Fig. 2. Transverse ultrasound images (A, C) of a collateral ligament of the distal interphalangeal joint at the level of the middle phalanx with two different beam orientations and the corresponding MR images at the same level (B, D). On the ultrasound images, the central aspect of the ligament changes in echogenicity on the basis of the ultrasound beam angle. The appearance is represen- tative of bundles of fibers with different orientations. This difference in fiber bundle orientation also results in the changes in signal intensity present on the MR images when the position of the limb is altered. The recognition of this ultrasound appearance was the result of the initial identification of the different fiber bundle orientation on MRI. Therefore, MRI was used to develop a more accurate method for evaluation of ultrasound images of the collateral ligaments of the distal interphalangeal joint at this level. Dorsal is to the left of each image.
Fig. 3. Transverse ultrasound images of the superficial and deep digital flexor tendons at the level of the metacarpus that demonstrate the principle of anisotropy, or the dependency of echogenicity on beam angle. A, Ultrasound beam is perpendicular to the tendons creating diffuse echogenicity within the tendon. B, Ultrasound beam is off-angle or oblique incidence to the tendons. In this image, the echogenicity of the tendons are reduced compared with that in A. However, the tendon margins remain echogenic. Medial is to the left of each image.
