Abstract text (incl. figure legends and references)
1 Introduction
Magnetic Resonance Microscopy (MRM) is limited in spatial resolution (a≈10µm) and sensitivity compared to optical microscopy (OM) or SEM but offers the advantages of being non-destructive, 3-Dimensional imaging of intact 3D biological tissue and a multifold variety of different contrast weightings (eg. T2/T1, magn. susceptibility, diffusivity, chemical information). These features qualify MRM for investigating biological samples and delineation of microstructural changes as a consequence of biological or mechanical impact.
The elementary functional element of the tendon is represented by collagen filaments. These are arranged to larger fiber bundles, surrounded by the MR-bright endotenon, in a hierarchical, self-similar way from primary to tertiary fiber bundles1. Strong mechanical load without rupture, e.g. by abrupt trauma in sport or repetitive dynamic fatigue overload might change the microstructural arrangement of fiber bundles permanently and therefore might result in increased risk for permanent fatigue aches and even rupture within routine sport activities.
2 Objectives
.1 Can high resolution MRM delineate permanent changes in the microstructure of tendons as a consequence of strong mechanical load?
.2 What are these microstructural changes and can they be related to the results of ex-vivo investigations on human tendons using OP or SEM? What bio-physical model could be used for the interpretation of the observed changes in T2* contrast MRM?
3 Materials/Methods
3.1 MR-microscopy
The spatial encoding in 2DFT-MR-imaging is obtained via slice selective excitation, phase- and frequency encoding of the sample emitted radiofrequency (rf) signal. The extraordinary high microscopic spatial resolution on an Ultra-High-Field (B=7T) human MR-scanner was achieved using a prototype strong magn. field gradient insert with sensitive rf-detectors2. 2D and 3D- T2*-weighted images (VS: 64x64x60µm3) were obtained with short encoding time TE for visualization of tendon tissue with short relaxation time T2*.
3.2 Tendon-tissue samples
Several samples (l≈5cm) from cattle Achilles tendon were halved into two parts; one segment was exposed to a mechanical load (F≈600N). The samples were scanned with orientation of the main tendon axis and mechanical load parallel to the outer static magnetic field B of the MR-scanner.
4 Results
An MR-microscopic axial slice out of a Multi-slice MR data set (fig.1) and a sagittal view reconstructed from a high-resolution isotropic 3D data set are shown exemplarily below (fig.2).
The MRM images were compared to polarization sensitive OM and SEM of the rat tendon under mechanical load3. The main direction of tendon fiber filaments changes for a length of about 60µm (crimp structure of tendon) until being oriented back again. The number and angle of this crimp misalignment reduces with load (flattening triangle model) and was also observed in MRM (disappearing of the hyperintense filaments) as a consequence of the Magic angle effect4 in MR.
5 Conclusion
Advanced MR-technology based on prototype insert hardware on UHF scanners is capable of the non-destructive visualization of permanent changes in the microstructure of tendons with load below rupture level. The MR appearance can be interpreted within a flattening alignment model of the tendon crimp structure. Thus, MR-microimaging offers perspectives for the early diagnosis of pre-damage to tendons and interpretation of undefined tendon aches.