Session: 1K

A dual-transducer system was used to test concepts for on-line guidance and lesion monitoring during ultrasound therapy. The system used a spherical-cap therapy transducer (4 MHz; 40-mm dia.; 90-mm focus) with a central, collinear transducer (7.5 MHz; 10-mm dia.) for A-mode examination. A digital system provided 3 operational modes, tested in in-vitro liver. Mode 1 senses in-situ therapy-beam harmonics that affect absorption and heating. The therapy transducer emits a 2-μs pulse at the same amplitude used in therapy; echoes from this pulse are received with the A-mode transducer and digitally filtered to produce A-scans at the therapy frequency (4 MHz) and its harmonics (8 MHz). Ratios of these echoes provide a semi-quantitative measure of therapy-beam harmonics in target tissues. In liver, this method successfully tracked second harmonic generation, which increased with increasing excitation voltage. This mode can also aid lesion detection: preand posttherapy harmonic data are compared to detect changes in B/A and to sense bubbles formed during therapy. Mode 2 uses these transducers to detect lesions by virtue of their increased stiffness. A 1-ms therapy-transducer pulse produces radiation force and internal tissue motion that is tracked by cross-correlating sequential A-scans. The procedure is repeated after therapy to detect decreased motion due to increased Young’s modulus. This mode successfully detected lesions in liver where motion decreased from 15 μm (pre-) to 9 μm (post-treatment). Mode 3 uses the A-mode transducer to sense lesions with increased attenuation and / or scattering. Sliding-window spectrum analysis is applied to RF A-mode data to determine the slope and midband fit of tissue spectra (in dB) vs range. Ratios of preand post-treatment data from posterior tissues document changes in cumulative attenuation in the treatment region. Scattering changes in lesions are also documented. All modes were successfully tested in-vitro and are incorporated in a new system using arrays to image features derived in each mode.