P1C-5 Transient Acoustic Radiation Force Elastography for HIFU Guidance and Monitoring

High intensity focused ultrasound (HIFU) has previously been used to destroy tumor tissue near the focus of the ultrasound beam. However, it can be difficult to predict in advance where the lesion associated with a given exposure will form. Furthermore, a means of verifying that the entire tumour mass has been ablated is required. In this study, a new technique was investigated in which the acoustic radiation force that can be generated by running the HIFU transducer for short (10 ms), sub-ablative exposures, was used to generate localised and transient stresses inside the medium. Commercial diagnostic ultrasound scanners and elastographic techniques for displacement tracking and strain estimation were then applied to image both the induced displacement and strain distributions. Each single sub- ablative pulse from the HIFU transducer was found to provide a useful way to help localise the focus of the ultrasound beam (i.e. before any lesions are formed). Multiple sub- ablative pulses on the other hand, where the position of the focus was translated between exposures and, at each focus position, the local induced strain was calculated, were shown to be useful for constructing a composite image of local strain inside the tissue. Such transient strain images show contrast for stiffness, which is considerably increased by thermal coagulation of tissue, and should therefore be helpful post-ablation, in revealing the location and extent of tissue damage. We conclude that elastographic displacement and strain imaging, in which transient acoustic radiation force is generated by the HIFU transducer, has considerable potential to be used in the guidance and monitoring of HIFU treatment.