The magic angle effect: A source of artifact, determinant of image contrast, and technique for imaging

Abstract

This review provides a formalism for understanding magic angle effects in clinical studies. It involves consideration of the fiber-to-field angle for linear structures such as tendons, ligaments, and peripheral nerves, disc-like and circular structures such as menisci and labra, as well as complex three-dimensional structures. There may be one or more fiber types with different orientations within each of these tissues. The orientation of these fibers to B 0 is crucial in determining their magic angle effect. Tissues may show a variety of appearances depending on their baseline T2, as well as the increase in T2 produced by the magic angle effect. The appearances are affected by TE, which affects both the general tissue signal level and the change in signal produced by the magic angle effect, fiber-to-slice orientation, and partial volume effects. Deliberate positioning of structures and tissues at particular orientations to B 0 can be used to increase the signal from tissues such as tendons and ligaments and so allow them to be imaged with conventional sequences. The technique can also be used to produce contrast between tissues with fibers that have different orientations to B 0 . IN PULSE SEQUENCES with a moderate or short TE, the signal intensity of tendons and ligaments depends on their orientation to the static magnetic field (B 0 ) (1-3). These highly ordered, collagen-rich tissues contain water that is bound to collagen. The protons within this water are subject to dipolar interactions whose strength depends on the orientation of the fibers to B 0 . These interactions usually result in rapid dephasing of the MR signal after excitation. As a consequence, tendons and ligaments typically produce little or no detectable MR signal and appear dark when imaged with conventional clinical pulse sequences. KeyThe dipolar interactions are modulated by the term 3cos 2 Ϫ 1, where is the angle the structures make with the magnetic field B 0 . When 3cos 2 Ϫ 1 ϭ 0 ( ϭ 55°, 125°, etc., approximately, the magic angle), dipolar interactions are minimized with the result that the T2 of these tissues is increased and signal intensity may become evident within them when they are imaged with conventional pulse sequences. The magnitude of the magic angle effect may be quite large. For example, Fullerton et al (1) have described an increase in T2 of Achilles tendon from 0.6 to 22 msec and Henkelman et al (2) have described an increase from 7 to 23 msec when the orientation of the tendon to B 0 was changed from 0°to 55°. With some commonly used pulse sequences this may correspond to the tendon signal intensity increasing from the bottom of the imaging gray scale to the top of it.In initial studies with clinical MRI the effect was seen principally in tendons that underwent a change of direction along their course such as the supraspinatus tendon near its insertion so that the fibers of one part of the tendon were oriented at or near 55°to B 0 . The high signal resulting from the increase in T2 in the part of the tendon at 55°to B 0...

Feasibility of T mapping for the evaluation of hip joint cartilage at 1.5T using a three‐dimensional (3D), gradient‐echo (GRE) sequence: A prospective study

“…Their study demonstrated a good correlation between T 2 values using GRASE and TSE. Interestingly, since the GRASE sequence includes refocusing pulses (22), readings and measurements are similar to T 2 relaxation. Additionally, their study also lacks histological validation and is a limiting factor.…”

“…It does have its own limitations such as sensitivity to susceptibility artifacts that are pronounced at the bone-cartilage interface or foreign body particles such postsurgical debris or metallic particles that may make accurate evaluation of cartilage difficult. Furthermore, the alpha-angle effect of increasing T 2 /T * 2 at an angle of ϳ55°to the main magnetic field has to be considered and monitored closely, especially in a spherically-shaped joint as the hip (22). The differences between the dGEMRIC and T* 2 technique regarding the interpretation of normal-appearing cartilage needs to be further elucidated.…”

Magic Angle Effect: A Relevant Artifact in MR Neurography at 3T?

“…7,8 The MA is associated with a hyperintense T2 signal-intensity alteration in tissues densely composed of collagen, was first observed in tendons and ligaments, and was later reported also to occur in peripheral nerves. [9][10][11][12] It is conceived as a T2 signal-intensity increase related to the suppression of dipole-dipole interactions of the hydrogen protons bound to quasicrystalline structures such as tropocollagen, the triple-helical main compound of various types of fibrillary collagen. 13 The dipole-dipole interaction be-tween the highly ordered protons produces local magnetic fields with a field strength in the z-direction:…”

The carpometacarpal joint of the thumb: MR appearance in asymptomatic volunteers

“…The variance of ligament SI of the AOL and POL in our study compared to the reported low SI of ligaments might be related to the magic angle effect, as the echo times of the coronal and sagittal intermediated-balanced FS sequences were of 25 and 24 ms, respectively. Because of the horizontal orientation of the IML, the mostly striated appearance does not assume to be related to the magic angle effect, and instead might be due to the ligament morphology [30,31].…”