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Impact of Gas Bubbles Generated During Interstitial Ablation on Elastographic Depiction of In Vitro Thermal Lesions

Departments of Medical Physics (T.V., U.T., J.A.Z.), Biomedical Engineering (T.V.), Electrical and Computer Engineering (U.T.), and Radiology (F.T.L.), The University of Wisconsin–Madison, Madison, Wisconsin USA.

Address correspondence and reprint requests to Tomy Varghese, PhD, Department of Medical Physics, The University of Wisconsin–Madison, 1530 Medical Sciences Center, 1300 University Ave, Madison, WI 53706 USA. E-mail: tvarghese{at}wisc.edu.

Objective. Artifacts from gas bubble formation during radio frequency ablation along with the poor intrinsic contrast between normal and treated regions (zone of necrosis) are considerable problems for the visualization of the necrotic region on conventional sonography. Sonographic elastography is very effective for visualizing the zone of necrosis, but it uses the same echo signals to estimate strain as those used to form gray scale images. Thus, the impact of gas bubbles on strain images or elastograms must be investigated. Methods. Radio frequency ablation was performed in vitro on liver tissue samples, approximately 40 x 40 x 20 mm, encased in 80-mm cubed gelatin phantoms. Elastograms generated at different instants during the ablation procedures were obtained on a real-time scanner with a 5-MHz linear array. Sequences of elastograms illustrate the growth of the thermal lesion. Results. Degradation of the distal boundary of the thermal lesion was observed. The degradation was confined to the lower-fifth quadrant of the thermal lesion. However, accurate estimates of lesion areas could still be obtained by extrapolation of the thermal lesion boundary. Conclusions. Elastograms of thermal lesions in vitro can be obtained during radio frequency ablation. Some loss of thermal lesion boundary information on strain images was observed in regions where attenuation due to gas bubbles reduced the signal-noise ratio of the echo signals.

Key Words: ablation • bubbles • elasticity • elastography • elastogram • imaging • radio frequency ablation • sonography • strain

Abbreviations: HIFU, high-intensity focused ultrasound • RF, radio frequency

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