Methods for determining focal point and delay for ultrasound arrays and multichannel electronics

Sammanfattning: High intensity focused ultrasound is a growing technique for tissue ablation, among other uses, and given its destructive capabilities, there is a need for control of where the energy is delivered. There exist a number of methods for focusing such ultrasound arrays, but these often assume prior knowledge of the impulse response, or require extensive full-system simulations. This thesis explores schemes for calculating impulse and frequency response of simple but still nonhomogeneous media, and implements different focusing methods, the spatiotemporal inverse filter, the Gerchberg-Saxton algorithm, and gradient descent, to test them. With a 128-channel transducer operating at 5 MHz, these techniques are carried out in a simulated 2D setting on water and concrete with first a straight edge and then an oblique one between the two media. With a focus depth of 5 cm, the techniques are able to clearly outperform the uncompensated results, and were able to produce feasible foci even for offset or multiple simultaneous foci locations. Although the optimization-based method did fail to produce adequate results for parts of the test, the overall investigation was seen as a successful venture, and that extension of the techniques to more complex media and 3D settings would be needed before any practical value can be realized.