Local Impedance Changes Research Articles

Enhancing boundary detection of radiofrequency ablation lesions through photoacoustic mapping

Atrial fibrillation (A-fib) is the most common type of heart arrhythmia, typically treated with radiofrequency catheter ablation to isolate the heart from abnormal electrical signals. Monitoring the formation of ablation-induced lesions is crucial for preventing recurrences and complications arising from excessive or insufficient ablation. Existing imaging modalities lack real-time feedback, and their intraoperative usage is in its early stages. A critical need exists for an imaging-based lesion indexing (LSI) method that directly reflects tissue necrosis formation. Previous studies have indicated that spectroscopic photoacoustic (sPA) imaging can differentiate ablated tissues from their non-ablated counterparts based on PA spectrum variation. In this paper, we introduce a method for detecting ablation lesion boundaries using sPA imaging. This approach utilizes ablation LSI, which quantifies the ratio between the signal from ablated tissue and the total tissue signal. We enhance boundary detection accuracy by adapting a regression model-based compensation. Additionally, the method was cross-validated with clinically used intraoperative monitoring parameters. The proposed method was validated with ex vivo porcine cardiac tissues with necrotic lesions created by different ablation durations. The PA-measured lesion size was compared with gross pathology. Statistical analysis demonstrates a strong correlation (R > 0.90) between the PA-detected lesion size and gross pathology. The PA-detected lesion size also exhibits a moderate to strong correlation (R > 0.75) with local impedance changes recorded during procedures. These results suggest that the introduced PA imaging-based LSI has great potential to be incorporated into the clinical workflow, guiding ablation procedures intraoperatively.

Deep Neural Network Predicts Ti‐6Al‐4V Dissolution State Using Near‐Field Impedance Spectra

AbstractRetrieval studies document Ti‐6Al‐4V selective dissolution within crevices of total hip replacement devices. A gap persists in the fundamental understanding of Ti‐6Al‐4V crevice corrosion in vivo and its impact on local impedance. Previous studies use nearfield electrochemical impedance spectroscopy (nEIS) for characterization of retrieved CoCrMo surfaces and phase angle symmetry‐based EIS (sbEIS) for rapid data acquisition. In this study, these methods are combined with a deep neural network to characterize the local impedance changes after selective dissolution. It is hypothesized that structural changes occurring during dissolution will manifest as property changes to the oxide film capacitance. First, after sustained cathodic activation, the Ti‐6Al‐4V β phase selectively dissolves from the surface. Next, nEIS acquires n = 100 control and n = 105 dissolved spectra. Over dissolved regions, oxide capacitance significantly increases (Log10Q = ‐4.17 versus ‐4.78 (Scm−2(s)α), p = 0.000). Using single frequency EIS (5000 Hz), a capacitance‐based scanning impedance microscopy method identifies dissolved regions within seconds. Finally, Bode phase plots of the 205 control and dissolved nEIS spectra are input into a deep neural network. After training with n = 180 spectra, the model predicts the surface state for n = 25 previously unseen nEIS spectra with 96% accuracy.

Local impedance changes are a suitable real time surrogate for lesion formation in RF-ablation

Abstract Funding Acknowledgements Type of funding sources: None. Background Radiofrequency current remains one of the most important techniques for catheter ablation. Creating durable lesions is key for sufficient and safe procedures. Several possibilities were tested for estimating the progress of lesion formation. For instance, force-time-integral, ablation index and lesion size index are used to assess quality of RF-application and, therefore, lesion size and durability. Nevertheless, recovered conduction after acute conduction block remains an issue in RF-ablation. The aim of this study was to evaluate local impedance (LI) changes as real time surrogate for lesion formation. Methods RF-lesions were created using an ex vivo porcine cardiac model and a force and local impedance sensing catheter. The experimental setup consisted of a saline-filled container, a dispersive electrode, a heated thermostat and a circulation pump to imitate in vivo conditions. Global impedance was kept at 120 Ohm as well as the temperature at 37°C. RF-lesions were created using identic values of RF duration and contact force. RF power (20W, 30W, 40W, 50W) and level of electrode-tissue-coupling were also systematically varied between minor and full coupling. In minor ETC-level, only the distal end of the catheter is in contact with the tissue. In full ETC-level, the whole catheter tip is in contact with the tissue. All parameters (power, temperature, global and local impedance, contact force, ETC, lesion size) were measured constantly during application of RF-current, enabling real-time correlation of RF parameters and lesion size. In case of an audible steam pop, RF application was stopped. Results Including a total of 8654 measurements into the analysis, maximum diameter and depth were 9.51 ± 1.91 mm and 7.29 ± 2.50 mm in average. Local impedance correlated well with lesion depth (r= 0,78, p<0,001), as well as lesion diameter (r=0,652, p<0,001, Figure 1). After significant declines of LI at the beginning of RF-application, it slowly approximates a minimum. Lesion size on the other hand, rises exponentially in first seconds and gets closer to a maximum after more than 25 seconds. This relationship is illustrated in Figure 2. With decreasing lesion growth at the end of RF-application, LI changes decrease to low level (Figure 3). Conclusion Lesion formation in RF-ablation is not linear during application of RF current. Local impedance changes seem to be a suitable real-time surrogate for assessing changes in lesion size during ablation.

Baseline local impedance and changes in local impedance are predictors of electrode tissue coupling in RF-ablation

Abstract Funding Acknowledgements Type of funding sources: None. Background Radiofrequency (RF) current remains the most ablation technology in the electrophysiologic laboratory. Durable lesion formation without collateral tissue damage is key for obtaining efficient and safe procedures. RF power, duration and contact force are considered as main determinants of lesion size. Recently published data show that electrode tissue coupling (ETC) is another important parameter of lesion formation. This ex vivo model was created to investigate local impedance changes and lesion size when alternating ETC-levels. Methods RF-lesions were created using a force and local impedance sensing catheter in an ex vivo porcine cardiac model. The experimental setup consisted of a saline-filled container, a dispersive electrode, a heated thermostat and a circulation pump to imitate in vivo conditions. Global impedance was kept at 120 Ohm as well as the temperature at 37°C. RF-lesions were created using identic values of RF duration and contact force. A RF power of 20W, 30W, 40W, and 50W was used. The ETC levels were systematically varied between minor and full coupling. In minor ETC-level, only the distal end of the catheter is in contact with the tissue. In full ETC-level, the whole catheter tip is in contact with the tissue. All parameters (power, temperature, global and local impedance, contact force, ETC, lesion size) were measured constantly during application of RF-current, enabling real-time correlation of RF parameters and lesion size. In case of an audible steam pop, RF application was stopped. Results 8654 measurements of lesion depth and diameter were analyzed after creating 72 lesions. Lesion depth and diameter in full ETC-level were significantly higher compared to minor ETC-level (s. Table 1). Interestingly, lesions in lower power levels (20-30 Watt), but full ETC-level became wider and deeper than lesions in high power levels (40-50 Watts), but minor ETC-level (s. Figure 1). These observations were made despite shorter RF-duration (54.00 ± 13.07 s vs. 47.11 ± 17.57 s, p=0.032) in lesions with full ETC-level due to higher incidence of steam pops (14 vs. 6 steam pops, p = 0.032). Baseline LI and LI-drop were significantly higher in full ETC-level (Figure 2), but no relevant difference was observed in baseline GI. Conclusion Baseline LI and LI changes during ablation provide information about ETC-levels and interaction of the RF ablation catheter with cardiac tissue. ETC-levels, as predicted by LI, had a significantly higher impact on RF lesion size than RF power or RF-duration. Therefore, LI should be monitored during RF ablation for estimating ETC-levels and lesion formation.

PO-03-066 LOCAL IMPEDANCE CHANGES ARE A SUITABLE REAL TIME SURROGATE FOR LESION FORMATION IN RF-ABLATION

Radiofrequency current remains one of the most important techniques for catheter ablation. Creating durable lesions is key for sufficient and safe procedures. Several possibilities were tested for estimating the progress of lesion formation. For instance, force-time-integral, ablation index and lesion size index are used to assess quality of RF-application and, therefore, lesion size and durability. Nevertheless, recovered conduction after acute conduction block remains an issue in RF-ablation. The aim of this study was to evaluate local impedance (LI) changes as real time surrogate for lesion formation. RF-lesions were created using an ex vivo porcine cardiac model and a contact force and LI sensing catheter. The experimental setup consisted of a saline-filled container, a dispersive electrode, a heated thermostat and a circulation pump to imitate in vivo conditions. Global impedance was kept at 120 Ohm as well as the temperature at 37°C. RF-lesions were created using identic values of RF duration and contact force. RF power (20W, 30W, 40W, 50W) and level of electrode-tissue-coupling were also systematically varied between minor and full coupling. All parameters (power, temperature, global and local impedance, contact force, ETC, lesion size) were measured constantly during 60 seconds of RF-application, enabling real-time correlation of RF parameters and lesion size. In case of an audible steam pop, RF application was stopped. Including a total of 8654 measurements into the analysis, maximum diameter and depth were 9.51 ± 1.91 mm and 7.29 ± 2.50 mm in average. LI correlated well with lesion depth (r= 0,78, p<0,001), as well as lesion diameter (r=0,652, p<0,001). After significant declines of LI at the beginning of RF-application, it slowly approximates a minimum. Lesion size on the other hand, rises exponentially in first seconds and gets closer to a maximum after more than 25 seconds. With decreasing lesion growth at the end of RF-application, LI changes decrease to low level (Figure 1). Lesion formation in RF-ablation is not linear during application of RF current. Local impedance changes seem to be a suitable real-time surrogate for assessing changes in lesion size during ablation.

PO-03-064 BASELINE LOCAL IMPEDANCE AND CHANGES IN LOCAL IMPEDANCE ARE PREDICTORS OF ELECTRODE TISSUE COUPLING IN RF-ABLATION

Radiofrequency (RF) current remains the most used ablation technology in the EP lab. Durable lesion formation without collateral tissue damage is key for obtaining efficient and safe procedures. RF power, duration and contact force (CF) are considered as main determinants of lesion size. Recently published data show that electrode tissue coupling (ETC) is another important parameter of lesion formation. This ex vivo model was created to investigate local impedance (LI) changes and lesion size when alternating ETC-levels. RF-lesions were created using a CF and LI sensing catheter in an ex vivo porcine cardiac model. The experimental setup consisted of a saline-filled container, a dispersive electrode, a heated thermostat and a circulation pump to imitate in vivo conditions. Global impedance was kept at 120 Ohm as well as the temperature at 37°C. RF-lesions were created using identic values of RF duration and contact force. RF power of 20W, 30W, 40W, and 50W was used. The ETC levels were systematically varied between minor and full coupling. In minor ETC-level, only the distal end of the catheter is in contact with the tissue. In full ETC-level, the whole catheter tip is in contact with the tissue. All parameters (power, temperature, global and local impedance, CF, ETC, lesion size) were measured constantly during RF-application, enabling real-time correlation of all parameters and lesion size. 8654 measurements of lesion depth and diameter were analyzed after creating 72 lesions. Lesion depth and diameter in full ETC-level were significantly higher compared to minor ETC-level (s. Table 1). Interestingly, lesions in lower power levels (20-30 Watt), but full ETC-level became wider and deeper than lesions in high power levels (40-50 Watts), but minor ETC-level (s. Figure 1). These observations were made despite shorter RF-duration (54.00 ± 13.07 s vs. 47.11 ± 17.57 s, p=0.032) in lesions with full ETC-level due to higher incidence of steam pops (14 vs. 6 steam pops, p = 0.032). Baseline LI and LI-drop were significantly higher in full ETC-level (Figure 1), but no relevant difference was observed in baseline GI. Baseline LI and LI changes during ablation provide information about ETC-levels and interaction of the RF ablation catheter with cardiac tissue. ETC-levels, as predicted by LI, had a significantly higher impact on RF lesion size than RF power or RF-duration. Therefore, LI should be monitored during RF ablation for estimating ETC-levels and lesion formation.Tabled 1Table 1. Lesion size and impedance changes in different ETC-levels.Minor ETC (n=36)Full ETC (n=36)p-valueMaximum diameter [mm]8.48 ± 1.6110.54 ± 1.63<0.001Maximum depth [mm]5.13 ± 0.999.45 ± 1.45<0.001Start LI [Ω]236.89 ± 26.42300.67 ± 21.61<0.001Start GI [Ω]150.61 ± 21.61151.33 ± 23.680.893LI-drop [Ω]-53.61-125.53<0.001GI-drop [Ω]-21.33-33.44<0.001 Open table in a new tab

In Silico Study of Local Electrical Impedance Measurements in the Atria - Towards Understanding and Quantifying Dependencies in Human.

Electrical impedance measurements have become an accepted tool for monitoring intracardiac radio frequency ablation. Recently, the long-established generator impedance was joined by novel local impedance measurement capabilities with all electrical circuit terminals being accommodated within the catheter. This work aims at in silico quantification of distinct influencing factors that have remained challenges due to the lack of ground truth knowledge and the superposition of effects in clinical settings. We introduced a highly detailed in silico model of two local impedance enabled catheters, namely IntellaNav MiFi OI and IntellaNav Stablepoint, embedded in a series of clinically relevant environments. Assigning material and frequency specific conductivities and subsequently calculating the spread of the electrical field with the finite element method yielded in silico local impedances. The in silico model was validated by comparison to in vitro measurements of standardized sodium chloride solutions. We then investigated the effect of the withdrawal of the catheter into the transseptal sheath, catheter-tissue interaction, insertion of the catheter into pulmonary veins, and catheter irrigation. All simulated setups were in line with in vitro experiments and in human measurements and gave detailed insight into determinants of local impedance changes as well as the relation between values measured with two different devices. The in silico environment proved to be capable of resembling clinical scenarios and quantifying local impedance changes. The tool can assists the interpretation of measurements in humans and has the potential to support future catheter development.

Correlation of local impedance, global impedance and contact force with resulting lesion size in RF ablation

Abstract Funding Acknowledgements Type of funding sources: None. Background During clinical application of RF-energy, several parameters are constantly observed, e.g. RF time, power, global impedance and temperature. The new parameter "local impedance" is gaining importance and might be a possible real-time marker for predicting long-lasting RF-lesions and increasing safety. The aim of this study was to investigate the correlation between local and global impedance as well as RF power and contact force. Methods RF-lesions were created using an ex vivo model with porcine cardiac preparations. These were put in a saline-filled container with a dispersive electrode. Global impedance was held at 120 Ohm by adjusting saline and water. Additionally, a heated thermostat and a circulation pump were installed to imitate blood flow. RF-lesions were produced with different settings of energy and contact force (20, 30, 40 and 50 watts; contact force of 0-5g, 10-15g and 20-25g). While creating the lesions, global and local impedance, temperature, energy and RF time were documented, as well as the current lesion width and depth. Local and global impedance drops were calculated as the difference between baseline impedance and current impedance. Results In total, 1223 measurements were made during application of RF-energy. 6 steam pops occurred. Contact force, local and global impedance changes showed highly significant correlations with lesion depth and diameter. Amongst analyzed values, local impedance drop showed highest correlation with lesion diameter and depth (r = 0,391 and 0,613; p&amp;lt;0,001). Visualized in Figure 1 and 2, global and local impedance correlate with lesion diameter and depth. Discussion Local impedance was found to be a suitable real-time marker for lesion size, showing significantly higher correlations with RF lesion size than contact force and global impedance. Further investigation is necessary to find a cut off for a safe but long-lasting RF-lesion.

PO-695-04 CORRELATION OF LOCAL IMPEDANCE, GLOBAL IMPEDANCE AND CONTACT FORCE WITH RESULTING LESION SIZE IN RF-ABLATION

During clinical application of RF-energy, several parameters are constantly observed, e.g. RF time, power, global impedance and temperature. The new parameter “local impedance” is gaining importance and might be a possible real-time marker for predicting long-lasting RF-lesions and increasing safety. The aim of this study was to investigate the correlation between local and global impedance as well as RF power and contact force. RF-lesions were created using an ex vivo model with porcine cardiac preparations. These were put in a saline-filled container with a dispersive electrode. Global impedance was held at 120 Ohm by adjusting saline and water. Additionally, a heated thermostat and a circulation pump were installed to imitate blood flow. RF-lesions were produced with different settings of energy and contact force (20, 30, 40 and 50 watts; contact force of 0-5g, 10-15g and 20-25g). While creating the lesions, global and local impedance, temperature, energy and RF time were documented, as well as the current lesion width and depth. Local and global impedance drops were calculated as the difference between baseline impedance and current impedance. - In total, 1223 measurements were made during application of RF-energy. 6 steam pops occurred. Contact force, local and global impedance changes showed highly significant correlations with lesion depth and diameter. Amongst analyzed values, local impedance drop showed highest correlation with lesion depth and diameter (p-value <0,001 each). Visualized in Figure 1 and 2, global and local impedance correlate with lesion diameter and depth. Figure 1. Correlation of lesion size and local impedance drop. Light blue = Depth. Darker Blue = diameter. Figure 2. Correlation of lesion size and global impedance drop. Light blue = Depth. Darker Blue = diameter. Local impedance was found to be a suitable real-time marker for lesion size, showing significantly higher correlations with RF lesion size than contact force and global impedance. Further investigation is necessary to find a cut off for a safe but long-lasting RF-lesion.

Effect and identification of parametric distributed uncertainties in longitudinal wave propagation

Uncertainties play an important role in dynamic systems regarding their vibration and wave propagation behaviour. Stochastic methods have been used to address the randomness incorporated in numerical models. The spectral element method (SEM) is suitable to perform vibration and wave propagation analysis based on large frequency ranges with accuracy and low computational cost. This paper explores the longitudinal wave propagation considering uncertainties in the media aside from demonstrating and quantifying the effect of randomness inherent in the material. The stochastic Love rod spectral elements are proposed, and the parameters were assumed to be spatially distributed alongside the structure expressed as a random field. It is expanded using the Karhunen-Loève spectral decomposition and memoryless transformation. The Wentzel-Kramers-Brillouin (WKB) approximation is a powerful tool to evaluate local impedance changes slowly. It is used to indicate and quantify a changing rate related to material properties varying along the rod. Numerical examples analyse wave propagation in a longitudinal waveguide with distributed parameters.

Laser ultrasonic imaging of wavefield spatial gradients for damage detection

This article describes a new damage visualization method to investigate and analyze propagating guided Lamb waves using analyses of wavefield spatial gradients. A laser ultrasonic interrogation system was used to create full-field ultrasonic data measurements for ultrasonic wavefield imaging. The laser scanning process was performed based on both a raster scan and a circle scan. From the high-resolution wavefield data, a spatial gradient–based image processing technique was developed using gradient vectors to extract features sensitive to defects. Local impedance changes at the damaged area would result in a local distortion of the waveform which was captured and quantified by the variation of the gradient vectors in the scanning area as time evolves. Such variation was accumulated over time with a statistical threshold filter to generate a gradient-orientation map for damage visualization. The proposed algorithm was capable of producing distinctive damage patterns when tested experimentally on a 3-mm aluminum plate with multiple simultaneous simulated defects. Compared to conventional techniques like local wavenumber estimation, the generation of the accumulated orientation map involves no filtering process in the frequency or wavenumber domain, at the expense of more accurate shaping of the defect. A spatial covariance analysis was adopted to locate damage from the results as well as to evaluate the correlation between different kinds of defects. Combining the proposed approach with conventional laser ultrasonic imaging techniques enables a fast and robust damage identification and characterization process which requires lower computational burden and practical operation.

Efficient and Robust Detection of Local Impedance Changes Using Selected Electrical Excitation Conditions

Bioelectrical impedance method is useful for evaluating tissue state such as muscle injury but there is a trade off between efficiency and robustness in detection of various local impedance changes. To solve this problem, we proposed a method using compressed electrical impedance sensing and discrimination analysis. We employed multiple excitation conditions like electrical impedance tomography, and investigated the effective excitation conditions. We then used both simulation and measurement data for efficient training process. In our experiment, we implemented a simulator and a measurement system with 16 electrodes and biological phantoms, and evaluated the discrimination rate for various impedance distribution by assuming muscle injury. As a result, we found that 6/120 excitation conditions are sufficient for detection of local impedance changes in a simple model and sufficient blending ratio of the measurement data to simulation data was 10.6% to perform 90% discrimination. The experimental result also indicated that the proposed method was robust for injury size, injury position, and body shape and 90% discrimination was achieved using 13/120 excitation conditions for complex models including skin, bone, and muscle area.

Antenna Investigation by a Thermoelastic Optical Indicator Microscope: Defects Measurement and 3D Visualization of Electromagnetic Fields

Using the thermoelastic optical indicator microscopy (TEOIM) technique, we perform the visualization of the near-field distribution of a patterned frequencymodulation (FM) antenna. For the defect characterization, subtraction of images is done, whereas a background image is used for the defect-free antenna image. The electromagnetic fields distribution in the overall antenna structure is changed due to the defects, and this indicates the local impedance changes of the antenna pattern. The sensitivity of the proposed inspection technique is estimated to be in orders of a few milliwatts. In addition, a 3D visualization of the antenna transmission is realized to describe the field intensity and distribution dependences on the distance from the antenna surface. This type of investigation with a visualization opportunity may become an important tool for engineers and researchers, and it can successfully be a supplement for existing measurement techniques.

Diffraction imaging for seal evaluation using ultra high resolution 3D seismic data

Cap rock is any impermeable or low permeability formation that may trap oil, gas or water, preventing it from migrating to the surface. Evaluation of sealing properties of the cap rock is a critical task for seismic exploration. It is important for oil and gas prospecting as well as for locating reservoirs appropriate for carbon dioxide storage. The latter has been proposed as one solution to global climate change caused by heat-trapping anthropogenic gasses in the atmosphere. A fluid escaped from the reservoir fills voids in the overlying strata thereby causing local changes of acoustic properties. We aim to explore such fluid-saturated areas and thus evaluate sealing properties of underlying cap rocks. We assume that local acoustic impedance changes caused by fluid migration generate seismic diffractions. We isolate diffracted waves from P-Cable seismic data acquired over the area of interest and perform diffraction analysis of the near-surface interval (above 200 m).In the diffraction image obtained, we interpret an extensive system of broad sinuous channels, narrow linear faults, and diffuse fracture networks. In addition, we detect high-diffractivity anomalies, which extend vertically and laterally. We note no evidence of these anomalies in a conventional seismic image and associated seismic attribute (similarity and fracture density) volumes. We associate this high diffractivity to a variable pore fluid composition and saturation, which we infer to be due to hydrocarbon migration from the underlying strata. Consequently, we conclude poor seal quality for the underlying cap rocks.During data processing, we test diffraction focusing analysis for velocity model building. This option appears quite valuable for P-Cable data processing because of short offset and limitations of the standard velocity analysis.

EXPIRATORY TIME CONSTANTS BY ELECTRICAL IMPEDANCE TOMOGRAPHY IN HYPOXEMIC AND HYPERCAPNIC ACUTE LUNG FAILURE - A FEASIBILITY STUDY

During relaxed breathing expiration can be compared to a RC-circuit with R being the airway resistance, C the respiratory system compliance and τ = R∙C the expiratory time constant. For the adult respiratory system, the normal τ is around 0.8 s. Different lung pathologies have different time constants. Two common disease types are the acute respiratory distress syndrome (ARDS) and the chronic obstructive pulmonary disease (COPD). ARDS is characterized by stiff or noncompliant lungs with low compliance and normal or lower resistance which results in shorter τ. COPD is characterized by impaired airflow or high airways resistance and high compliance which results higher τ. τ reveals information about respiratory mechanics and the time required for the lungs to empty. Traditional pulmonary function tests provide global information only. EIT is a non-invasive real-time imaging technology which determines changes of lung volumes on a regional basis assuming that local impedance changes are proportional to local changes in lung volume. Pikkemaat [1] introduced the method to calculate regional τ, what we improved and applied in 10 patients.

Statistically-based damage detection in geometrically-complex structures using ultrasonic interrogation

This article introduces a novel approach to damage detection in the context of a structural health monitoring system. A statistical model of the ultrasonic guided wave interrogation process was developed and used to formulate a likelihood ratio test. From the likelihood ratio test, the optimal detector for distinguishing damaged and undamaged states of the structure was derived and found to be a metric related to signal energy. That result was confirmed using a data-driven approach based on using receiver-operating characteristic curves to compare the energy metric to other metrics found in the literature. The data in this study were generated from instrumenting two separate, geometrically complex structures with ultrasonic sensor–actuators. A three-story bolted-frame structure was constructed in the laboratory to test the approach for connections that produce highly uncertain wave paths, with damage being introduced through local impedance changes and bolt loosening. The second test structure was a section of a fuselage rib taken from a commercial aircraft in which holes and cracks were introduced to provide a testbed with a high degree of realism. The detection performance in both structures was quantified and presented. Finally, different sensor fusion strategies were implemented and the ability of such techniques to increase the statistical distinction between damaged and undamaged cases was quantified.

Electrical Impedance Tomography

t. This paper surveys some of the work our group has done in electrical impedance tomography.

The integrative properties of spiny distal dendrites

The dendritic spines of many central neurons are generally thought to modulate the ability of individual synaptic conductances to depolarize the dendritic shaft. A compartmental analysis using typical spine dimensions shows that spine neck resistances are probably far too low to support such a function, because low conductance synapses act as time-varying current sources. However, the collective presence of all spines on a dendrite significantly modifies the electrical properties of the branch in ways which have previously been overlooked. In particular, they lower its input impedance and length constant, reducing the amplitude of the unitary excitatory postsynaptic potential as well as the strength of spatial summation. This enables a dendrite to integrate large numbers of synaptic inputs while occupying minimal volume. In this way, dendritic spines are analogous to axonal myelin, which also alters transcellular impedance in order to maximize neurite function and minimize volume. Unlike membrane resistance changes, spines have little effect on the membrane time-constant so they maintain a long window for temporal summation. Though spine shape and neck resistance do not significantly affect dendritic potentials, spine area does. Therefore, while changes in spine morphology probably do not directly potentiate the strength of individual synapses, changes in spine density can regulate the synaptic excitability of an entire dendrite. The shortened length-constant of the spiny dendrite requires excitable membranes to be located in distal dendrites. These, in turn, eliminate many of the electrotonic nonlinearities associated with summation in long, thin processes, and make all distal synapses equipotent. The short length-constant also enhances the sensitivity of dendritic spikes to local impedance changes while decreasing the sensitivity to distant impedance changes. This would enable a neuron to effectively use inhibitory synapses or branch points to regulate propagation through its spiny dendritic tree. A model neuron is developed in which dendritic spines, excitable membranes, and dendritic branching combine to form a two-stage filter, which serves as a synaptic input coincidence detector with adjustable gain. Gain is regulated by potassium conductances which modulate branch point safety factor. The model is consistent with the notion of functional independence of distal dendrites and demonstrates that certain aspects of dendritic spiking which have previously been thought to require membrane hot-spots can also result from geometrical properties. It is suggested that the activation of spiny neurons may depend as much on the density as on the number of active synapses, and that spiny neurons may tend to have discrete output states whereas nonspiny neurons may be more continuous.

Electrocutaneous threshold changes produced by electric shock.

ABSTRACTThree experiments were performed to investigate the effect of strong shock on absolute electrocutaneous thresholds. Experiment 1 confirms the existence of a reliable elevation of absolute threshold after strong shock. Experiment 2 shows the effect to be highly localized to the site of the strongly shocked electrode. It also shows that threshold elevation does not follow the same time course as do local vascular and impedance changes. Experiment 3 used skin treatment to alter the local vascular and impedance levels and demonstrated that threshold elevation occurs after strong shock even after these measures have been stabilized. These data indicate that the threshold increase due to electric shock is probably analogous to auditory fatigue but does not rule out the possible involvement of more central control factors.

Local vascular and impedance changes induced by electric shock.

Local vascular and impedance changes induced by electric shock.