How does light of various wavelengths move through tissue?
How does tissue fluorescence at various wavelengths?
How does light elastically scatter from tissue?
(elastic scattering spectroscopy)
How does light inelastically scatter?
(IR Raman scattering)
How do photons propagate coherently through tissue?
How do coherent photons reflect from tissue?
(optical coherence tomography)
How is light absorbed by tissues to generate sound waves?
(photoacoustic spectroscopy and imaging)
Light can be delivered/collected via optical fibers which can access remote sites within the body via needles or endoscopic catheters. Such remote assessment is a key strength of optical techniques.
Light does not penetrate deeply in tissues. Visible light penetrates only a few mm through tissues. Near infrared light penetrates only a few cm through tissues. This is both a strength and a weakness. It is a weakness because light can only interrogate limited volumes of tissues. It is a strength because much of the body consists of thin tissue layers, therefore optical techniques are well-suited for localized interrogation of tissue layers. Many medical decisions are based on the status of thin tissue layers.
Medical applications fall into two general categories:
Assessing tissues without removal of tissue is a great advantage. A tissue biopsy removes tissue which yields a wound that requires healing. Healing may cause short-term discomfort to the patient and may yield a scar that interferes with later re-examination or simply looks bad. Multi-wavelength spectroscopy of tissues allows quantification of the tissue components contributing to the overall optical spectrum of a particular tissue site. Hence, a tissue can be discriminated by its components without requiring tissue removal for analysis. Such optical assessment is sometimes called an optical biopsy.
Such spectral analysis can be implemented using absorption spectroscopy, fluorescence spectroscopy, IR Raman spectroscopy, and other spectroscopic techniques. There are steady-state, time-resolved, and frequency-domain techniques.
Some clinical examples of spectroscopy currently being developed:
Detection of an object within a tissue (e.g., a tumor) or mapping of functional status within a tissue (e.g., blood perfusion) requires more than just spectroscopy. The spatial distribution of a measured parameter must be mapped. Measurements at multiple positions provide the "triangulation" needed to yield a mapping which is the 3D reconstruction of a tissue based on some spectroscopically defined mode of contrast. The mode of contrast can be based either absorption, scattering, fluorescence, or IR Raman scattering.
Combining spectroscopy with imaging yields a spectrally weighted image. Optical images are usually fuzzy, but they have spectral content and can yield functional mappings. For example: