First validation of a model-based hepatic percutaneous microwave ablation planning on a clinical dataset.

A model-based planning tool, integrated in an imaging system, is envisioned for CT-guided percutaneous microwave ablation. This study aims to evaluate the biophysical model performance, by comparing its prediction retrospectively with the actual ablation ground truth from a clinical dataset in liver. The biophysical model uses a simplified formulation of heat deposition on the applicator and a heat sink related to vasculature to solve the bioheat equation. A performance metric is defined to assess how the planned ablation overlaps the actual ground truth. Results demonstrate superiority of this model prediction compared to manufacturer tabulated data and a significant influence of the vasculature cooling effect. Nevertheless, vasculature shortage due to branches occlusion and applicator misalignment due to registration error between scans affects the thermal prediction. With a more accurate vasculature segmentation, occlusion risk can be estimated, whereas branches can be used as liver landmarks to improve the registration accuracy. Overall, this study emphasizes the benefit of a model-based thermal ablation solution in better planning the ablation procedures. Contrast and registration protocols must be adapted to facilitate its integration into the clinical workflow.

Morphometric characterization and temporal temperature measurements during hepatic microwave ablation in swine.

PURPOSE: Heat-induced destruction of cancer cells via microwave ablation (MWA) is emerging as a viable treatment of primary and metastatic liver cancer. Prediction of the impacted zone where cell death occurs, especially in the presence of vasculature, is challenging but may be achieved via biophysical modeling. To advance and characterize thermal MWA for focal cancer treatment, an in vivo method and experimental dataset were created for assessment of biophysical models designed to dynamically predict ablation zone parameters, given the delivery device, power, location, and proximity to vessels. MATERIALS AND METHODS: MWA zone size, shape, and temperature were characterized and monitored in the absence of perfusion in ex vivo liver and a tissue-mimicking thermochromic phantom (TMTCP) at two power settings. Temperature was monitored over time using implanted thermocouples with their locations defined by CT. TMTCPs were used to identify the location of the ablation zone relative to the probe. In 6 swine, contrast-enhanced CTs were additionally acquired to visualize vasculature and absence of perfusion along with corresponding post-mortem gross pathology. RESULTS: Bench studies demonstrated average ablation zone sizes of 4.13±1.56cm2 and 8.51±3.92cm2, solidity of 0.96±0.06 and 0.99±0.01, ablations centered 3.75cm and 3.5cm proximal to the probe tip, and temperatures of 50 ºC at 14.5±13.4s and 2.5±2.1s for 40W and 90W ablations, respectively. In vivo imaging showed average volumes of 9.8±4.8cm3 and 33.2±28.4cm3 and 3D solidity of 0.87±0.02 and 0.75±0.15, and gross pathology showed a hemorrhagic halo area of 3.1±1.2cm2 and 9.1±3.0cm2 for 40W and 90W ablations, respectfully. Temperatures reached 50ºC at 19.5±9.2s and 13.0±8.3s for 40W and 90W ablations, respectively. CONCLUSION: MWA results are challenging to predict and are more variable than manufacturer-provided and bench predictions due to vascular stasis, heat-induced tissue changes, and probe operating conditions. Accurate prediction of MWA zones and temperature in vivo requires comprehensive thermal validation sets.

Model-based hepatic percutaneous microwaveablation planning. First validation on a clinical dataset.

A model-based planning tool, integrated in an imaging system, is envisioned for CT-guided percutaneous microwave ablation. This study aims to evaluate the biophysical model performance, by comparing its prediction retrospectively with the actualablation ground truth from a clinical data set in liver. The biophysical model uses a simplified formulation of heat depositionon the applicator and a heat sink related to vasculature to solve the bioheat equation. A performance metric is defined toassess how the planned ablation overlaps the actual ground truth. Results demonstrate superiority of this model predictioncompared to manufacturer tabulated data and a significant influence of the vasculature cooling effect. Nevertheless, vasculatureshortage due to branches occlusion and applicator misalignment due to registration error between scans affects the thermalprediction. With a more accurate vasculature segmentation, occlusion risk can be estimated, whereas branches can be usedas liver landmarks to improve the registration accuracy. Overall, this study emphasizes the benefit of a model-based thermalablation solution in better planning the ablation procedures. Contrast and registration protocols must be adapted to facilitate itsintegration into the clinical workflow.

Effective models of microwave antennae for ablation treatment planning.

The level of detail of typical numerical models of microwave tumor ablations poses a challenge to the development of generic, model based treatment planning tools aiming at real time performance. The present contribution describes a flexible and accurate approximation of the microwave heat absorption that aims at mitigating these issues.

Fluorescence modeling of droplets intersecting a focused laser beam.

Laser-induced fluorescence (LIF) has been previously used in various forms to characterize droplets in a spray, according to either size or temperature. A rigorous examination is presented of the LIF signal obtained when a water droplet seeded by Rhodamine 6G passes through one or two highly focused laser beams, i.e., with a beam waist of the order of the droplet diameter or smaller. The calculations are performed with a fluorescence model based on the generalized Lorenz-Mie theory and on ray tracing methods, assuming that the droplet is spherical and nonabsorbing. A parametric study reveals a size dependent signal as the droplet passes through the focused beam(s). The observed signal features suggest several avenues for the use of LIF scattering for size and velocity measurements of individual droplets.

Numerical analysis of diameter influence on droplet fluorescence.

Laser-induced fluorescence (LIF) is used in planar droplet sizing, assuming that the signal integrated over the droplet is proportional to its volume. Nevertheless, this assumption is rigorously valid in nonabsorbing mixtures. We performed an examination of the LIF signal with a fluorescence model, based on the Lorenz-Mie theory and on ray-tracing methods, for n-heptane droplets seeded by 3-pentanone. A parametrical study quantifies the bias caused not only by the absorption of the laser, but also by shadow zones in the droplets, which do not contribute to the fluorescence signal. Moreover, the effect of the first- and higher-order internal reflections is examined. The results of this study have immediately implications for the design of measurement techniques.