The magic angle effect in musculoskeletal MR imaging.

The "magic angle" effect in magnetic resonance (MR) imaging is caused by changes in the dipolar interactions between water hydrogen protons that are loosely bound along collagen fibrils in organized tissue such as tendon or articular cartilage. When tendons are aligned at 55 degrees to the main magnetic field, the T2 relaxation time is lengthened, causing focal increased signal on short echo time MR images. Tendons in the ankle, wrist, and rotator cuff of the shoulder are common sites to observe this effect. Distinguishing magic angle effect from pathologic signal abnormalities due to degeneration or partial tears requires close comparison between T1- and T2-weighted images, as well as evaluation for secondary signs of injury such as tendon thickening or associated fluid.