A Review of Mechanisms of Contrast for Diffuse Optical Imaging of Cancer

Abstract

Diffuse optical imaging and spectroscopy have been extensively used to detect, diagnose and monitor cancer. This paper is a review not of cancer in general, but of the biological changes that occur that can provide contrast for near infrared (NIR) techniques. Cancer is defined by characteristics such as the development of new blood vessels, changes in metabolism and the ability to invade surrounding tissue. The sensitivity of NIR techniques to haemoglobin, water and lipid concentrations, tissue oxygen saturation and cell density means that the pathological changes that occur in malignant tissues can provide contrast between healthy and pathological tissue. Each of these contrast mechanisms are discussed in turn, with a short introduction to the physiological changes that occur, and then a review of the NIR techniques that can be used to investigate these changes and findings from published papers. The role of exogenous contrast agents with NIR techniques in cancer diagnosis and assessment are also discussed. By understanding the characteristics of cancer and the changes that occur, we can interpret our results from NIR with greater confidence and accuracy and develop new techniques that target these characteristics. Information on angiogenesis or hypoxia can be clinically useful, especially when planning treatment (hypoxia) or predicting outcome to chemotherapy (haemoglobin concentration can be used as a measure of vessel density). By understanding the characteristics of cancer and the changes that occur, we can interpret our results from NIR with greater confidence and accuracy and develop new techniques that target these characteristics.