Effect of Overpressure on Acoustic Emissions and Treated Tissue Histology in ex Vivo Bulk Ultrasound Ablation.

Bulk ultrasound ablation is a thermal therapy approach in which tissue is heated by unfocused or weakly focused sonication (average intensities on the order of 100 W/cm2) to achieve coagulative necrosis within a few minutes exposure time. Assessing the role of bubble activity, including acoustic cavitation and tissue vaporization, in bulk ultrasound ablation may help in making bulk ultrasound ablation safer and more effective for clinical applications. Here, two series of ex vivo ablation trials were conducted to investigate the role of bubble activity and tissue vaporization in bulk ultrasound ablation. Fresh bovine liver tissue was ablated with unfocused, continuous-wave ultrasound using ultrasound image-ablate arrays sonicating at 31 W/cm2 (0.9 MPa amplitude) for either 20 min at a frequency of 3.1 MHz or 10 min at 4.8 MHz. Tissue specimens were maintained at a static overpressure of either 0.52 or 1.2 MPa to suppress bubble activity and tissue vaporization or at atmospheric pressure for control groups. A passive cavitation detector was used to record subharmonic (1.55 or 2.4 MHz), broadband (1.2-1.5 MHz) and low-frequency (5-20 kHz) acoustic emissions. Treated tissue was stained with 2% triphenyl tetrazolium chloride to evaluate thermal lesion dimensions. Subharmonic emissions were significantly reduced in overpressure groups compared with control groups. Correlations observed between acoustic emissions and lesion dimensions were significant and positive for the 3.1-MHz series, but significant and negative for the 4.8-MHz series. The results indicate that for bulk ultrasound ablation, where both acoustic cavitation and tissue vaporization are possible, bubble activity can enhance ablation in the absence of tissue vaporization, but can reduce thermal lesion dimensions in the presence of vaporization.

In vivo thermal ablation monitoring using ultrasound echo decorrelation imaging.

Previous work indicated that ultrasound echo decorrelation imaging can track and quantify changes in echo signals to predict thermal damage during in vitro radiofrequency ablation (RFA). In the in vivo studies reported here, the feasibility of using echo decorrelation imaging as a treatment monitoring tool was assessed. RFA was performed on normal swine liver (N = 5), and ultrasound ablation using image-ablate arrays was performed on rabbit liver implanted with VX2 tumors (N = 2). Echo decorrelation and integrated backscatter were computed from Hilbert transformed pulse-echo data acquired during RFA and ultrasound ablation treatments. Receiver operating characteristic (ROC) curves were employed to assess the ability of echo decorrelation imaging and integrated backscatter to predict ablation. Area under the ROC curves (AUROC) was determined for RFA and ultrasound ablation using echo decorrelation imaging. Ablation was predicted more accurately using echo decorrelation imaging (AUROC = 0.832 and 0.776 for RFA and ultrasound ablation, respectively) than using integrated backscatter (AUROC = 0.734 and 0.494).

Treatment of rabbit liver cancer in vivo using miniaturized image-ablate ultrasound arrays.

In the preclinical studies reported here, VX2 cancer within rabbit liver has been treated by bulk ultrasound ablation employing miniaturized image-ablate arrays. Array probes were constructed with 32 elements in a 2.3 × 20 mm(2) aperture, packaged within a 3.1 mm stainless steel tube with a cooling and coupling balloon for in vivo use. The probes were measured capable of 50% fractional bandwidth for pulse-echo imaging (center frequency 4.4 MHz) with >110 W/cm(2) surface intensity available at sonication frequencies 3.5 and 4.8 MHz. B-scan imaging performance of the arrays was measured to be comparable to larger diagnostic linear arrays, although nearfield image quality was reduced by ringdown artifacts. A series of in vivo ablation procedures was performed using an unfocused 32-element aperture firing at 4.8 MHz with exposure durations 20-70.5 s and in situ spatial average, temporal average intensities 22.4-38.5 W/cm(2). Ablation of a complete tumor cross-section was confirmed by vital staining in seven of 12 exposures, with four exposures ablating an additional margin >1 mm beyond the tumor in all directions. Analysis suggests a threshold ablation effect, with complete ablation of tumor cross-sections for exposures with delivery of >838 J acoustic energy. The results show feasibility for in vivo liver cancer ablation using miniaturized image-ablate arrays suitable for interstitial deployment.