Hydrophone scans of pressure patterns from a phased array transducer for high intensity focused ultrasound

The purpose of the research presented herein is to verify ultrasound beam simulations of a 256-element phased-array (PA) transducer for use in High Intensity Focused Ultrasound (HIFU) treatments. PA transducers are useful for their ability to electronically steer the focus by altering the signal phases on each element. Experimental measurements were performed using a hydrophone to scan in two-dimensional planes perpendicular to the ultrasound beam. An x-y stepper motor platform was assembled to map the pressure pattern within a tank of water. The pulsed signals received at discrete distances were processed by a peak-detector circuit that an A/D converter and a MATLAB computer program could sample. The resulting data were then plotted as a scaled image, or analyzed for power calculations using the hydrophone's power calibration. The actual pressure pattern showed close correlation to simulations using the Hybrid Angular Spectrum method. It was found that the beam's focal diameter was within 5% of the theoretical prediction.;Although an advantage in treatment flexibility, the electronic steering capability for a PA transducer comes at a cost. The disadvantage is that not all the power produced by the PA is applied at the focus, and even less when the focus is steered from its geometric focus. From power calculations in an area about the focus, the ratio of power in a 15-mm by 15-mm area to a 65-mm by 65-mm area when unsteered was 0.736 for experimental (versus 0.583 for simulation). When steered 5 mm, 10 mm and 15 mm in x and y, the power ratio decreased even more. The experimental values showed more power loss (30 % difference or more) than the simulations due partly to electrical cross-talk in the transducer channels and to experimental variability. Further investigation into the effects of electrical cross-talk is needed to adequately compare power measurements when steered.;Simulations do not (at this point in time) account for beam scattering that occurs in tissue. As another experiment, three agar phantoms were used to scatter the beam. The resulting focal diameter increased according to the phantom thickness and the acoustic characteristics of the phantoms.;The development of the accurate scanning system is useful in the characterization of HIFU transducers. The results obtained have been helpful in verifying the simulations and will lead to improvements in how simulations can be adjusted to improve HIFU treatments in the future.