Optical assessment of middle ear inflammation

Jung, David S. (2016) Optical assessment of middle ear inflammation. PhD thesis, University of Nottingham.

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This thesis describes the development of an optical device to assess the inflammatory state of the middle ear mucosa through the ear canal, after ventilation tube insertion in otitis media with effusion in children.

An optical phantom of the middle ear was developed in order to allow repeatable experiments. The phantom consists of eardrum and mucosa while all other structures are neglected. The optical properties of the phantom were determined based on literature review and experiments on an animal model. The middle ear mucosa phantom is based on a polyester resin, with dyes added to introduce absorption and a titanium dioxide based white colour to introduce scattering. Four phantom are created to model healthy, intermediate, and diseased mucosa. Several approaches are taken for the eardrum phantom, either a grid glass diffuser or a resin phantom with scattering introduced by fibre glass is used.

Middle ear inflammation affects the mucosa while the eardrum might not be affected. Hence, the mucosa must be assessed and signals resulting from the eardrum, blocking the direct light path, filtered, in order to reduce background signals. During literature research and preliminary experiments, multi-wavelength measurements were selected to assess the mucosa and confocal techniques to allow measurements through the eardrum. The tissue is illuminated with two wavelengths and the reflected signal analysed. Appropriate selection of the wavelengths at characteristic point of the absorption spectrum of blood allows assessment of the inflammation via the blood concentration in tissue. The confocal idea was adopted leading to the “anti-confocal” system, where a central stop replacing the pinhole rejects light from the plane in focus rather than rejecting all out of focus light. With the eardrum in focus and a stop radius larger than the confocal pinhole radius (r_stop=0.48mm), most light from the eardrum is rejected (reduced to 0.2%) while signal from the mucosa are still detected (reduced to 25.6%), according to simulations.

Simulations of the anti-confocal system showed an increase of the signal level by a factor of 3.2 or a 1.5 times higher background rejection ratio (SBR) compared to the conventional confocal system, when keeping the respective other value constant. This advantage still holds and is even improved in some cases when increasing the scattering coefficient (from 11 up to 44mm-1), reducing the scattering anisotropy (from 0.99 to 0.6), changing the distance between eardrum and mucosa (0.5 to 8mm), inaccurate focus (up to 3mm out of focus), and changed NA (0.055-0.27). Further, best wavelengths for measurements of the blood concentration and thus inflammation of the mucosa have been determined in simulations to be 730 and 546nm. In the investigated range of wavelengths (500 to 940nm), the relation of near infra-red signal at 730nm to green reflection signal at 546nm gives the highest response to a change in the total blood level in tissue while showing a low response to changes in blood oxygenation.

The anti-confocal system was built as bench-top system and characterised. Instead of using a physical stop, a CCD camera was used and anti-confocal filtering done during post-processing, by selecting certain pixel on the camera. Experimental results confirmed the simulations and showed an increased signal and easier use of the anti-confocal system compared to the confocal system as no exact focus is required. An anti-confocal stop with 0.48mm radius showed best performance, showing a high contrast and low variation during the measurement. Measurements were possible with increased scattering (simulated by the stronger scattering grid glass diffuser) and attenuation (simulated by absorbing dyes added to the resin eardrum phantom) of the eardrum, increased distance between eardrum and mucosa (2-6mm), defocus of the system, and altered orientation of the phantom surface (0-10deg) with differences in the mucosal blood level still detectable. But the measured inflammation index is influenced by the transmission properties of the eardrum. While the influence of absorption can be accounted for by confocal detection of the properties of the eardrum during the same measurement, improved signal processing and modelling of light propagation are necessary to account for changed scattering of the eardrum.

Tests on the hand of healthy volunteers showed that the proposed system is able to detect a change in the concentration of haemoglobin of living tissue measured through an eardrum simulating scattering layer. The next steps are the improvement of signal processing to account for changes of the measured inflammation index due to scattering of the eardrum. Once this is achieved, the optical system can be minimised to allow measurements on the ear and pilot trials for evaluation and calibration of the system.

Item Type: Thesis (University of Nottingham only) (PhD)
Supervisors: See, Chung W.
Somekh, Michael G.
Crowe, John A.
Birchall, John P.
Keywords: medical and biological imaging; middle ear imaging; otitis media; glue ear; middle ear inflammation; otolaryngology; anti-confocal; imaging through turbid media; optical phantom; monte-carlo simulation; spectroscopy
Subjects: R Medicine > RC Internal medicine
Faculties/Schools: UK Campuses > Faculty of Engineering
Item ID: 38529
Depositing User: Jung, David
Date Deposited: 30 Jan 2017 10:01
Last Modified: 05 Jun 2018 16:02
URI: https://eprints.nottingham.ac.uk/id/eprint/38529

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