Monitoring microwave ablation for liver tumors with electrode displacement strain imaging

Minimally invasive ablative therapies have become important alternatives to surgical treatment of both hepatocellular carcinoma (HCC) and liver metastases. Image based guidance and monitoring are therefore essential. Although ultrasound (US) imaging suffers from inadequate echogenic contrast between ablated and normal tissue, US based elasticity imaging has shown remarkable ability to depict ablated regions and delineate margins. The purpose of this study is to apply “electrode displacement elastography,” or EDE for monitoring clinical microwave ablation (MWA) treatments for HCC and liver metastases. EDE images were acquired from 10 patients who underwent MWA for their liver tumors. The MWA system used was a Neuwave Medical Certus 140 (Madison, WI, USA) operating at 2.45 GHz. The MWA power and duration was adjusted for each patient, with typical values of 65 watts and 5 minutes. A Siemens S2000 scanner equipped with a curvilinear array transducer (VFX 6C1) pulsed at 4 MHz was used to acquire radiofrequency echo data. Electrode displacement was applied manually by the physician. A multi-seed two-dimensional tracking algorithm, with kernel dimensions of 0.096 mm × 3 A-lines was used to estimate local displacements between consecutive data frames. Strain images were computed as the gradient of the local displacement estimates. The average contrast of the ablated region was 0.23±0.07 (0.14-0.35) on B-mode images and 0.73±0.08 (0.56-0.82) on EDE. The average contrast improvement with EDE over B mode was about 230%. The average tumor size was 2.2±0.8 (0.7-3.5) cm on pre-treatment diagnostic images (CT or MRI). The average size of the ablated region was 3.8±0.7 (2.6-4.9) cm on EDE, with an average ablation margin of 1.6 cm which is within the clinically suggested ablated margin (>0.5 cm).