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Magic angle effect (MRI artifact)

At the time the article was created Frank Gaillard had no recorded disclosures.

At the time the article was last revised Raymond Chieng had no financial relationships to ineligible companies to disclose.

• Magic angle artifact
• Magic angle artefact
• Magic angle effect

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.

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Magic angle artefact mri

A homework problem often assigned in college physics courses is to compute the local magnetic field at a distance (r) and angle ( θ ) from from a magnetic dipole source (μ). The full derivation is in a reference below, but here is the result for the z-component:

This equation shows the expected dependence on the strength of the dipole (μ) and inversely with the third power of distance (1/r³). What may be a little surprising is the angular dependence given by the term (3 cos²θ − 1). It is fairly easy to see from the diagram above that the z-component of the dipolar field (B μz ) would be maximally positive directly in front of the dipole and maximally negative immediately to either side. Somewhere between these positive and negative "lobes" B μz must pass through zero. This occurs at angles θ m where (3 cos²θ m − 1) = 0. These θ m values are known as the magic angles . They occur at approximately 54.7°, 125.3° (=180°−54.7°), and so forth around the circle.

When two nuclei interact through a dipole-dipole mechanism, the z-component of the dipolar field from one nucleus alters the local field experienced by the second nucleus. In other words, B μz adds or subtracts to the B o field experienced by the other nucleus, causing it to precess slightly faster or slower than it otherwise would. As a result, the second nucleus gains or loses phase and undergoes T2 relaxation. This effect is particularly strong when molecular motion is slow and the B μz field is relatively static.

Dipolar interaction as a function of angle between spins. In a clinical MRI context this can be viewed as 1/T2 vs tendon orientation relative to B_o.

When the arrangement of the two nuclei approach the magic angle, the effect of B μz is minimized and the T2 value is increased. As shown in the graph (left), this angular dependence changes gradually over a wide range of angles rather than dropping off precipitously at the magic angle. The dipolar interaction does not return to normal at 90° because of the combined effects of the "dips" at 54.7° and 125.3°.

The magic angle effect is important in the clinical MR imaging of certain tissues that are highly structured and are oriented obliquely to the main magnetic field (especially tendons, cartilage, and peripheral nerves). The MR signal may spuriously increase near the magic angle mimicking pathology.

Due to their dense molecular structure and strong static dipolar interactions, tendons possess short intrinsic T2 values (1-10 ms). The signal of tendons is uniformly low on all conventional MR sequences due to this rapid T2 decay. When oriented at the magic angle, however, the T2 of tendons rises to about 20 ms with a corresponding increase in their signal intensity.

Although the magic angle phenomenon affects T2 relaxation (not T1), the increase in signal intensity of tendons will only be appreciated if TE is kept reasonably short. When TE exceeds 40 ms the tendon signal will again be so low that the T2 prolongation will not be recognized. Paradoxically, therefore, the magic angle phenomenon in tendons will be most noticeable on T1- and proton-density-weighted images (using short of medium TE's) than on heavily T2-weighted sequences (with long TE's).

A basic science application of this phenomenon is known as Magic angle spinning (MAS) , a technique used in solid-state MR spectroscopy laboratories. The sample is placed in an air-turbine device angled 54.7° with respect to the main magnetic field and spun at rates up to 70 kHz. The angulation and spinning averages out the dipole-dipole couplings to zero allowing better appreciation of features of the NMR spectrum (such as secondary quadripolar couplings and chemical shift anisotropies) for determining molecular structures.

Advanced Discussion (show/hide)»

Although mostly reported in regard to tendons, the magic angle phenomenon can also be seen in peripheral nerves that contain longitudinally-oriented type I collagen fibers in the epineurium. Nerves, however, possess several imaging differences compared to tendons.

First, the magic angle effects of T2 prolongation in peripheral nerves can be appreciated on STIR or T2-weighted SE sequences (even with TE's longer than 60 ms). Secondly, the change in signal may occur more gradually and over a wider range of orientations in peripheral nerves compared to tendons. This different imaging behavior likely relates to the fact that the peripheral nerves contain substantial material besides collagen and with average baseline T2 values higher than tendons.

Finally, it should be noted that the magic angle effects will occur in a different location and plane of imaging if MRI is performed in a vertical bore scanner (having a different orientation of B_o with respect to the regional anatomy).

MRI Magic Angle Artifact

The magic angle artifact is an MRI artifact that occurs due to the phenomenon of nuclear dipole-dipole interactions. It appears as a bright signal on T1-weighted images and can occur in tissues with highly organized collagen fibers, such as tendons, ligaments, and cartilage.

When the tissues are aligned at approximately 55 degrees to the main magnetic field (known as the magic angle), there is an increase in the signal intensity on T1-weighted images, leading to a bright or hyperintense appearance. This effect is caused by the interaction between the collagen fibers and the magnetic field, resulting in shortening of the T2 relaxation time. The resulting image can have a characteristic appearance, which includes:

Bright and sharp signal intensity: The affected tissue may appear bright and sharply defined on the MRI scan, which can mimic the appearance of a true abnormality.

Linear or curvilinear patterns: The artifact may produce linear or curvilinear patterns that appear superimposed over the underlying tissue, which can be mistaken for a real structure or lesion.

Signal voids: In some cases, the artifact can cause signal voids or areas of low signal intensity that may be mistaken for true pathology.

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Here are some strategies to minimize or avoid Magic Angle Artifact

Changing the imaging sequence: Using sequences such as STIR (Short T1 Inversion Recovery) or FSE (Fast Spin Echo) can reduce the artifact.

Using a lower field strength: The artifact can be reduced by using lower field strengths, such as 1.5T instead of 3T.

Adjusting imaging parameters: Changing the angle of the imaging plane or increasing the repetition time (TR) and echo time (TE) can help avoid this artifact.

Using a higher b-value: In diffusion-weighted imaging, increasing the b-value can reduce the Magic Angle Artifact.

Patient repositioning: Repositioning the patient can help reduce the effect of the Magic Angle Artifact by changing the angle between the collagen fibers and the imaging plane.

References:

• Thawait GK, Subhawong TK, Thawait SK, et al. Magic angle effect: a cause of increased signal intensity in the normal lateral patellar tendon on short-tau inversion recovery (STIR) images. Skeletal Radiol. 2012;41(5):545-551. doi:10.1007/s00256-011-1274-

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