Thermal characteristics of microwave ablation in the vicinity of an arterial bifurcation

Management of adreno-cortical adenomas using microwave ablation: study of the effects of the fat tissue

BACKGROUND AND OBJECTIVES

Adrenocortical neoplasms are the main causes of secondary hypertension and related comorbidities including hypokalemia and cardiovascular diseases. Conventional techniques for the management of this condition are often invasive and not resolutive. Recent studies proposed microwave thermal ablation (MWA) to eradicate adrenocortical adenomas arising in proximity to sensitive structures. This study explores a new MWA approach to selectively direct the electromagnetic energy into the target and shield the surrounding tissues. The new solution relies on the anatomical and dielectric characteristics of the adrenal gland and the surrounding fat capsule.

METHODS

A 3 D model of the adrenal gland is developed, and a cooled microwave applicator is placed parallel to the interface between the fat and adrenal tissue. Numerical simulations are conducted at 2.45 GHz accounting for two energy delivery settings, two orientations of the applicator and blood perfusion of the tissues. Ex vivo and in vivo ablation procedures are conducted on ovine adrenal glands. Histology analysis completes the experimental studies.

RESULTS

Numerical results show asymmetric ablation profiles in ex vivo and in vivo conditions. The asymmetry ratio is influenced by the procedure settings and orientation of the applicator. Ablation zones obtained experimentally agree with those predicted by the numerical simulations. Histology analysis confirms irreversible cellular changes only in the adrenal tissue close to the applicator.

CONCLUSIONS

The outcomes show that the dielectric contrast between the fat layer and tissue target can be a tool in MWA to shape ablation zones to protect the surrounding structures from excessive temperature increases.

Numerical study on the effect of bifurcation vessel parameters on microwave ablation of lung tissue

Background: In order to study the effect of bifurcation vessels parameters on the temperature field and coagulation zone of microwave ablation on lung tissue. Methods: The finite element method was used to establish the simulation model. The angle of bifurcation vessel model was 60°. The position of the antenna and the main blood vessel are parallel, and the distance between them was 5, 10 and 15 mm, respectively. Temperature field distribution was obtained at 2450 MHz, 50 W and 300 s. The blood flow velocity was set to 0.1 and 0.2 m/s. Results: The results showed when the antenna was 5 mm away from the bifurcation vessel and the velocity was 0.1 m/s, the position of x = 8.4 mm achieved the complete necrosis at 220 s, while the fraction of necrotic tissue at the symmetry point x = 1.6 mm was 0.2 at 300 s. For the distance was 10 mm and the velocity was 0.1 m/s, the fraction of necrotic tissue at x = 3 mm that near the bifurcation vessel was 0.53 and was 0.69 at the symmetry point x = 17 mm. When the antenna is 15 mm away from the vessel, the fraction of necrotic tissue of symmetrical points on both sides of the antenna obtained after ablation were the same. Conclusions: The distance between the antenna and the bifurcation vessel over 15 mm, the blood flow has no effect on the coagulation zone. Besides, the distance between bifurcation vessel and antenna possesses a greater influence on the temperature distribution and coagulation zone than the blood flow velocity.

Numerical Analysis of Human Cancer Therapy Using Microwave Ablation

Microwave ablation is one type of hyperthermia treatment of cancer that involves heating tumor cells. This technique uses electromagnetic wave effects to kill cancer cells. A micro-coaxial antenna is introduced into the biological tissue. The radiation emitted by the antenna is absorbed by the tissue and leads to the heating of cancer cells. The diffuse increase in temperature should reach a certain value to achieve the treatment of cancer cells but it should be less than a certain other value to avoid damaging normal cells. This is why hyperthermia treatment should be carefully monitored. A numerical simulation is useful and may provide valuable information. The bio-heat equation and Maxwell’s equations are solved using the finite element method. Electro-thermal effects, temperature distribution profile, specific absorption rate (SAR), and fraction of necrotic tissue within cancer cells are analyzed. The results show that SAR and temperature distribution are strongly affected by input microwave power. High microwave power causes a high SAR value and raises the temperature above 50 °C, which may destroy healthy cells. It is revealed that with a power of 10 W, the tumor cells will be killed without damaging the surrounding tissue.

Computational study of the effects of arterial bifurcation on the temperature distribution during cryosurgery

BACKGROUND

Thermally significant blood flows into locally cooled diseased tissues and warm them during cryosurgery so that the iceball is often hard to cover the whole diseased volume. This paper is aimed at investigating the effects of large arterial bifurcation on the temperature distribution during cryosurgery through simulation method.

METHODS

A parametric geometry model is introduced to construct a close-to-real arterial bifurcation. The three-dimensional transient conjugate heat transfer between bifurcated artery and solid tissues with phase change during cryosurgery is performed by finite volume method.

RESULTS

The discussion was then made on the effects of the relative position between cryoprobe and artery bifurcation, the inlet velocity of root artery and the layout of multiple cryoprobes on the temperature distribution and iceball evolution. The results show that the thermal interaction between blood flow and iceball growth near bifurcation is considerable complex. The thermal effects of bifurcation could modulate the iceball morphology, severely weaken its freezing volume and prevent the blood vessel from being frozen.

CONCLUSION

The present work is expected to be valuable in optimizing cryosurgery scheme of the situation that the bifurcated artery is embedded into the disease tissue.

The vascular cooling effect in hepatic multipolar radiofrequency ablation leads to incomplete ablation ex vivo

PURPOSE

Major limitations of conventional RFA are vascular cooling effects. However, vascular cooling effects are supposed to be less pronounced in multipolar RFA. The objective of this ex vivo study was a systematic evaluation of the vascular cooling effects in multipolar RFA.

MATERIALS AND METHODS

Multipolar RFA with three bipolar RFA applicators was performed ex vivo in porcine liver (applicator distance 20 mm, energy input 40 kJ). A saline-perfused glass tube ('vessel') was placed parallel to the applicators in order to simulate a natural liver vessel. Five applicator-to-vessel geometries were tested. A liquid-filled glass tube without perfusion was used as a dry run. Ablations were orthogonally cut to the applicators at a defined height. Cooling effects were analysed qualitatively and quantitatively along these cross sectional areas.

RESULTS

Thirty-six ablations were performed. A cooling effect could be seen in all ablations with perfused vessels compared to the dry run. While this cooling effect did not have any influence on the ablation areas (859-1072 mm(2) versus 958 mm(2) in the dry run, p > 0.05), it had a distinctive impact on ablation shape. A vascular cooling effect could be observed in all ablations with perfusion directly around the vessel independent of the applicator position compared to the dry run (p < 0.01).

CONCLUSIONS

A vascular cooling effect occurred in all multipolar RFA with simulated liver vessels ex vivo independent of the applicator-to-vessel geometry. While the cooling effect did not influence the total ablation area, it had a distinctive impact on the ablation shape.

Design, development and microwave inter-comparison of dual slot antenna configurations for localized hepatic tumor management

Slot antennas are generally preferred for localized liver cancer treatment modalities due to desired radiation characteristics. An iterative thermal/microwave numerical routine is used to analyze regular and miniature slot antenna configurations at 5.8 GHz. A thermal/microwave solver determines the specific absorption rate to malignant tissues as a pre- processing step to compute microwave solution in terms of propagation wave number, return loss and insertion loss. The regular and miniature dual slots antenna geometries were then developed to estimate the return loss characteristics against antennas slot lengths at a constant frequency of 5.8 GHz. Results reveal that the regular geometry has return loss less than -5 dB as compared to <-25 dB return loss for miniature slot antenna configuration. Furthermore, 5.8 GHz antenna geometry provides physical size reduction up to 50 %, lower fabrication cost and is a better minimally invasive choice due to further packed thermal ablation spots.

A meshless point collocation treatment of transient bioheat problems

A meshless numerical method is proposed for the solution of the transient bioheat equation in two and three dimensions. The Pennes bioheat equation is extended in order to incorporate water evaporation, tissue damage, and temperature-dependent tissue properties during tumor ablation. The conductivity of the tissue is not assumed constant but is treated as a local function to simulate local variability due to the existence of usually unclear interfacing of healthy and pathological segments. In this way, one avoids the need for accurate identification of the boundaries between pathological and healthy regions, which is a typical problem in medical practice, and sidesteps, evidently, the corresponding mathematical treatment of such boundaries, which is usually a tedious procedure with some inevitable degree of approximation. The numerical results of the new method for test applications of the bioheat transfer equation are validated against analytical predictions and predictions of other numerical methods. 3D simulations are presented that involve the modeling of tumor ablation and account for metabolic heat generation, blood perfusion, and heat ablation using realistic values for the various parameters. An evaluation of the effective medium approximation to homogenize conductivity fields for use with the bioheat equation is also provided.

MRI-based finite element simulation on radiofrequency ablation of thyroid cancer

In order to provide a quantitative disclosure on the RFA (radiofrequency ablation)-induced thermal ablation effects within thyroid tissues, this paper has developed a three-dimensional finite element simulation strategy based on a MRI (magnetic resonance imaging)-reconstructed model. The thermal lesion's growth was predicted and interpreted under two treatment conditions, i.e. single-cooled-electrode modality and two-cooled-electrode system. The results show that the thermal lesion's growth is significantly affected by two factors including the position of RF electrode and thermal-physiological behavior of the breathing airflow. Additional parametric studies revealed several valuable phenomena, e.g. with the electrode's movement, thermal injury with varying severity would happen to the trachea wall. Besides, the changes in airflow mass produced evident effects on the total heat flux of thyroid surface, while the changes in breathing frequency only generated minor effects that can be ignored. The present study provided a better understanding on the thermal lesions of RFA within thyroid domain, which will help guide future treatment of the thyroid cancer.

Can interventional ablation be applied to the treatment of arterial aneurysm?

Interventional therapy is commonly applied to the treatment of arterial aneurysm. Thermal ablation features rapid and minimal invasive treatment. A hypothesis of combining these two techniques was proposed to treat arterial aneurysm. An antenna is delivered with a catheter into the aneurysm sac and heated with microwave or RF, and hence induces instant coagulation and thrombosis of blood flow in the aneurysm sac so that the treatment motivation is achieved. The initiation of this hypothesis is to overcome the disadvantages of biological/mechanical mismatch between the interventional devices and the arteries in the interventional therapy, also utilize the advantages of rapid treatment and minimal invasion in the thermal ablation procedure.

Theoretical Modeling for Hepatic Microwave Ablation

Thermal tissue ablation is an interventional procedure increasingly being used for treatment of diverse medical conditions. Microwave ablation is emerging as an attractive modality for thermal therapy of large soft tissue targets in short periods of time, making it particularly suitable for ablation of hepatic and other tumors. Theoretical models of the ablation process are a powerful tool for predicting the temperature profile in tissue and resultant tissue damage created by ablation devices. These models play an important role in the design and optimization of devices for microwave tissue ablation. Furthermore, they are a useful tool for exploring and planning treatment delivery strategies. This review describes the status of theoretical models developed for microwave tissue ablation. It also reviews current challenges, research trends and progress towards development of accurate models for high temperature microwave tissue ablation.

Theoretical modeling for hepatic microwave ablation

Thermal tissue ablation is an interventional procedure increasingly being used for treatment of diverse medical conditions. Microwave ablation is emerging as an attractive modality for thermal therapy of large soft tissue targets in short periods of time, making it particularly suitable for ablation of hepatic and other tumors. Theoretical models of the ablation process are a powerful tool for predicting the temperature profile in tissue and resultant tissue damage created by ablation devices. These models play an important role in the design and optimization of devices for microwave tissue ablation. Furthermore, they are a useful tool for exploring and planning treatment delivery strategies. This review describes the status of theoretical models developed for microwave tissue ablation. It also reviews current challenges, research trends and progress towards development of accurate models for high temperature microwave tissue ablation.

Finite-element analysis and in vitro experiments of placement configurations using triple antennas in microwave hepatic ablation

This study presents analyses of triple-antenna configurations and designs for microwave (MW) hepatic ablation using 3-D finite-element (FE) analyses verified by in vitro experiments. Treatment of hepatic cancer often requires removal or destruction of large volume lesions. Using multiple antennas offers a potential solution for creating ablation zones with larger dimensions, as well as varied geometrical shapes. We performed both 3-D FE analyses and in vitro experiments using three identical open-tip MW antennas simultaneously, placing them in three types of configurations-"linear array," "triangular," and "T-shaped" arrangements. We compared coagulation volumes created, as well as temperature distribution characteristics, from the three-antenna arrangements after power delivery of 50 W for 60 s. We also performed additional tests using nonidentical antennas (open tip, slot, and slot with insulating jacket) for the three configurations. The results illustrate that arranging antennas in the "T-shaped" pattern destroyed more unwanted tissues than those found when using "linear array" and "triangular" arrangements, with maximum coagulation width and depth of 46 and 81 mm, respectively, and coagulation volume of 30.7 cm(3) . In addition, using nonidentical triple antennas caused variations in coagulation zone characteristics, and thus, the technique could be applied to treatment situations where nonsymmetric coagulation zones are required.

Medical application oriented blood flow simulation

In order to show the application of computational fluid dynamics in biomedical engineering, some numerical simulations of blood flow in arteries, such as hemodynamics of bypass graft for stenosed arteries, hemodynamics of stented aneurysm at the aortic arch, hemodynamics of bypass treatment for DeBakey III aortic dissection, and influence of blood flow on the thermal characteristics of microwave ablation, which were performed by the authors, were reviewed. These simulations can be a powerful tool for the computer assisted surgery in medical application.