August 9, 2022

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Impedance Analyzers Fundamentals Explained

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Electrical Impedance Tomography for Cardio-Pulmonary Monitoring


Electrical Impedance Tomography (EIT) is a bedside monitor that visually examines the local environment as well as conceivably lung perfusion distribution. In this article, we review and analyzes the methodological and clinical aspects of the thoracic EIT. Initially, investigators addressed the possibility of using EIT to assess regional ventilation. Present research is focused on its clinical applications to determine the extent of lung collapse, increased tidal flow, and lung overdistension. The goal is to monitor positive end expiratory pressure (PEEP) and Tidal volume. In addition, EIT may help to detect pneumothorax. Recent studies looked at EIT as a tool to assess regional lung perfusion. Indicate-free EIT tests could be enough to continuously monitor cardiac stroke volume. A contrast agent such as saline might be required in order to determine the regional lung perfusion. This is why EIT-based surveillance of regional airflow and lung perfusion could reveal the perfusion match and local ventilation which could be beneficial in the treatment of patients suffering from chronic respiratory distress syndrome (ARDS).

Keywords: Electrical impedance tomography bioimpedance, image reconstruction Thorax; regional ventilation and regional perfusion monitoring.

1. Introduction

Electrical impedance tomography (EIT) is one of the non-radiation functional imaging technique that permits non-invasive monitoring of bedside regional lung ventilation and arguably perfusion. Commercially available EIT devices were introduced for clinical application of this technique and the thoracic EIT is widely used for both pediatric and adult patients 1., 1.

2. Basics of Impedance Spectroscopy

Impedance Spectroscopy can be defined as the electrical response of biological tissue to an externally applied alternating electric current (AC). It is normally measured using four electrodes, where two are utilized for AC injection and the other two are used to measure voltage 3,4. Thoracic EIT measures the regional Impedance Spectroscopy of the thoracic region and can be seen as an expansion of the four electrode principle to the image plane spanned by an electrode belt [ 11. Dimensionally, electrical inductance (Z) is similar to resistance, and the appropriate International System of Units (SI) unit is Ohm (O). It can be described as a complex number in which the actual part is resistance, and the imaginary component is called reactance. It quantifies effects resulting from either inductance or capacitance. The amount of capacitance is determined by biomembranes’ features of the tissues such as ion channel, fatty acids, and gap junctions. Resistance is mainly determined by the composition and the amount of extracellular fluid [ 1., 22. When frequencies are below 5 kilohertz (kHz) electricity runs through extracellular fluid and is heavily dependent on the properties of the resistive tissues. At higher frequencies up to 50 kHz. electrical currents are slightly slowed down at cell membranes , leading to an increase in tissue capacitive properties. At frequencies above 100 kHz electricity can pass through cell membranes and reduce the capacitive portion 22. Therefore, the effects that determine the impedance of tissue depend on the used stimulation frequency. Impedance Spectroscopy is usually given as conductivity or resistance, which will normalize conductance or resistance in relation to the unit’s area and length. The SI units that correspond to it is Ohm-meter (O*m) for resistivity and Siemens per meters (S/m) for conductivity. The resistance of thoracic tissue varies between 150 O*cm of blood and up to 700 o*cm for lung tissue that has been deflated, and up to 2400 O*cm for an inflated lung tissue ( Table 1). In general, the tissue’s resistance or conductivity will vary based on amount of fluid and the ion concentration. For breathing, it depends on the volume of air inside the alveoli. Though most tissues exhibit an isotropic response, heart and muscle in particular exhibit anisotropic properties, which means that resistance strongly depends on the direction from which they are measured.

Table 1. Thoracic tissues have electrical resistance.

3. EIT Measurements and Image Reconstruction

For EIT measurements electrodes are positioned around the thorax in a transverse plan which is typically located in the 4th to the 5th intercostal areas (ICS) near that line called parasternal [5]. As a result, changes in impedance can be measured in areas of the lower part of the right and left lungs, and also within the heart region ,2]. It is possible to position the electrodes below the 6th ICS might be difficult as the diaphragm and abdominal content frequently enter the measurement plane.

Electrodes can be self-adhesive or single electrodes (e.g. electrocardiogram, ECG) that are placed individually with equal spacing between the electrodes, or they are integrated into electrode belts ,2[ 1,2]. Also, self-adhesive stripes are available for a more user-friendly application ,21,2. Chest tubes, chest wounds bandsages that are not conductive or wire sutures can hinder or negatively impact EIT measurements. Commercially available EIT devices typically utilize 16 electrodes, but EIT devices with 8 or 32 electrodes are also available (please consult Table 2 for details) It is recommended to consult Table 2 for more details. ,2[ 1,2.

Table 2. Commercially available electrical impedance (EIT) gadgets.

During an EIT test, low AC (e.g. five mA at a frequency of 100 kHz) are applied to various pairs of electrodes and the produced voltages are measured using the remaining other electrodes [ 6. The bioelectrical resistance between the injecting and the electrode pairs that measure is calculated from the known applied current as well as the measured voltages. Most often, adjacent electrode pairs are used for AC application in a 16-elektrode set-up, while 32-elektrode systems often apply a skip pattern (see Table 2) to increase the distance between the electrodes used for injecting current. The resulting voltages can be measured by using the remaining electrodes. At present, there is an ongoing debate about the various kinds of current stimulation, as well as their distinct advantages and disadvantages [77. To get a complete EIT data set of bioelectrical measurements both the injecting and measuring electrode pairs are continuously rotating around the entire thorax .

1. Current application and voltage measurements within the thorax, using an EIT system consisting of 16 electrodes. In a matter of milliseconds all the active voltage electrodes and the active voltage electrodes will be repeatedly turned in the area of the thorax.

The AC utilized during EIT tests are safe for use on body surfaces and remain undetected by the individual patient. For safety reasons, the use of EIT in patients with electrically active devices (e.g., cardiac pacemakers or cardioverter-defibrillators) is not recommended.

The EIT data set that is stored during one cycle from AC applications is technically referred to as an image frame. It includes voltage measurements needed to produce this initial EIT image. Frame rate refers to the number of EIT frames recorded in a second. Frame rates at least 10 images/s are required to monitor ventilation and 25 images/s in order to monitor the cardiac function or perfusion. Commercially accessible EIT devices run frame rates ranging from 40 to 50 images/s, as illustrated in

To produce EIT images from recorded frames, a process known as image reconstruction method is used. Reconstruction algorithms attempt to solve the reverse problem of EIT that is recuperation of the conductivity distribution inside the thorax based on the voltage measurements taken at the electrodes on the thorax surface. In the beginning, EIT reconstruction assumed that electrodes were placed on a circular or ellipsoid plane. However, newer techniques incorporate information about the anatomical shape of the thorax. Currently, EIT reconstruction algorithms such as the Sheffield back-projection algorithm and the finite element algorithm (FEM) using a linearized Newton–Raphson algorithm [ ] as well as the Graz consensus reconstruction algorithm for EIT (GREIT) [10typically used.

On the whole, EIT images have a similarity to a computed two-dimensional (CT) image. These images are typically rendered in a way that the operator looks at the cranial and caudal regions when analysing the image. In contrast to CT images, unlike a CT image An EIT image does not display the form of a “slice” but an “EIT sensitivity region” [1111. The EIT sensitivity region is a lens-shaped intra-thoracic area with impedance-related changes that contribute to EIT picture generationThe EIT image is generated by impedance changes. The shape and size of the EIT sensitivity region depend on the dimensions, the bioelectric characteristics, and the appearance of the Thorax as in the used voltage measurement and current injection pattern [12The shape and thickness of the EIT sensitivity region is determined by the voltage measurement pattern [.

Time-difference image is a technique that is used for EIT reconstruction in order to display changes in conductivity instead of total conductivity. The time-difference EIT image compares changes in impedance with the baseline frame. This allows you to observe time-dependent physiological processes like lung ventilation or perfusion [22. Color-coding for EIT images is not unicoded but typically shows the change in impedance in relation to a reference level (2). EIT images are typically coded using a rainbow-color scheme with red representing the high in relative intensity (e.g., during inspiration) with green being a medium relative impedance and blue the smallest relative impedance (e.g. when expiration is in progress). For clinical applications An interesting approach is to use color scales ranging from black (no impedance changes) through blue (intermediate impedance changes) and white (strong impedance shift) to code ventilation or between black and white, in order to code mirror perfusion.

2. Different available color codings of EIT images in comparison to the CT scan. The rainbow-color scheme makes use of red to indicate the highest relative impedance (e.g. when inspiration occurs) Green for a medium relative impedance, and blue to indicate the least relative imperceptibility (e.g. when expiration is in progress). A more recent color scale uses instead black for no impedance changes) and blue for an intermediate impedance variation, and white for the strongest impedance variation.

4. Functional Imaging and EIT Waveform Analysis

Analyzing Impedance Analyzers data is performed using EIT waveforms created inside individual image pixels within the form of a sequence of raw EIT images over the course of time (Figure 3). The term “region of interest” (ROI) can be defined to describe activity in the individual pixels of the image. In each ROI, the image shows changes in the region’s conductivity over time , resulting from respiration (ventilation-related signal, VRS) or heart activity (cardiac-related signal CRS). Additionally, electrically conductive contrast-agents like hypertonic saltsaline may be used to produce an EIT signal (indicator-based signal, IBS) and may be linked to perfusion in the lung. The CRS could be a result of both the heart and lung region, and is possibly due to lung perfusion. The precise origins and components are not understood fully 1313. Frequency spectrum analysis is frequently employed to distinguish between ventilationor cardiac-related changes in the impedance. Impedance changes that do not occur regularly could result from changes in the settings of the ventilator.

Figure 3. EIT form and function EIT (fEIT) image are derived from the Raw EIT images. EIT waveforms can be defined pixels-wise or based on a area that is of particular interest (ROI). Changes in conductivity are naturally triggered by breathing (VRS) or heart activity (CRS) but they can also be created artificially, e.g. through IBS (IBS) for perfusion measurement. FEIT images present specific physiological parameters of the region such as perfusion (Q) and ventilation (V) (V) and perfusion (Q), extracted from the raw EIT images by applying a mathematical procedure over time.

Functional EIT (fEIT) images are generated using a mathematical process on an array of raw images together with the appropriate pixel EIT waveforms [14]. Since the mathematical procedure is used to determine a physiologically relevant parameter for each pixel, physiological regional characteristics such as regional ventilation (V) and respiratory system compliance as also local perfusion (Q) can be assessed and display (Figure 3). The information derived collected from EIT waveforms , as well as concurrently registered airway pressure measurements can be used to determine the lung compliance and the lung’s opening and closing times at each pixel, using variations of impedance and pressure (volume). Comparable EIT measurements taken during stepwise inflation and deflation of the lungs allow the displaying of pressure-volume curves at scales of pixel. Depending on the mathematical operation different kinds of fEIT photographs could be used to analyze different functions in the cardiopulmonary system.

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