Harmonic Motion Imaging – Applications in the Detection of Stiffer Masses

The technique of Harmonic Motion Imaging (HMI) utilizes the localized stimulus of the oscillatory ultrasonic radiation force as produced by two overlapping beams of distinct frequencies, and estimates the resulting harmonic displacement in the tissue in order to assess its underlying mechanical properties. In this paper, we studied the relationship between the measured displacement and the gel/tissue stiffness. Two focused transducers with a 100 mm focus were used at the frequencies of 3.7500 MHz and 3.7502 MHz (or, 3.7508 MHz depending on the case), respectively, in order to produce an oscillatory motion at 200 Hz in the gel (or, 800 Hz in the tissue). A 1.1 MHz diagnostic transducer (Imasonics, Inc.) was also focused at 100 mm and acquired RF signals of 5 ms in total duration (PRF =3.5 kHz) at 100 MHz sampling frequency during radiation force application. First, three acrylamide gels of 50 x 50 mm were prepared at concentrations of 4%, 8% and 16%. The resulting displacement was estimated using crosscorrelation techniques between successively acquired RF signals with a 2 mm window and 80% window overlap at 1260 W/cm. A 1-D normal indentation instrument (TeMPeST) applied oscillatory loads at 0.5-200 Hz with a 5 mm-diameter flat indenter. Then, 12 displacement measurements in six ex vivo porcine muscle specimens (2 measurements/case) were made using 1260 W/cm, before and after ablation for 10s at 1260 W/cm. In all gel cases, the harmonic displacement was found to steadily decrease with gel concentration. The TeMPeST measurements showed that the elastic moduli for the 4%, 8% and 16% gels equaled 3.93+/-0.06kPa, 17.1+/-0.2kPa and 75+/-2kPa, respectively; demonstrating, thus, that the HMI displacement estimate depends directly on the gel stiffness. Finally, in the ex vivo tissues, the mean displacement amplitude showed a two-fold decrease between non-ablated and ablated tissue; depicting thus the stiffness dependence of the HMI response in tissues.