Introduction

Water associated with a collagen matrix (tendons, meniscus, etc.) produces “magic angle” effects on MR imaging(1). The conventional isotropic liquid approach describing T2 relaxation is not sufficient to understand the behavior of transverse magnetization in this situation. The NMR lineshape is not a single (or double) Lorenzian, but rather a more complicated shape that is a strong function of fiber to field orientation and state of hydration (Fig. 1). NMR researchers use anisotropic solvents to provide residual dipolar couplings in biological molecules, allowing the measurement of intramolecular proton distances for structure determination(2). The behavior of water trapped in a collagen matrix can be thought of as a solute in an ordered solvent. Orientation variations of the collagen fibrils, when combined with the longer correlation times of water hydrogen bonded to the protein matrix produce wide and non-Lorentzian linewidths. The non-isotropic averaging of the dipolar interactions on the water molecule results in an apparent ordering direction that points along the collagen fibers and produces the (3cos2ϴ-1) dependence in the frequency dispersion. This has a strong influence on the transverse magnetization (coherence) lifetime. Understanding this dipolar fiber to field mechanism allows interpretation of MR images in collagen containing tissues or even mapping out fiber structure (DAFI technique)(3).

Material and Methods

A short length of frozen human Achilles tendon was immersed in Fomblin and contained in a10 ml plastic syringe barrel. The tissue fiber direction was orthogonal to the cylinder axis, allowing rotation in small custom solenoid T/R coils. This sample rotation allowed images to be obtained as the fiber to field axis was varied in steps of 30 deg. 3D GRE images were obtained with 0.2 mm isotropic resolution at both 11.7 and 3.0 T using a Bruker BioSpec 117/16USR animal scanner and a GE HDx clinical scanner. Imaging parameters were identical on both systems: TR 24 ms, TE 4.0 ms. 150x150x150 acq matrix, flip angle 5 deg, 2 NEX. Images were registered using FLIRT software (FSL, Oxford). ImageJ software was used to obtain intensity measurements in selected ROIs on congruent images at 6 orientations and 2 field strengths.

Results

Fig. 2A,2B are 3T and 11.7 T images of the tendon with the field direction parallel (0 deg) to the fiber direction. The tendon has overall low intensity except for some imbedded structures (endotenon, crimped fibers). Rotating the sample to make the fiber to field direction 60 deg produces images having high signal intensity (Fig 3A,3B). Intensity was plotted as a function of rotation angle (Fig. 4). In all orientations the appearance of the 3T and 11.7T images were remarkably similar.

Conclusion

What does T2* mean exactly for complex collagen NMR lineshapes(4). It is helpful to keep in mind solid state or liquid crystalline systems where NMR spectra have well defined dipolar couplings. With water hydrogen bonded in the collagen matrix, residual dipolar coupling seem to be the main source of “magic angle” effects seen in MRI. These do not scale with field and are consistent with our data. A recent idea is that T2 varies with orientation when water is entrapped in elliptical nano-cavities(5). It is not clear how this mechanism competes with the anisotropic averaging in a strong hydrogen bonding environment.

Acknowledgements

No acknowledgement found.