A Review Of Impedance Tomography

Electrical Impedance Tomography for Cardio-Pulmonary Monitoring

Abstract

Electrical Impedance Tomography (EIT) is an instrument for monitoring bedside that provides non-invasive visualisation of local ventilation and even lung perfusion. This article reviews and analyzes the methodological and clinical aspects of the thoracic EIT. Initially, researchers were concerned about the possibility of using EIT to determine regional ventilation. Research is currently focused on clinical applications of EIT for assessing 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 research has evaluated EIT as a way to assess regional lung perfusion. Indicator-free EIT measurements may be sufficient to continuously measure the heart stroke volume. A contrast agent such as saline may be necessary for assessing regional perfusion of the lungs. This is why EIT-based monitoring of respiratory ventilation and lung perfusion could reveal local perfusion and oxygenation, which can be helpful in the treatment of patients suffering from acute respiratory distress syndrome (ARDS).

Keywords: electrical impedance tmography Bioimpedance; image reconstruction; thorax; regional ventilation, regional perfusion; monitoring

1. Introduction

The electrical impedance imaging (EIT) is an non-radiation functional imaging method that permits an uninvasive monitoring of respiratory ventilation in the region and possibly perfusion. Commercially accessible EIT devices were introduced for clinical application of this technique, and the thoracic EIT can be used with safety in both adult and pediatric patients [ 1, [ 1, 2].

2. Basics of Impedance Spectroscopy

Impedance Spectroscopy may be described as the voltage response of biological tissue to an externally applied alternating electricity (AC). It is usually achieved using four electrodes. Two are used for AC injection, and the remaining two are used for measuring voltage 3,4. Thoracic EIT measures the regional variation of the intra-thoracic bioimpedance. It is seen in the same way as applying the four electrode principle to the image plane spanned through the electro belt 1]. In terms of dimension, electrical impedance (Z) is similar to resistance, and the corresponding International System of Units (SI) unit is Ohm (O). It can be described as a complex number in which the real component is resistance and the imaginary part is called the reactance, which evaluates the effects that result from capacitors or the effect of inductance. Capacitance is a function of biomembranes’ characteristics of a tissue , including ion channels and fatty acids as well as gap junctions. However, resistance is mainly determined by the composition and quantity of extracellular fluid 1., 2[ 1, 2]. When frequencies are below 5 kilohertz (kHz) electricity travels through extracellular fluids and is primarily dependent upon the properties of the resistive tissues. For higher frequencies that exceed 50 kHz, electrical currents are slightly diverted at cells’ membranes which causes an increase of capacitive tissue properties. If frequencies are higher than 100 kHz electrical current can flow through cell membranes, and diminish the capacitive portion 21 2. So, the results that determine tissue impedance strongly depend on the used stimulation frequency. Impedance Spectroscopy is usually given as conductivity or resistivity, which regulates conductance or resistance according to unit length and area. The SI equivalent units comprise Ohm-meter (O*m) for resistivity and Siemens per meters (S/m) (S/m) for conductivity. The tissue’s resistance varies from 150 o*cm for blood and up to 700 o*cm for air-filled lung tissue, and as high as 2400 O*cm when dealing with an inflated lung tissue ( Table 1). In general, the tissue’s resistance or conductivity depends on the amount of fluid and the ion concentration. In terms of those in the lungs, it also depends on the volume of air that is present in the alveoli. While most tissues exhibit anisotropic behavior, the heart and muscles in skeletal tend to be anisotropic, this means that resistivity is heavily dependent on the direction from which you measure it.

Table 1. Electrical resistance of thoracic tissues.

3. EIT Measurements and Image Reconstruction

To perform EIT measurements electrodes are placed around the thorax in a transverse plane that is usually located in the 4th through 5th intercostal spaces (ICS) near Parasternal Line [55. The changes in impedance can be assessed in the lower lobes of the right and left lungs, as well as in the region of the heart ,2[ 1,2]. To position the electrodes above the 6th ICS could be difficult since abdominal content and the diaphragm are frequently inserted into the measurement plane.

Electrodes are self-adhesive electrodes (e.g. electrocardiogram ECG) that are positioned individually in a similar spacing between electrodes, or they are integrated into electrode belts ,2]. Additionally, self-adhesive strips are offered for a more user-friendly application [ ,2[ 1,2]. Chest tubes, chest wounds (non-conductive) bandages or sutures for wires can significantly impact EIT measurements. Commercially available EIT equipment typically uses 16 electrodes. However, EIT systems that have 8 as well as 32 electrodes are available (please see Table 2 for information) It is recommended to consult Table 2 for more details. ,21.

Table 2. Electronic impedance (EIT) gadgets.

During an EIT test, low AC (e.g. 5 1 mA at 100 kHz) is applied through various electrode pairs, and the resulting voltages are measured using the other electrodes 6. The bioelectrical resistance between the injecting and the electrode pairs used to measure the voltage is calculated based on the applied current and measured voltages. Most commonly the electrodes adjacent to each other are utilized for AC application in a 16-elektrode set-up and 32-elektrode systems typically use a skip pattern (see the table 2.) in order to extend the space between the electrodes for current injection. The resulting voltages are measured using all the electrodes. Presently, there’s an ongoing discussion about different types of current stimulation and the advantages and disadvantages of each [77. To acquire a complete EIT data set that includes bioelectrical tests, the injecting and the electrode pairs used for measuring are constantly rotated throughout the entire thorax .

1. Current measurements and voltage measurements around the thorax with an EIT system with 16 electrodes. In only a few milliseconds each of the electrodes for current and an active voltage electrode can be turned in the area of the thorax.

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

This EIT data set captured during a single cycle that is recorded during one cycle of AC software is called an image frame. It includes voltage measurements used to create that raw EIT image. The term frame rate reflects the number of EIT frames that are recorded every second. Frame rates of at least 10 images/s are essential to monitor ventilation , and 25 images/s for monitoring the cardiac function or perfusion. Commercially available EIT equipment uses frame rates ranging from 40 to 50 images/s, as is shown in

To create EIT images from recorded frames, an algorithm known as image reconstruction method is used. Reconstruction algorithms attempt to solve the inverse problem of EIT, which is the restoration of the conductivity pattern in the thorax using the voltage measurements recorded at the electrodes that are on the thorax surface. In the beginning, EIT reconstruction assumed that electrodes were placed on an ellipsoid, circular or circular plane, while newer algorithms use information about anatomy of the thorax. In the present, it is the Sheffield back-projection algorithm [ along with the finite elements method (FEM) built on a linearized Newton and Raffson algorithm ], and the Graz consensus reconstruction algorithm for EIT (GREIT) [10is frequently employed.

It is generally true that EIT images have a similarity to a two-dimensional computed tomography (CT) image: these images are typically rendered in a way that the viewer is looking at the cranial and caudal regions when studying the image. In contrast to a CT image one can observe that an EIT image doesn’t show an actual “slice” but an “EIT sensitivity region” [11]. The EIT sensitivity region is a lens-shaped intra-thoracic volume from which impedance changes contribute to EIT imaging process [11]. The size and shape of EIT sensitization region is determined by the dimensions, the bioelectrical properties, and also the structure of the chest as well with the type of current injection and voltage measurement pattern [12It is important to note that the shape of the thoracic thorax can.

Time-difference-based imaging is a process that is employed in EIT reconstruction, which displays changes in conductivity instead of relative conductivity of the levels. The time-difference EIT image compares the variation in impedance with a baseline frame. This provides the chance to track the time-dependent physiological changes like lung ventilation or perfusion [22. Color coded EIT images isn’t uniform, but usually displays the change in intensity to a baseline level (2). EIT images are generally coded using a rainbow-color scheme with red indicating the highest in relative intensity (e.g. when inspiration occurs) and green for a middle relative impedance, while blue is the lowest impedance (e.g. for expiration). For clinical purposes the best option is to use color scales that range from black (no change in impedance) and blue (intermediate impedance change), and white (strong impedance shift) to code ventilation . from black to white, and red for mirror perfusion.

2. Different color codes that are available for EIT images in comparison to the CT scan. The rainbow-color scheme is based on red for the most powerful in terms of relative intensity (e.g., during inspiration), green for a medium relative impedance, and blue, for the lowest ratio of impedance (e.g. during expiration). The newer color scales employ instead of black, which has no impedance change), blue for an intermediate change in impedance and white for the highest impedance change.

4. Functional Imaging and EIT Waveform Analysis

Analyzing Impedance Analyzers data is based on EIT waves that are generated in the individual pixels of a series of raw EIT images over long periods of (Figure 3). A region of interest (ROI) can be defined to summarize activity in individual pixels of the image. In each ROI the image shows the changes in conductivity of the region over time , resulting from respiration (ventilation-related signal, VRS) and cardiac activities (cardiac-related signal, CRS). Additionally, electrically conductive contrast agents like hypertonic saline could be used to produce the EIT waveform (indicator-based signal, IBS) and is linked to perfusion in the lung. The CRS can be traced to both the lung and the cardiac region and may be partly due to lung perfusion. The exact source and composition are not well understood. 13]. Frequency spectrum analysis is often utilized to differentiate between ventilationand cardiac-related changes in impedance. Impedance changes outside of the periodic cycle could result from changes in settings for the ventilator.

Figure 3. EIT waveforms , as well as the functional EIT (fEIT) photos are derived from EIT raw EIT images. EIT waveforms are defined in a pixel-wise manner or based on a specific region of interest (ROI). Conductivity changes result naturally from the process of ventilation (VRS) or cardiac activity (CRS) however they could be artificially induced, e.g. or through bolus injection (IBS) for the purpose of measuring perfusion. FEIT images depict various physiological parameters in the region including ventilation (V) or perfusion (Q) which are extracted from raw EIT images by applying an algorithmic operation over time.

Functional EIT (fEIT) images are created by applying a mathematical calculation on the raw images and the corresponding EIT form [14]. Since the mathematical procedure is used to determine a physiologically relevant parameter for each pixel, physiological regional characteristics like regional ventilation (V) and respiratory system compliance, as in addition to respiratory system compliance as well as regional perfusion (Q) can be measured as well as displayed (Figure 3). Data from EIT waveforms , as well as concurrently registered airway pressure values can be utilized to calculate the lung compliance as well as the lung’s opening and closing times for each pixel by calculating changes of pressure and impedance (volume). Comparable EIT measurements taken during gradual inflation and deflation of the lungs enable the display of volume-pressure curves at an individual level. Based on the mathematical operation, different types of fEIT photos could reflect different functional characteristics for the cardio-pulmonary system.

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