Magic angle effect (MRI artifact)

The magic angle is an MRI artifact that occurs in sequences with a short TE (less than 32 ms) - T1 weighted, proton density weighted, and gradient echo sequences.

It is confined to regions of tightly bound collagen at 54.74° from the main magnetic field (B₀), and appears hyperintense, thus potentially being mistaken for tendinopathy.

Normal

In tightly-bound collagen, water molecules are restricted usually causing very short T2 times, accounting for the lack of signal.

Artifact

When molecules lie at 54.74°, there is lengthening of T2 times with corresponding increase in signal. Thus in short TE sequences, the T2 signal does not decay significantly before the scanner picks up the signal. On the other hand, in long TE sequences (like T2 weighted sequences), by the time the scanner picks up the signal, the T2 signal has already decayed. 

The reason for this change is due to quantum mechanics: in the set of equations that describe the interaction of spins (their Hamiltonian), there are several terms that are orientation-dependent. Normally, these orientations are averaged over as protons tumble around thermally, but in sites with long-range order, these terms can be important. In the case of structured collagen, lots of water binds to the outside of the protein, and therefore exhibits an orientation-dependent effect.

Typical sites include:

• proximal part of the posterior cruciate ligament (PCL)

• infrapatellar tendon at the tibial insertion

• peroneal tendons as they hook around the lateral malleolus

• cartilage, e.g. femoral condyles

• supraspinatus tendon

• triangular fibrocartilage complex (if the patient is imaged with the arm elevated)

It appears that the effects are reduced in a 3 T MRI system compared to a 1.5 T system.

Other non-pathological causes of high signal within tendons include near tendon insertions and/or where the tendon normally fans out or merges with other tendons.

Remedy

It only occurs in short TE sequences (e.g. T1, PD, GRE). Sequences with a longer TE (e.g. T2) can be used to avoid this artifact.

See also

• anisotropy (ultrasound artifact)