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Role of optical diagnostics in the early detection of oral mucosal cancers


Dr T Balasubramanian


Introduction:


When light passes through tissue it undergoes various modifications depending on tissue biological properties. Analysis of this light will provide vital insights into the physiological condtion of the tissue at a molecular level. This property of light is made use of in early diagnosis of superficial mucosal malignant lesions. Studies have demonstrated that many of the biological events leading on to development of malignancies also alter the optical properties of the tissue causing them to produce characteristic optical signatures which can be easily recorded. Various types of light based detection systems have been developed to identify these optical signatures.


Advantages of optical diagnostic system:


  1. It is non invasive

  2. It can precisely identify suspicious lesions for specific biopsy procedures to be performed

  3. It has the potential to provide real time tissue assessment


Types of optical diagnostic system:


  1. Spectoscopy

  2. Fluorescence spectroscopy

  3. Elastic scattering spectroscopy

  4. Raman spectroscopy

  5. Fluorescence imaging

  6. Optical coherence tomography

  7. Narrow band imaging

  8. Multimodal optical imaging


Spectroscopy:


This method is used to analyse the changes undergone by light passing through tissue. The tissue while undergoing changes from normal to neoplasia undergoes complex molecular transformation. These transformations modifies the manner in which light is absorbed / reflected from the tissue. While performing spectroscopic analysis of tissue, the light of specific wave length is delivered to the tissue via an optical probe. The spectral pattern produced by the light reacting with the tissue is analysed for light – tissue interactions.


Fluorescence spectroscopy:

This study is based on biological emission of fluorescent light when the tissue studied is exposed to ultraviolet / short wavelength visible light. Light is composed of energy packages known as photons. Tissues on being exposed to light may cause absorption / reflection / scattering of photons. Tissues are known to contain light reactive biomolecules known as fluoropores. These fluoropores absorb energy from light and emit fluorescent light of lower energy and longer wave length. Study of this emitted light from tissue will provide valuable insight of biological status of the tissue.

The concentration and distribution of fluorophores vary as the tissue undergoes malignant transformation. This becomes clearly evident in the spectroscopic pattern produced.


Elastic scattering spectroscopy:


This is also known as diffuse reflectance spectroscopy. Here white light (400 – 700 micrometer) is used to illuminate the tissue under study. Photons from this visible light gets reflected from the tissue without a change in wavelength / energy. The reflected light can be measured to look for changes. This reflected light is known as scattering. Tissues with malignant potential show different scattering pattern.


Raman spectroscopy:

This method can be used to provide detailed information about molecular composition of tissue. Raman spectra is generated by inelastic scattering of light which is highly molecule specific. The light source used is usually in the infra red spectrum. Infrared light on striking the tissue undergoes wavelength shift which is caused by energy transfer between incident photon and tissue molecules. This subtle shift in wavelength of light can be spectroscopically analysed.


Fluorescence imaging:

These system classically use the principle of spectroscopy to capture fluorescence emission from a larger surface area of tissue. The image is usually captured in real time using a camera. The tissue is illuminated with light source generating light in the uv range. These systems are capable of capturing images from large tissue areas.


Optical coherance tomography:

This imaging technique provides high resolution cross sectional images of the tissue studied. The technology involved is most similar to B scan (Ultrasound). This technique detects back scattered light waves after the tissue is probed with low power infrared light source. The optical probe used is about 2 mm in diameter. The tissue in study can be scanned in transverse / linear / radial fashion.


Narrow band imaging:

This technique helps in visualizing microvasculature of surface mucosa. Optical interference filters in red / green / blue range are used to limit the penetration of tissue by light. Hemoglobin is known to absorb blue light. Just by studying tissue absorption of blue spectrum of light an indirect assessment of vascularity of the tissue studied can be made. Since malignant transformation of tissue is assocaited with formation of new blood vessels this imaging modality shows clearly the malignant transformation of tissue.

Multimodal optical imaging:

These devices make use of 2 / more technologies descussed above. These devices can also be used to assess tissue in real time. Since it involves more than one spectroscopic imaging technique the results are fairly accurate.

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