Localized harmonic motion imaging for Focused Ultrasound Surgery targeting and treatment outcome

Focused Ultrasound Surgery (FUS) has been proposed as a noninvasive thermal therapy for diverse applications. FUS can generate a well-localized and fast rise of temperate that induces tissue coagulation. Recently, an in vivo real-time ultrasound-based monitoring technique that uses localized harmonic motion (LHM) to detect changes in tissues during FUS has been proposed to control the coagulation. This technique can potentially be used for targeting and post-treatment imaging. This will provide for an alternative imaging method to Magnetic Resonance Imaging which is currently the FDA-approved method for FUS treatment targeting and control. In the present study, we evaluated the feasibility of using LHM to detect changes in tissues stiffness and use it for FUS therapy targeting and follow up. A single-element FUS transducer (80-mm focal length, 100-mm diameter, 1.485 MHz) was used for inducing a localized harmonic motion. A separate ultrasound diagnostic transducer (5 MHz) was used to track tissue motion and the motion was estimated using cross-correlation techniques. Silicon phantom studies were performed in order to determine the hardness difference and size of inclusion that was possible to detect. On these phantoms, it was possible to detect inclusions as small as 4 mm. Seven New Zealand rabbits had VX2 tumors implanted on their thighs and LHM was induced and measured at the tumor region before and after FUS was carried out. LHM amplitude registered after FUS decreased all times and it was possible to detect lesions. Tumors were discerned from the surroundings as a reduction on LHM amplitude as compared to the surroundings. The sensitivity was low as well when small tumors were imaged.