Investigations into high resolution optical fiber probe for targeted illumination and imaging around opaque obstacles
Date of Issue2018
School of Mechanical and Aerospace Engineering
Controlled illumination at targetted locations (targeted illumination) and imaging at high switching speed have found significant applications in different frontiers of science and technology such as optogenetics, photodynamic therapy (PDT), neurophotonics, and medical imaging. Although there are numerous optical systems reported with targeted illumination ability, most of these are modified standard (bench top) microscopes, hence not suitable for in vivo applications. Therefore, there is a critical need to develop optical imaging systems with targeted illumination suitable for in vivo studies. The small diameter, user-defined length and high mechanical flexibility of fiber optic imaging bundle (FOIB) facilitates its positioning at remote and difficult-toaccess in vivo sites. Recently, few FOIB based studies are reported, which have shown adaptation of targeted illumination. However, image resolution and switching speed achieved with these methods are found to be limited to 10 11m and 20 Hz, respectively. In order to investigate biological or chemical processes, the methodology adopted or equipment used should be able to provide higher resolution, faster-switching speed and an option for targeted illumination and imaging. Moreover, images obtained with FOIB probes are always affected by pixelation noise which deteriorates the image resolution and contrast. Though there are many algorithms to remove pixelation noise, objective comparison of these algorithms are not possible due to the non-availability of a common test image database. Also, there are no theoretical models available currently to simulate the image guidance through FOm. From this perspective, one of the objectives of this thesis is aimed at developing a high-speed imaging probe using a combination of FOffi and Digital Micromirror Device (DMD) with targeted illumination and imaging feature for potential in vivo applications. Compared to the earlier FOffi based probes, the newly developed probe has shown improved lateral and axial image resolution of2.7 Jlm and 5.5 J.Lm, respectively. This imaging system also provides a larger field of view (200 J.Lm X 200 J.Lm) at high resolution compared to earlier reported targeted FOffi probes. The developed FOffi probe's illumination switching speed is defined by the DMD, which is 10,000 Hz. The multiline digitally controlled laser source allows the FOffi probe to illuminate the targeted regions with different wavelengths. Further, an objective comparison of different depixelation methods for FOIB imaging is also performed as part of the investigation. A theoretical model for FOffi based imaging is developed and used to generate images of simulated fiber pixelated 1mages. The parameters such as packing fraction, fiberlet to fiberlet distance, fiberlet core diameter, core-cladding properties and light guiding properties of fiberlet are considered in the theoretical formulations. These studies have led to the development of a Fiber Pixelated Image Database (FPID), which now serves as a free open source common test image sample database for researchers working on the development of novel depixelation methods. The effect of variable pinhole size on the imaging properties is also studied using the proposed probe. An illustrative demonstration is carried out for four different imaging approaches using this targeted FOffi probe. These approaches are 'targeted confocal imaging', 'multi-directional scanning in targeted sample regions', 'targeted time averaged imaging for contrast enhancement', and 'single shot multitarget multispectral imaging', which were found to improve resolution, contrast, and imaging speed. The potential of the FOffi probe is demonstrated by synchronous multispectral spatiotemporal illumination of targeted mouse kidney cells. Additionally, the efficiency of this FOffi probe for tracking and targeted illumination of dynamic (moving) particles is demonstrated as a proof of concept. This developed FOIB probe ·has also been demonstrated as a portable targeted illumination source for the standard bench-top microscope. The second major objective of this thesis is aimed at imaging around opaque obstacles. Since there are numerous injuries (cuts) reported due to blind injection or improper position of the surgical tool, caused due to the blocking of the field of view of the sample by opaque surgical tools during surgery, an imaging system capable of imaging around obstacle would be helpful in avoiding such injuries. Hence in this thesis, a detailed research is carried out to come up with an optical system to image around obstacles. In this thesis, use of an axicon lens is explored for imaging around opaque obstacles. The simulation of axicon lens to perform imaging around an opaque obstacle is demonstrated using Zemax software. This is further validated experimentally by imaging around different thick opaque obstacles of different shape and thickness, such as Allen key, syringe needle, metallic pin, hair, and thread. Finally, the proof of concept of this method is demonstrated by imaging around a surgical needle during needle injection procedure. It is envisaged that the invaluable advantages provided by the targeted probe along with its specialty features can make a great impact in the research and developmental arena such as optogenetics to selectively activate neuronal cells or cell organelles. Further the work done on the imaging round opaque obstacles are expected to contribute significantly during surgical procedures in the near future.