Radoslav Bortel

Approximation of statistical distribution of magnitude squared coherence estimated with segment overlapping

This paper suggests an approximation to the statistical distribution of a magnitude squared coherence (MSC) function estimated with segment overlapping. So far, the statistical distribution is known for an MSC computed without segment overlapping only. However, as the overlapped segmentation provides a more accurate MSC estimate, its statistical distribution is desired to allow its evaluation. This paper provides an approximation of the cumulative density function, probability density function, confidence limit formula, confidence interval computation and a procedure for the comparison of two MSC estimates, all for an MSC accurately estimated with the segment overlapping. Additionally, the benefits of the knowledge of the approximated statistics are illustrated on EEG-EMG (i.e. cortico-muscular) MSC estimates.

EEG-EMG coherence enhancement

The paper introduces a new approach to the estimation of the EEG-EMG coherence, which is used to examine the functional connection between a human brain and muscles. A typical EEG-EMG coherence estimation, with a magnitude squared coherence (MSC) barely exceeding 0.15, is enhanced so that MSC reaches or even goes above 0.5. The proposed method is mathematically analyzed, and its properties are discussed. Additionally, the paper includes several EEG-EMG coherence analysis results, with MSC exceeding 0.5.

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

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.

Regularization Techniques in Realistic Laplacian Computation

This paper explores regularization options for the ill-posed spline coefficient equations in the realistic Laplacian computation. We investigate the use of the Tikhonov regularization, truncated singular value decomposition, and the so-called lambda-correction with the regularization parameter chosen by the L-curve, generalized cross-validation, quasi-optimality, and the discrepancy principle criteria. The provided range of regularization techniques is much wider than in the previous works. The improvement of the realistic Laplacian is investigated by simulations on the three-shell spherical head model. The conclusion is that the best performance is provided by the combination of the Tikhonov regularization and the generalized cross-validation criterion-a combination that has never been suggested for this task before.

Potential approximation in realistic Laplacian computation

OBJECTIVE This paper aims to improve the shortcomings of the extant methodologies for realistic Laplacian (RL) computation, and correct the erroneous claims published in the past. METHODS We implemented several variants of RL computation methods, using various potential approximation techniques and different regularization approaches. The individual variants of the RL computation were tested using simulations based on a realistic head model computed with the boundary element method (BEM). The results which disagreed with previously published works were further analyzed, and the reasons for the disagreement were identified. RESULTS We identified the best regularization techniques for the surface potential approximation, and we showed that once these techniques are used there is often little difference between various potential approximations, which is in contrast with previous claims that promoted the radial basis function (RBF) approximation. Further, our analysis shows that the RBF approximation suffers from Runge phenomenon, which cannot be mitigated simultaneously for both deep and shallow sources; therefore, its good performance is guarantied only if a priori knowledge about the source depth is available. CONCLUSIONS The previously published methodology for RL computation was not optimal. Improvements are possible if the newly suggested approach is used. SIGNIFICANCE The methodology presented in our paper allows more efficient utilization of the RL, providing a useful tool for processing of high density EEG recordings. Presented techniques allow to achieve high EEG spatial resolution, and avoid unnecessary spatial blurring caused by the problems in the previously published RL methodology.

Electrode Position Scaling in Realistic Laplacian Computation

This note discusses the effects of the electrode position scaling on the realistic Laplacian (RL) computation. It is shown that when the RL is estimated with the help of Tikhonov regularization and the generalized cross-validation (GCV) criterion, improper electrode position scaling may influence the GCV criterion, which results in the decrease of RL precision. We identify what the proper scaling should be, and we provide a closer examination of how the GCV criterion is affected by the electrode position scaling.

Fast Communication: Approximation of the null distribution of the multiple coherence estimated with segment overlapping

In this fast communication we suggest an approximation of the null distribution of the multiple coherence (MC) estimated with segment overlapping. The approximation is based on the formulas known for the non-overlapped segmentation, but the parameter corresponding to the number of segments is altered. The suggested approximation is statistically tested through a Monte Carlo simulation, and it is shown that its precision is quite high for a considerable range of MC parameters.

Analysis of minimally invasive directional antennas for microwave tissue ablation

Abstract 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.

UHF RFID tag design for disaster management

This paper deals with the Ultra High Frequency (UHF) Radio Frequency Identification (RFID) tag design as a part of RFID localization system primarily designated for mass disasters. The design of RFID tag considers many casualties in an area of hundreds square meters, human body impedance, battery lifetime, maximal output power, resistant encapsulation etc. All requirements are taken into account in the design of several versions of RFID tag and the best one is chosen. Subsequently the design is manufactured and measured. Results show the designed RFID tag prototype is suitable for application in a disaster management.

Design of RFID outdoor localization system: RFID locator for disaster management

This paper presents an RFID outdoor localization system that is able to localize hundreds of active transponders in the area of one square kilometer using three RFID receiving stations. The RFID outdoor locator is designed for the tracking of casualties during mass disasters. Active transponders are included in triage tags that are fastened to all casualties during the first wave of rescue operations. Direction of arrival of each transponder and GPS position of three receiving stations determine the position of each active transponder in the area. The location of the transponder is displayed on a mobile terminal in maps. The paper describes a design and simulations of such system. The precision of localization of realized prototypes of the system is measured. The results and applications using the proposed RFID outdoor locator are discussed.