Volume 8 (2017): Issue 1 (January 2017)

Electrical impedance tomography (EIT) is relatively new. It is a verypromising technique to be developed especially in the medical field. The advantages of EIT are that it is non-ionizing, simple, and portable and that it produces a high contrast image. Unfortunately, this modality does not have the capability to generate a highresolution image. Almost all imaging modalities has both advantages and disadvantages. Combining one modality with another is hence expected to cover the weaknesses of each other. The problem is how to develop the concepts, measurement systems and algorithm of dual modalities, particularly electrical and acoustical. The electrical modality can produce high contrast and the acoustical modality can produce high resolution. Combination of these will enhance the image resolution of EIT. High image resolution from the ultrasound reflection tomography is used as the prior information to improve the image resolution of the EIT. Finite Element Model (FEM) can be arranged by non-uniform elements, which are adapted to the boundary. Element models with higher density are arranged at the boundaries to obtain improvements of resolution and the model elements with lower density arranged at other locations to reduce the computational cost. The dual modality EIT with Ultrasound Reflection (EIT-UR) can produce high resolution and contrast image. The resolution improvement can also accelerate the convergence of the Newton-Raphson reconstruction methods.

An animal model of deep brain stimulation (DBS) was used in in vivo studies of the encapsulation process of custom-made platinum/iridium microelectrodes in the subthalamic nucleus of hemiparkinsonian rats via electrical impedance spectroscopy. Two electrode types with 100-μm bared tips were used: i) a unipolar electrode with a 200-μm diameter and a subcutaneous gold wire counter electrode and ii) a bipolar electrode with two parallelshifted 125-μm wires. Miniaturized current-controlled pulse generators (130 Hz, 200 μA, 60 μs) enabled chronic DBS of the freely moving animals. A phenomenological electrical model enabled recalculation of the resistivity of the wound tissue around the electrodes from daily in vivo recordings of the electrode impedance over two weeks. In contrast to the commonly used 1 kHz impedance, the resistivity is independent of frequency, electrode properties, and current density. It represents the ionic DC properties of the tissue. Significant resistivity changes were detected with a characteristic decrease at approximately the 2nd day after implantation. The maximum resistivity was reached before electrical stimulation was initiated on the 8th day, which resulted in a decrease in resistivity. Compared with the unipolar electrodes, the bipolar electrodes exhibited an increased sensitivity for the tissue resistivity.

In neurosurgery intensive care units, cerebrovascular reactivity tests for neuromonitoring are used to evaluate the status of cerebral blood flow autoregulation; lack of autoregulation indicates a poor patient outcome. The goal of neuromonitoring is to prevent secondary injuries following a primary central nervous system injury, when the brain is vulnerable to further compromise due to hypoxia, ischemia and disturbances in cerebral blood flow and intracranial pressure. Ideally, neuromonitoring would be noninvasive and continuous. This study compares cerebrovascular reactivity monitored by rheoencephalography, a noninvasive continuous monitoring modality, to cerebrovascular reactivity measured by currently used neuromonitoring modalities: transcranial Doppler, near infrared spectroscopy and laser Doppler flowmetry. Fourteen healthy volunteer subjects were measured. The tests used for comparison of cerebrovascular reactivity were breath-holding, hyperventilation, CO₂ inhalation, the Valsalva maneuver, and the Trendelenburg and reverse Trendelenburg positions. Data for all modalities measured were recorded by computers and processed off line. All measured modalities reflected cerebrovascular reactivity with variabilities. Breath-holding, CO₂ inhalation, and the Valsalva maneuver caused CO₂ increase and consequent brain vasodilatation; hyperventilation caused CO₂ decrease and brain vasoconstriction. The Trendelenburg and reverse Trendelenburg positions caused extracranial blood volume changes, which masked intracranial cerebrovascular reactivity. The hyperventilation test proved ineffective for measuring cerebrovascular reactivity with rheoencephalography due to respiratory artifacts. Some discrepancies among the

In this study the transformed theory is applied to derive the dielectric characteristics of cells, considering the electrorotation (ER) peak frequency. In current studies, estimations of low frequency, which are credible for the values less than 1 mS/m for medium conductivity, are used to obtain the corresponding permittivity and conductivity of cells. Unlike the presented works, the transformed theory applies the comprehensive statement for corresponding permittivity and conductivity of cells. In the transformed theory, the membrane and interior characteristics could be obtained from the high and the low frequencies of peak ER, for all values of conductivity of medium. Characteristics of cells are obtained via optimization of an equation for the conductivity of medium regarding the peak ER frequency. The optimization process is performed applying genetic algorithm due to its swift adaptation to the problem and faster convergence.

This paper describes a new combined impedance plethysmographic (IPG) and electrical bioimpedance spectroscopic (BIS) instrument and software that allows noninvasive real-time measurement of segmental blood flow and changes in intracellular, interstitial, and intravascular volumes during various fluid management procedures. The impedance device can be operated either as a fixed frequency IPG for the quantification of segmental blood flow and hemodynamics or as a multi-frequency BIS for the recording of intracellular and extracellular resistances at 40 discrete input frequencies. The extracellular volume is then deconvoluted to obtain its intra-vascular and interstitial component volumes as functions of elapsed time. The purpose of this paper is to describe this instrumentation and to demonstrate the information that can be obtained by using it to monitor segmental compartment volumes and circulatory responses of end stage renal disease patients during acute hemodialysis. Such information may prove valuable in the diagnosis and management of rapid changes in the body fluid balance and various clinical treatments.

Current guidelines do not recommend bioelectrical impedance analysis (BIA) in patients with implanted cardiac devices. There is no data on the influence of such devices over the parameters assessed by BIA. We aimed to assess the influence of cardiac devices on the parameters assessed by BIA as well as to evaluate the likelihood of electromagnetic interference of BIA in patients with implanted cardiac devices. Sixty-two consecutive patients over 18 years of age who underwent single (PM) or multisite (CRT) pacemaker or defibrillator (ICD) implantation were included. Body composition assessment was done using a single frequency device, on both right and left sides, before and after cardiac device implantation. During BIA analysis after device implantation, we did real-time telemetry to assess electromagnetic interference. Patients were 67+14 years old and 51.6% male. PM was implanted in 52 patients (83.9%), ICD in 7 (11.3%), ICD with CRT in 2 (3.2%) and CRT in 1 (1.6%). During real-time telemetry, there was no electromagnetic interference including interruption of telemetry. Default device programming did not change after BIA assessment. After surgery, resistance and fat mass were smaller, while cellular mass, fat-free mass, metabolic rate and total body water/ body weight increased, on right and left sides measurements. We concluded that decreased resistance and related parameters after device implantation were probably influenced to a change in hydration status, regardless of the implanted device. Bioimpedance analysis is safe in patients with an implanted cardiac device.

The electrical impedance method of peripheral vein detection is a novel approach, which offers the advantages of not being expensive and the capability of minimizing and reducing the difficulty of achieving intravenous access in many patients, especially pediatric and obese patients. The electrical impedance method of peripheral vein detection is based on the measurement of electrical impedance using the 4-electrode technique by applying a known alternating current of frequency 100 kHz and constant amplitude to a set of current electrodes and measuring the resulting surface potential at two separate electrodes. This paper presents the results of investigations to estimate the efficiency of this method.

A mechanistic mathematical model for electrical impedance spectroscopy (EIS) measurements of human skin is analyzed, leading to a reduced model and approximate solutions. In essence, the model considers a complex-valued Laplace equation in the frequency domain for the alternating current from a circular EIS probe passing through the layers – stratum corneum, viable skin and adipose tissue – of human skin in the frequency range 1 kHz – 1 MHz. The reduced model, which only needs to be solved numerically for the viable skin with modified boundary conditions, is verified with the full set of equations (non-reduced model): good agreement is found with a maximum relative error of less than 3%. A Hankel transform of the reduced model allows for approximate solutions of not only the measured impedance but also the point-wise potential distribution in the skin. In addition, the dimensionless numbers governing the EIS are elucidated and discussed.

Classification of the RBCs with their shapes, volumes and volume fractions is an important indicator for the normality of the healthy body. RBCs in the plasma are simulated electrically as a conductor solution with insulated particles moving through the plasma. Consequently, the impedance of the plasma-RBCs is proportional to the number and the volume of the RBCs within the plasma. This paper presents a new proposed method for studying the characteristics of the RBCs by using a surface acoustic wave sensor. Because of the free motion of the RBCs during the test of the erythrocytes sedimentation rate, the concentration of the RBCs varies from one layer to another. Consequently, the output waveform of the surface acoustic wave sensor changes from one time to another related to the behavior of the RBCs. This method shows its ability to classify not only the volume fraction and volumes of the RBCs, but also the different types of the RBCs.

Bioimpedance is an electrical property, which is measured to indicate related parameters and diagnose several diseases of the body. The heart pulsatile is a blood flow with periodic variations as a result of heart beats. The main objective of this article is studying the effect of the heart pulsatile on the measurements of artery bioimpedance. However, neglecting the heart pulsatile leads to error in calculations of many applications based on artery bioimpedance measurement such as glucose monitoring, stenosis, and cholesterol detection. Furthermore, the studying of the heart pulsatile effect could be developed to measure the heart rate as a novel method based on bioimpedance phenomena. A simple model of electrodes and composite layers (skin, fat, muscle, and artery) is simulated using COMSOL. In this work, a model of noninvasive electrodes for measuring an artery bioimpedance is described to show the best method to take into consideration the effect of heart pulsatile.

Heart failure is a chronic disease marked by frequent hospitalizations due to pulmonary fluid congestion. Monitoring the thoracic fluid status may favor the detection of fluid congestion in an early stage and enable targeted preventive measures. Bioelectrical impedance spectroscopy (BIS) has been used in combination with the Cole model for monitoring body composition including fluid status. The model parameters reflect intracellular and extracellular fluid volume as well as cell sizes, types and interactions. Transthoracic BIS may be a suitable approach to monitoring variations in thoracic fluid content.

Electrical impedance tomography (EIT) is a relatively new imaging technique. It has the advantages of low cost, portability, non-invasiveness and is free from radiation effects. So far, this imaging technique has shown satisfactory results in functional imaging. However, it is not yet fully suitable for anatomical imaging due to its poor spatial resolution. In this paper, we review the basic directions of research in the area of the spatial resolution of the EIT systems. The improvements to the hardware and the software developments are highlighted. Finally, possible techniques to enhance the spatial resolution of the EIT systems using array processing beamforming methods are discussed.