Publications by authors named "Jan Sebek"

12 Publications

  • Page 1 of 1

Transcervical microwave ablation in type 2 uterine fibroids via a hysteroscopic approach: analysis of ablation profiles.

Biomed Phys Eng Express 2021 Jun 1;7(4). Epub 2021 Jun 1.

Department of Electrical and Computer Engineering, Kansas State University, Manhattan, Kansas, United States of America.

Type 2 uterine fibroids are challenging to resect surgically as ≥ 50% volume of myoma lies within the myometrium. A hysteroscopic approach for ablating fibroids is minimally-invasive, but places a considerable burden on the operator to accurately place the ablation applicator within the target. We investigated the sensitivity of transcervical microwave ablation outcome with respect to position of the ablation applicator within 1 - 3 cm type 2 fibroids.A finite element computer model was developed to simulate 5.8 GHz microwave ablation of fibroids and validated with experiments intissue. The ablation outcome was evaluated with respect to applicator insertion angles (30°, 45°, 60°) , depth and offset from the fibroid center (±2 mm for 3 cm fibroid and ±1 mm for 1 cm fibroid) with 35 W and 15 W applied power for 3 cm and 1 cm fibroids, respectively. Power deposition was stopped when thermal dose of 40 cumulative equivalent minutes at 43 °C (CEM43) was accrued in adjacent myometrium.Within the range of all evaluated insertion angles, depths and offsets, the ablation coverage was less sensitive to variation in angle as compared to depth and offset, and ranged from 34.9 - 83.6% for 3 cm fibroid in 140 - 400 s and 34.1 - 67.9% for 1 cm fibroid in 30 - 50 s of heating duration. Maximum achievable ablation coverage in both fibroid cases reach ∼ 90% if thermal dose is allowed to exceed 40 CEM43 in myometrium.The study demonstrates the technical feasibility of transcervical microwave ablation for fibroid treatment and the relationship between applicator position within the fibroid and fraction of fibroid that can be ablated while limiting thermal dose in adjacent myometrium.
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http://dx.doi.org/10.1088/2057-1976/abffe4DOI Listing
June 2021

Microwave ablation of lung tumors: A probabilistic approach for simulation-based treatment planning.

Med Phys 2021 May 8. Epub 2021 May 8.

Department of Electrical and Computer Engineering, Kansas State University Manhattan, KS, 66506, USA.

Purpose: Microwave ablation (MWA) is a clinically established modality for treatment of lung tumors. A challenge with existing application of MWA, however, is local tumor progression, potentially due to failure to establish an adequate treatment margin. This study presents a robust simulation-based treatment planning methodology to assist operators in comparatively assessing thermal profiles and likelihood of achieving a specified minimum margin as a function of candidate applied energy parameters.

Methods: We employed a biophysical simulation-based probabilistic treatment planning methodology to evaluate the likelihood of achieving a specified minimum margin for candidate treatment parameters (i.e., applied power and ablation duration for a given applicator position within a tumor). A set of simulations with varying tissue properties was evaluated for each considered combination of power and ablation duration, and for four different scenarios of contrast in tissue biophysical properties between tumor and normal lung. A treatment planning graph was then assembled, where distributions of achieved minimum ablation zone margins and collateral damage volumes can be assessed for candidate applied power and treatment duration combinations. For each chosen power and time combination, the operator can also visualize the histogram of ablation zone boundaries overlaid on the tumor and target volumes. We assembled treatment planning graphs for generic 1, 2, and 2.5 cm diameter spherically shaped tumors and also illustrated the impact of tissue heterogeneity on delivered treatment plans and resulting ablation histograms. Finally, we illustrated the treatment planning methodology on two example patient-specific cases of tumors with irregular shapes.

Results: The assembled treatment planning graphs indicate that 30 W, 6 min ablations achieve a 5-mm minimum margin across all simulated cases for 1-cm diameter spherical tumors, and 70 W, 10 min ablations achieve a 3-mm minimum margin across 90% of simulations for a 2.5-cm diameter spherical tumor. Different scenarios of tissue heterogeneity between tumor and lung tissue revealed 2 min overall difference in ablation duration, in order to reliably achieve a 4-mm minimum margin or larger each time for 2-cm diameter spherical tumor.

Conclusions: An approach for simulation-based treatment planning for microwave ablation of lung tumors is illustrated to account for the impact of specific geometry of the treatment site, tissue property uncertainty, and heterogeneity between the tumor and normal lung.
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http://dx.doi.org/10.1002/mp.14923DOI Listing
May 2021

NIRS-based monitoring of kidney graft perfusion.

PLoS One 2020 2;15(12):e0243154. Epub 2020 Dec 2.

Transplant Surgery Department, Institute for Clinical and Experimental Medicine, Prague, Czech Republic.

Introduction: Acute early vascular complications are rare, but serious complications after kidney transplantation. They often result in graft loss. For this reason, shortening the diagnostic process is crucial. Currently, it is standard procedure to monitor renal graft perfusion using Doppler ultrasound (DU). With respect to acute vascular complications, the main disadvantage of this type of examination is its periodicity. It would be of great benefit if graft blood perfusion could be monitored continuously during the early postoperative period. It appears evident that a well-designed near infrared spectroscopy (NIRS) monitoring system could prove very useful during the early post-transplantation period. Its role in the immediate diagnosis of vascular complications could result in a significant increase in graft salvage, thus improving the patient's overall quality of life and lowering morbidity and mortality for renal graft recipients. The aim of this study was to design, construct and test such a monitoring system.

Materials And Methods: We designed a rough NIRS-based system prototype and prepared a two-stage laboratory experiment based on a laboratory pig model. In the first stage, a total of 10 animals were used to verify and optimize the technical aspects and functionality of the prototype sensor by testing it on the animal kidneys in-vivo. As a result of these tests, a more specific prototype was designed. During the second stage, we prepared a unique laboratory model of a pig kidney autotransplantation and tested the system for long-term functionality on a group of 20 animals. Overall sensitivity and specificity were calculated, and a final prototype was prepared and completed with its own analytic software and chassis.

Results: We designed and constructed a NIRS-based system for kidney graft perfusion monitoring. The measurement system provided reliable performance and 100% sensitivity when detecting acute diminished blood perfusion of the transplanted kidneys in laboratory conditions.

Conclusion: The system appears to be a useful tool for diagnosing diminished blood perfusion of kidney transplants during the early postoperative period. However, further testing is still required. We believe that applying our method in current human transplantation medicine is feasible, and we are confident that our prototype is ready for human testing.
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http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0243154PLOS
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7710057PMC
January 2021

Bronchoscopically delivered microwave ablation in an porcine lung model.

ERJ Open Res 2020 Oct 13;6(4). Epub 2020 Oct 13.

Dept of Electrical and Computer Engineering, Kansas State University Manhattan, Manhattan, KS, USA.

Background: Percutaneous microwave ablation is clinically used for inoperable lung tumour treatment. Delivery of microwave ablation applicators to tumour sites within lung parenchyma under virtual bronchoscopy guidance may enable ablation with reduced risk of pneumothorax, providing a minimally invasive treatment of early-stage tumours, which are increasingly detected with computed tomography (CT) screening. The objective of this study was to integrate a custom microwave ablation platform, incorporating a flexible applicator, with a clinically established virtual bronchoscopy guidance system, and to assess technical feasibility for safely creating localised thermal ablations in porcine lungs .

Methods: Pre-ablation CTs of normal pigs were acquired to create a virtual model of the lungs, including airways and significant blood vessels. Virtual bronchoscopy-guided microwave ablation procedures were performed with 24-32 W power (at the applicator distal tip) delivered for 5-10 mins. A total of eight ablations were performed in three pigs. Post-treatment CT images were acquired to assess the extent of damage and ablation zones were further evaluated with viability stains and histopathologic analysis.

Results: The flexible microwave applicators were delivered to ablation sites within lung parenchyma 5-24 mm from the airway wall a tunnel created under virtual bronchoscopy guidance. No pneumothorax or significant airway bleeding was observed. The ablation short axis observed on gross pathology ranged 16.5-23.5 mm and 14-26 mm on CT imaging.

Conclusion: We have demonstrated the technical feasibility for safely delivering microwave ablation in the lung parenchyma under virtual bronchoscopic guidance in an porcine lung model.
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http://dx.doi.org/10.1183/23120541.00146-2020DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7553114PMC
October 2020

Assessment of thermal damage to myometrium during microwave ablation of uterine fibroids.

Annu Int Conf IEEE Eng Med Biol Soc 2020 07;2020:5263-5266

Thermal ablation techniques are increasingly used for the treatment of symptomatic uterine fibroids. Thermal protection of myometrial tissue adjacent to the fibroid from ablation is critical to maximally preserve the uterus. This study presents a bench top experimental setup, using ex vivo bovine muscle as a surrogate tissue, for evaluating collateral thermal damage in tissues during fibroid ablation. The study reports on the effect of applicator insertion angles (67.5° and 90°) into a mock fibroid on the efficacy of treatment. 6 experiments were performed (3 for each insertion angle) with 30 W applied power at 2.45 GHz. The heating duration was restricted to the time at which a thermal dose of 10 cumulative equivalent minutes at 43 °C (10 CEM 43) was accrued at the boundary of the mock fibroid. Results showed that the volume of ablation inside the mock fibroid dropped considerably from 66% to 17% when the applicator insertion angle was changed from 90º to 67.5º, suggesting that insertion angle plays an important role during microwave ablation of fibroid. The proposed setup provides a method for validating computational models for accurate and safe delivery of ablation to target tissues in fibroid treatment.
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http://dx.doi.org/10.1109/EMBC44109.2020.9176092DOI Listing
July 2020

Broadband Dielectric Properties of Ex Vivo Bovine Liver Tissue Characterized at Ablative Temperatures.

IEEE Trans Biomed Eng 2021 01 21;68(1):90-98. Epub 2020 Dec 21.

Objective: To investigate the thermal and frequency dependence of dielectric properties of ex vivo liver tissue - relative permittivity and effective conductivity - over the frequency range 500 MHz to 6 GHz and temperatures ranging from 20 to 130 °C.

Methods: We measured the dielectric properties of fresh ex vivo bovine liver tissue using the open-ended coaxial probe method (n = 15 samples). Numerical optimization techniques were utilized to obtain parametric models for characterizing changes in broadband dielectric properties as a function of temperature and thermal isoeffective dose. The effect of heating tissue at rates over the range 6.4-16.9 °C/min was studied. The measured dielectric properties were used in simulations of microwave ablation to assess changes in simulated antenna return loss compared to experimental measurements.

Results: Across all frequencies, both relative permittivity and effective conductivity dropped sharply over the temperature range 89 - 107 °C. Below 91 °C, the slope of the effective conductivity changes from positive values at lower frequencies (0.5-1.64 GHz) to negative values at higher frequencies (1.64-6 GHz). The maximum achieved correlation values between transient reflection coefficients from measurements and simulations ranged between 0.83 - 0.89 and 0.68 - 0.91, respectively, when using temperature-dependent and thermal-dose dependent dielectric property parameterizations.

Conclusion: We have presented experimental measurements and parametric models for characterizing changes in dielectric properties of bovine liver tissue at ablative temperatures.

Significance: The presented dielectric property models will contribute to the development of ablation systems operating at frequencies other than 2.45 GHz, as well as broadband techniques for monitoring growth of microwave ablation zones.
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http://dx.doi.org/10.1109/TBME.2020.2996825DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7680390PMC
January 2021

Directional Microwave Ablation: Experimental Evaluation of a 2.45-GHz Applicator in Ex Vivo and In Vivo Liver.

J Vasc Interv Radiol 2020 07 11;31(7):1170-1177.e2. Epub 2020 Mar 11.

Department of Electrical and Computer Engineering, Kansas State University, Manhattan, Kansas. Electronic address:

Purpose: To experimentally characterize a microwave (MW) ablation applicator designed to produce directional ablation zones.

Materials And Methods: Using a 14-gauge, 2.45-GHz side-firing MW ablation applicator, 36 ex vivo bovine liver ablations were performed. Ablations were performed at 60 W, 80 W, and 100 W for 3, 5, and 10 minutes (n = 4 per combination). Ablation zone forward and backward depth and width were measured and directivity was calculated as the ratio of forward to backward depth. Thirteen in vivo ablations were performed in 2 domestic swine with the applicator either inserted into the liver (80 W, 5 min, n = 3; 100 W, 5 min, n = 3; 100 W, 10 min, n = 2) or placed on the surface of the liver with a nontarget tissue placed on the back side of the applicator (80 W, 5 min, n = 5). The animals were immediately euthanized after the procedure; the livers were harvested and sectioned perpendicular to the axis of the applicator. In vivo ablation zones were measured following viability staining and assessed on histopathology.

Results: Mean ex vivo ablation forward depth was 8.3-15.5 mm. No backward heating was observed at 60 W, 3-5 minutes; directivity was 4.7-11.0 for the other power and time combinations. In vivo ablation forward depth was 10.3-11.5 mm, and directivity was 11.5-16.1. No visible or microscopic thermal damage to nontarget tissues in direct contact with the back side of the applicator was observed.

Conclusions: The side-firing MW ablation applicator can create directional ablation zones in ex vivo and in vivo tissues.
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http://dx.doi.org/10.1016/j.jvir.2020.01.016DOI Listing
July 2020

Broadband lung dielectric properties over the ablative temperature range: Experimental measurements and parametric models.

Med Phys 2019 Oct 10;46(10):4291-4303. Epub 2019 Aug 10.

Department of Electrical and Computer Engineering, Kansas State University, 1701D Platt st., Manhattan, KS, 66506, USA.

Purpose: Computational models of microwave tissue ablation are widely used to guide the development of ablation devices, and are increasingly being used for the development of treatment planning and monitoring platforms. Knowledge of temperature-dependent dielectric properties of lung tissue is essential for accurate modeling of microwave ablation (MWA) of the lung.

Methods: We employed the open-ended coaxial probe method, coupled with a custom tissue heating apparatus, to measure dielectric properties of ex vivo porcine and bovine lung tissue at temperatures ranging between 31 and 150  C, over the frequency range 500 MHz to 6 GHz. Furthermore, we employed numerical optimization techniques to provide parametric models for characterizing the broadband temperature-dependent dielectric properties of tissue, and their variability across tissue samples, suitable for use in computational models of microwave tissue ablation.

Results: Rapid decreases in both relative permittivity and effective conductivity were observed in the temperature range from 94 to 108  C. Over the measured frequency range, both relative permittivity and effective conductivity were suitably modeled by piecewise linear functions [root mean square error (RMSE) = 1.0952 for permittivity and 0.0650 S/m for conductivity]. Detailed characterization of the variability in lung tissue properties was provided to enable uncertainty quantification of models of MWA.

Conclusions: The reported dielectric properties of lung tissue, and parametric models which also capture their distribution, will aid the development of computational models of microwave lung ablation.
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http://dx.doi.org/10.1002/mp.13704DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6893909PMC
October 2019

Suppression of overlearning in independent component analysis used for removal of muscular artifacts from electroencephalographic records.

PLoS One 2018 14;13(8):e0201900. Epub 2018 Aug 14.

Dept. of Circuit Theory, Czech Technical University, Faculty of Electrical Engineering, Prague, Czech Republic.

This paper addresses the overlearning problem in the independent component analysis (ICA) used for the removal of muscular artifacts from electroencephalographic (EEG) records. We note that for short EEG records with high number of channels the ICA fails to separate artifact-free EEG and muscular artifacts, which has been previously attributed to the phenomenon called overlearning. We address this problem by projecting an EEG record into several subspaces with a lower dimension, and perform the ICA on each subspace separately. Due to a reduced dimension of the subspaces, the overlearning is suppressed, and muscular artifacts are better separated. Once the muscular artifacts are removed, the signals in the individual subspaces are combined to provide an artifact free EEG record. We show that for short signals and high number of EEG channels our approach outperforms the currently available ICA based algorithms for muscular artifact removal. The proposed technique can efficiently suppress ICA overlearning for short signal segments of high density EEG signals.
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http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0201900PLOS
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6091961PMC
February 2019

Computational modeling of 915 MHz microwave ablation: Comparative assessment of temperature-dependent tissue dielectric models.

Med Phys 2017 Sep 7;44(9):4859-4868. Epub 2017 Aug 7.

Department of Electrical and Computer Engineering, Kansas State University, Manhattan, KS, 66506, USA.

Purpose: The objective of this study is to develop a computational model for simulating 915 MHz microwave ablation (MWA), and verify the simulation predictions of transient temperature profiles against experimental measurements. Due to the limited experimental data characterizing temperature-dependent changes of tissue dielectric properties at 915 MHz, we comparatively assess two temperature-dependent approaches of modeling of dielectric properties: model A- piecewise linear temperature dependencies based on existing, but limited, experimental data, and model B- similar to model A, but augmented with linear decrease in electrical conductivity above 95 °C, as guided by our experimental measurements.

Methods: The finite element method was used to simulate MWA procedures in liver with a clinical 915 MHz ablation applicator. A coupled electromagnetic-thermal solver incorporating temperature-dependent tissue biophysical properties of liver was implemented. Predictions of the transient temperature profiles and ablation zone dimensions for both model A and model B were compared against experimental measurements in ex vivo bovine liver tissue. Broadband dielectric properties of tissue within different regions of the ablation zone were measured and reported at 915 MHz and 2.45 GHz.

Results: Model B yielded peak tissue temperatures in closer agreement with experimental measurements, attributed to the inclusion of decrease in electrical conductivity at elevated temperature. The simulated transverse diameters of the ablation zone predicted by both models were greater than experimental measurements, which may be in part due to the lack of a tissue shrinkage model. At both considered power levels, predictions of transverse ablation zone diameters were in closer agreement with measurements for model B (max. discrepancy of 5 mm at 60 W, and 3 mm at 30 W), compared to model A (max. discrepancy of 9 mm at 60 W, and 6 mm at 30 W). Ablation zone lengths with both models were within 2 mm at 30 W, but overestimated by up to 10 mm at 60 W.

Conclusions: The inclusion of decreased electrical conductivity above 95 °C, implemented with model B as guided by our experimental measurements, may be a good approach for approximating the dynamic changes that occur during MWA at 915 MHz. Although a step toward more effectively modeling MWA at 915 MHz, further investigation of the transition in dielectric properties with temperature and tissue shrinkage, especially at high temperatures is needed for more accurate simulations.
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http://dx.doi.org/10.1002/mp.12359DOI Listing
September 2017

Analysis of minimally invasive directional antennas for microwave tissue ablation.

Int J Hyperthermia 2017 02 5;33(1):51-60. Epub 2016 Jul 5.

a Department of Electrical and Computer Engineering , Kansas State University , Manhattan , Kansas , USA.

Purpose: Microwave ablation (MWA) applicators capable of creating directional heating patterns offer the potential of simplifying treatment of targets in proximity to critical structures and avoiding the need for piercing the tumour volume. This work reports on improved directional MWA antennas with the objectives of minimising device diameter for percutaneous use (≤ ∼13 gauge) and yielding larger ablation zones.

Methods: Two directional MWA antenna designs, with a modified monopole radiating element and spherical and parabolic reflectors are proposed. A 3D-coupled electromagnetic heat transfer with temperature-dependent material properties was implemented to characterise MWA at 40 and 77 W, for 5 and 10 min. Simulations were also used to assess antenna impedance matching within liver, kidney, lung, bone and brain tissue. The two antenna designs were fabricated and experimentally evaluated with ablations in ex vivo tissue at the two power levels and treatment durations (n = 5 repetitions for each group).

Results: The computed specific absorption rate (SAR) patterns for both antennas were similar, although simulations indicated slightly greater forward penetration for the parabolic antenna. Based on simulations for antennas inserted within different tissues, the proposed antenna design appears to offer good impedance matching for a variety of tissue types. Experiments in ex vivo tissue showed radial ablation depths of 19 ± 0.9 mm in the forward direction for the applicator with spherical reflector and 18.7 ± 0.7 mm for the applicator with parabolic reflector.

Conclusion: These results suggest the applicator may be suitable for creating localised directional ablation zones for treating small and medium-sized targets with a percutaneous approach.
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http://dx.doi.org/10.1080/02656736.2016.1195519DOI Listing
February 2017

Sensitivity of microwave ablation models to tissue biophysical properties: A first step toward probabilistic modeling and treatment planning.

Med Phys 2016 May;43(5):2649

Department of Electrical and Computer Engineering, Kansas State University, Manhattan, Kansas 66506.

Purpose: Computational models of microwave ablation (MWA) are widely used during the design optimization of novel devices and are under consideration for patient-specific treatment planning. The objective of this study was to assess the sensitivity of computational models of MWA to tissue biophysical properties.

Methods: The Morris method was employed to assess the global sensitivity of the coupled electromagnetic-thermal model, which was implemented with the finite element method (FEM). The FEM model incorporated temperature dependencies of tissue physical properties. The variability of the model was studied using six different outputs to characterize the size and shape of the ablation zone, as well as impedance matching of the ablation antenna. Furthermore, the sensitivity results were statistically analyzed and absolute influence of each input parameter was quantified. A framework for systematically incorporating model uncertainties for treatment planning was suggested.

Results: A total of 1221 simulations, incorporating 111 randomly sampled starting points, were performed. Tissue dielectric parameters, specifically relative permittivity, effective conductivity, and the threshold temperature at which they transitioned to lower values (i.e., signifying desiccation), were identified as the most influential parameters for the shape of the ablation zone and antenna impedance matching. Of the thermal parameters considered in this study, the nominal blood perfusion rate and the temperature interval across which the tissue changes phase were identified as the most influential. The latent heat of tissue water vaporization and the volumetric heat capacity of the vaporized tissue were recognized as the least influential parameters. Based on the evaluation of absolute changes, the most important parameter (perfusion) had approximately 40.23 times greater influence on ablation area than the least important parameter (volumetric heat capacity of vaporized tissue). Another significant input parameter (permittivity) had 22.26 times higher influence on the deviation of ablation edge shape from a sphere than one of the less important parameters (latent heat of liver tissue vaporization).

Conclusions: Dielectric parameters, blood perfusion rate, and the temperature interval across which the tissue changes phase were found to have the most significant impact on MWA model outputs. The latent heat of tissue water vaporization and the volumetric heat capacity of the vaporized tissue were recognized as the least influential parameters. Uncertainties in model outputs identified in this study can be incorporated to provide probabilistic maps of expected ablation outcome for patient-specific treatment planning.
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http://dx.doi.org/10.1118/1.4947482DOI Listing
May 2016