TwinTree Insert

17-06 The Magic Angle Effect

he magic angle artifact is not an artifact but a true elec­tro­phy­si­o­lo­gi­cal phe­no­me­non created by dipole coupling, similar to the quadrupolar dips de­scrib­ed in Chapter 4.

The magic angle is a precisely defined angle of approximately 54.7° (in most pub­­li­ca­tions rounded to 55°) to a strong exter­nal magnetic field at which two nuclei with a dipolar coupling vector have zero dipolar coupling. This phenomenon is applied in solid-state NMR spectroscopy as the magic angle spinning technique to re­move or re­duce dipolar couplings and thus increasing spectral resolution [⇒ Berendsen 1962].

When tendons in an MR imaging equip­ment are aligned at an angle of 54.7° re­la­­tive to B₀, the dipole coupling descends to zero. This was first reported in 1985 by Ful­ler­ton and col­lea­gues [⇒ Fullerton 1985]; they described an increase in T2 of the achil­les tendon from 0.6 to 22 ms.

The effect produces bright spots in col­lagen fi­bers of tendons, ligaments, periph­eral nerves and other tissues which may lead to false-po­si­ti­ve results in musculo-skeletal MR studies [⇒ Peto 1990, ⇒ Fullerton 2007].

Using different pulse sequence parame­ters or changing the patient's position and repeating the examination can help distin­guishing a magic angle effect from patho­logical changes (Figure 17-19).

Figure 17-19:
Black patellar tendon aligned — more or less — parallel to B₀ due to short T2/T2* with a bright lesion in the tendon just below the patella. This small bright area is not a magic angle effect (or “artifact”), but a chronic degeneration of the tendon (tendinosis).