Recent Advances in the Image-Guided Tumor Ablation of Liver Malignancies: Radiofrequency Ablation with Multiple Electrodes, Real-Time Multimodality Fusion Imaging, and New Energy Sources

Abstract

Radiofrequency ablation (RFA) has emerged as an effective loco-regional treatment modality for malignant hepatic tumors. Indeed, studies have demonstrated that RFA of early stage hepatocellular carcinomas can provide comparable overall survival to surgical resection. However, the incidence of local tumor progression (LTP) after RFA is significantly higher than that of surgical resection. Thus, to overcome this limitation, multiple electrode radiofrequency (RF) systems that use a multi-channel RF generator have been developed, and they demonstrate better efficiency in creating larger ablation zones than that using the conventional RFA with a single electrode. Furthermore, RFA with multiple electrodes can allow the "no-touch" ablation technique which may also help to reduce LTP. Another technique that would be helpful in this regard is multi-modality-ultrasound fusion imaging, which helps to not only more accurately determine the target lesion by enabling the RFA of small, poorly visible or invisible tumors, but also improve the monitoring of procedures and determine the appropriateness of the ablation margin. In addition, new energy sources, including microwave and cryoablation, have been introduced in imaging-guided tumor ablation. In this review, these recently introduced ablation techniques and the results of the most current animal and clinical studies are discussed.

Keywords: Cryoablation; Fusion imaging technique; Hepatocellular carcinomas (HCCs); Liver; Malignant hepatic tumors; Microwave; Multiple electrodes; Radiofrequency ablation.

Figures

Fig. 1. Diagram showing use of three electrodes with various RF energy delivery methods.

A. Single switching monopolar mode: RF energy is delivered to one of three electrodes at given time, and switched to adjacent electrode. Ground pad serves as passive electrode. B. Dual switching monopolar mode: RF energy is simultaneously delivered to pair of electrodes among three inserted electrodes at given time, and switched to pair of electrodes. Ground pad serves as passive electrode. C. Switching bipolar mode: pair of electrodes is activated, and then switched to another pair of electrodes: one electrode serves as active electrode and other as passive electrode. Green circle: inserted electrode; red circle: active electrode; blue circle and blue bar: passive electrode; and light orange or eclipse: ablation zone. RF = radiofrequency

Fig. 2. Switching monopolar RFA with multiple electrodes for medium-sized HCCs.

A. Contrast-enhanced arterial phase transverse CT image displays 4.2 cm enhancing mass in segment VII of liver. B. Portal venous phase transverse CT image shows wash-out of tumor, which indicated HCC. C, D. Under US guidance, three electrodes with 3-cm active tip (arrows) are placed across index tumor. E. After RF energy delivery with switching monopolar mode, echo-cloud of microbubbles are generated, encompassing index tumor. F. One-month follow-up contrast-enhanced portal venous phase transverse CT image displays complete ablation of index tumor without evidence of viable residual tumor. CT = computed tomography, HCCs = hepatocellular carcinomas, RFA = radiofrequency ablation, US = ultrasound

Fig. 3. RFA for HCC using multiple electrodes with “no-touch” technique under fusion imaging guidance.

A. Contrast-enhanced arterial phase transverse CT image displays 1.5-cm enhancing nodular lesion (arrow) in segment VI of liver, which suggests HCC. B. Fusion imaging technique between real-time working US and reference arterial phase CT images clearly display low echoic target lesion on US image. C. Under fusion imaging guidance, two electrodes (arrows) are inserted outside of target tumor (*). Tumor itself is not violated during electrode insertion. D. After RF energy delivery, echo-cloud of micro-bubbles is created around target tumor. E. Echo-cloud of microbubbles completely covers target tumor, which suggests successful ablation of target tumor. F. Immediate follow-up contrast-enhanced portal venous phase transverse CT image demonstrates complete destruction of target tumor with sufficient ablation margin.

Fig. 4. Process of fusion imaging between real-time US and reference MR images.

A. Plane registration: after loading of reference MR images to US equipment that implemented fusion imaging technique; plane registration was done. In this case, sagittal plane was used for plane registration. B. Point registration: after plane registration, point registration was performed by selecting same anatomic landmark (proximal right portal vein in this case) on both real-time working US and reference MR images. C. Target tumor marking: after point registration, targeted tumor was marked on reference MR image. Location of targeted tumor on real-time working US was immediately visualized in corresponding location. MR = magnetic resonance

Fig. 5. RFA of small invisible tumor on conventional B-mode US under fusion imaging guidance.

A. Subtraction image from arterial phase gadoxetic acid-enhanced MR image to precontrast MR image demonstrates 1.2-cm arterial enhancing nodular lesion in segment V subcapsular portion of liver (arrow). There is non-enhancing area (*) due to previous ablation therapy. B. Hepatobiliary phase image displays low signal intensity within tumor (arrow), which suggests HCC. Previous ablation zone also exhibited low signal intensity (*). C. On conventional B-mode US image, there is no focal lesion corresponding to HCC that is visible on gadoxetic acid-enhanced MR. D. Under fusion imaging guidance between real-time working US and reference hepatobiliary phase MR images, location of index tumor is determined, and electrode is inserted. E. After RF energy delivery, echo-cloud of micro-bubbles is created and encompasses index tumor. F. Immediate follow-up contrast-enhanced portal venous phase transverse CT image displays complete destruction of target tumor with sufficient ablation margin.