On-line assessment of HIFU beams and lesion monitoring using dual-transducer modes

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 with a central, A-mode transducer. Three operational modes were successfully tested in in-vitro liver specimens. 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 and its harmonics. Mode 2 detects 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 motion changes due to formed lesions. Mode 3 uses the A-mode transducer (pulse-echo) to sense lesions with increased attenuation and scattering using sliding-window spectrum analysis.