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Polarisation-sensitive optical coherence tomography of Collagen

Sergey Gangnus, Nadya Ugryumova, Don Attenburrow, Peter Winlove and Steve Matcher.

Collagen is the most important structural protein in the body. It acts as the “scaffolding” of the extra-cellular matrix and gives skin, tendon, cartilage and intervertebral discs their mechanical strength. Alterations in its physical properties underlie many important pathological processes in the body such as scar tissue formation, osteoarthritis and back pain.

The physical structure of Collagen

The collagen molecule is physically anisotropic. This gives collagen-rich tissues interesting optical properties such as strong linear birefringence and a high non-linear susceptibility. Polarization-sensitive optical coherence tomography combines polarimetry with low coherence reflectometry to provide high-resolution optically sectioned images of the birefringence of intact tissues.



Collagen structure in tendon at various spatial scales

(none)

Polarisation-sensitive optical coherence tomography


Schematic diagram of a bulk optics PS-OCT system.


Photograph of a bulk optics PS-OCT system.

OCT of intervertebral discs in the spine

Intervertebral discs can be studied using PS-OCT. The outer region of the disc (the annulus fibrosus AF) consists of layers, each several hundreds of microns thick, in which the alignment direction of the collagen fibres alternates.


A section of the human spine showing the position of an intervertebral disc.
[New York Neurological Institute]
A cross-section of an intervertebral disc showing the the annulus fibrosus (AF) [Duke University]

The biomechanics of the intervertebral disc.


The intervertebral disc forms the flexible joint between adjacent vertebrae in the spine. It also acts to resist compressive forces, by acting as a fluid-filled pressure vessel. When the disc is compressed, it responds by bulging outwards. The collagen fibres in the outer layer are inclined to the vertical, such that they stretch when the disc is compressed. Since collagen fibres are very inextensible, this prevents the disc from compressing.

Standard OCT image
of annulus fibrosus


A standard OCT image of the optical reflectivity of intervertebral disc reveals little contrast. Image size is 1 mm × 1 mm

Polarisation-sensitive OCT image
of annulus fibrosus


PS-OCT images of optical retardance show strong banding; this is the signature of optical birefringence. Phase-resolved PS-OCT can determine the optical fast-axis as a function of position and hence map the spatial variation of collagen orientation. This has applications in the study of diseases such as scoliosis, in which the discs are abnormally curved.

Articular cartilage and osteoarthritis

Another tissue whose collagen structure is of great interest is articular cartilage. Articular cartilage covers the sliding surfaces of the bone in articular joints. Cartilage is composed chiefly of collagen fibres, proteoglycans and water. Osteoarthritis is associated with irreversible degradation of the cartilage and a consequent disruption of the normal collagen structure. The ability to measure cartilage birefringence non-invasively may lead to new tools for detecting early-stage osteoarthritic change via arthroscopic examination.

PS-OCT images of equine articular cartilage.


8mm x 1mm retardance image of visually normal cartilage.


A similar image but at a site displaying a macroscopic, visual lesion. Note the disruption of the regular banding seen in the specimen of normal tissue.

References

“The collagen structure of equine articular cartilage, characterized using polarization sensitive optical coherence tomography”, Ugryumova N, Winlove CP, Attenburrow DA and Matcher SJ, J. Phys. D. (in press).

“The collagen structure of bovine intervertebral disc studied using polarization sensitive optical coherence tomography”, Matcher SJ, Winlove CP and Gangnus SV, Phys. Med. Biol. 49, 1295 - 1306 (2004).

“Monte Carlo modelling of polarized light propagation in biological tissues”, Gangnus SV, Matcher SJ and Meglinski IV, Laser Physics 14 (6), 886-891 (2004)

This page was written by Steve Matcher of the University of Exeter Biomedical Physics Group.

Feb 2005.

                                                                                                                                                                                                                                                                       
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