Magic Angle Effect

For Medical Students

What It Shows

A tissue of ordered parallel collagen has a signal that depends on the angle its fibres make with the static magnetic field. 5.2.12 covers the basic principles.

At one particular orientation that signal rises, so a normal tendon or ligament appears abnormally bright. 5.5.10 covers the tendons.

It matters because the appearance is indistinguishable from degeneration or a partial tear on the sequences it affects. 5.2.15 covers proton density imaging.

How It Is Acquired

It appears on sequences with a short echo time, which is where this relaxation mechanism dominates. 5.2.15 covers proton density imaging.

It disappears on sequences with a long echo time, which is the direct test for it. 5.2.14 covers T2 imaging.

It affects only ordered collagen, so a tendon or a ligament rather than a muscle or a marrow. 5.5.9 covers the ligaments.

In the knee it is seen where a structure curves away from the field direction, which is why specific sites recur. 5.5.10 covers the tendons.

It is unrelated to anisotropy on ultrasound, which produces the same class of error from different physics. 5.2.10 covers anisotropy.

Where It Misleads

A normal tendon is reported as tendinopathy or a partial tear. 5.5.10 covers the tendons.

A normal collateral ligament is reported as sprained. 5.9.1 covers the medial collateral anatomy.

The brightness near a cruciate's tibial attachment is read as a tear. 5.7.1 covers the normal cruciate.

A popliteus tendon is affected by its oblique course and reported as abnormal. 5.10.2 covers popliteus injuries.

The appearance is confirmed by a second reader on the same sequence rather than tested on another. 5.2.14 covers T2 imaging.

MRCS Level

How It Is Acquired

In a viva this is the effect to name when asked how to confirm an unexpected bright tendon.

The expected points are the short echo time, the long echo time test, the restriction to ordered collagen, the site dependence, and the ultrasound analogue.

Say that the finding should be checked on a long echo time sequence. 5.2.14 covers T2 imaging.

Say that the effect is an orientation phenomenon rather than a tissue change. 5.2.12 covers the basic principles.

Say that anisotropy on ultrasound is the same class of error. 5.2.10 covers anisotropy.

How Good It Is

This effect is well described, easily tested and still a recurring source of false positive reports.

Its best feature, from a reader's point of view, is that one additional sequence settles it definitively. 5.2.14 covers T2 imaging.

It is limited as a problem to ordered collagen on short echo time sequences, which makes it predictable rather than general. 5.5.9 covers the ligaments.

No figures appear for accuracy on this page, under decision 028, and no angle is stated either: the orientation at which the effect peaks is a published quantity and this environment cannot check it.

Where It Misleads

The commonest failure is reporting from the short echo time sequence without checking the long one. 5.2.15 covers proton density imaging.

The effect is unknown to the reader, so the appearance is taken at face value. 5.1.6 covers imaging pitfalls.

A clinically normal tendon is investigated further on the strength of it. 5.1.4 covers imaging pathways.

The same site is affected on serial studies and the finding appears stable, which reads as real. 5.20.8 covers common reporting errors.

It is confused with anisotropy, which has a different test. 5.2.10 covers anisotropy.

FRCS Level

How Good It Is

At decision level this page prevents an operation on a normal tendon.

A reported partial tendon tear in a knee with normal function and no tenderness should be checked against the long echo time sequence before anything else. 5.2.14 covers T2 imaging.

A reported collateral sprain with no history and a stable knee is the same pattern. 5.9.2 covers medial collateral sprains.

A finding at a known affected site in a structure that curves is a reason for suspicion rather than for action. 5.10.2 covers popliteus injuries.

The check costs one sequence review and no additional imaging, which makes it the highest value habit in this chapter. 5.20.1 covers structured reporting.

Where It Misleads

The commonest error is accepting a tendon or ligament abnormality seen on one weighting. 5.2.15 covers proton density imaging.

The clinical picture is not used to challenge the report. 5.1.2 covers clinical examination versus imaging.

A patient is followed up for a finding that is an artefact. 5.1.4 covers imaging pathways.

A tendon procedure is offered for it. 5.4.20 covers tendon procedures.

The effect is quoted as an explanation without the confirming sequence being looked at. 5.2.14 covers T2 imaging.

Fellowship Level

Where It Misleads

At fellowship level this effect is the cleanest example in the section of a known, testable, preventable error that persists.

Everything needed to avoid it is established and has been for decades, and reports of tendinopathy in normal tendons continue, which locates the problem in practice rather than in knowledge. 5.20.8 covers common reporting errors.

The structures it affects in the knee are the ones ultrasound images best, so the two modalities' characteristic false positives land on the same tendons. 5.2.10 covers anisotropy.

Ultra short echo time sequences image ordered collagen directly and behave differently again, so the effect's relevance will change rather than disappear. 5.18.4 covers ultra short echo time imaging.

The Magic Angle MRI Academy at 5.19 carries the specialty treatment, so this page stays a physics page and the chapter there takes the effect as an unrealised measurement rather than only as an error. 5.19.1 covers the physics of magic angle imaging.

The transferable lesson is the one this chapter keeps returning to: before believing an appearance, ask what the instrument contributed to it. 5.2.12 covers the basic principles.