Tissue Characterization by Low-Frequency Acoustic Waves Generated by a Single High-Frequency Focused Ultrasound Beam.

The mechanical properties of biological tissues are fingerprints of certain pathologic processes. Ultrasound systems have been used as a non-invasive technique to both induce kilohertz-frequency mechanical vibrations and detect waves resulting from interactions with biological structures. However, existing methodologies to produce kilohertz-frequency mechanical vibrations using ultrasound require the use of variable-frequency, dual-frequency and high-power systems. Here, we propose and demonstrate the use of bursts of megahertz- frequency acoustic radiation to observe kilohertz-frequency mechanical responses in biological tissues. Femoral bones were obtained from 10 healthy mice and 10 mice in which osteoporosis had been induced. The bones' porosity, trabecular number, trabecular spacing, connectivity and connectivity density were determined using micro-computed tomography (µCT). The samples were irradiated with short, focused acoustic radiation pulses (f = 3.1 MHz, t = 15 µs), and the low-frequency acoustic response (1-100 kHz) was acquired using a dedicated hydrophone. A strong correlation between the spectral maps of the acquired signals and the µCT data was found. In a subsequent evaluation, soft tissue stiffness measurements were performed with a gel wax-based tissue-mimicking phantom containing three spherical inclusions of the same type of gel but different densities and Young's moduli, yet with approximately the same echogenicity. Conventional B-mode ultrasound was unable to image the inclusions, while the novel technique proposed here showed good image contrast.