Mario Gallati

SPH Simulation of Sediment Flushing Induced by a Rapid Water Flow

This paper shows an advanced application of the smoothed particle hydrodynamics (SPH) method to the numerical modeling of noncohesive sediment flushing aiming at the setup of a reliable engineering tool for the prediction of the coupled water-sediment dynamics at the bottom of an artificial reservoir. Both liquid and granular materials are modeled as weakly compressible viscous fluids, whose motion results from the numerical solution of the continuity and momentum equations discretized according to standard SPH formulation. The effect of two alternative erosion criteria on the description of the failure mechanism of bottom sediments is analyzed. These criteria are based, respectively, on Mohr-Coulomb yielding criterion and Shields theory. A sensitivity analysis is performed in order to assess, for both criteria, the influence of the model parameters on the simulation of the erosion process; the method is eventually validated by comparing numerical results with the experimental data obtained in a two-dimen...

SPH Modeling of Solid Boundaries Through a Semi-Analytic Approach

Abstract: This paper presents a general semi-analytic approach for modeling solid boundaries in the SPH method: boundaries are here considered as a material continuum with a suitable distribution of velocity and pressure; their contributions to each term of the SPH mass and momentum equations can be expressed in terms of a suitable integral extended to the part of the sphere of influence of the particle delimited by the boundary surface. Analytical details with reference to a slightly compressible viscous Newtonian fluid in three dimensions are given. The validity of the method is checked by comparing the obtained numerical results with available experimental data in a benchmark flow case.

Fiber-optic chirped FBG for distributed thermal monitoring of ex-vivo radiofrequency ablation of liver

A linearly chirped fiber Bragg grating (LCFBG) has been used as a temperature sensor for online monitoring of radiofrequency thermal ablation (RFTA). The LCFBG acts as a distributed sensor, with spatial resolution of 75 μm. A white-light setup that records the LCFBG spectrum estimates the temperature profile in real time. Three RFTA experiments have been performed ex-vivo on porcine liver measuring the radial temperature distribution during the heating process. The analysis of thermal maps quantifies the spatial heat distribution along the measurement axis and determines the ablation efficiency.

Fiber-optic combined FPI/FBG sensors for monitoring of radiofrequency thermal ablation of liver tumors: ex vivo experiments.

We present a biocompatible, all-glass, 0.2 mm diameter, fiber-optic probe that combines an extrinsic Fabry-Perot interferometry and a proximal fiber Bragg grating sensor; the probe enables dual pressure and temperature measurement on an active 4 mm length, with 40 Pa and 0.2°C nominal accuracy. The sensing system has been applied to monitor online the radiofrequency thermal ablation of tumors in liver tissue. Preliminary experiments have been performed in a reference chamber with uniform heating; further experiments have been carried out on ex vivo porcine liver, which allowed the measurement of a steep temperature gradient and monitoring of the local pressure increase during the ablation procedure.

Optical fiber sensors-based temperature distribution measurement in ex vivo radiofrequency ablation with submillimeter resolution

Abstract. Radiofrequency thermal ablation (RFTA) induces a high-temperature field in a biological tissue having steep spatial (up to 6°C/mm) and temporal (up to 1°C/s) gradients. Applied in cancer care, RFTA produces a localized heating, cytotoxic for tumor cells, and is able to treat tumors with sizes up to 3 to 5 cm in diameter. The online measurement of temperature distribution at the RFTA point of care has been previously carried out with miniature thermocouples and optical fiber sensors, which exhibit problems of size, alteration of RFTA pattern, hysteresis, and sensor density worse than 1  sensor/cm. In this work, we apply a distributed temperature sensor (DTS) with a submillimeter spatial resolution for the monitoring of RFTA in porcine liver tissue. The DTS demodulates the chaotic Rayleigh backscattering pattern with an interferometric setup to obtain the real-time temperature distribution. A measurement chamber has been set up with the fiber crossing the tissue along different diameters. Several experiments have been carried out measuring the space-time evolution of temperature during RFTA. The present work showcases the temperature monitoring in RFTA with an unprecedented spatial resolution and is exportable to in vivo measurement; the acquired data can be particularly useful for the validation of RFTA computational models.

Hydrodynamic Characterization of a Nozzle Check Valve by Numerical Simulation

The ability to obtain correct estimates of the hydraulic characteristics of a nozzle check valve by finite-volume numerical simulation is discussed. The evaluation of the numerical results is performed by comparison of the computed pressure drops inside the valve with experimental measurements obtained on an industrial check valve. It is shown that, even with high mesh refinement, the obtained result is highly dependent on the choice of the turbulence model. The renormalization group theory (RNG) k-e model proves to be the more accurate to describe the flow inside the valve, which is characterized by repeated flow decelerations and accelerations and by boundary layer development under adverse pressure gradient. Pressure-drop and flow coefficients computed by adopting the RNG model agree well with the experimental values at different positions of the plug. The opening transient of the valve is also analyzed by an unsteady flow simulation where the motion of the plug is taken into account. The characteristic curve of the valve obtained in steady flow conditions is finally compared with the transient opening characteristic, highlighting a temporary increase in the pressure drop, which occurs because of a large unsteady separation region downstream of the plug.

Effect of Hyperbarism on Radiofrequency Ablation Outcome

OBJECTIVE Our objective was to investigate whether increases in atmospheric or local tissue pressure would affect the outcome of radiofrequency ablation procedures and the size of the created thermal lesions. MATERIALS AND METHODS Thermal lesions were produced in specimens of explanted bovine liver inside a hyperbaric chamber at 101 (atmospheric), 141, 202, 273, and 364 kPa using radiofrequency power settings of 20, 30, 40, and 50 W. In subsequent in vivo experiments, thermal lesions were produced in the livers of anesthetized pigs with or without occlusion of the hepatic vein draining the ablation site. RESULTS At each radiofrequency power setting, progressive increases in applied pressure were paralleled by decreases in minimum impedance and increases in maximum tissue temperatures at the electrode tip (reflecting tissue-fluid boiling points), delivery time, total energy delivered, and thermal lesion volumes. Similar increases were observed in radiofrequency ablation procedures performed in vivo under occlusion of the vein draining the ablation site. CONCLUSION By elevating the tissue-fluid boiling point, increased pressure delays the desiccation of tissue in contact with the radiofrequency electrode tip and the related sharp increase in impedance. The result is prolonged delivery of larger amounts of radiofrequency energy and larger thermal lesions.

On the Simulation of Radio Frequency Thermal Lesions in Porcine Liver

In the present paper we present some results of our research on the RFTA (Radio Frequency Thermal Ablation) simulation: in particular the steps performed for the validation of the numerical model. We performed a series of RFTA experiments on pig liver tissue as accurate as possible, at different powers, measuring the time evolution of temperature in selected points and impedances, to be taken as a reference to compare the output of different numerical models. We present the results obtained with a basic simulation model (shortly described) assessing the need to account for the change of the tissue electrical resistivity with temperature. We then underline the need of getting a deeper insight in the phenomena that bring to the power suspension thus limiting the extension of the necrotized tissue. At the end we suggest a line for the future development of the research.

Distributed fiber-optic sensors for thermal monitoring in radiofrequency thermal ablation in porcine phantom

In this paper, we report for the first time the application of two distributed fiber-optic sensing systems in medical radiofrequency thermal ablation (RFTA). Measurement systems are based on a distributed temperature sensor based on high-speed detection of Rayleigh signature, and a linearly chirped fiber Bragg grating (LCFBG) sensor that detects temperature distribution on 1.5 cm length. Both technologies are capable of achieving sub-0.1mm spatial resolution. The sensing systems have been applied to monitor the temperature pattern induced by RFTA, measuring temperature gradients in excess of 5°C/mm. All tests have been performed on porcine liver tissue, the phantom of human liver. The results show the premises for the realization of a distributed sensor installed on a RFTA device, capable of real-time prediction and estimation of the ablation effect.

Optical fiber biocompatible sensors for monitoring selective treatment of tumors via thermal ablation

Thermal ablation (TA) is an interventional procedure for selective treatment of tumors, that results in low-invasive outpatient care. The lack of real-time control of TA is one of its main weaknesses. Miniature and biocompatible optical fiber sensors are applied to achieve a dense, multi-parameter monitoring, that can substantially improve the control of TA. Ex vivo measurements are reported performed on porcine liver tissue, to reproduce radiofrequency ablation of hepatocellular carcinoma. Our measurement campaign has a two-fold focus: (1) dual pressure-temperature measurement with a single probe; (2) distributed thermal measurement to estimate point-by-point cells mortality.