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Purpose To study the correlation between the overall coagulation zone (A) attained in percutaneous microwave ablation (MWA) of hepatocellular carcinomas (HCC) and: (1) the hyperdense zone (C) visible in the central part of zone A on post-treatment unenhanced CT scans; (2) the deposited energy; (3) the coagulation zones observed on ex-vivo bovine liver. Materials and methods The post-procedural computed tomography (CT) scans of HCCs treated with a single energy deployment through the same 2450 MHz MWA system were retrospectively analyzed, retrieving the dimensions of A and C zones and the deposited energy (E). Ex-vivo bovine liver MWA with the same system were performed and analyzed to determine the same quantities by gross-pathologic examination and CT imaging. Results A total of 101 HCC treatments were analyzed. The average coagulation volumes increased linearly with deposited energy (1.11 cc/kJ, R2 = 0.90, 4.2 kJ ≤ E ≤ 48 kJ), similarly to ex-vivo findings (1.38 cc/kJ, R2 =0.97, 7.2 kJ ≤ E ≤ 144 kJ). The long axis (L) and short axis (D) of zones A and C held a fairly constant ratio both in-vivo (LC/LA=0.43 ± 0.13; DC/DA=0.42 ± 0.10) and ex-vivo (LC/LA = 0.49 ± 0.07; DC/DA = 0.28 ± 0.06). Conclusions The average dimensions of the ablation zone induced by the considered system on HCC increase linearly with the deposited energy and are fairly well predicted by the corresponding ex-vivo dimensions. The ratio between each linear dimension of A and C zones was found to be roughly constant over a large deposited energy span, both ex-vivo and in-vivo.

To investigate liver stiffness changes—evaluated by point shear wave elastography (pSWE)—in controlled hyperthermia and microwave ablation (MWA) in an ex vivo animal model. Five samples of ex vivo bovine liver were uniformly heated to temperatures ranging from 40 to 100 °C. B-mode ultrasound imaging and pSWE were acquired simultaneously, and shear wave velocity (SWV) was measured in a region of interest (ROI). The threshold value of SWV at 60 °C (avg60) was identified. Subsequently, MWA was performed in 11 liver samples at 60 W until avg60 + 0.5 m/s was reached. SWV was measured in ROIs at 10–40 mm from the antenna feed. The correlation of mean values of SWV with location (within, border, or outside necrotic area) at gross pathology was evaluated. In controlled hyperthermia experiments, a steep transition in liver stiffness was observed at 63.0 ± 2.4 °C (SWV 3.54 ± 0.68 m/s). Avg60 was of 2.5 m/s. In 8/9 MWA experiments, interrupted when SWV of 3 m/s was measured, the ROI was at the inner side of the necrotic area border at pathology (accuracy 89%). No correlation between SWV values for outside, border, and within necrosis could be identified. pSWE can provide a velocity threshold predictive of the presence of coagulation necrosis during MWA in ex vivo liver model. However, pSWE is not able to reliably capture changes in stiffness within, at the border, and outside the necrotic zone in this experimental model.

The aim of this study was to compare the performance of a microwave ablation (MWA) apparatus in preclinical and clinical settings.The same commercial 2.45 GHz MWA apparatus was used throughout this study. In total 108 ablations at powers ranging from 20 to 130 W and lasting from 3 to 30 min were obtained on ex vivo bovine liver; 28 ablations at 60 W, 80 W and 100 W lasting 5 and 10 min were then obtained in an in vivo swine model. Finally, 32 hepatocellular carcinomas (HCCs) and 19 liver metastases in 46 patients were treated percutaneously by administering 60 W for either 5 or 10 min. The treatment outcome was characterised in terms of maximum longitudinal and transversal axis of the induced ablation zone.Ex vivo ablation volumes increased linearly with deposited energy (rFor the selected MW ablation device, ex vivo data on bovine liver was more predictive of the actual clinical performance on liver malignancies than an in vivo porcine model. Equivalent MW treatments yielded a significantly different response for HCC and metastases at higher deposited energy, suggesting that outcomes are not only device-specific but must also be characterised on a tissue-by-tissue basis.

The aim of this study was to develop a predictive model of the shrinkage of liver tissues in microwave ablation.Thirty-seven cuboid specimens of ex vivo bovine liver of size ranging from 2 cm to 8 cm were heated exploiting different techniques: 1) using a microwave oven (2.45 GHz) operated at 420 W, 500 W and 700 W for 8 to 20 min, achieving complete carbonisation of the specimens, 2) using a radiofrequency ablation apparatus (450 kHz) operated at 70 W for a time ranging from 6 to 7.5 min obtaining white coagulation of the specimens, and 3) using a microwave (2.45 GHz) ablation apparatus operated at 60 W for 10 min. Measurements of specimen dimensions, carbonised and coagulated regions were performed using a ruler with an accuracy of 1 mm. Based on the results of the first two experiments a predictive model for the contraction of liver tissue from microwave ablation was constructed and compared to the result of the third experiment.For carbonised tissue, a linear contraction of 31 ± 6% was obtained independently of the heating source, power and operation time. Radiofrequency experiments determined that the average percentage linear contraction of white coagulated tissue was 12 ± 5%. The average accuracy of our model was determined to be 3 mm (5%).The proposed model allows the prediction of the shrinkage of liver tissues upon microwave ablation given the extension of the carbonised and coagulated zones. This may be useful in helping to predict whether sufficient tissue volume is ablated in clinical practice.

Purpose: The aim of this study was to numerically and experimentally characterise the influence of tissues dimensions on the size and shape of microwave-induced ablation zones. Materials and methods: A 2.45GHz interstitial antenna was introduced into ex vivo bovine liver samples, delivering 60W for 10min; then the dimensions of the coagulated area were measured. Ablations were performed both in large samples (termed unrestricted tissue) for characterising the tissue response, and in thin samples, whose dimensions in the plane perpendicular to the antenna were smaller than the short axis of the ablated area obtained in unrestricted samples. In the numerical study the electromagnetic field emitted from the antenna and the corresponding temperature increase were evaluated in both unrestricted and thin tissue samples. Results: When the height of the tissue was smaller than the ablation diameter measured in unrestricted samples, a 7.5% increase in length of the ablated zone was experimentally observed. When both the height and width were lower than the diameter measured in unrestricted samples, an elongation of about 23.4% was experimentally obtained. The numerical study showed that the boundary conditions between the target tissue and the surrounding materials are critical. Conclusions: The ex vivo performances of microwave ablation devices are notably influenced by the shape and dimension of the tissues where the procedure takes place. Accordingly, dedicated interventional protocols should be developed for treatment planning on targets of different shape and size. © 2015 Informa UK Ltd. All rights reserved.

Results: Overall, substantial shrinkage of 52-74% of initial tissue volume was noted. The shrinkage was non-uniform over time and space, with observed asymmetry favouring the radial (23-43 % range) over the longitudinal (21-29%) direction. Algorithmic relationships for the shrinkage as a function of time were demonstrated. Furthermore, the smallest cubes showed more substantial and faster contraction (28-40% after 1 min), with more considerable volumetric shrinkage (>10%) in muscle than in liver tissue. Additionally, CT imaging demonstrated initial expansion of the tissue volume, lasting in some cases up to 3 min during the microwave ablation procedure, prior to the contraction phenomenon.Conclusions: In addition to an asymmetric substantial shrinkage of the ablated tissue volume, an initial expansion phenomenon occurs during MW ablation. Thus, complex modifications of the tissue close to a radiating antenna will likely need to be taken into account for future methods of real-time ablation monitoring.Purpose: The aim of this study was to characterise changes in tissue volume during image-guided microwave ablation in order to arrive at a more precise determination of the true ablation zone.Materials and methods: The effect of power (20-80 W) and time (1-10 min) on microwave-induced tissue contraction was experimentally evaluated in various-sized cubes of ex vivo liver (10-40 mm ± 2 mm) and muscle (20 and 40 mm ± 2 mm) embedded in agar phantoms (N = 119). Post-ablation linear and volumetric dimensions of the tissue cubes were measured and compared with pre-ablation dimensions. Subsequently, the process of tissue contraction was investigated dynamically during the ablation procedure through real-time X-ray CT scanning.

The frequency shift of the transfer function of single layer composite materials has been analyzed and tested. The efiects are studied by means of planar pseudo-elliptical fllters in Ka waveguide. The fllters, consisting of a frequency selective surface placed perpendicularly to the waveguide axis, have been realized by a high resolution photolithographic technique. Deviations of the experimental transfer functions from the simulation are analyzed with particular emphasis to the efiect of metal thickness. The flnite thickness of the metal constituting the frequency selective surface causes a shift of the transfer function towards high frequencies (blueshift), attributed to dipole-dipole interaction in the metal layer. Such an efiect is only partially predicted by full wave analysis based on flnite element method. The increase of the thickness determines a reduction of the attenuation for thickness values between 10 and 100 skin depths.

Purpose To evaluate the damage caused by microwave ablation to vessels inside and outside the ablation zone in an in vivo swine model. Materials and Methods Four pigs underwent microwave liver ablation with a 2.45-GHz generator and a 14-gauge water-cooled antenna with a miniature choke. Each animal underwent four 15-minute microwave ablations (two at 40 W, two at 60 W). Mean minimum and maximum diameters of ablation areas were calculated on gross pathologic and histologic examination. At minimum, a whole-mount section and two to four specimens were obtained from each ablation and stained with hematoxylin and eosin. Specimens were analyzed to verify the presence of damaged vessels in and outside the ablation area. Results Mean ablation diameters at gross pathologic examination, including the hemorrhagic halo, were 3.1 cm ± 0.5 at 40 W and 3.6 cm ± 1.1 at 60 W. All ablation zones presented a characteristic pattern consisting of three concentric zones: (i) a central area consisting of coagulative necrosis (mean maximum diameter, 8.5 mm ± 2.6), (ii) a larger area characterized by irreversibly damaged hepatocytes (mean maximum thickness, 11.7 mm ± 3.4), and (iii) a hemorrhagic halo. Twenty-one veins outside the ablation zone were evaluated (mean diameter, 5.6 mm), three of which (14%) showed diffuse endothelial damage. All three represented a continuation of a portal vein within the ablation area. Conclusions In a small percentage of microwave ablation cases, endothelial damage can extend from a portal vessel included in the ablation zone to a segment of the vessel situated outside the ablation zone. Further investigation of the clinical significance of this finding is needed.

To test the in vivo effects (toxicity, completeness of necrosis, dimensions of the lesion) of microwave thermoablation on porcine kidneys, using the Amica Probe v3 (Hospital Service SpA, Aprilia, Italy), in a refrigerated 17-G microwave applicator, that can be used to induce a spherical necrotic area.Six pigs were used; each kidney was treated, with no kidney pedicle clamping, by microwave thermoablation at least in three different zones with different exposure times and power, during open surgery. Twelve kidneys had 32 microwave thermoablations overall. The kidneys were then surgically removed, and necrotic lesions measured and evaluated microscopically. The sphericity index (SI) was also calculated to evaluate lesion reproducibility. Areas of renal tissue that were missed were then microscopically evaluated by NADH in vivo staining.In all, 32 thermoablations were applied; the mean (sd) lesion diameter ranged from 1.2 (0.3) to 4.2 (0.1) cm and changed in relation to both power and time of exposure. The 50-W power particularly induced necrotic renal lesions ranging from 1.9 (0.2) to 4.2 (0.1) cm as a function of the time of exposure and the optimal SI (1.04). Pathological evaluation showed no skipped areas in the context of the lesion, or healthy kidney tissue damage close to necrotic lesions.Thermoablation with the Amica probe is safe and showed excellent in vivo effects in this porcine model. Increasing the exposure time at 50 W power could be a useful percutaneous minimally invasive treatment for small solid masses (4.2 cm), avoiding the risk of missing tumour areas or kidney parenchymal damage from microwave treatment.

Negative refractive index materials tuned at n = -1 are believed to realize perfect lensing of real and evanescent modes. Metamaterials are the natural candidates to realize negative refractive index by the inversion of the effective dielectric permittiviy and magnetic permeability. The effect of the density on the tuning in the proximity of n = -1 is studied in order to find viable solutions to the issue of discretization of the lattice which correspondingly produces steps in the electromagnetic parameters of metamaterials. We study the microwave frequency negative refractive index of a metamaterial as a function of the density of the lattice period. The negative refractive index is realized by means of a waveguide filled with a split ring resonator lattice, exploited below the cut off frequency of the waveguide. We discuss the pass-band behaviour and the collective effects on the negative refractive index.

A method to determine the dielectric permittivity of materials is presented. Such a method exploits the use of a low-cost frequency selective structure (FSS), transversally placed in a waveguide in the proximity of the sample under test. The presence of the additional dielectric placed close to the FSS leads to a shift of bandpass and bandstop resonance frequencies. The relationship between the frequency shift and the permittivity of the dielectric under test allows the determination of the unknown permittivity. Such a procedure is particularly suitable for measuring the dielectric permittivity of thin slabs.