Optical coherence tomography

Schematic of Optical Coherence Tomography.

The Matcher group at Sheffield develop novel biophotonic tools to aid the characterisation of biological tissues. Photonics offers many unique advantages over other imaging techniques for studying biological systems, including structural and functional contrast, cellular level resolution, speed, non-destructiveness and cost. The group´s work focuses on techniques offering high resolution such as optical coherence tomography (OCT) and non-linear laser scanning microscopy.

Facilities also include Doppler, polarisation and spectroscopic OCT systems, supercontinuum light sources offering cellular resolution imaging, an ultrafast laser for two-photon/second-harmonic microscopy and equipment for determining tissue optical properties. The group works closely with the Tissue Engineering group within the Kroto Institute (Prof S MacNeil and Prof R Smallwood) and has collaborative links with the III-V device fabrication group within Dept of Electrical and Electronic Engineering (Dr R Hogg) and the Dental School (Dr A Crawford).

Optical coherence tomography (OCT) is an optical analogue of ultrasound imaging but offering axial resolution of 1-10 microns over depths up to 2 mm in biological tissues. Our swept-source system is based on a 1300 nm frequency-swept laser, fibre-optic interferometer and telecentric beam scanning optics (see schematic).

Skin constructs were fabricated by seeding de-epithelialized acellular dermis (DED) with keratinocytes and fibroblasts. The constructs were then grown in cell culture medium at an air-liquid interface for up to 21 days. OCT imaging was performed at a series of time-points from day 1 to day 21. At day 21 OCT (below left) demonstrated the formation of neo-epidermis that correlated with invasive histological assessment (below right).

Formation of new epidermis followed by OCT.

2009 also saw the group complete the construction of a swept-source polarization-sensitive OCT system. This is an enhanced version of an earlier design that produces “phase-retardance” images in addition to the structural images illustrated above-left. Such images provide information on the presence of directionally organised collagen, via its linear birefringence.

Retardance images of cartilage providing information on protein fibre orientation.

The two images above show retardance images of the same site on equine articular cartilage taken with two different illumination directions. The strong banding pattern visible on the left is not a structural feature but instead reveals the presence of strong linear birefringence. The absence of the banding on the right image, despite the measurement site being the same, provides information on how the collagen fibres are oriented in 3-D (the fibres must be nearly parallel to the beam direction to produce low birefringence).

We are working to develop this idea into a tool that can characterise the ECM geometry of articular cartilage samples and thus guide the design of scaffolds to produce tissue engineered cartilage replacements.

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Steve Matcher

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