The influence of flip angle on the magic angle effect.

Overview

OBJECTIVE: To assess the impact of flip angle with gradient sequences on the "magic angle effect". We characterized the magic angle effect in various gradient echo sequences and compared the signal-to-noise ratios present on these sequences with the signal-to-noise ratios of spin echo sequences. DESIGN: Ten normal healthy volunteers were positioned such that the flexor hallucis longus tendon remained at approximately at 55 degrees to the main magnetic field (the magic angle). The tendon was imaged by a conventional spin echo T1- and T2-weighted techniques and by a series of gradient techniques. Gradient sequences were altered by both TE and flip angle. Signal-to-noise measurements were obtained at segments of the flexor hallucis longus tendon demonstrating the magic angle effect to quantify the artifact. Signal-to-noise measurements were compared and statistical analysis performed. Similar measurements were taken of the anterior tibialis tendon as an internal control. RESULTS AND CONCLUSIONS: We demonstrated the magic angle effect on all the gradient sequences. The intensity of the artifact was affected by both the TE and flip angle. Low TE values and a high flip angle demonstrated the greatest magic angle effect. At TE values less than 30 ms, a high flip angle will markedly increase the magic angle effect.