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Background, Motivation and Objective: High-intensity focused ultrasound (HIFU) has primarily been performed using a specialized ultrasound transducer for therapy and either a separate transducer for imaging or a different imaging modality. Accurate localization and monitoring of HIFU treatment is important to improving the efficacy of the treatment as well as minimizing complications. The goal of this work was to demonstrate the feasibility of using a diagnostic ultrasound system to perform spot ablations in liver with concurrent Acoustic Radiation Force Impulse (ARFI) stiffness imaging in order to monitor lesion formation. Statement of Contribution/Methods: A diagnostic ultrasound system (Siemens Antares? and CH6-2 curvilinear array) was used to both: (1) ablate ex vivo liver samples with a custom M-mode sequence and (2) to monitor the resulting tissue stiffening with 2-D ARFI imaging. Ablation patterns were generated using a grid of varying numbers of heating locations. Results: ARFI images showed irreversible liver stiffening with heating that corresponded to discolored regions in gross pathology. Images were taken before and after ablation, as well as in 5-second intervals during ablation to monitor the increase in stiffness contrast and extent with time. No gaseous body formation was observed during the ablations in B-mode, thus lesions were not visualized in matched B-mode images. In order to mimic the presence of a stiffer tumor, bovine muscle tissue was inserted into a liver sample, and subtraction images (pre-post ARFI) clearly distinguished the ablated (stiffened) liver tissue from the stiffer bovine tissue. Discussion and Conclusions: This study demonstrated the ability of a diagnostic system using custom beam sequences to localize an ablation site, heat the site to the point of irreversible damage, and monitor the formation of the ablation lesion. Future work will involve testing this treatment system in vivo.