Magic angle

This article is about the magic angle as defined in the field of nuclear magnetic resonance spectroscopy. For the magic angle as defined in the field of electron energy-loss spectroscopy see magic angle (EELS).

The magic angle is a precisely defined angle, the value of which is approximately 54.7°. The magic angle is a root of a second-order Legendre polynomial, , and so any interaction which depends on this second-order Legendre polynomial vanishes at the magic angle. This property makes the magic angle of particular importance in solid-state NMR spectroscopy.

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Mathematical definition

  The magic angle θ_m is

,

where arccos and arctan are the inverse cosine and tangent functions respecively.

θ_m is the angle between the space diagonal of a cube and any of its three connecting edges, see image.

Magic angle and dipolar coupling

In nuclear magnetic resonance (NMR) spectroscopy, the dipolar coupling D in a strong magnetic field depends on the orientation of the internuclear vector with the external magnetic field by

Hence, two nuclei with a internuclear vector at an angle of θ_m to a strong external magnetic field, have zero dipolar coupling, D(θ_m)=0. Magic angle spinning is a technique in solid-state NMR spectroscopy which employs this principle to remove or reduce dipolar couplings, thereby increasing spectral resolution.

Application to medical imaging: The magic angle artifact

The magic angle artifact refers to the increased signal on sequences with short echo time (e.g., T1 or PD Spin Echo sequences ) in MR images seen in tissues with well-ordered collagen fibers in one direction (e.g., tendon or articular hyaline cartilage)^[1]. This artifact occurs when the angle such fibers make with the magnetic field is equal to θ_m.

References

1. ^ Bydder M, Rahal A, Fullerton G, Bydder G (2007). "The magic angle effect: a source of artifact, determinant of image contrast, and technique for imaging". Journal of magnetic resonance imaging 25 (2): 290-300. PMID 17260400.