What is OCT?

1. OPTICAL COHERENCE TOMOGRAPHY Specimen Figure 6. Progression of glaucomatous optic nerve head Figure 7. Visual field loss corresponding to glaucomatous optic nerve head Figure 1. Comparison of OCT with various imaging techniques Figure 3. Microscopy OCT Figure 4. Components of OCT 1 mm Volumetric multiple wavelength ultrahigh resolution imaging in glaucomatous retina Marilyn Puah Standard Clinical Transverse Scanning ULTRASOUND 100 mm Backscattered Intensity High Frequency RESOLUTION (log) 10 mm Axial Position (Depth) Biomedical Imaging Group, School of Optometry and Vision Sciences 1 mm 2D Grey Scale or False Color Image CONFOCAL MICROSCOPY • Eye • Retina of eye consists of 10 layers • Nerve fibres in the eye exit via the optic nerve head to the brain • A flat disc or disc or shallow depth is indication of a healthy optic nerve head • Principle of OCT • Measures time delay and magnitude of optical echoes at different transverse positions • Dimensions of structures at different depths are based on “echo” time for light to be backscattered • By scanning optical beam in the transverse direction and performing successive axial scan measurements, cross sectional image is produced • A 2D grey scale or false colour image is produced • What is OCT? • Ultrahigh resolution optical coherence tomography (UHR OCT) is an upcoming new technology that allows non-invasive, optical medical diagnostic imaging. • Analogue to ultrasound but it allows real-time in situ imaging with higher resolution of 1 to 15 microns. • It enables a three-dimensional in vivo direct visualization of the microstructure of the retina, allowing both qualitative and quantitative changes to be seen which are beneficial in diagnosing optical diseases especially in early stages. • OCT measures by echo time delay and intensity of back reflected light 100 mm 1 mm 1 cm 10 cm IMAGE PENETRATION (log) Figure 5. Cross section of the eye, optic nerve head and retina • What is glaucoma? • Glaucoma is one common disease of the eye in human • Characterized by increase in intraocular pressure and atrophy in the optic nerve head with corresponding visual field loss which is usually not noticeable by patient until a late stage • Optic atrophy results in a larger cup disc size and depth, which is seen as a dip from the cross section of the optic nerve head. • Several studies done have proposed that structural changes of the optic nerve head and nerve fibre layer will precede before visual field loss appear. • Therefore it is crucial to detect it early so that appropriate treatment can be done to prevent further damage to the optic nerves. Figure 2. How OCT generates images • Methods • In vitro measurements of glaucomatous animal retinas were taken with microscope OCT • Allows three-dimensional imaging and visualisation of the morphological changes of the optic nerve head • Images’ intraretinal contrast were then compared to histology • Allows the improvement of OCT system in regards to resolution, contrast and penetration depth for future studies • 3 pairs of tree shrews’ eyes and 5 pairs of rats’ eyes were imaged • Glaucoma introduced in the left eye and right eye as a control • All retinas were processed using ImageJ • Best 3 pairs of eyes were chosen for this presentation • Comparison with other imaging techniques • Ultrasound allows increased penetration depth for imaging with higher frequency sound waves • But resolution is reduced due to increased ultrasonic attenuation • Confocal microscopy allows submicron resolution • But image penetration is limited • OCT strikes a balance between confocal microscopy and ultrasound imaging • OCT allows image penetration up to 2-3mm and resolution ranging from 15 to 1µm which is dependent on bandwidth of light source Results Tree shrew CTS1 normal RE Rat CR312 normal RE Rat CR337 normal RE Rat CR312 glaucomatous LE Tree shrew glaucomatous LE Rat CR337 glaucomatous LE • Discussion • Glaucoma introduced in left eye • Optic atrophy and morphological changes seen in left eye • Tree shrews’ and rats’ retinas differ • Hyaloid artery interfere with ONH imaging in tree shrews • Orbital fats caused a layer oil film, disrupting images • Careful handling of retinas required • Hyaloid artery interferes with imaging • Detection of glaucoma in animals’ retina correspond to that in human • Future outlook • Visualise structure without sacrificing animal • Extend knowledge to human • Beneficial in early detection of glaucoma • Pick up other ocular diseases due to early morphological changes • Applies to other areas of biomedical research • Acknowledgements • I would like to thank Prof Wolfgang, Dr Boris Povazay and the OCT team for their help and guidance throughout the 6 weeks of the placement which made the experience amazing • I would also like to extend my upmost gratitude to CUROP and Cardiff University for funding me during the 6 weeks of placement • Reflections • Provide an insight to research work • Exposed to new technology • Rewarding, fruitful experience • Allowed to be part of a promising research team References Figure 5 taken from http://www.healthyeyes.org.uk/uploads/pics/Eye-1.jpg and http://www.e-sunbear.com/images/glopticnerve.jpg and http://www.answersingenesis.org/Home/Area/Magazines/tj/images/v13n1retina3.gif Figure 6 taken from http://www.meei.harvard.edu/patient/images/optic-nerve-head.jpg Figure 7 taken from http://www.intecheye.com/Upload/glaucoma1.jpg • Limitations of OCT • Optical imaging restricted to surface tissues but not affected in the eye • Less penetration than ultrasound • Optical artefacts interfere with quality of images