THE MAGIC-ANGLE PHENOMENON IN TENDONS - EFFECT OF VARYING THE MR ECHOTIME

Increased signal intensity on magnetic resonance (MR) imaging of tendo ns arising from the magic angle phenomenon is well recognized. This st udy aimed to evaluate the effect of varying the echo time (TE) upon te ndon signal intensity, and to determine if a modified TE value produce s acceptable T-1 and proton density (PD) weighted images. Fresh bovine tendons were imaged in a 1.5 T MR scanner using spin echo (SE) T-1 an d PD weighted sequences and utilizing a number of different coils. For each set of sequences, the tendon was orientated at 55 degrees to the main magnetic field (B-0) and imaged using constant TR and incrementa l TE values. Signal intensity was measured on images at each TR/TE val ue and compared with the signal intensities of tendons orientated al 0 degrees to B-0, obtained using minimum TE values. This experiment was repeated with a 1.0 T MR scanner and utilizing a spine coil. The Achi lles tendon of a human volunteer was similarly imaged using a general purpose flex coil. For bovine and human tendons orientated at 55 degre es to B-0, the signal intensities decreased exponentially with increas ing TE. A critical TE value exceeding 37 ms, for each sequence, reduce d the signal intensities to the levels obtained with the tendons orien tated at 0 degrees to B-0, such that the magic angle phenomenon could be avoided. Although there was variability of the signal intensities w ith different coils, the critical TE value remained constant and the a natomical clarity was not degraded. The critical TE value was unaltere d using two MR scanners of different field strengths.