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Although high-intensity focused ultrasound (HIFU) has exciting potential for noninvasively treating tumors and cardiac diseases, its clinical acceptance is hindered by the lack of a reliable and cost-effective method of noninvasively guiding and monitoring the treatment. The present study investigated the feasibility of employing optoacoustic imaging (OAI) for guiding and monitoring HIFU therapy. OAI combines molecular specificity provided by optical imaging and the resolution provided by diagnostic ultrasound. A 3-D optoacoustic imaging system was used to visualize thermal lesions produced in excised tissue specimens and in vivo mice using high intensity focused ultrasound (HIFU). A 7.5 MHz surgical, focused transducer with a radius of curvature of 35 mm and an aperture of 23 mm generated HIFU. A pulsed laser, which could operate at 755 nm and 1064 nm, illuminated the specimens. Tomographic images were obtained using a 64 element curved array while the specimens were rotated incrementally. Images were acquired before and after HIFU exposure. The images were then combined to reconstruct 3-D volume images (voxel resolution 0.5 mm). Optoacoustic images using 1064-nm illumination provided visualization of HIFU lesions. The location and the extent of the lesions were confirmed upon dissection. These preliminary results demonstrate the potential of optoacoustic imaging to assess and monitor HIFU therapy. [ABSTRACT FROM AUTHOR]