The long-term objective of the proposed research is to develop a novel diagnostic tool for microscopic imaging of superficial lesions that can be accessed either non-invasively or minimally invasively. The proposed technique, high frequency photoacoustic tomography (PAT), can image intact biological tissues at the microscopic scale in vivo based on optical contrast. Optical contrast is sensitive to the molecular conformation of biological tissues and is related to physiological states such as the oxygenation of hemoglobin. Applications include the diagnostics of, for example, skin cancer, cervical cancer, esophageal lesions, and vulnerable atherosclerotic plaques. PAT can also potentially be used to spatially resolve molecular beacons for high-resolution and high-sensitivity imaging. The proposed PAT combines the contrast advantage of optical imaging with the resolution advantage of ultrasound imaging. The proposed technology does not depend on ballistic or backscattered light as optical coherence tomography (OCT) does. Any light, including both singly and multiply scattered photons, contributes to the imaging signal; as a result, the imaging depth in PAT is excellent. The resolution is diffraction limited by the high-frequency photoacoustic waves rather than the optical diffusion; consequently, the resolution of PAT is excellent. Furthermore, PAT is free of the speckle artifacts present in OCT and pulse-echo ultrasonography, two analogous technologies. PAT images of unprecedented high quality have been obtained by the Texas A&M groups. In the proposed research, PAT and high-frequency ultrasound imaging will be performed on the same cross sections of tissue. This multi-modality approach will allow us to obtain both optical contrast and acoustic contrast, and the dual contrasts are expected to provide greater diagnostic value. A unique strength of the proposed research is the multi-disciplinary team and the support of the NIH Resource on Medical Ultrasonic Transducer Technology, which has been funded to pursue state-of-the-art research in the development of high-frequency ultrasound transducers/arrays. The specific aims of the proposed research, in which the animal experiments will have dual foci--the imaging of skin cancers (animal model 1) and the imaging of atherosclerotic plaques (animal model 2), are as follows: Aim 1. Develop a 30-MHz PAT system to image biological tissues with both high optical contrast and high resolution in real-time acquisition. Aim 2. Characterize the imaging capability of the proposed hand-held probe by imaging skin cancers in vivo in a mouse model (animal model 1). Aim 3. Characterize the imaging capability of the proposed hand-held probe by imaging atherosclerotic plaques ex vivo in a rabbit model (animal model 2).