Abstract The goal of this project is to develop a clinical real-time spectroscopic photoacoustic/ultrasound (PAUS) system for molecular guidance of interventional procedures through a partnership between UW and GE Research. Recently, we proposed a new, fast-sweep concept for PA imaging. To put this concept into practice, we first developed a unique, compact, diode-pumped, tunable (700 -900 nm) laser operating at very high (up to 1000 Hz) repetition rates and relatively low (~ 1 mJ) pulse energies, and a fiber-optic delivery system to sequentially couple laser pulses into the imaging probe. In addition to US B-mode, and all other US modes, the system simultaneously produces real-time (50 Hz) spectroscopic PA images, which were combined for the first time for real-time PAUS imaging. A unique feature is automatic on-line laser-fluence compensation and motion correction, enabling quantitative optical absorption spectroscopy at every image pixel. Spectroscopy can identify substances opaque to US based on their molecular constituents (drugs/contrast agents), and quantify tissue functional changes (e.g., blood oxygenation and its concentration) within the image; in addition, manipulation with a needle is better visualized with PA. UW will work with GE Research to integrate spectroscopic PAUS into a high-end US scanner to create a clinical-grade PAUS system, and test whether it can improve interventional procedure guidance in general and, particularly, in ethanol (EA) ablation therapies of recurrent thyroid tumors. The prognosis for most people with thyroid cancer after primary treatment is very good, but the recurrence rate or persistence can be up to 30%. If recurrent cancer is confirmed, image-guided nonsurgical procedures such as EA or radio frequency ablation (RFA) are commonly used alternatives to more invasive procedures. Although US helps position EA and RFA needles, on-line imaging of the ablative area and confirmation of ablation remain difficult for US. When the recurrent nodule (especially the capillary network in it) is not entirely treated, the cancer will return with possible metastasis. We hypothesize here that real-time spectroscopic PAUS will improve the efficacy of ablation procedures and dramatically reduce procedure repetitions. If successful in this initial stage, the project will move to a clinical trial to both guide and validate ablative therapies and explore real-time spectroscopic PAUS for other interventional procedures. SA1 will integrate our unique laser and scanning fiber-optic delivery system with a clinical GE US scanner for real-time spectroscopic PAUS. Then, SA2 will develop real-time signal processing tools for motion correction and fluence compensation and imaging protocols for spectroscopic PAUS. SA3 will focus on optimizing the PAUS system using phantom and ex vivo studies. Finally, in SA4 the developed PAUS system will be used to test the clinical applicability of PAUS guidance with three in vivo models, including ...