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bio-optics Gemma Church explains how LightTools, from Synopsys, makes tissue modelling more accessible to design next-gen diagnostics


T


he interaction of light with biological tissue is notoriously difficult to model. ‘Most users


of simulation design packages are accustomed to working with materials that are either very well characterised or easy to measure. Biological tissue is neither,’ Katherine Calabro, senior R&D engineer at Synopsys, explained. ‘It is unfortunately not


realistic to import a single material profile with static optical properties and assume it’s “close enough”. Tissue is not a material that has been manufactured to certain specifications in a controlled environment, and as such requires a greater level of knowledge and analysis on the part of the designer,’ she added. Synopsys is addressing


this issue in its LightTools illumination design software. By providing researchers with a tool to accurately model the interaction between light and biological tissue, they hope it will lead to the development of new diagnostic devices and methods.


Brilliant bio-optics The physics of light interaction with tissue is generally well understood, where scattering and absorption are the two dominant interactions. The effect of scattering can even be further broken down into how often the light is


20 Electro Optics April 2021


Providing researchers with a tool to accurately model the interaction between light and biological tissue could enable new diagnostic devices and methods


scattered in tissue and also in what direction. This angular distribution of scattering is known as the phase function. Auto-fluorescence is another effect, where light is naturally emitted by some biological tissues.


‘The difficulty, however,


is knowing how to measure each of these effects separately,’ Calabro explained. ‘Ultimately, the Holy Grail of a lot of biomedical optics research is knowing how to take an optical signal that is simultaneously affected by all the above interactions, and then decomposing it into separate measurements.’ Traditionally, biological tissue measurements were taken on animal tissues using standard measurement techniques like spectrophotometers. But this work was limited by the fact that the tissue is no longer living. Factors such as the oxygenation of blood, or water draining from the tissue, for example, were not taken into account – but these could have a dramatic effect on the scattering and absorption


“The greatest challenge in biological tissue modelling is variability. Not only measurement error and variability, but biological variability”


spectra in living tissue. ‘That is why we have seen such incredible work being done to develop devices and measurement techniques that are able to make measurements in the living body in a repeatable and reliable way,’ Calabro said. Much progress has been made in this area. Early tissue measurements were only done at a single wavelength. Now, measurements are conducted across the UV to the IR range of the electromagnetic spectrum. These full absorption spectra


can then be further decomposed into the concentrations of chromophores in the tissue: blood volume, blood oxygenation, water content, fat


content and so on, allowing optical measurements to be made to identify physiological markers. Simulation and modelling


are also established methods in the field of biomedical optics. A codebase named Monte Carlo for Multi-Layered media (MCML), for example, is a widely shared and used resource for biomedical optics researchers, helping them estimate optical interactions in tissue. Many of the analysis


techniques used to separate complex optical measurements into individual scattering and absorption components use data simulated by these Monte Carlo programs. But the greatest challenge


in the field of biological tissue modelling is variability, according to Calabro. ‘Not only measurement error and variability, but biological variability. Optical measurements can vary widely between different patients, but can also vary widely in the same organ in the same individual. Further,


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