PROPAGATION AND BACKPROPAGATION FOR ULTRASONIC WAVE-FRONT DESIGN

Wave backpropagation is a concept that can be used to calculate the ex citation signals for an array with programmable transmit waveforms to produce a specified field that has no significant evanescent wave comp onents. This concept can also be used to find the field at a distance away from an aperture based on measurements made in the aperture. For a uniform medium, three methods exist for the calculation of wave prop agation and backpropagation: the diffraction integral method, the angu lar spectrum method, and the shift-and-add method. The boundary condit ions that are usually implicitly assumed by these methods are analyzed , and the relationship between these methods are explored. The applica tion of the angular spectrum method to other kinds of boundary conditi ons is discussed, as is the relationship between wave backpropagation, phase conjugation, and the time-reversal mirror. Wave backpropagation is used, as an example, to calculate the excitation signals for a rin g transducer to produce a specified pulsatile plane wave with a limite d spatial extent.