How can the depth of a localised fluorescence inclusion be estimated from the recorded fluorescence spectrum?

Fluorescence is, apart from visual inspection, probably the most well evaluated optical technique under development for tissue characterisation. The technique is based on laser light excitation. Parts of the light illuminating the tissue will be absorbed by various tissue chromophores. For some of these events, parts of the excess internal energy gained due to absorbed photons, will be released again by emission as fluorescence photons. There is always a loss in energy in this process, meaning that the emitted light is less energetic than the excitation light - there is a wavelength-shift in the emitted light. The spectrum of the emitted fluorescence is characteristic for the chromophore involved. Thus fluorescence spectra from tissue containing different chromophores could be distinguished. Fluorescence spectroscopy could thus potentially be used to identify the tissue type under examination. The group in Lund was one of the pioneering groups in this field and we have been developing fluorescence techniques for medical applications, mainly malignant tumour and cardiovascular tissue lesion identification, for more than 10 years now. Much of the research today involves clinical studies to determine the sensitivity and selectivity in detecting such lesions in different clinical specialities. We also perform research from a methodological point of view. We have identified that a recorded fluorescence spectrum can vary considerably depending on the detection geometry. This has been identified to be due to reabsorption of light as it passes through the tissue towards the detector. This effect complicates the comparison of results from different groups, but can also be used as an advantage. The fluorescence spectrum for a small localised fluorescent inclusion inside tissue will depend on how deeply the inclusion is located.

The aim of this project is to make an estimation of how accurately and robustly the depth of an inclusion can be determined from the spectral information.

The intention is that the project should include the following:

• Review the field of fluorescence for tissue identification and fluorescence molecular imaging. To do this properly, you should do a literature search. This review should be presented in the introduction to your final report. Look specifically for papers dealing with geometry dependence of the detected fluorescence signal as well as depth-resolved fluorescence spectroscopy measurements. Discuss the possibility to make depth estimates of an inclusion.
• Discuss the research and/or clinical need for utilising fluorescence spectroscopy for depth estimations of small inclusions.
• You should measure the optical properties at some fluorescence wavelengths of muscle tissues (during the laboratory exercises). Optical properties at shorter wavelengths are difficult to measure, and could instead be found in the literature.
• You should model a tissue with a small inclusion at several different depths to verify that you could predict the depth of the inclusion from the recorded spectral shape. Consider carefully what wavelengths to include in the modeling.
• If possible - it would be interesting if you could perform some measurements to verify the behaviour of your simulations (during the lab).   
• Discuss how such a method could be combined with fluorescence tomographic techniques to improve the estimations of position, size and shape of an inclusion.This is a question presently investigated and discussed at scientific conferences, meaning that you are not expected to find the ultimate answer.
• Discuss the importance of your findings in relation to available studies on tissue fluorescence for diagnostic purposes.

In the project at least these parameters should be considered: The light penetration for the wavelengths of interest, the tissue fluorescence properties, the detection geometry

Suggested key-words: fluorescence, laser-induced fluorescence, tissue diagnostics, malignant tumour, cancer