Imaging Techniques to Monitor Photosensitizer and sO2 Levels During Photodynamic Therapy of Actinic Keratoses

Inclusion Criteria: * Participants of at least 18 years of age, with at least 10 AK lesions on the arms or legs, and with two AK lesions close enough to be seen together within a selected region of interest. * Female participants must not become pregnant during the study. The effects of 5-aminolevulinic acid (LevulanTM) on the human fetus are unknown. For this reason, participants of child-bearing potential must agree to use contraception. However, we note here that the vast majority of participants with chronic sun-induced AK lesions are beyond the age of menopause. * Participants must be able to understand and willing to sign a written informed consent document. Exclusion Criteria: * Pregnant or nursing. * Using any topical treatment on their AKs; must stop at least one month prior. * Currently undergoing treatment for other cancers with medical or radiation therapy. * Participants with a known hypersensitivity to 5-aminolevulinic acid or any component of the study material. * Participants with history of a photosensitivity disease, such as porphyria cutanea tarda.