Nonlinear and Non-Stationary Signal Analysis of Ultrasound Contrast Agents

Abstract

Ultrasound contrast agents (UCA) are encapsulated gas bubbles which are utilized in medical ultrasound imaging for their nonlinear scattering signatures. The subharmonic response and threshold of UCAs have been investigated at the standard diagnostic frequencies (1-10 MHz) and more recently, in the high frequency ultrasound regime ( > 15 MHz ). The optimal design for the subharmonic response of UCAs for high frequency applications is an on-going research topic. Furthermore, studies have shown that a chirp (frequency modulated) excitation significantly in-creases the subharmonic signals from UCAs. Previous studies have used a 20 dB spectral criteria for the detection of a subharmonic event. However, this may have a limitation for non-stationary, chirp excitations. The subharmonic response of UCAs from a high frequency, chirp pulse is investigated numerically and experimentally using a spectral analysis and the Empirical Mode Decomposition (EMD) method. The EMD decomposes a signal into its intrinsic mode function (IMF) in which the IMFs form an orthogonal basis of the signal. The scattered pressure signal from a mono-frequency (40 MHz) and chirp excitation (30-50 MHz) is calculated numerically. Subharmonic thresholds determined from the relative peak level from the power spectrum are compared to those from a novel method based on the energy ratio of the second IMF component. The instan-taneous phases and frequencies were obtained from the respective IMF components. Enhanced subharmonic responses are related to the phase locking between the fundamental and subharmonic components. A lower thresholds were found for the chirp excitations. Also, thresh-old values were compared with a previously derived theoretical expression. Limitations of the spectral threshold criteria are quantified for short, non-stationary excitations. Furthermore, these methods are implemented on the experimental data from two types of polymer UCAs (Phillips and Point Biomedical) excited by a mono-frequency (20 MHz) and linear chirp (15-25 MHz) pulse for incident pressures from 0.5 MPa to 2.4 MPa. Subharmonic thresholds of 1.0 MPa and 2.0 MPa for the Phillips UCA and Point Biomedical UCA, respectively, for a chirp excitation was determined. The subharmonic response of two interacting UCAs was investigated numerically. The changes in the subharmonic response are related to the phase difference between the two funda-mental responses with the EMD method. In a separate research study, the acoustic emission from an ice fracture was related to its mechanical properties and the associated acoustic signals were analyzed using the EMD and Teager-Kaiser Energy Operator.