Quantitative assessment of thermal lesion stiffness in the liver: Initial ex vivo results

The purpose of this study was to investigate the feasibility of a shear wave-based elastography prototype on Philips' iU22 ultrasound system for measuring liver tissue stiffness before and after RFA (Radio Frequency Ablation) therapy. A special pulse sequence that generates and tracks shear waves was implemented on a curvilinear transducer C5-1, with RF data acquisition and off-line processing. The system was regulated as safe for use in human. Two RFA lesions were generated in ex vivo porcine liver by a commercial RF ablation electrode (Rita Medical Systems, CA). Lesions were created around depth 40 mm beneath the liver surface. The target temperature was set to 90°C and 95°C, and ablation lasted about 5 and 6 minutes for two lesions, respectively. Elasticity measurements were made pre-RFA at two sites where lesions were centered. Post RFA, the needle tines were retrieved to prevent interference with the shear wave propagation. To avoid residual bubbles, post-RFA elasticity measurement was performed at an image plane about 1 mm away in elevation from the ablation center plane containing the needle shaft. After each elasticity experiment, the liver was cut open approximately at the imaging plane to confirm the pathological changes caused by RFA. For statistical analysis, shear mechanical properties were estimated and averaged in a region with area about 10 mm x 4 mm at the center of the lesions. In this ex vivo study, the shear wave peak displacement at the push focus was about 10 μm pre-RFA. It was significantly reduced to 2 μm post-RFA, signaling the underlying elevated tissue stiffness. Quantitative reconstruction further confirmed the necrosis-induced stiffness increase. Pre-RFA, the shear modulus was 1.56 ± 0.19 kPa and 1.15 ± 0.25 kPa for lesion #1 and #2 respectively; post-RFA, the shear modulus was 29.48 ± 7.28 kPa and 26.80 ± 9.11 kPa. An increase of one order of magnitude in shear modulus was observed for both lesions. Similar stiffness contrast also existed between the ablated and neighboring non-treated region post-RFA for both lesions. These early results demonstrate the feasibility of the prototype for quantifying elasticity of thermal lesions in liver. Further technical development such as imaging capability and automated analysis tools is required to enable the visualization of the ablation zone and assessment of the therapy procedure.