Importance of the enhanced cooling system for more spherical ablation zones: Numerical simulation, ex vivo and in vivo validation.

INTRODUCTION: This study aimed to investigate the efficacy of a small-gauge microwave ablation antenna (MWA) with an enhanced cooling system (ECS) for generating more spherical ablation zones. METHODS: A comparison was made between two types of microwave ablation antennas, one with ECS and the other with a conventional cooling system (CCS). The finite element method was used to simulate in vivo ablation. Two types of antennas were used to create MWA zones for 5, 8, 10 min at 50, 60, and 80 W in ex vivo bovine livers (n = 6) and 5 min at 60 W in vivo porcine livers (n = 16). The overtreatment ratio, ablation aspect ratio, carbonization area, and other characteristcs of antennas were measured and compared using numerical simulation and gross pathologic examination. RESULTS: In numerical simulation, the ECS antenna demonstrated a lower overtreatment ratio than the CCS antenna (1.38 vs 1.43 at 50 W 5 min, 1.19 vs 1.35 at 50 W 8 min, 1.13 vs 1.32 at 50 W 10 min, 1.28 vs 1.38 at 60 W 5 min, 1.14 vs 1.32 at 60 W 8 min, 1.10 vs 1.30 at 60 W 10 min). The experiments revealed that the ECS antenna generated ablation zones with a more significant aspect ratio (0.92 ± 0.03 vs 0.72 ± 0.01 at 50 W 5 min, 0.95 ± 0.02 vs 0.70 ± 0.01 at 50 W 8 min, 0.96 ± 0.01 vs 0.71 ± 0.04 at 50 W 10 min, 0.96 ± 0.01 vs 0.73 ± 0.02 at 60 W 5 min, 0.94 ± 0.03 vs 0.71 ± 0.03 at 60 W 8 min, 0.96 ± 0.02 vs 0.69 ± 0.04 at 60 W 10 min) and a smaller carbonization area (0.00 ± 0.00 cm2 vs 0.54 ± 0.06 cm2 at 50 W 5 min, 0.13 ± 0.03 cm2 vs 0.61 ± 0.09 cm2 at 50 W 8 min, 0.23 ± 0.05 cm2 vs 0.73 ± 0.05 m2 at 50 W 10 min, 0.00 ± 0.00 cm2 vs 1.59 ± 0.41 cm2 at 60 W 5 min, 0.23 ± 0.22 cm2 vs 2.11 ± 0.63 cm2 at 60 W 8 min, 0.57 ± 0.09 cm2 vs 2.55 ± 0.51 cm2 at 60 W 10 min). Intraoperative ultrasound images revealed a hypoechoic area instead of a hyperechoic area near the antenna. Hematoxylin-eosin staining of the dissected tissue revealed a correlation between the edge of the ablation zone and that of the hypoechoic area. CONCLUSIONS: The ECS antenna can produce more spherical ablation zones with less charring and a clearer intraoperative ultrasound image of the ablation area than the CCS antenna.

Dynamic changes in liver volume calculated using a three-dimensional visualisation system after microwave ablation of hepatocellular carcinomas.

OBJECTIVES: To investigate the changes in liver volume and function after microwave ablation (MWA) of hepatocellular carcinomas (HCCs). MATERIALS AND METHODS: We retrospectively analysed 76 patients with 106 nodules who underwent MWA for HCCs ≤5 cm between January 2015 and September 2017. Liver and ablation volumes were calculated using a three-dimensional visualisation system on MRI. Multiple regression analysis was used to estimate the association between the ablation volume and liver volume changes. Deformable image registration (DIR) was performed to confirm the influence of liver volume changes on curative effect evaluation after ablation. RESULTS: The initial liver and tumour volumes were 1262.1 ± 259.91 cm3 (range: 864.9-1966.8) and 2.5 cm3 (interquartile range [IQR]: 1.3-8.8), respectively. Compared to the initial liver volumes, the entire live volume (ELV) increased by 10.1% ± 8.93% (range: -4.9% to 46.68%) on the 3rd day after ablation. Subsequently, it recovered to initial level at the 3rd month and maintained its level during the 1-year follow-up. The median total ablation volume was 34.9 cm3 (IQR: 20.4-65.4) on the 3rd day after ablation, which decreased by 71.2% (IQR: 57.4%-78.1%) 1 year after ablation. Alanine aminotransferase (ALT), aspartate aminotransferase (AST) and total bilirubin (T-Bil) peaked within 3 days after MWA and recovered to normal within 1 month. The ablation volume proportion of the ELV was an independent risk factor for the increase in the ELV and AST, ALT and T-Bil levels within 3 days after ablation. When DIR was conducted to fuse ablation zone and tumour, the reshaped tumour volumes were enlarged by 40% because of the increase in ELV. CONCLUSIONS: MWA of HCCs based on the Milan criteria could induce temporary increases in ELV and RLV within 3 days after ablation, but both parameters recovered to initial levels 3 months after ablation. This indicates that MWA of early-stage HCCs would not lead to liver volume loss and could potentially protect liver function. The liver cannot be treated as an incompressible organ after ablation, and the appropriate deformation constraint should be designed for DIR to evaluate ablation margin accurately.

Fast calculation of 3D radiofrequency ablation zone based on a closed-form solution of heat conduction equation fitted by ex vivo measurements.

Fast calculation or simulation of the ablation zone induced by radiofrequency ablation (RFA) has a critical role in hepatic RFA planning and therapy. However, it remains challenging to approximate the ablation zone in real time, especially when more than one probe is involved in one ablation session. This paper presents a novel computational technique to calculate the 3D ablation zone of one probe RFA and two-probe switching RFA. The main idea is to get an approximate solution of the temperature distribution from a simplified Pennes bioheat equation, and further fit the solution to the coagulation measurements on ex vivo porcine liver. With a closed-form solution of temperature distribution, the calculation of the ablation zone is as simple as the commonly used ellipsoidal model, but it allows a more realistic prediction of combined ablation zones with different inter-probe spacing. The new approximation technique could potentially replace the original ellipsoidal model in the intervention planning step.

Pre-operative Detection of Liver Fibrosis in Hepatocellular Carcinoma Patients Using 2D Shear Wave Elastography: Where to Measure?

The aim of this study was to pre-operatively investigate the diagnostic performance of 2D shear wave elastography (2D-SWE) for staging liver fibrosis and inflammation in patients with hepatocellular carcinoma (HCC) who then undergo surgery and to determine the optimal locations for measurement. In total, 106 patients were enrolled in this prospective study from March 2017 to May 2018. Two-dimensional SWE was used to measure liver stiffness (LS) in each patient 0-1, 1-2 and 2-5 cm from the tumor border (groups 1, 2 and 3, respectively). Spearman's correlation was used to evaluate the relationships between LS and hepatic fibrosis and between LS and inflammation. Receiver operating characteristic curve (ROC) analysis was used to evaluate the diagnostic accuracy of 2D-SWE. The technical success rate of SWE in tissue distant from the tumor (group 3) was significantly higher than that in peri-tumoral tissue (groups 1 and 2) (p < 0.001). Moreover, the area under the ROC for diagnosing cirrhosis (F4) and severe inflammation (A3) was higher for group 3 than for groups 1 and 2. Our results suggest that 2D-SWE is a helpful approach to assessment of hepatic fibrosis in HCC patients before hepatic resection. We found that to achieve a superior success rate and preferable diagnosis accuracy for patients with HCC, LS measurement should be performed 2-5 cm from the tumor margin.

An analytical solution for temperature distributions in hepatic radiofrequency ablation incorporating the heat-sink effect of large vessels.

Fast prediction of the local thermal field induced by radiofrequency ablation (RFA) plays a critical role in hepatic RFA therapy. At present, it is still a challenging task to calculate and visualize the temperature distribution of RFA in real-time, especially when the heat-sink effect of adjacent large vessels is taken into account. To achieve this, the current investigation presented an analytical solution to calculate the temperature in RFA with an execution time of 0.05 s for three dimensional thermal field reconstruction. The presented temperature distribution is a combination of temperatures in homogeneous tissue and a quantification of the heat-sink effect of adjacent blood vessels. Temperatures in homogeneous tissue is calculated from a simplified Pennes bioheat equation, where several weighting parameters in the temperature expression are determined based on some reference point temperatures from the numerical simulation. The heat-sink effect is quantified based on a temperature factor, which measures the temperature difference between the vessel and the heated tissue, and a distance factor, which measures the distance to the vessel. The proposed method is validated to be able to gain similar temperature distributions to the numerical simulation but with its computational time being orders of magnitude smaller than that of numerical simulation, which improves the efficiency of interactive planning of RFA.

Semiautomatic Radiofrequency Ablation Planning Based on Constrained Clustering Process for Hepatic Tumors.

Radiofrequency ablation (RFA) is currently one of the most effective methods for minimally invasive treatment of hepatic tumors. Planning the probe placements is an essential and challenging step in RFA treatment. To completely destroy the tumor with minimum amount of affected native tissue, a new RFA planning system is proposed in this paper. In the proposed planning system, the minimum number of ablations and a conical insertion region for each ablation session are determined automatically. Based on the geometric character of the tumor, a novel clustering algorithm is developed to allow a better layout of the overlapping ablations. For each case, we force the clustering process under the constraint of a manually defined puncture scope, such that all of the needle trajectories are gathered in a reasonable region. Moreover, the proposed planning system enables the clinician to manually choose a proper insertion path inside the conical insertion region to avoid penetrating large vessels or ribs, which is critical in RFA treatment. The proposed planning system was evaluated on 18 CT scan images and two clinical cases. Results implied that the planning system could provide feasible and accurate RFA treatment plans for hepatic tumors.