Some new angles on the magic angle: what MSK radiologists know and don't know about this phenomenon

Magic angle effects (MAE) are well-recognized in musculoskeletal (MSK) MRI. With short TE acquisitions, the signal intensity of tendons, ligaments, and menisci depend on their orientation relative to the main magnetic field (B₀). An interactive resident physics teaching module simulating MR imaging of a tendon forced us to identify and correct several misconceptions we had about MAE. We suspected these misconceptions were shared by other MSK radiologists.

We surveyed members of the Society of Academic Bone Radiologists (SABR) regarding which pulse sequences, acquisition parameters, tissues and angles relative to B₀ were most likely to produce MAE.

Survey respondents knew that MAE strongly depend on TE and commonly appear on T1W, FSE and PD sequences, but were less aware that MAE may also appear on T2W, STIR and DWI sequences. They knew of MAE effects in tendons, ligaments and cartilage, but were less aware of those in entheses, peripheral nerves and intervertebral discs. Respondents underestimated the wide angular range (full-width at half-maximum ≈ 40^∘) over which significant MAE can be seen with short TE.

Collagen-containing tissues with parallel molecular alignment exhibit increased signal intensity when oriented at 55^∘ relative to B₀. Experienced MSK radiologists were found to underestimate the combinations of image parameters, pulse sequences, tissues and collagen orientations in which significant MAE may be seen. Our survey results highlight the need for ongoing MR physics education for practicing radiologists.