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Methods for the bulk ablation of soft tissue using intense ultrasound, with potential applications in the thermal treatment of focal tumors, are presented. An approximate analytic model for bulk ablation predicts the progress of ablation based on tissue properties, spatially averaged ultrasonic heat deposition, and perfusion. The approximate model allows the prediction of threshold acoustic powers required for ablation in vivo as well as the comparison of cases with different starting temperatures and perfusion characteristics, such as typical in vivo and ex vivo experiments. In a full three-dimensional numerical model, heat deposition from array transducers is computed using the Fresnel approximation and heat transfer in tissue is computed by finite differences, accounting for heating changes caused by boiling and thermal dose-dependent absorption. Similar ablation trends due to perfusion effects are predicted by both the simple analytic model and the full numerical model. Comparisons with experimental results show the efficacy of both models in predicting tissue ablation effects. Phenomena illustrated by the simulations and experiments include power thresholds for in vivo ablation, differences between in vivo and ex vivo lesioning for comparable source conditions, the effect of tissue boiling and absorption changes on ablation depth, and the performance of a continuous rotational scanning method suitable for interstitial bulk ablation of soft tissue.

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Minimally invasive, miniature (2.2- × 50-mm aperture, 3.3-mm diameter) dual-mode linear arrays have been developed into low-cost disposable probes with high acoustic power output (120 W/cm2 at the source), high transmit efficiency (>65% typical), and good imaging performance (50% fractional bandwidth, >100-mm-deep field of view). These therapy/imaging probes have been integrated into a flexible intense ultrasound surgery platform which also includes conventional diagnostic imaging probes. A system architecture has been developed which includes a 64-channel therapy driver with software selection of array aperture and phasing (λ/16), frequency (0.5–8 MHz), drive amplitude (5 W/channel, nominal), rotational steering (±180 deg), and temporal sequencing/switching of imaging/therapy/monitoring modes. System software includes graphical and text-based script mode control of therapeutic treatment. Real-time monitoring of electric power per channel, temperature sensors, and thermal effects provide a range of feedback and safety. Numerous system and probe technological issues such as electrical interconnect and matching, acoustic coupling, thermal control, and maintaining probe efficiency have been addressed. The array-based imaging/therapy system has produced encouraging results in preclinical studies of bulk tissue ablation and imaging.

A potential alternative to extracorporeal, noninvasive HIFU therapy is minimally invasive, interstitial ultrasound ablation that can be performed laparoscopically or percutaneously. Research in this area at Guided Therapy Systems and Ethicon Endo-Surgery has included development of miniaturized (∼3 mm diameter) linear ultrasound arrays capable of high power for bulk tissue ablation as well as broad bandwidth for imaging. An integrated control system allows therapy planning and automated treatment guided by real-time interstitial B-scan imaging. Image quality, challenging because of limited probe dimensions and channel count, is aided by signal processing techniques that improve image definition and contrast. Simulations of ultrasonic heat deposition, bio-heat transfer, and tissue modification provide understanding and guidance for development of treatment strategies. Results from in vitro and in vivo ablation experiments, together with corresponding simulations, will be described. Using methods of rotational scanning, this approach is shown to be capable of clinically relevant ablation rates and volumes.

For thermal treatment of soft tissue, an alternative to HIFU is bulk ablation using unfocused or weakly focused intense ultrasound fields. This approach offers faster ablation of large tissue volumes and can be performed in minimally invasive (e.g., laparoscopic or percutaneous) procedures. Here, methods for image-guided ablation of large tissue volumes using compact dual-modality (image and treat) ultrasound arrays are reported including tissue modification caused by the thermal therapy. The dual-modality arrays developed have 16-64 elements spanning apertures of 2-8 mm in elevation and 24-48 mm in azimuth. These devices can provide both therapeutically significant power (e.g. source intensity > 80 W/cm 2 at 3.1 MHz) and broad bandwidth (e.g. 50% with a center frequency of 3.5 MHz) for imaging. Imaging challenges associated with limited probe dimensions and channel count are met using signal processing techniques that improve definition and contrast, allowing high-quality B-scan images and useful monitoring information to be obtained during therapy planning and treatment. Using linear and rotational scanning methods, large tissue volumes (20-60 cc) can be treated. The approach can be applied for ablation of other soft tissue pathologies, e.g., kidney, heart, uterus, brain, GI, etc.