Quantitative Tissue Characterization with Optical Coherence Tomography
Authors:   , Kübler Johannes 
Published: February 2025

Eyesight is essential for daily life, from reading and recognizing faces to navigating and
appreciating the world around us. Vision loss profoundly impacts quality of life, making
eye health crucial for maintaining independence and well-being. Retinal diseases, in
particular, can lead to irreversible vision loss.
The eye’s optical properties make it particularly well-suited for noninvasive assessment.
Optical coherence tomography (OCT), which provides cross-sectional views of tissue, has become an essential tool for evaluating morphological defects and structural changes, such as retinal layer thinning. However, obtaining more precise quantitative results, like attenuation coefficient mapping, requires meticulous calibration of parameters such as focus position and Rayleigh length. Additionally, accurate lateral scaling of retinal OCT scans necessitates an estimate of the eye’s axial length. This thesis aims to refine these calibrations and further explore properties such as chromatic focal shift and dispersion.
An overview of the anatomy, physiology, and key pathologies of the human eye, with a
focus on the retina, is given in Chapter 1.
The theoretical foundation for OCT is briefly recapitulated in Chapter 2. Special
attention is given to the role of dispersion and aspects for attenuation coefficient imaging.
Methods for extracting the confocal function, which is essential for accurately determining
the attenuation coefficient from OCT data, are investigated in Chapter 3. Extensive measurements are conducted using different focus depths for both phantom models and in vivo retinal scans. The results demonstrate the feasibility of determining the focus plane and Rayleigh length from OCT scans acquired with different focus settings for samples with various concentrations of scatterers, enabling accurate and reproducible
attenuation coefficient measurements. The methods are also applied to scans of a layered phantom and in vivo retinal data.
The limitation of requiring different focus settings for confocal function extraction
is addressed in Chapter 4. By exploiting the chromatic focal shift of the OCT system, different focus positions are achieved for images reconstructed from sub-spectra. The
coherent addition of sub-spectrum confocal function corrected B-scans to produce high-resolution attenuation coefficient maps is demonstrated. The approach is validated in
both phantom models and in vivo retinal data, showing good agreement with theoretical
predictions. Additionally, the experimental determination of the chromatic focal shift of
an OCT system in vivo is demonstrated.
The final experimental Chapter 5 introduces a novel method for estimating the axial
length of the human eye using spectral domain OCT retinal scans. The approach considers the chromatic dispersion caused by the anterior segment and vitreous of the eye, relating axial shifts in sub-spectral scans to sample thicknesses. After validation with a model eye, the method is applied to in vivo data. Challenges such as inter-subject
variability and the need for precise chromatic dispersion data for ocular tissues within the relevant OCT spectral range are discussed. Using the dispersion of water, the refractive indices of an established eye model are interpolated from visible wavelengths to the infrared wavelengths of the OCT system and dispersion properties of the human lens are estimated.
The thesis concludes with Chapter 6 by discussing the results, emphasizing the potential of OCT data analysis to enhance quantitative accuracy through calibration. By compensating for system-specific parameters—including those related to the imaged
eye, which is integral to the imaging system in in vivo studies—these approaches can
reduce the need for separate calibration measurements that are often time-consuming,
labor-intensive, or require costly additional equipment.