Formation of desired waveform and focus structure by Time Reversal Acoustic focusing system

Abstract

The ultrasound focusing systems based on Time Reversal Acoustic (TRA) principles are capable of delivering ultrasound energy to the specific region in a highly inhomogeneous medium with focusing efficacy virtually unachievable using conventional phased array transmitters. Furthermore, numerous reflections from various boundaries, which distort focusing in conventional ultrasound focusing systems and are viewed as a significant technical hurdle, lead to improvement of the focusing ability of the TRA system. We present a novel approach to TRA focusing of ultrasound that allows: (i) calculation of the TRA focused ultrasound waveform and spatial distribution and (ii) formation of the TRA focused signal with desired waveforms. This approach is based on the measurements of impulse responses of the TRA focusing system (TRA FS) transducers. The proposed method requires creating a library of impulse responses recorded for each transducer of the TRA FS to short pulses radiated from a grid of points in the projected focal area. The developed approach for TRA waveform prediction was tested on a 5 channel TRA FS based on aluminum resonator with attached piezotransducers. The TRA FS provided focusing of ultrasound in the frequency band of 100-1000 kHz. The calculated and experimentally measured waveforms and spatial distributions were practically identical. The formation of ultrasonic signals with various predefined envelopes was demonstrated experimentally. We formed triangular, rectangular, and amplitude modulated tone burst signals with different frequencies in the focal region. Experiments were conducted for focusing of ultrasound to several points simultaneously and formation of focal area with the shape of an elongated ellipse was demonstrated. © 2006 IEEE.