Respiratory motion during high-intensity focused ultrasound (HIFU) therapy reduces the efficiency of the treatment in abdominal organs. Hence, for the purpose of motion-correction during HIFU therapy, two different ultrasound-based 3-D motion-tracking methods were presented and adapted to the existing Philips Sonalleve MR-HIFU platform. The displacement estimation accuracies of these two techniques were determined using a metal pin and an in vitro tissue sample as targets. The measurement data was collected on all movement directions using element clusters consisting of one, three and 32 transmitting transducer elements. Simulations of the acoustic fields were also performed in order to discuss the theoretical limitations of the motion-tracking with the existing HIFU system.The displacement estimation accuracies did not differ significantly for the two different techniques introduced but was rather dependent on the transmitting element cluster size and the orientation of the ultrasound beam axis. Using smaller number of elements and defining the beam axis angle accurately in the calculation algorithm yielded more stable results. The motion-tracking using the in vitro tissue sample was significantly more difficult to achieve than with the metal pin target. This was due to the incorrect time-shift values given by the cross-correlation algorithm. Hence, the failed channels had to be manually excluded from the calculations in order to yield the correct displacement estimation values.The introduced motion-tracking methods cannot be readily used as such during clinical HIFU therapy treatment, because the incorrect time-shift values had to be manually excluded from the calculation algorithm. This process could probably be automated by observing the peak count and the amplitude of the cross-correlation curves. However, this method was not verified in this thesis and hence requires further research about its feasibility.
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