achilles tendon related to the flexion angle of ankle (Figure 8). In addition to it, there was the finding of calcified insertion of the Achilles tendon to the upper calcaneus, which was not observed in MRI and was more clearly visualized than those in US images.

A normal Kager’s triangle demonstrates high signal intensity on MRI in T1WI, whereas the normal Achilles tendon demonstrates low signal intensity. There is a solitary vertical line of high signal intensity in the midsubstance of the tendon, which probably represents the site where the soleus and gastrocnemius tendons area apposed to one another, or else it represents avascular channel (Figure 9).

On US images, the normal Achilles tendon appears echogenic (bright) with an organized fibrillar ultrastructure. The paratenon envelops the tendon as a thin, echogenic tissue layer clearly distinct from the tendon under dynamic imaging (Figure 10).

Quantitative analysis

Our computerized method provided quantitative kinetic information, such as flexion angle and the shape of Kager’s triangle. Figure 11 shows the relationship between the area of Kager’s triangle and flexion angle of the ankle for three subjects provided by MRI and our methods, respectively. In all normal subjects, the area of Kager triangle increased from extension to flexion position, and after reaching the peak, it decreased toward the end of flexion position. The similar curves were observed both in MRI and functional radiography. Figure 12 shows the mapping images of Kager’s triangle. The areas were successfully determined in all subjects. The mapping images of Kager’s triangle were very useful for orthopedic surgeons and physical therapists to evaluate the condition of the Achilles tendon. Ultrasound showed real-time dynamic images, however, any quantitative information could not be provided because of lack of some part of Kager’s triangle.

Although the present method is associated with radiation dose, improved FPD technology realized extremely low dose imaging, less than that of conventional ankle radiography complied with IAEA guidance level. In addition, a recently-developed portable dynamic FPD facilitated easy-to-use approach. Thus, functional radiography is expected to play a significant role in daily clinical situations. Further studies are required to investigate the ability to detect abnormalities in patients with Achilles tendon ruptures.