Feasibility of Extracted-Overlay Fusion Imaging for Intraoperative Treatment Evaluation of Radiofrequency Ablation for Hepatocellular Carcinoma

Abstract

Background and aims: Extracted-overlay fusion imaging is a novel computed tomography/magnetic resonance-ultrasonography (CT/MR-US) imaging technique in which a target tumor with a virtual ablative margin is extracted from CT/MR volume data and synchronously overlaid on US images. We investigated the applicability of the technique to intraoperative evaluation of radiofrequency ablation (RFA) for hepatocellular carcinoma (HCC).

Methods: This retrospective study analyzed 85 HCCs treated with RFA using extracted-overlay fusion imaging for guidance and evaluation. To perform RFA, an electrode was inserted targeting the tumor and a virtual 5-mm ablative margin overlaid on the US image. Following ablation, contrast-enhanced US (CEUS) was performed to assess the ablative margin, and the minimal ablative margins were categorized into three groups: (I) margin <0 mm (protrusion), (II) margin 0 to <5 mm, and (III) margin ≥5 mm. Margin assessment was based on the positional relationship between the overlaid tumor plus margin and the perfusion defect of the ablation zone. Tumors in group I underwent repeat ablation until they were in groups II or III. The final classifications were compared with those obtained by retrospectively created fusion images of pre- and post-RFA CT or MR imaging (CT-CT/MR-MR fusion imaging).

Results: Treatment evaluation was impossible using CEUS in six HCCs because the tumors were located far below the body surface. Of the remaining 79 HCCs, the categorizations of minimal ablative margins between CEUS extracted-overlay fusion imaging and CT-CT/MR-MR fusion imaging were in agreement for 72 tumors (91.1%) (Cohen's quadratic-weighted kappa coefficient 0.66, good agreement, p<0.01).

Conclusions: Extracted-overlay fusion imaging combined with CEUS is feasible for the evaluation of RFA and enables intraoperative treatment evaluation without the need to perform contrast-enhanced CT.

Keywords: Contrast-enhanced ultrasonography; Extracted-overlay fusion imaging; Hepatocellular carcinoma; Multimodality fusion imaging; Radiofrequency ablation.

Figures

Fig. 1

Extracted-overlay fusion imaging in a 66-year-old woman with HCC in segment VIII. The tumor identified on the hepatobiliary-phase Gd-EOB-DTPA-MR image was used as the reference image and was overlaid on the real-time ultrasonography (US) image. a Conventional MR-US fusion imaging. b The extracted-overlay fusion image overlies the target tumor only. c The extracted-overlay fusion image overlies the target tumor (pink portion) and a 5-mm virtual ablative margin (blue portion) is added.

Fig. 2

Flow chart of the treatment procedures in this study.

Fig. 3

Categorization of minimal ablative margins after RFA using extracted-overlay fusion imaging combined with CEUS. a Group I: margin <0 mm (tumor extends outside the ablation zone); (b) group II: margin 0 to <5 mm; (c) group III: margin ≥5 mm.

Fig. 4

Categorization of minimal ablative margins after RFA by CT-CT/MR-MR fusion imaging. a Group I: margin <0 mm (tumor extends outside the ablation zone); (b) group II: margin 0 to <5 mm; (c) group III: margin ≥5 mm. MR-MR fusion images are presented in this figure.

Fig. 5

Cumulative local tumor progression rate of two groups categorized by intraoperative CEUS using extracted-overlay fusion imaging. The 1-year cumulative local tumor progression rates in groups II and III were 8.9% and 0.0%, respectively.