Time delay spectrometry methods for broadband characterization of plastics and tissue-mimicking materials for high intensity therapeutic ultrasound

Abstract

Because of the nonlinear nature of High Intensity Therapeutic Ultrasound (HITU), tissues and tissue-mimicking materials must be characterized over a broad frequency band. Time Delay Spectrometry (TDS) measures the frequency response of a system or sample by applying a swept frequency source and using a tracking receiver to select only those through-transmission signals that have the desired time delay. Due to the greater time-bandwidth product, TDS provides considerable signal-to-noise improvement over Fourier transform techniques. This system has been used previously to provide broadband measurements of complex hydrophone sensitivity. We have further refined our TDS system to provide frequency-dependent attenuation and phase velocity characterization of materials over the range from 1 MHz to 19 MHz. We have compared the results to predicted dispersion curves based on the Kramers-Kronig relations. The measurements of low-density polyethylene (LDPE) and a HITU-compatible tissue-mimicking material agree to within 1% for both TDS and PR acquisition methods, but the TDS measurements provide superior bandwidth over the PR. Measured phase velocity agrees to within 1% with Kramers-Kronig-based dispersion curves from 1 MHz to 19 MHz.