Physics of Magic Angle Imaging

For Medical Students

What It Shows

Ordered collagen has a signal that depends on the angle its fibres make with the scanner's static field. 5.2.12 covers the basic principles.

The dependence peaks at one particular orientation, where a normal tendon or ligament brightens. 5.2.20 covers the magic angle effect.

That brightness is produced by the geometry of the examination rather than by anything wrong with the tissue, which is why it is grouped with the artefacts. 5.2.19 covers the artefacts.

This chapter takes the same dependence as information. Nothing else in a standard study reports the direction a fibre runs. 5.5.1 covers the standard protocol.

How It Is Acquired

Nothing is acquired for the effect on purpose, so it appears wherever anatomy and positioning put fibres near the critical orientation. 5.5.2 covers the sports knee protocol.

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

It fades on sequences with a long echo time, and that is the test which separates it from disease. 5.2.14 covers T2 imaging.

It is confined to tissue whose collagen is ordered and parallel, so a tendon or a ligament rather than a muscle or marrow. 5.5.10 covers the tendons.

The leg is positioned for access and for the patient's comfort, so the orientation that produced the appearance is set by convenience and written down nowhere. 5.20.1 covers structured reporting.

What Normal Looks Like

A tendon lying close to the field direction is dark along its whole length. 5.5.10 covers the tendons.

The same tendon curving away from that direction brightens over the segment where it curves. 5.5.9 covers the ligaments.

A structure with disordered collagen shows no such change with position. 5.5.11 covers the synovium.

Muscle is unaffected, which is what makes the effect a marker of fibre order rather than of tissue type. 5.5.6 covers the bones.

What Abnormal Looks Like

The appearance that matters is a bright segment in a structure that is normal everywhere else. 5.7.1 covers the normal ACL.

Disease brightens a structure on long echo time sequences as well, so it survives the test that orientation fails. 5.2.14 covers T2 imaging.

An injury usually brings company, such as fluid, marrow change or a torn neighbour, and orientation brings none. 5.13.1 covers bone marrow oedema.

A bright segment that sits at the same place on every scan of every patient is geometric rather than pathological. 5.20.8 covers common reporting errors.

MRCS Level

How It Is Acquired

In a viva the expected answer separates the effect from disease by acquisition rather than by appearance.

The expected points are the short echo time, the long echo time test, the restriction to ordered collagen, the site dependence, and the absence of accompanying findings.

Say that the orientation is set by how the leg was positioned. 5.5.1 covers the standard protocol.

Say that the effect and ultrasound anisotropy are the same class of error from different physics. 5.2.10 covers anisotropy.

Say that the confirming sequence is a change of weighting rather than a second reader. 5.2.14 covers T2 imaging.

How Good It Is

As an error the effect is well described and easy to test. As a measurement it is unusable in the form a routine study provides, and that gap is the subject of this chapter.

Its best feature is that one extra sequence settles the question without more imaging. 5.2.15 covers proton density imaging.

Its limit is that the variable driving it, the angle between fibre and field, is never recorded, so the appearance cannot be compared between studies or between patients. 5.20.1 covers structured reporting.

Positioning the knee deliberately would turn the effect into a measurement, and standard practice positions for access. 5.18.5 covers weight-bearing MRI.

No figures appear for accuracy on this page, under decision 028, and no angle, echo time or field strength appears either: the orientation at which the effect peaks and the timings that show it are published quantities and this environment cannot check them.

FRCS Level

How Good It Is

At decision level the effect's value is negative and large: it prevents operations on normal tendons and ligaments.

A reported partial tear in a structure that functions normally is checked against the long echo time sequence before anything else happens. 5.2.14 covers T2 imaging.

A reported collateral sprain with no history and a stable knee follows the same route. 5.9.2 covers MCL sprains.

The check costs a sequence review and no further imaging, which makes it the cheapest safeguard in the section. 5.1.6 covers imaging pitfalls.

Its positive value, as a probe of collagen order, is not yet available at decision level, and this chapter says so on every page rather than implying otherwise. 5.1.7 covers evidence-based imaging.

Where It Misleads

The commonest failure is believing a short echo time appearance without testing it. 5.2.15 covers proton density imaging.

The reader does not know the effect, so the appearance is taken at face value. 5.1.6 covers imaging pitfalls.

The appearance recurs at the same site on serial studies and stability reads as reality. 5.20.8 covers common reporting errors.

The effect is named as the explanation without the confirming sequence being opened. 5.2.14 covers T2 imaging.

The reverse error is also made: a real injury at an affected site is dismissed as orientation. 5.10.2 covers popliteus injuries.

Fellowship Level

Where It Misleads

At fellowship level this page is about an unrealised measurement rather than an error, because the error is already solved by a sequence any department can run.

The physics has been understood for decades and the clinical use has not followed, which places the obstacle in positioning and documentation rather than in knowledge. 5.19.9 covers the clinical translation.

A study that does not record the knee's position cannot be reanalysed later, however good the later method is, so the information is lost at acquisition rather than at reporting. 5.18.1 covers quantitative MRI.

Ultra short echo time sequences image ordered collagen directly and change what this effect is for rather than removing it. 5.18.4 covers UTE imaging.

The chapter's working rule is that an orientation dependent appearance is uninterpretable unless the orientation is recorded, and every page here returns to it. 5.19.2 covers the collagen architecture.