Looking out the optical window physical principles and instrumentation of imaging in photodynamic therapy

As a light-based treatment modality, photodynamic therapy (PDT) is inherently conducive to integration with optical imaging. e central principle of PDT is to leverage photochemistry that occurs following activation of a photosensitizing chemical [photosensitizer (PS)] using a light source of the appropriate wavelength and mode of delivery to achieve destruction of target tissues (Dougherty et al. 1998). Importantly, some degree of specicity is achieved as PSs have been almost universally observed to exhibit quasi-selective accumulation in neoplastic tissues, going back to the early observations of Policard, who studied accumulation of hematoporphyrin in rat sarcomas (Policard 1924). Since clinical PS should have little or no dark toxicity, an additional degree of selectivity is aorded by restriction of light to the target tissue. is basic photodynamic process, the fundamentals of which are discussed more extensively throughout this volume, has been developed and adapted for treatment of numerous cancer and noncancer pathologies at diverse anatomical sites using appropriate chemical modications of the PS and innovative light delivery strategies. Importantly, the same photosensitizing agents employed in PDT for targeted tissue destruction also have a nite probability to undergo a radiative transition back to the ground state following light absorption. In other words, PS can act as both therapeutic agents and diagnostic uorophores. erefore, upon illumination, longer wavelength uorescence emission is generated from the malignant tissues in which the PS accumulates, thus marking the tumor location and margins otherwise dicult to visualize. is process has been extensively leveraged to conrm PS uptake and localization and to guide surgical resection as discussed at length in the literature and reviewed elsewhere (Celli et al. 2010).