One of the best things about MSK MRI is even though you need to have a good understanding of the radiological anatomy, the response to injury is similar if not the same in most joints. So often you can learn the response in one area and apply it to every other joint. How do tendons respond to injury? Its pretty limited. The options are to gather some fluid, become small, big, bright, really bright or just disappear.
Normal tendons are uniformly black on T1/T2/PD scans ( Medial Ankle Tendons). A minor amount of fluid in the tendon sheath is normal.
Tenosynovitis: Fluid in the tendon sheath. A minor amount of fluid in the tendon sheath such as in the normal example at the beginning, is ok. However this amount of fluid ( yellow arrow) indicates inflammatory changes of the tendon sheath. The tendon itself may be normal. In the example above, the Tibialis Posterior tendon ( anterior most tendon) is mildly enlarged ( compare to Normal example).
Tendinosis: Increase in Tendon size and or Increase in Tendon Signal. Significantly increase in size of the Tibialis Posterior tendon ( yellow arrow). There is also mild increased signal in the tendon but the signal is not the same as fluid signal, indicating tendinosis rather than a tear. Note significant tenosynovitis.
Partial Tear: Split Tendon. The tibialis posterior tendon is split in two ( double yellow arrows). Single tendon before and after the split. Mild increased signal before and after split indicates tendinosis.
Partial Tear: Fluid signal within the tendon indicates a tear of the tendon fibres. Once something is torn there cant be an empty space, something has to fill it, and it gets filled initially with fluid. Note that the signal intensity is much higher than tendinosis.
Complete Tear: There is complete loss of tendon fibres and the area is filled with fluid. In the shoulder above the yellow arrow indicates the tear filled with fluid. The orange arrow indicates tendinosis which is of a less bright signal. Complete rupture of the biceps tendon with an empty tendon sheath ( yellow arrow image on right).
Paratenonitis: For tendons such as the Achilles that dont have a tendon sheath, inflammatory changes spread into the adjacent fat ( Yellow arrow high signal in pre Achilles fat).
Thats it. Pretty simple and repetitive pattern which can be applied to any tendon.