Asymmetric Signal Intensity in Normal Collateral Ligaments of the Distal Interphalangeal Joint in Horses With a Low‐field Mri System Due to the Magic Angle Effect

Abstract

Increased signal intensity in one of the collateral ligaments of the distal interphalangeal (DIP) joint of sound horses in images acquired using a low-field magnet with vertical orientation of the magnetic field was investigated as a possible manifestation of the magic angle effect. Three isolated equine digits were imaged using the following pulse sequences: (1) spin echo TI, (2) turbo spin echo proton density and T2, and (3) 3D gradient echo T1, in different positions by mildly changing the orientation of the long axis of the digit, in the dorsal plane, relative to the magnetic field. The signal intensity in a ligament was significantly increased when the ligament orientation relative to the magnetic field was 55 +/- 10 degrees. The signal intensity was markedly increased in pulse sequences with short echo time (TE) 5.0, 4.9, and 3.9 times increased, respectively, for 3D gradient echo T1, spin echo T1, and turbo spin echo proton density) and to a lesser extent with pulse sequences with a longer TE (1.8 times increased for turbo spin echo T2). These changes are characteristic of the magic angle effect. Because of the anatomic orientation of the collateral ligaments of the DIP joint, a slight deviation of the long axis of the digit in the dorsal plane, from the ideal horizontal position, will induce an increased signal intensity that can be confused with desmitis. Careful positioning of the foot in magnetic resonance imaging systems where B0 is perpendicular to the long axis of the digit is critical to prevent the occurrence of the magic angle effect.

Characterization of the Magic Angle Effect in the Equine Deep Digital Flexor Tendon Using a Low‐field Magnetic Resonance System

“…The limb should be perfectly horizontal, so that the long axis of the digit is perpendicular to the magnetic field. 1 Changes in the deep digital flexor tendon, similar to those described in this study, are also encountered using standing MR systems. However, angulations, such as the ones used in this study, are unlikely to occur with a standing horse, except if major mediolateral foot imbalance is present.…”

“…Ã The imaging and positioning protocols have been described previously. 1 Briefly, the feet were initially scanned as if the horse were in lateral recumbency, with the sagittal plane of the limb horizontal and thus perpendicular to the magnetic field. Then, imaging was repeated with different angulations of the sagittal plane relative to horizontal, with a maximum angulation of 201.…”

“…Previously, as we were investigating the magic angle effect in the collateral ligaments of the distal interphalangeal joint with a low-field MR unit, we observed focal signal intensity changes in the deep digital flexor tendon. 1 Thereafter, we hypothesized that the signal observed in the deep digital flexor tendon could be related to the magic angle effect, and we subsequently reevaluated data from the previous study for changes in signal intensity in the deep digital flexor tendon.…”

Magnetic resonance protocols in equine lameness examination, used sequences, and interpretation

“…In standing low-field MRI basic protocol may consist of T1 GE, T2 FSE and STIR in sagittal, transverse and frontal planes (Mair and Kinns 2005). When looking for magic angle effect in collateral ligaments of distal interphalangeal joint Spriet et al (2007) have chosen sequences as SE T1, PD TSE, T2 TSE, T1 3D GE dorsal and transverse views. For diagnosing navicular bursa adhesions Holowinski et al (2012) have recommended to use images in sagittal and perpendicular to the deep digital flexor tendon transverse planes favorably in PD, STIR and T2w sequences (Fig.…”

Magnetic Resonance Imaging of the Deep Digital Flexor Tendon in the equine foot

“…Möglicher-weise lässt sich dieses Phänomen durch einen Artefakt, den "magic angle effect", erklären (Busoni and Snaps 2002). Bei dem für die vorliegende Arbeit genutzten MRT kommt es im Bereich der TBS nicht zu jenem Artefakt, da bei diesem Magneten das Magnetfeld horizontal zur Gliedmaße verläuft (Spriet et al 2007, Murray 2008.…”