Purpose: To characterize ultrasonographic (US) findings after irreversible electroporation (IRE) to determine the utility of these findings in the accurate assessment of ablation margins. Materials and Methods: The institutional animal care and use committee approved the study. IRE ablation (n = 58) was performed in vivo in 16 pig livers by using two 18-gauge electroporation electrodes with 2-cm tip exposure, 1.5- or 2.0-cm interelectrode spacing, and an electroporation generator. Energy deposition was applied at 2250-3000 V (pulse length, 50-100 μsec; pulse repetition, 50-100 ). Ablations were performed under US guidance. Images were obtained during ablation and at defined intervals from 1 minute to 2 hours after the procedure. Zones of ablation were determined at gross and histopathologic examination of samples obtained from animals sacrificed 2-3 hours after IRE. Dimensions of the histologic necrosis zone and US findings were compared and subjected to statistical analysis, including a Student t test and multiple linear regression. Results: Within 20-50 pulse repetitions of IRE energy, the ablation zone appeared as a hypoechoic area with well-demarcated margins. During the next 8-15 minutes, this zone decreased in size from 3.4 cm ± 0.5 to 2.5 cm ± 0.4 and became progressively more isoechoic. Subsequently, a peripheral hyperechoic rim measuring 2-7 mm (mean, 4 mm ± 1) surrounding the isoechoic zone developed 25-90 minutes (mean, 41 minutes ± 19) after IRE. The final length of the treatment zone, including the rim, increased to 3.3 cm ± 0.6. The final dimensions of the outer margin of this rim provided greatest accuracy (1.7 mm ± 0.2) and tightest correlation (r 2 = 0.89) with gross pathologic findings. Histologic examination demonstrated widened sinusoidal spaces that progressively filled with spatially distributed hemorrhagic infiltrate on a bed of hepatocytes with pyknotic nuclei throughout the treatment zone. Conclusion: US findings in the acute period after IRE are dynamic and evolve. The ablation zone can be best predicted by measuring the external hyperechoic rim that forms 90-120 minutes after ablation. This rim is possibly attributable to evolving hemorrhagic infiltration via widened sinusoids.