The optical properties of normal and diseased tissues are usually different despite the large variation of values in optical properties of the normal tissues alone. A case in point is the cancerous tissues that manifest significant architectural changes at the cellular and subcellular levels, and the cellular components that cause elastic scattering have dimensions typically on the order of visible to near-IR wavelengths. Some tumours are associated with vascularization, where blood causes increased light absorption. The optical contrast agents may also be made use of to enhance the optical contrast between normal and abnormal tissues.
Since tissues are optically turbid media that are highly scattering, light is quickly diffused inside tissues as a result of frequent scattering. The strong scattering has made optical detection of biological tissues challenging. A typical scattering coefficient for visible light in biological tissues is 100 cm-1 in comparison with 0.2 cm-1 for x-rays used in biomedical diagnostics. Light transmitted through the tissues is classified into three categories:
- Ballistic light
- Quasi-ballistic light
- Diffuse light
Ballistic light experiences no scattering by tissue and thus travels straight through the tissue. Ballistic light carries direct imaging information like the x-ray radiation. Quasi-ballistic light experiences minimal scattering and carries some imaging information. Multiply-scattered diffuse light carries little direct imaging information and overshadows ballistic or quasi-ballistic light in thick biological tissue.
One of the methods employed for optical tomography is termed to as “early-photon imaging”. If diffuse light is rejected, and ballistic or quasi-ballistic light is collected, buried objects can be detected. This technique uses a short-pulse laser (< 1 ps pulse width) to illuminate the tissue. Only the initial portion of transmitted light is allowed to pass to a light detector, and the late-arriving light is gated off by a fast optical gate. Since the ballistic or quasi-ballistic photons travel along the shortest path length, they arrive at the detector sooner than diffuse photons. If only ballistic light is detected, the technique is called ballistic imaging. It has been established that ballistic imaging is feasible only for tissue of thickness less than 1.4 mm or 42 mean free paths. Most ballistic imaging techniques have achieved approximately 30 mean free paths. Hence this, technique is appropriate for thin tissue samples but suffers loss of signal and resolution for thick tissues as a result of the strong scattering of light by the tissue. Please note that, this is a field that is in active research.
Related: Comparison of Imaging Modes – Ultrasound, CT & MRI
Application
Because the optical information is established by the molecular conformations of biological tissues, optical imaging is expected to provide sensitive signs for early cancer detection and monitoring.
Also Read: The Measurement of Tissue Optical Properties
