Juho Väisänen

The influence of age and skull conductivity on surface and subdermal bipolar EEG leads

Bioelectric source measurements are influenced by the measurement location as well as the conductive properties of the tissues. Volume conductor effects such as the poorly conducting bones or the moderately conducting skin are known to affect the measurement precision and accuracy of the surface electroencephalography (EEG) measurements. This paper investigates the influence of age via skull conductivity upon surface and subdermal bipolar EEG measurement sensitivity conducted on two realistic head models from the Visible Human Project. Subdermal electrodes (a.k.a. subcutaneous electrodes) are implanted on the skull beneath the skin, fat, and muscles. We studied the effect of age upon these two electrode types according to the scalp-to-skull conductivity ratios of 5, 8, 15, and 30 : 1. The effects on the measurement sensitivity were studied by means of the half-sensitivity volume (HSV) and the region of interest sensitivity ratio (ROISR). The results indicate that the subdermal implantation notably enhances the precision and accuracy of EEG measurements by a factor of eight compared to the scalp surface measurements. In summary, the evidence indicates that both surface and subdermal EEG measurements benefit better recordings in terms of precision and accuracy on younger patients.

Sensitivity and Spatial Resolution of Transvenous Leads in Implantable Cardioverter Defibrillator

It has been previously documented that the main features and sensing performance of electrograms (EGMs) recorded in implantable cardioverter defibrillators (ICDs) depend on lead configuration. Although this dependence has been ascribed to differences in lead sensitivity and spatial resolution, the quantification of these two properties on ICD has not yet been attempted. In this paper, an operative framework to study the spatial resolution of ICD transvenous leads is presented. We propose to quantify the spatial resolution of ICD transvenous leads based on a new characterization called lead resolution volume (ResV). We analyzed the sensitivity distribution and the ResV of two unipolar (tip-can and coil-can ) and two bipolar (true or tip-ring and integrated or tip-coil) ICD transvenous lead configurations. A detailed 3-D model of the human thorax based on the visible human man dataset was used to compute the lead sensitivity and computer simulations of simple cardiac dynamics were used to quantify the ResV. Differences in the sensitivity distribution throughout the ventricular myocardium (VM) were observed for each lead configuration. In our computer model of the human thorax, the ResV was found to comprise 7%, 35%, 45%, and 70% of the VM for true bipolar, integrated bipolar, tip-can unipolar, and coil-can unipolar ICD leads, respectively. Furthermore, our analysis shows that the spatial resolution depends on both lead sensitivity and cardiac dynamics, and therefore, it can vary for different heart rhythms.

Effects of lactide monomer on the hydrolytic degradation of poly(lactide-co-glycolide) 85L/15G

The hydrolytic degradation of oriented poly(L-lactide-co-glycolide) 85 L/15 G (PLGA 85/15) sample materials with various amounts of lactide monomer was monitored in vitro at 37 °C. The materials were manufactured from medical grade PLGA 85/15 by a two-step melt extrusion-die drawing process. Results showed that the hydrolytic degradation rate depended highly on the lactide monomer content, which in turn influenced the retention of mechanical properties, mass loss, crystallinity, and dimensional stability. Even small quantities of lactide monomer (0.05-0.20 wt%) affected especially the retention of mechanical properties, which started to decline rapidly upon the inherent viscosity reaching 0.6-0.8 dl/g due to hydrolytic degradation. Based on our hydrolytic degradation data, we constructed a simplified mathematical model of degradation-related strength retention and recommend it as a functional quality control tool for melt-processed biodegradable medical devices manufactured from poly(L-lactide-co-glycolide) 85 L/15 G.

New method for analysing sensitivity distributions of electroencephalography measurements

In this paper, we introduce a new modelling related parameter called region of interest sensitivity ratio (ROISR), which describes how well the sensitivity of an electroencephalography (EEG) measurement is concentrated within the region of interest (ROI), i.e. how specific the measurement is to the sources in ROI. We demonstrate the use of the concept by analysing the sensitivity distributions of bipolar EEG measurement. We studied the effects of interelectrode distance of a bipolar EEG lead on the ROISR with cortical and non-cortical ROIs. The sensitivity distributions of EEG leads were calculated analytically by applying a three-layer spherical head model. We suggest that the developed parameter has correlation to the signal-to-noise ratio (SNR) of a measurement, and thus we studied the correlation between ROISR and SNR with 254-channel visual evoked potential (VEP) measurements of two testees. Theoretical simulations indicate that source orientation and location have major impact on the specificity and therefore they should be taken into account when the optimal bipolar electrode configuration is selected. The results also imply that the new ROISR method bears a strong correlation to the SNR of measurement and can thus be applied in the future studies to efficiently evaluate and optimize EEG measurement setups.

Averaging in vitro cardiac field potential recordings obtained with microelectrode arrays

Extracellular field potential (FP) recordings with microelectrode arrays (MEAs) from cardiomyocyte cultures offer a non-invasive way of studying the electrophysiological properties of these cells at the population level. Several studies have examined the FP properties of cardiomyocytes of various origins, including stem cell-derived cardiomyocytes. This focus reflects growing importance and interest in the field of MEA. High-quality cardiac FP signals are often difficult to obtain, especially from stem cell-derived cardiomyocyte cultures, which represent an important new field in cardiac electrophysiology. One way to improve the quality of these recordings is to average the cardiac FP signals. To date, however, no studies have examined the effect of averaging on cardiac FP signals. We report here that cardiac FP averaging can yield higher-quality signals than original individual FPs, and therefore promise more accurate detection of different phases and analysis of the cardiac FP signal. Averaged signals improved the signal-to-noise ratio (SNR), and obtaining reliable averages required approximately 50 cardiac cycles. We therefore propose that routine cardiac FP averaging can serve as a tool to compare the effects of different experimental conditions or stimuli on the properties of cardiac FPs.

Evaluation of an implantable ECG monitoring device in vitro and in vivo

An implantable wireless, inductively powered ECG-monitoring device has been tested both in vitro and in vivo. Measurements were done in saline, on the body surface and in cows. Measurement results are reported and some considerations about the device suitability for a long-time monitoring are made.

Finite difference and lead field methods in designing implantable ECG monitor

To minimize time-consuming and expensive in vitro and in vivo testing, information regarding the effects of implantation and the implants on measurements should be available during the designing of active implantable devices measuring bioelectric signals such as electrocardiograms (ECG). Modeling offers a fairly inexpensive and effective means of studying and demonstrating the effects of implantation on ECG measurements prior to any in vivo tests, and can thus provide the designer with valuable information. Finite difference model (FDM) and lead field approaches offer straightforward and effective modeling methods supporting the designing of active implantable ECG devices. The present study demonstrates such methods in developing and studying ECG implants. They were applied in demonstrating the effects of implant dimensions and of electrode implantation on the measurement sensitivity of the ECG device. The results of the simulations indicated that the interelectrode distance is the factor of the implant design determining the lead sensitivity. Other parameters related implant dimensions and shape have minor effect on the morphology of the ECG or on the average sensitivity of the measurement. This is shown for example when the interelectrode distance was reduced to 1/3 of original the average lead sensitivity decreased by 69.1% while larger relative changes in other dimensions produced clearly smaller changes. It was also observed here that implanting the electrodes deeper under the skin has major effects on the local sensitivities in heart muscle and thus affect to the morphology of the ECG. The study indicated also that non-conducting medium (i.e. implant insulated body) between the electrodes increases the sensitivity on heart muscle compared to cases where only electrodes are implanted.

Numerical analysis of the resolution of surface electrocardiographic lead systems

Non-invasive electrocardiographic (ECG) techniques for assessing the electrical activity of selected regions within the cardiac muscle can benefit from suitable positioning of surface electrodes. This positioning is usually guided heuristically and complemented by clinical and experimental studies, but there is a lack of general methods to characterize quantitatively the ability of a given electrode configuration to focus on selected regions of the heart. In this study we explore an approach to the characterization of the resolution of surface ECG systems based on the concept of Resolution Mass (RM). By integrating bioelectric signal modeling and numerical methods, we explore, in an application example, the location and size of the RM for a multielectrode ECG system. The concept of RM combined with bioelectric signal modeling and numerical methods constitutes a powerful tool to investigate the resolution properties of surface ECG systems.

Prediction of implantable ECG lead systems by using thorax models

New implantable ECG devices may provide more stable and noiseless measurements compared to body surface ECG measurements. When the electrodes are moved to inside of the body the way the ECG measurement is done is changing. Modeling can be an effective way to study effects of implantation to the capacity of electrodes to measure ECG compared to surface measurements. This work introduces a project where effects of electrode implantation to the magnitude and direction of lead sensitivity to detect cardiac source, lead field, was studied with a model of the thorax as a volume conductor. The study was based on 3D finite difference method (FDM) featuring visible human man. The results of the study indicate that the effect of electrode implantation under the skin (5-15 mm) to the way they measure ECG is rather small. Magnitude change is dependent of the studied lead and the change of the sensitivity to hearts equivalent sources in direction of lead field is minor.

Correlation between signal-to-noise ratios and region of interest sensitivity ratios of bipolar EEG measurements

We have developed a parameter, which describes how well the measurement is concentrated on the region of interest source area compared to other source areas in the volume conductor. The parameter concept is called the region of interest sensitivity ratio (ROISR). We assume that ROISR is also connected to the SNR of the measurement. The objective of the present study was to investigate the assumed correlation between the ROISR and SNR of the measurement with three-layer spherical head model. We studied how the source distribution and orientation affect the correlation and thus how applicable the ROISR is in analysing the sensitivity distributions of measurements. We simulated bipolar EEG-evoked potential measurements with 16 combinations of four-source distribution and four-source orientation models. The results indicate that the ROISR correlates with the SNR of the measurement with all tested source distributions and orientations. Thus the ROISRs concept can be applied to analyse measurement setups by modelling and analysing the sensitivity distributions.