Magic angle effect in magnetic resonance imaging of the Achilles tendon and enthesis.

Collagen fibers in tendons and entheses are highly ordered. The protons within the bound water are subject to dipolar interactions whose strength depends on the orientation of the fibers to the static magnetic field B(0). Clinical pulse sequences have been employed to investigate this magic angle effect of the Achilles tendon, but only limited to imaging appearance with a signal void at many angular orientations due to its short T2. Here we investigated the magic angle effect of the Achilles tendons and entheses on a clinical 3-T scanner using clinical sequences as well as an ultrashort TE sequence with a minimal TE of 8 micros. Qualitative and quantitative investigation of the angular-dependent imaging appearance, T1 and T2* values were performed on five ankle specimens. There was a significant increase in signal intensity for all pulse sequences near the magic angle. Mean T2* for tendon increased from 1.94+/-0.28 ms at 0 degrees relative to the B(0) field to 15.25+/-2.13 ms at 55 degrees, and mean T1 increased from 598+/-37 ms at 0 degrees to 621+/-44 ms at 55 degrees. There was less magic angle effect for enthesis whose mean T2* increased from 4.12+/-0.37 ms at 0 degrees to 12.46+/-1.78 ms at 55 degrees, and mean T1 increased from 685+/-41 ms at 0 degrees to 718+/-56 ms at 55 degrees.