Angle dependence of signal intensity of a bovine tendon at spin echo sequence

It has been reported that the signal intensity of dense collagen fibers markedly increases in some tissues on MRI, when collagen fibers are oriented at about 55deg against the static magnetic field. This angle is sometimes called the magic angle. The magic angle phenomenon is caused by magnetic dipolar-dipolar interaction between water protons. As this phenomenon has been reported to be related to the T2 relaxation rate only, the signal intensity may be influenced by the T2 relaxation rate at the spin echo (SE) sequence if other parameters are fixed. Information regarding the increasing ratio of signal intensity depending on the angle may be clinically important. A bovine tendon was rotated horizontally in a static magnetic field and images were obtained with SE or fast spin echo (FSE) sequences as follows: (a) SE (TR/TE=2000/15, 30, 45, 60), (b) SE (TR/TE=300, 500, 700/14), (c) FSE (TR/TE=3000/15, 105 eff., echo train length=14). By comparing the theoretical curves and data, it was confirmed that the signal intensity was related to the second power of the local magnetic field strength. The increasing ratio of signal intensity of the bovine tendon at 55deg (magic angle) compared with 0deg was about 3 times (TE=30). The most influential TE was about 20-40 msec. The increasing ratio of signal intensity did not depend on TR change or the FSE technique. (author)