Towards low cost multiple reference optical coherence tomography for in vivo and NDT applications
Dsouza, Roshan Isdore
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Developing a cost-effective, compact, and easy-to-use optical coherence tomography (OCT) platform based on smartphones for personal care and point-of-care (POC) diagnostics can enable rapid and accurate diagnosis with reduced cost and time. However, current embodiments of commercially available OCT systems are bulky, expensive and only suitable for stationary use in clinical applications. This thesis describes the design and implementation of a novel, low-cost OCT technology called multiple reference OCT (MR-OCT) for next-generation smartphone based personal care and POC applications. MR-OCT is an extension of time-domain OCT (TD-OCT), with the addition of a partial mirror (PM) placed a small distance in front of the reference mirror (RM). The combination of the PM and the oscillating RM generates a composite reference signal based on multiple reflections between the partial mirror and the oscillating reference mirror to provide reference light corresponding to increasing depths. This arrangement enables MR-OCT to be implemented at low cost and as a miniature device. The results demonstrated in this thesis show the feasibility of MR-OCT and demonstrate a wide range of industrial, medical and consumer level applications. The first generation MR-OCT setup utilised a piezoelectric transducer (PZT) as a depth scanning mechanism. Although, a PZT transducer provides excellent linearity, the cost and size of the PZT controller is a limiting factor for producing a low-cost and compact MR-OCT system. Furthermore, PZT devices require a high driving voltage, which makes them less suitable for consumer applications. For this reason, the second generation MR-OCT setup utilised a voice coil motor (VCM) which was extracted from a CD/DVD-ROM. The dimensions of the VCM are approximately 15 mm × 15 mm and can be purchased for about 0.50 Euro. Experimental implementation of the MR-OCT system was based on a free-space Michelson interferometer. The VCM was driven at a frequency of 600 Hz and utilised forward and backward scans to obtain a total scan rate of 1200 A-lines per seconds. The system achieved an axial resolution of about 13 µm and a transverse resolution of about 27 µm (in air). Of the multiple reflections generated by the MR-OCT arrangement, up to 12 orders of reflections were investigated. The usefulness of orders higher than 12 was limited due to the transmission ratio of the PM. The measured sensitivity for the first and ninth orders were 89 dB and 78 dB with a phase stability of Δφ1 = 0.21 ± 0.14 rad and Δφ9 = 1.15 ± 0.7 rad. The capabilities of the system were demonstrated with various biological and nondestructive testing (NDT) samples. Fingerprint scanning on human subjects reveals both surface and sub-surface structure of the fingertip. This enables improved authentication by verifying compatibility between the conventional surface fingerprint and the sub-surface fingerprint and by also detecting liveness through a phase-sensitive method. NDT applications include the evaluation of coatings, quality control on pharmaceutical products such as capsules or tablets, and monitoring the deposition process of layers during the production of graphical displays and monitors. To demonstrate some POC applications, the MR-OCT system was combined with a dermascope to provide depth-resolved information from skin structures. The system can simultaneously register both the superficial dermascope image and the depth-resolved OCT subsurface information by an interactive beam steering method. A medical practitioner is able to obtain the depth-resolved information at the point of interest by simply using the mouse cursor. The proposed approach of combining a dermascope with a low-cost OCT provides a unique powerful optical imaging modality for dermatological applications. The results showed MR-OCT has the potential to detect structural changes with high accuracy and could become a powerful new tool for clinical applications. Furthermore, the moderate scan speeds of MR-OCT do not demand high specification computational platforms and mobile systems may be sufficient for its needs. In summary, this thesis describes a means to provide a 100 fold improvement in the size and cost of an OCT system and several applications are reported. Furthermore, this design is capable of miniaturization to a level to allow it to be integrated into a future generation of smartphones.