Note that this tendon is perfectly normal; this increased signal is purely an artifact.
Ho, ho, ho it's magic....
Some of you may be old enough remember that lyric from the 1974 song "Magic", by the Scottish band Pilot.
A busy musculoskeletal radiologist could sing this lyric several times a day. Not talkin' about love, son, rather the magic angle effect.
The magic angle effect is a well-know phenomenon that results in artifactual increased signal in structures with ordered collagen, such as tendons, fibrocartilage, and hyaline cartilage.
Physicists tell us that this is because when collagen is oriented at 55 degrees to the main magnetic field of the magnet, dipole-dipole interactions go to zero, resulting in a prolongation of T2 relaxation time.
In English, that means that collagen-containing structures can exhibit increased signal when they are at certain angles with respect to the main magnetic field.
A picture is worth a thousand words. Here is a coronal gradient echo image showing magic angle artifact in the extensor pollicis longus (EPL) tendon (yellow arrows), which is oriented at 55 degrees to the main magnetic field:
This phenomenon can be seen in multiple areas of the body. Common areas to see this artifact in high-field scanners include the posterior horn of the lateral meniscus, the patellar tendon, tendons in the ankle, and the glenoid labrum. This can confound accurate interpretation of these areas.
Finally, collagen fibers are oriented in various directions in tendons and cartilage. Thus, although magic angle effects tend to be greatest when the structure is at 55 degrees to the magnetic field, they can be seen at other angles as well.