Thermal Volumes Research Articles

Predicting liver ablation volumes with real-time MRI thermometry

Background & AimsMRI guidance offers better lesion targeting for microwave ablation of liver lesions with higher soft-tissue contrast, as well as the possibility of real-time thermometry. This study aims to evaluate the correlation of real-time MR thermometry-predicted lesion volume with the ablation zone in postprocedural first-day images. Materials and MethodsThis single center retrospective analysis evaluated prospectively collected patients who underwent MRI-guided microwave ablation with real-time thermometry between December 2020 and July 2023. All procedures were performed in general anesthesia on a 1.5T MRI scanner. Real-time thermometry data were acquired using multi-slice gradient-echo EPI sequences, and thermal dose (CEM43 of 240 minutes as a threshold) maps were created. The volume of tissue exposed to lethal thermal dose in MR thermometry (thermal dose) was compared with the ablation zone volume in portal phase T1w MRI on postprocedural first-day using Pearson correlation test, and visual quantitative assessment by radiologists was performed to evaluate the similarity of shapes and volumes. ResultsOut of 30 patients with 33 lesions with thermometry images, six (18.1%) lesions were excluded due to artifacts limiting interpretation of thermal dose volume. Twenty-four patients with 27 lesions (20 male, age 63.1±9.1) were evaluated for the volume correlation. The volume of thermal dose-predicted lesions and the postprocedural first-day ablation zones showed a strong correlation (R=0.89, p<0.001). Similarly, visual similarity of MR thermometry-predicted shape and the ablation zone shape was graded as perfect in 23 (85.1%) lesions. ConclusionsReal-time thermal dose-predicted volumes show very good correlation with the ablation zone volumes in images obtained one day after the procedure, which could reduce the local recurrence rates with the possibility of reablating lesions within the same procedure.

Transducer module apodization for reducing bone heating during focused ultrasound uterine fibroid ablation

During MR-guided focused ultrasound (MRgFUS) for uterine fibroids, thermoablation of tissue near spine/hips is challenging due to bone heating that can cause patient pain and potentially damage nerves. Here we investigate transducer module apodization for maximizing the focal-to-bone heating ratio (ΔT ;ratio) in silico using a 6144-element flat fully populated phased array operating at 0.5 MHz (Arrayus Technologies, Inc.). Acoustic and thermal simulations were performed using anatomies of ten patients who underwent MRgFUS ablation for uterine fibroids with this device as part of a clinical trial (NCT03323905). Transducer modules (64 elements/module) whose beams intersected no-pass regions were identified, their amplitudes were reduced by varying blocking percentage levels, and the resulting temperature field distributions were evaluated across multiple sonications per patient. For all simulated sonications transducer module blocking improved ΔT ;ratio compared to no blocking. In 42% of sonications, full module blocking maximized ΔT ;ratio, with mean improvements of 97% ± 55% and 47% ± 36% in hip and spine compared to no blocking, at the cost of increased focal thermal volumes and acoustic power levels. In the remaining sonications, partial module blocking provided increased ΔT ;ratio values (39% ± 45% in hip, 19% ± 15% in spine targets) relative to full blocking. The optimal blocking percentage varied depending on the specific treatment geometry.

Characterizing reversible, irreversible, and calcium electroporation to generate a burst-dependent dynamic conductivity curve

The relationships between burst number, reversible, irreversible, and calcium electroporation have not been comprehensively evaluated in tumor tissue-mimics. Our findings indicate that electroporation effects saturate with a rate constant (τ) of 20 bursts for both conventional and high frequency waveforms (R2 > 0.88), with the separation between reversible and irreversible electroporation thresholds converging at 50 bursts. We find the lethal thresholds for calcium electroporation are statistically similar to reversible electroporation (R2 > 0.99). We then develop a burst-dependent dynamic conductivity curve that now incorporates electroporation effects due to both the electric field magnitude and burst number. Simulated ablation and thermal damage volumes vary significantly between finite element models using either the conventional or new burst-dependent dynamic conductivity curve (p < 0.05). Lastly, for clinically relevant protocols, thermal damage is indicated to not begin until 50 bursts, with maximum nonthermal ablation volumes at 100 bursts (1.5–13% thermal damage by volume). We find that >100 bursts generated negligible increases in ablation volumes with 40–70% thermal damage by volume at 300 bursts. Our results illustrate the need for considering burst number in minimizing thermal damage, choosing adjuvant therapies, and in modeling electroporation effects at low burst numbers.

Development and validation of a radiofrequency ablation treatment planning system for vertebral metastases.

Bone-targeted radiofrequency ablation (RFA) is widely used in the treatment of vertebral metastases. While radiation therapy utilizes established treatment planning systems (TPS) based on multimodal imaging to optimize treatment volumes, current RFA of vertebral metastases has been limited to qualitative image-based assessment of tumour location to direct probe selection and access. This study aimed to design, develop and evaluate a computational patient-specific RFA TPS for vertebral metastases. A TPS was developed on the open-source 3D slicer platform, including procedural setup, dose calculation (based on finite element modelling), and analysis/visualization modules. Usability testing was carried out by 7 clinicians involved in the treatment of vertebral metastases on retrospective clinical imaging data using a simplified dose calculation engine. In vivo evaluation was performed in a preclinical porcine model (n = 6 vertebrae). Dose analysis was successfully performed, with generation and display of thermal dose volumes, thermal damage, dose volume histograms and isodose contours. Usability testing showed an overall positive response to the TPS as beneficial to safe and effective RFA. The in vivo porcine study showed good agreement between the manually segmented thermally damaged volumes vs. the damage volumes identified from the TPS (Dice Similarity Coefficient = 0.71 ± 0.03, Hausdorff distance = 1.2 ± 0.1mm). A TPS specifically dedicated to RFA in the bony spine could help account for tissue heterogeneities in both thermal and electrical properties. A TPS would enable visualization of damage volumes in 2D and 3D, assisting clinicians in decisions about potential safety and effectiveness prior to performing RFA in the metastatic spine.

A search for thermal gyro-synchrotron emission from hot stellar coronae

ABSTRACT We searched for thermal gyro-synchrotron radio emission from a sample of five radio-loud stars whose X-ray coronae contain a hot ($T_e\gt 10^7\,$K) thermal component. We used the JVLA to measure Stokes I and V/I spectral energy distributions (SEDs) over the frequency range 15 – 45 GHz, determining the best-fitting model parameters using power-law and thermal gyro-synchrotron emission models. The SEDs of the three chromospherically active binaries (Algol, UX Arietis, HR 1099) were well-fit by a power-law gyro-synchrotron model, with no evidence for a thermal component. However, the SEDs of the two weak-lined T Tauri stars (V410 Tau, HD 283572) had a circularly polarized enhancement above 30 GHz that was inconsistent with a pure power-law distribution. These spectra were well-fit by summing the emission from an extended coronal volume of power-law gyro-synchrotron emission and a smaller region with thermal plasma and a much stronger magnetic field emitting thermal gyro-synchrotron radiation. We used Bayesian inference to estimate the physical plasma parameters of the emission regions (characteristic size, electron density, temperature, power-law index, and magnetic field strength and direction) using independently measured radio sizes, X-ray luminosities, and magnetic field strengths as priors, where available. The derived parameters were well-constrained but somewhat degenerate. The power-law and thermal volumes in the pre-main-sequence stars are probably not co-spatial, and we speculate they may arise from two distinct regions: a tangled-field magnetosphere where reconnection occurs and a recently discovered low-latitude poloidal magnetic field, respectively.

An hour-ahead predictive control strategy for maximizing natural ventilation in passive buildings based on weather forecasting

Due to the increasing demands of energy conservation and emission reduction, passive buildings with extra-low energy consumption and near zero energy consumption have attracted more attentions as a promising sustainable building type. Due to passive techniques such as air-tightness, passive buildings have a special demand of mechanical fresh air systems after closing windows. So it is indispensable to explore elaborate control strategies for maximizing natural ventilation, for the purpose of postponing the use of fossil energy. By taking meteorological forecast parameters as the input, an hour-head model-based predictive control (MPC) strategy of natural ventilation was proposed in this paper. Specifically, simulated multi-dimensional datasets obtained by setting different control actions under the same disturbances were obtained as the database for data-driven prediction models, which take weather forecast parameters as the input and labels of thermal comfort levels and air volumes as the output. It has been verified that although both with extended working hours and more stable comfort levels, the MPC strategy with priority to thermal comfort is superior to that with priority to fresh air volume. Furthermore, when compared with the traditional control strategy, ventilation hours via this MPC strategy could be increased by 56.3% from 366 h to 572 h. With this MPC model, the self-adaptive adjustment of window openness could be achieved by taking hour-ahead weather forecasting parameters as the input, thus to facilitate the climatic self-adaptive intelligent operation of passive buildings in future.

Ground motions induced by pore pressure changes at the Szentes geothermal area, SE Hungary

Excessive thermal water volumes have been extracted from porous sedimentary rocks in the Hungarian part of the Pannonian Basin. Thermal water production in Hungary increased significantly from the early 1970s. Regional-scale exploitation of geothermal reservoirs without re-injection resulted in basin-scale pressure drop in the Upper Pannonian (Upper Miocene) sediments, leading to compaction. This compaction resulted in ground subsidence primarily through poro-elastic coupling.We investigated surface deformation at the Szentes geothermal filed, SE Hungary, where the largest pressure decline occurred. Subsequently, hydraulic head recovery in the western part of the geothermal reservoir was initiated in the mid-1990s. We obtained data from the European Space Agency’s Envisat satellites to estimate the ground motions for the period of November 2002–December 2006. We applied inverse geomechanical modeling to estimate reservoir properties and processes. We constrained the model parameters using the Ensemble Smoother with Multiple Data Assimilation, which allowed us to incorporate large amounts of surface movement observations in a computationally efficient way. Ground movements together with the modeling results show that uplift of the Szentes geothermal field occurred during the observation period. Since no injection wells were operated at Szentes before 2018, and production temperatures remained relatively constant through the entire production period, we explain ground uplift with pore pressure increase due to natural recharge. The estimated decompaction coefficients of the reservoir system characterizing the elastic behavior of the Szentes geothermal reservoir varies between ~ 0.2 × 10–9 and 2 × 10–9 Pa−1. Compaction coefficients of the reservoir system corresponding to the earlier depressurization period, from ~ 1970 to the mid-1990s, may be significantly larger due to the potential inelastic behavior and permanent compaction of clay-rich aquitards. The improved parametrization enables better forecasting of the reservoir behavior and facilitates the assessment of future subsidence scenarios that are helpful for the establishment of a sustainable production scheme.

Tuning the Pennes Perfusion Rate to Model Large Vessel Cooling Effects in Hepatic Radiofrequency Ablation.

Radio frequency ablation (RFA) has become a popular method for the minimally invasive treatment of liver cancer. However, the success rate of these treatments depends heavily on the amount of experience the clinician possesses. Mathematical modeling can help mitigate this problem by providing an indication of the treatment outcome. Thermal lesions in RFA are affected by the cooling effect of both fine-scale and large-scale blood vessels. The exact model for large-scale blood vessels is advection-diffusion, i.e., a model capable of producing directional effects, which are known to occur in certain cases. In previous research, in situations where directional effects do not occur, the advection term in the blood vessel model has been typically replaced with the Pennes perfusion term, albeit with a higher-than-usual perfusion rate. Whether these values of the perfusion rate appearing in literature are optimal for the particular vessel radii in question, has not been investigated so far. This work aims to address this issue. An attempt has been made to determine, for values of vessel radius between 0.55 mm and 5 mm, best estimates for the perfusion rate which minimize the error in thermal lesion volumes between the perfusion-based model and the advection-based model. The results for the best estimate of the perfusion rate presented may be used in existing methods for fast estimation of RFA outcomes. Furthermore, the possible improvements to the presented methodology have been highlighted.

Comparison of computer simulations and clinical treatment results of magnetic resonance-guided focused ultrasound surgery (MRgFUS) of uterine fibroids.

PurposeMagnetic resonance‐guided focused ultrasound surgery (MRgFUS) can be used to noninvasively treat symptomatic uterine fibroids by heating with focused ultrasound sonications while monitoring the temperature with magnetic resonance (MR) thermometry. While prior studies have compared focused ultrasound simulations to clinical results, studies involving uterine fibroids remain scarce. In our study, we perform such a comparison to assess the suitability of simulations for treatment planning.MethodsSonications (N = 67) were simulated retrospectively using acoustic and thermal models based on the Rayleigh integral and Pennes bioheat equation followed by MR‐thermometry simulation in seven patients who underwent MRgFUS treatment for uterine fibroids. The spatial accuracy of simulated focus location was assessed by evaluating displacements of the centers of mass of the thermal dose distributions between simulated and treatment MR thermometry slices. Temperature–time curves and sizes of 240 equivalent minutes at 43°C (240EM43) volumes between treatment and simulation were compared.ResultsThe simulated focus location showed errors of 2.7 ± 4.1, −0.7 ± 2.0, and 1.3 ± 1.2 mm (mean ± SD) in the anterior–posterior, foot–head, and right–left directions for a fibroid absorption coefficient of 4.9 Np m–1 MHz–1 and perfusion parameter of 1.89 kg m–3 s–1. Linear regression of 240EM43 volumes of 67 sonications of patient treatments and simulations utilizing these parameters yielded a slope of 1.04 and a correlation coefficient of 0.54. The temperature rise ratio of simulation to treatment near the end of sonication was 0.47 ± 0.22, 1.28 ± 0.60, and 1.49 ± 0.71 for 66 sonications simulated utilizing fibroid absorption coefficient of 1.2, 4.9, and 8.6 Np m–1 MHz–1, respectively, and the aforementioned perfusion value. The impact of perfusion on peak temperature rise is minimal between 1.89 and 10 kg m–3 s–1, but became more substantial when utilizing a value of 100 kg m–3 s–1.ConclusionsThe results of this study suggest that perfusion, while in some cases having a substantial impact on thermal dose volumes, has less impact than ultrasound absorption for predicting peak temperature elevation at least when using perfusion parameter values up to 10 kg m–3 s–1 for this particular array geometry, frequencies, and tissue target which is good for clinicians to be aware of. The results suggest that simulations show promise in treatment planning, particularly in terms of spatial accuracy. However, in order to use simulations to predict temperature rise due to a sonication, knowledge of the patient‐specific tissue parameters, in particular the absorption coefficient is important. Currently, spatially varying patient‐specific tissue parameter values are not available during treatment, so simulations can only be used for planning purposes to estimate sonication performance on average.

Thin-film composite membranes based on hyperbranched poly(ethylene oxide) for CO2/N2 separation

Cross-linked amorphous poly(ethylene oxide) (XLPEO) is one of the leading membrane materials for post-combustion CO 2 capture. For example, XLPEO prepared from poly(ethylene glycol) methyl ether acrylate (PEGMEA) exhibited CO 2 permeability of 570 Barrer and CO 2 /N 2 selectivity of 41 at 35 °C. However, these XLPEOs cannot be dissolved in coating solutions, making it impossible to be fabricated into thin-film composite (TFC) membranes using state-of-the-art manufacturing processes. In this study, we synthesized high molecular weight yet soluble HPEO via atom transfer radical polymerization (ATRP). These polymers were thoroughly characterized and compared with XLPEO, including thermal transitions, free volumes, and pure-gas sorption and permeation properties. A polymer with the best combination of CO 2 permeability (540 Barrer) and CO 2 /N 2 selectivity (43) was fabricated into defect-free TFC membranes with a thickness as thin as 506 ± 44 nm. When challenged with simulated flue gas containing water vapor at 35 °C for over 100 h, the membrane shows stable CO 2 permeance of 850 GPU and CO 2 /N 2 selectivity of 37, comparable to the leading commercial membranes for carbon capture. • Hyperbranched PEO (HPEO) was synthesized by atom transfer radical polymerization. • HPEO 2 shows gas transport properties similar to those prepared by photopolymerization. • HPEO 2 can be fabricated into thin film composite membranes of 500 nm. • HPEO 2 membranes show one of the best CO 2 /N 2 separation properties reported. • Membranes demonstrate robust carbon capture performance with model flue gas.

Noble gas control of diamond-like content and compressive stress in carbon films by arc-mixed mode high power impulse magnetron sputtering

Conventional DC magnetron sputter deposition from a carbon target with argon as the sputtering gas is limited by a low deposition rate, and the resultant coatings have low diamond-like sp 3 content. Here we study arc-mixed mode high power impulse magnetron sputtering (HiPIMS) of carbon using He, Ne, Ar, Xe and Kr gases and show an increase in deposition rate is achieved by using noble gases heavier than argon. On the other hand, a higher sp 3 fraction is achieved by using noble gases lighter than argon. The higher deposition rate of the heavier noble gases is attributed to the higher sputtering yield and an earlier arc onset owing to their lower ionization potential . The higher sp 3 fraction achieved by lighter noble gases is attributed to stress generation by knock-on collisions at the surface of the depositing film, in the absence of stress relief created by large thermal spike volumes. When neon was used as the sputtering gas, the inert gas content was higher than for any other noble gas. Our results lead to opportunities for grading the sp 3 content with depth simply by changing the gas composition, allowing fabrication of buried conductive channels in ta-C, Metal-Insulator-Metal (MIM) (low sp 3 /high sp 3 /low sp 3 ) structures and biosensor films (high sp 3 /low sp 3 ). • Arc-mixed mode HiPIMS deposition of carbon films with He, Ne, Ar, Xe, and Kr noble gases. • The highest sp 3 fraction is achieved by He-Ne mixtures, attributed to effective momentum transfer. • The density and compressive stress in the films decrease with increasing size of noble gas atoms. • The results suggest creating buried conduction channels by changing the sputter gas composition.

Analysis of thermal energy storage tanks and PV panels combinations in different buildings controlled through model predictive control

The present study analyses the performance of a heating system controlled by a model predictive control strategy, where the impact of different combinations of thermal energy storage tank volumes and installed PV power capacities are analysed. The novelty of the paper lies in studying both economic and energy impacts of each equipment combination in different locations, buildings, and indoor occupancy schedules. The payback period, the reduction of the electricity grid consumption, and the behaviour of the coefficient of performance of the heat pump are studied in detail for all cases. Results point out that from an economic point of view, to invest in a thermal energy storage tank provides shorter payback periods in comparison to scenarios with PV panels, due to the high price of the solar elements. However, the energy performance analysis highlighted that the use of PV panels contributes to achieve up to 34%, 54%, and 90% of reduction of the electricity grid consumption in Helsinki, Strasbourg, and Athens, respectively. Finally, it is worth noting that the increase of the thermal energy storage volume improves the coefficient of performance of the heat pump.

Multi-Tissue Analysis on the Impact of Electroporation on Electrical and Thermal Properties.

Tissue electroporation is achieved by applying a series of electric pulses to destabilize cell membranes within the target tissue. The treatment volume is dictated by the electric field distribution, which depends on the pulse parameters and tissue type and can be readily predicted using numerical methods. These models require the relevant tissue properties to be known beforehand. This study aims to quantify electrical and thermal properties for three different tissue types relevant to current clinical electroporation. Pancreatic, brain, and liver tissue were harvested from pigs, then treated with IRE pulses in a parallel-plate configuration. Resulting current and temperature readings were used to calculate the conductivity and its temperature dependence for each tissue type. Finally, a computational model was constructed to examine the impact of differences between tissue types. Baseline conductivity values (mean 0.11, 0.14, and 0.12S/m) and temperature coefficients of conductivity (mean 2.0, 2.3, and 1.2 % per degree Celsius) were calculated for pancreas, brain, and liver, respectively. The accompanying computational models suggest field distribution and thermal damage volumes are dependent on tissue type. The three tissue types show similar electrical and thermal responses to IRE, though brain tissue exhibits the greatest differences. The results also show that tissue type plays a role in the expected ablation and thermal damage volumes. The conductivity and its changes due to heating are expected to have a marked impact on the ablation volume. Incorporating these tissue properties aids in the prediction and optimization of electroporation-based therapies.

Electro-Thermal Therapy Algorithms and Active Internal Electrode Cooling Reduce Thermal Injury in High Frequency Pulsed Electric Field Cancer Therapies.

Thermal tissue injury is an unintended consequence in current irreversible electroporation treatments due to the induction of Joule heating during the delivery of high voltage pulsed electric fields. In this study active temperature control measures including internal electrode cooling and dynamic energy delivery were investigated as a process for mitigating thermal injury during treatment. Ex vivo liver was used to examine the extent of thermal injury induced by 5000V treatments with delivery rates up to five times faster than current clinical practice. Active internal cooling of the electrode resulted in a 36% decrease in peak temperature vs. non-cooled control treatments. A temperature based feedback algorithm (electro-thermal therapy) was demonstrated as capable of maintaining steady state tissue temperatures between 30 and 80 °C with and without internal electrode cooling. Thermal injury volumes of 2.6cm3 were observed for protocols with 60 °C temperature set points and electrode cooling. This volume reduced to 1.5 and 0.1cm3 for equivalent treatments with 50 °C and 40 °C set points. Finally, it was demonstrated that the addition of internal electrode cooling and active temperature control algorithms reduced ETT treatment times by 84% (from 343 to 54s) vs. non-cooled temperature control strategies with equivalent thermal injury volumes.

ВЛИЯНИЕ ПАНДЕМИИ COVID-19 НА МИРОВОЙ РЫНОК ЭНЕРГЕТИЧЕСКОГО УГЛЯ

The article highlights the changes in the global thermal coal market associated with the consequences of the COVID-19 pandemic, examines the dynamics of import volumes by key countries - consumers of thermal coal and export volumes by major exporting countries, as well as price changes in the main international markets. Analysis of changes in the global steam coal market caused by the consequences of the COVID-19 pandemic and a number of other objective factors demonstrates a change in the competitive positions of the largest exporting countries, which may have long-term consequences. In addition, the article examines the continuing trend of dividing the thermal coal market into the high-calorie and low-calorie thermal coal markets, which have multidirectional development prospects.

The effects of electrical and thermal boundary condition on the simulation of radiofrequency ablation of liver cancer for tumours located near to the liver boundary

Effects of different boundary conditions prescribed across the boundaries of radiofrequency ablation (RFA) models of liver cancer are investigated for the case where the tumour is at the liver boundary. Ground and Robin-type conditions (electrical field) and body temperature and thermal insulation (thermal field) conditions are examined. 3D models of the human liver based on publicly-available CT images of the liver are developed. An artificial tumour is placed inside the liver at the boundary. Simulations are carried out using the finite element method. The numerical results indicated that different electrical and thermal boundary conditions led to different predictions of the electrical potential, temperature and thermal coagulation distributions. Ground and body temperature conditions presented an unnatural physical conditions around the ablation site, which results in more intense Joule heating and excessive heat loss from the tissue. This led to thermal damage volumes that are smaller than the cases when the Robin type or the thermal insulation conditions are prescribed. The present study suggests that RFA simulations in the future must take into consideration the choice of the type of electrical and thermal boundary conditions to be prescribed in the case where the tumour is located near to the liver boundary.

Magnetic resonance-guided laser interstitial thermal therapy for the treatment of non-lesional insular epilepsy in pediatric patients: thermal dynamic and volumetric factors influencing seizure outcomes.

To investigate the safety and efficacy of stereoelectroencephalography (sEEG) directed magnetic resonance-guided laser interstitial thermal therapy (MRgLITT) in medically refractory insular epilepsy in pediatric patients, define the relationship between ablation volumes and seizure control, and analyze the relationship between thermal energy and ablation volumes. A single-institution, retrospective review of pediatric patients with insular epilepsy who underwent sEEG directed MRgLITT over a 10-month period was performed. Perioperative, imaging, and outcome data were analyzed. Seizure outcomes were determined based on Engel score (Engel I versus Engel II-IV). Insula and ablation volumes were measured, and the proportion of insula volume ablated was calculated. Thermal energy was calculated in joules. Four patients underwent sEEG directed MRgLITT of insular epileptogenic foci. The ablation volume was higher in patients with Engel I outcome (3.93cm3) compared to Engel II-IV outcome (1.02cm3). The proportion of ablation to insula volume was lowest in patients with Engel II-IV outcome (25.09%). The mean energy requirement to create a unit volume of ablation in the insula is 1205.86J. A linear trend was noted between thermal ablation energy and ablation volume (R2 = 0.884). Over a mean follow-up period of 104days, three patients were seizure-free (Engel I), and one patient saw significant improvement in seizure frequency (Engel III). The proportion of insula ablated, as well as the volume of ablation, are related to seizure outcome with increasing ablation volumes corresponding to improved seizure control. Further analysis of insula laser ablation thermal dynamics and volumes is needed.

The Performance of Higher Frequency Microwave Ablation in the Presence of Perfusion

In this paper, we investigate the impact of perfusion on the performance of microwave ablation across a large frequency range. We designed multiple microwave ablation antennas to operate in liver tissue at discrete frequencies in the range 1.9-18 GHz. We performed electromagnetic simulations to calculate microwave power absorption patterns. Five-minute, 25W ablation experiments were performed at each frequency in perfused and nonperfused ex vivo porcine livers, and thermal lesion dimensions were measured. The volume of greatest microwave power absorption shrinks by two orders of magnitude as the frequency is increased from 1.9 to 18GHz. Mean thermal lesion volumes are consistent across the frequency range for a given perfusion state and are about three times smaller under active perfusion. Typical thermal lesion diameters (perpendicular to the antenna axis) were 24mm and 16mm for nonperfused and perfused ablations, respectively. No significant differences in axial ratio were observed among different frequency groups in active-perfusion experiments. Higher-frequency microwave ablation produces thermal lesions with volumes comparable to those achieved at lower frequencies, even in strongly perfused environments. Higher-frequency microwave ablation is appealing because it allows for more flexibility in antenna design. A critical issue concerning the feasibility of higher frequency microwave ablation, considering its strong dependence on heat diffusion to grow thermal lesions, is its performance in strongly perfused environments. This paper shows that higher frequency microwave ablation achieves thermal lesions comparable to those from microwave ablation performed at conventional frequencies in both non- and strongly perfused environments.

Analysis of the impact of storage conditions on the thermal recovery efficiency of low-temperature ATES systems

Aquifer thermal energy storage (ATES) is a technology with worldwide potential to provide sustainable space heating and cooling using groundwater stored at different temperatures. The thermal recovery efficiency is one of the main parameters that determines the overall energy savings of ATES systems and is affected by storage specifics and site-specific hydrogeological conditions. Although beneficial for the optimization of ATES design, thus far a systematic analysis of how different principal factors affect thermal recovery efficiency is lacking. Therefore, analytical approaches were developed, extended and tested numerically to evaluate how the loss of stored thermal energy by conduction, dispersion and displacement by ambient groundwater flow affect thermal recovery efficiency under different storage conditions. The practical framework provided in this study is valid for the wide range of practical conditions as derived from 331 low-temperature (<25°C) ATES systems in practice.Results show that thermal energy losses from the stored volume by conduction across the boundaries of the stored volume dominate those by dispersion for all practical storage conditions evaluated. In addition to conduction, the displacement of stored thermal volumes by ambient groundwater flow is also an important process controlling the thermal recovery efficiencies of ATES systems. An analytical expression was derived to describe the thermal recovery efficiency as a function of the ratio of the thermal radius of the stored volume over ambient groundwater flow velocity (Rth/u). For the heat losses by conduction, simulation results showed that the thermal recovery efficiency decreases linearly with increasing surface area over volume ratios for the stored volume (A/V), as was confirmed by the derivation of A/V-ratios for previous ATES studies. In the presence of ambient groundwater flow, the simulations showed that for Rth/u<1year, displacement losses dominated conduction losses. Finally, for the optimization of overall thermal recovery efficiency as affected by these two main processes, the optimal design value for the ratio of well screen length over thermal radius (L/Rth) was shown to decrease with increasing ambient flow velocities while the sensitivity for this value increased. While in the absence of ambient flow a relatively broad optimum exists around an L/Rth-ratio of 0.5–3, at 40m/year of ambient groundwater flow the optimal L/Rth-value ranges from 0.25 to 0.75. With the insights from this study, the consideration of storage volumes, the selection of suitable aquifer sections and well screen lengths can be supported in the optimization of ATES systems world-wide.

Solar energy use in district heating systems. A case study in Latvia

A significant increase in the use of solar energy can be observed. Solar energy is not, however, widely used in Latvia. The aim of this paper is to devise an algorithm to evaluate the possibility of integrating solar energy into the district heating system (DHS) of Latvia for both space heating and the supply of domestic hot water. The methodology was tested on a district heating system in a municipality in Latvia with 20 000 inhabitants. The existing DHS operation data were processed for the analysis and optimization of the system. Optimization of the system includes energy efficiency measures (EEM). Results show the possible energy production from solar energy depends on the area of different solar collector fields and thermal water storage tank volumes, as well as the probable costs of the system. It would be necessary to install solar collection fields of 9000 m2 to 72 900 m2 with a properly sized TES in order to produce from 6000 MWh/year up to 23 700 MWh/year. With this system it would be possible to supply 10% to 78% of the total heat demand with solar energy in any given DHS. If EEM have also been implemented, solar energy share can be increased.