Extended Depth of Field Optical Projection Tomography

Abstract

A limiting factor with regard to resolution in OPT is the limited depth of field (DoF) due to light detection with a Gaussian beam profile. The further a source in the sample is removed from the centre of rotation in the focal plane, the more distorted the image is in tangential direction due to the limited DoF. The goal of this research is to extend the depth of field in Optical Projection Tomography. The DoF limitations due to diffraction are mainly caused by the Gaussian beam shape and its inherent limitations such as a small high intensity spot and a small region of focus. An alternative to Gaussian beams is found sporadically in the literature in the form of non-diffracting beams. Non-diffracting beams are beams that propagate without diffraction and show regenerative properties after obstruction. The Bessel beam is a non-diffracting beam that is rotationally symmetric and displays a transversal high-intensity core. It can be generated without energy loss with a lens shaped like a rotationally symmetric prism, called an axicon.
A propagation simulation of the axicon generated Bessel beam, using the Hankel transform as a rotationally symmetric alternative to the 2D Fourier transform, is performed and used to verify the analytic description of the axicon generated Bessel beam. A numerical OPT simulation shows that OPT reconstructions of point sources show virtually no blurring, but do show concentric rings due to the intensity distribution of the Bessel beam. These rings can be removed by deconvolution of the projection or deconvolution of the reconstruction of simulated OPT results with the imaging point spread function (PSF) of the axicon-generated Bessel beam. The PSF describes the response of the imaging system to a point source.
Practical work is presented with the imaging set-up of an axicon with an objective lens as described earlier. The resolution of the PSF is analysed over paraxial distance from the objective lens for both coherent and incoherent illumination. The same is done for a resolution target in transmission. Comparison of the Bessel system with Gaussian models show that the DoF increase shown by the Bessel system is significant in all cases. It is found that for sources with spatially narrow intensity distributions (near-point source) the PSF resolution matches theoretical predictions. However, as the spatial light source distribution increases slightly, Bessel distributions overlap spatially. This creates artefacts and deteriorates the resolution
It is concluded that extended depth of field in OPT can be achieved with non-diffracting axicon generated Bessel beams. However, for objects larger than point sources the resolution deteriorates. Furthermore, the means of illumination are a major influence on the resulting images when using an axicon.
Further research on the optimization of illumination is recommended. Additionally, recommendations for further research on the significance of self-regeneration are made. Recommended applications for use of Bessel beams in optical imaging are those where a large DoF is desired and high resolution is less of a priority, or imaging and OPT of very sparse but large samples.