Directional microwave ablation in spine: experimental assessment of computational modeling.

BACKGROUND: Despite the theoretical advantages of treating metastatic bone disease with microwave ablation (MWA), there are few reports characterizing microwave absorption and bioheat transfer in bone. This report describes a computational modeling-based approach to simulate directional microwave ablation (dMWA) in spine, supported by ex vivo and pilot in vivo experiments in porcine vertebral bodies. MATERIALS AND METHODS: A 3D computational model of microwave ablation within porcine vertebral bodies was developed. Ex vivo porcine vertebra experiments using a dMWA applicator measured temperatures approximately 10.1 mm radially from the applicator in the direction of MW radiation (T1) and approximately 2.4 mm in the contra-lateral direction (T2). Histologic assessment of ablated ex vivo tissue was conducted and experimental results compared to simulations. Pilot in vivo experiments in porcine vertebral bodies assessed ablation zones histologically and with CT and MRI. RESULTS: Experimental T1 and T2 temperatures were within 3-7% and 11-33% of simulated temperature values. Visible ablation zones, as indicated by grayed tissue, were smaller than those typical in other soft tissues. Posthumous MRI images of in vivo ablations showed hyperintensity. In vivo experiments illustrated the technical feasibility of creating directional microwave ablation zones in porcine vertebral body. CONCLUSION: Computational models and experimental studies illustrate the feasibility of controlled dMWA in bone tissue.

Transmission Coefficient-Based Monitoring of Microwave Ablation: Development and Experimental Evaluation in Ex Vivo Tissue.

OBJECTIVES: To assess the feasibility of monitoring transient evolution of thermal ablation zones with a microwave transmission coefficient-based technique. METHODS: Microwave ablation was performed in ex vivo bovine liver with two 2.45 GHz directional antennas. A custom apparatus was developed to enable periodic switching between "heating mode" when power from the generator was coupled to the antennas, and "monitoring mode", when antennas were coupled to a network analyzer for broadband transmission coefficient ( s21) measurements. Experiments were performed with applied powers ranging between 30-50 W per antenna for 53-1219 s. Transient s21 spectra over the course of ablations were analyzed to determine feasibility of predicting extent of ablation zones and compared against ground truth assessment from images of sectioned tissue. A linear regression-based mapping between the two datasets was derived to predict ablation extent. RESULTS: Normalized average transmission coefficient initially rapidly decreased and then increased before asymptotically approaching steady state, with the transition time ranging between 53 s (45 W) and 109 s (30 W). Analysis of ground truth ablation zone images indicated time to complete ablation of 230-350 s. The relative prediction error for time to complete ablation derived from the s21 data was in the range of 1.6%-2.3% compared to ground truth. CONCLUSION: We have demonstrated the feasibility of monitoring transient evolution of thermal ablation zones using microwave transmission coefficient measurements in ex vivo tissue. SIGNIFICANCE: The presented technique has potential to contribute towards addressing the clinical need for a method of monitoring evolution of thermal ablation zones.

Shaping the future of microwave tumor ablation: a new direction in precision and control of device performance.

Microwave ablation (MWA) is becoming an increasingly important minimally invasive treatment option for localized tumors in many organ systems due to recent advancements in microwave technology that have conferred many advantages over other tumor ablation modalities. Despite these improvements in technology and development of applicators for site-specific tumor applications, the vast majority of commercially available MWA applicators are generally designed to create large-volume, symmetric, ellipsoid or spherically-shaped treatment zones and often lack the consistency, predictability, and spatial control needed to treat tumor targets near critical structures that are vulnerable to inadvertent thermal injury. The relatively new development and ongoing translation of directional microwave ablation (DMWA) technology, however, has the potential to confer an added level of control over the treatment zone shape relative to applicator position, and shows great promise to expand MWA's clinical applicability in treating tumors in challenging locations. This paper presents a review of the industry-standard commercially available MWA technology, its clinical applications, and its limitations when used for minimally-invasive tumor treatment in medical practice followed by discussion of new advancements in experimental directional microwave ablation (DMWA) technology, various techniques and approaches to its use, and examples of how this technology may be used to treat tumors in challenging locations that may otherwise preclude safe treatment by conventional omni-directional MWA devices.

Directional Microwave Ablation: Experimental Evaluation of a 2.45-GHz Applicator in Ex Vivo and In Vivo Liver.

PURPOSE: To experimentally characterize a microwave (MW) ablation applicator designed to produce directional ablation zones. MATERIALS AND METHODS: Using a 14-gauge, 2.45-GHz side-firing MW ablation applicator, 36 ex vivo bovine liver ablations were performed. Ablations were performed at 60 W, 80 W, and 100 W for 3, 5, and 10 minutes (n = 4 per combination). Ablation zone forward and backward depth and width were measured and directivity was calculated as the ratio of forward to backward depth. Thirteen in vivo ablations were performed in 2 domestic swine with the applicator either inserted into the liver (80 W, 5 min, n = 3; 100 W, 5 min, n = 3; 100 W, 10 min, n = 2) or placed on the surface of the liver with a nontarget tissue placed on the back side of the applicator (80 W, 5 min, n = 5). The animals were immediately euthanized after the procedure; the livers were harvested and sectioned perpendicular to the axis of the applicator. In vivo ablation zones were measured following viability staining and assessed on histopathology. RESULTS: Mean ex vivo ablation forward depth was 8.3-15.5 mm. No backward heating was observed at 60 W, 3-5 minutes; directivity was 4.7-11.0 for the other power and time combinations. In vivo ablation forward depth was 10.3-11.5 mm, and directivity was 11.5-16.1. No visible or microscopic thermal damage to nontarget tissues in direct contact with the back side of the applicator was observed. CONCLUSIONS: The side-firing MW ablation applicator can create directional ablation zones in ex vivo and in vivo tissues.