Technologies for Multiphoton 3D Endomicroscopy using MEMS Scanners
Creators: name: Birla, Mayur
Publisher: name: My University
Year: 2021
Subjects: subject: two photon
subject: MEMS mirror
subject: Electrothermal actuator
subject: Endomicroscopy
subject: phase estimation
subject: Biomedical Engineering
subject: FOS: Medical engineering
subject: Engineering (General)
subject: Mechanical Engineering
subject: FOS: Mechanical engineering
subject: Engineering
subject: Health Sciences

description: There has been growing interest in developing endomicroscopy systems as scientific tools for in-vivo imaging in small animals like mice. A well-designed endomicroscope for in-vivo imaging in mice could be used in assessing drug efficacy and study transient effects due to drug-inducing agents. Early-stage disease development could be studied by investigating molecular changes that precede the morphological changes in tissue. In-vivo assessment of the targeted biomarkers expressed within a tissue plays a vital role in such studies. An imaging modality capable of targeted imaging is well suited for this application. A major goal of this dissertation research is to develop such a miniature hand-held system capable of targeted multi-photon 3D imaging in small animals. Among a variety of laser-based imaging modalities, the multi-photon imaging technique has a high spatial resolution, reduced photodamage, and is capable of targeted imaging. These techniques are single-pixel imaging systems and require single or multiple scanners to steer the focal point in the lateral and/or axial directions. MEMS scanners are potentially well-suited to perform 3D laser focal point steering given stringent constraints on space in a typical endomicroscopy system. In this work, we present (i) an electrothermal scanning micro-mirror design with an unusually short thermal time constant that enhances dynamic operation near resonance; (ii) an algorithm for high-accuracy mechanical phase estimation using an image-based sharpness and blur metric; (iii) a two-photon 3D endomicroscopy instrument for imaging in small animals. The resonant frequency of a multi-axis MEMS scanner can drift by several degrees of phase angle due to environmental perturbations. This drift, in turn, produces a change in phase delay between mirror motion and the periodic input driving signal, adversely affecting the image quality. We demonstrate an image sharpness-based variance metric and blur-based threshold metric are well-suited to the compensation of unknown phase perturbations in Lissajous scanning with MEMS scanner. Sample images from a fluorescent confocal endomicroscope using a 2-axis parametrically-resonant micro-mirror are presented. Among a variety of transduction mechanisms, electrothermal scanners have demonstrated the largest total displacements and operate at low voltage, but are typically limited in speed by thermal response time and the amount of heat that can rapidly be dissipated in a small instrument. The electrothermal scanner uses a lattice-like honeycombed shaped silicon-dioxide support structure that results in extremely small mirror mass, both inertial and from a thermal capacity standpoint, with little increase in mirror curvature. The actuator features an unusually short thermal time constant that enhances dynamic operation near resonance. We demonstrate sample 3D multi-photon images of pollen grains taken at varying depths by scanning the electrothermal mirror in a benchtop imaging system. Motivated by the promising results from the benchtop system, we developed a handheld two-photon 3D endomicroscope for small animal models of disease. The system performance is optimized by ray-trace simulations and making design trade-offs during the process. A robust mechanical design is developed for easy alignment and packaging of optical elements, and minimizing the effect of dimensional and alignment tolerance. The performance of packaged distal probe is evaluated by capturing confocal reflectance images. Sample two-photon images taken using the miniature freespace coupled instrument are presented. Finally, we discuss potential methods to increase the versatility of the instruments, for instance, integrating a fiber for light delivery into the probe.