Electrical Impedance Tomography for Cardio-Pulmonary Monitoring

Abstract

Electrical Impedance Tomography (EIT) is a bedside monitoring device that does not require any surgery to see the local airflow 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. These studies focus 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 studies have evaluated EIT as a tool to measure regional lung perfusion. The absence of indicators in EIT tests could be enough to measure continuously the cardiac stroke volume. Utilizing a contrast agent, such as saline, may be required to measure regional perfusion of the lungs. Therefore, EIT-based monitoring of regional respiration as well as lung perfusion can be used to assess the perfusion match and local ventilation which could be beneficial in treating patients with acute respiratory distress syndrome (ARDS).

Keywords: electrical impedance tomography bioimpedance; reconstruction of images Thorax; regional vent as well as monitoring regional perfusion.

1. Introduction

EI tomography (EIT) is one of the radiation-free functional imaging modality that provides non-invasive bedside monitoring of both regional lung ventilation and , possibly perfusion. Commercially available EIT devices were introduced for the clinical use of this technique, and the thoracic EIT can be used with safety in both adult and pediatric patients [ 1., 2.

2. Basics of Impedance Spectroscopy

Impedance Spectroscopy may be described as the electrical response of biological tissue to an externally applied alternating electronic current (AC). It is usually achieved using four electrodes. Two are utilized to inject AC injection, and the remaining two for voltage measurement [ 3,]. Thoracic EIT measures the regional variation of the intra-thoracic bioimpedance. It is seen as an expansion of the four electrode principle to the image plane spanned through the electro belt 1]. Dimensionally, electrical resistance (Z) is the same as resistance , and the appropriate International System of Units (SI) unit is Ohm (O). It is easily expressed as a complex number where the real portion is resistance, and the imaginary component is known as reactance, which quantifies effects resulting from resistance or capacitance. Capacitance is dependent on biomembranes’ characteristics of the tissue , which includes ion channels, fatty acids, and gap junctions. The resistance is mostly determined by the structure and the amount of extracellular fluid [ 1, 22. Below 5 kilohertz (kHz) (kHz), electrical current travels through extracellular fluids and is predominantly dependent on the resistance characteristics of tissues. In higher frequencies above 50 kHz. electrical currents are slightly slowed down at the cell membranes resulting in an increase in tissue capacitive properties. When frequencies exceed 100 kHz electrical currents can travel through cell membranes and reduce the capacitive portion 21. So, the results that determine the impedance of tissue depend on the utilized stimulation frequency. Impedance Spectroscopy is typically described as conductivity or resistance, which equalizes conductance and resistance to unit length and area. The SI equivalent units consist of Ohm-meter (O*m) for resistivity and Siemens per meters (S/m) on 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, tissue resistivity or conductivity will vary based on level of fluids and ions. For the lungs, it is dependent on the quantity of air inside the alveoli. While most tissues exhibit anisotropic behaviour, the heart and skeletal muscle behave anisotropic, this means that resistivity is heavily dependent on the direction in which it’s measured.

Table 1. The electrical resistivity of the 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) at Parasternal Line [55. Subsequently, the changes of impedance can be measured in areas of the lower part of the left and right lungs and also in the region of the heart ,21. The placement of the electrodes below the 6th ICS might be difficult as abdominal content and the diaphragm occasionally enter the measurement area.

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(1). Self-adhesive stripes are also readily available for a user-friendly application [ ,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, but EIT systems with 8 or 32 electrodes are available (please consult Table 2 for more details) For more information, refer to Table 2. ,2[ 1,2.

Table 2. Electric impedance tomography (EIT) devices.

In an EIT test, low AC (e.g. the smallest value of 5 microamps at 100 kHz) is applied to different electrodes, and the generated voltages are measured with the remaining other electrodes [ ]. Bioelectrical impedance between the injecting and the electrodes that are measuring is calculated using the applied current as well as the observed voltages. Most commonly nearby electrode pairs are used for AC application within a 16-elektrode configuration however 32-elektrode systems usually use a skip pattern (see Table 2.) to increase the distance between the electrodes used for injecting current. The resulting voltages can be measured with other electrodes. At present, there is an ongoing discussion on different current stimulation patterns and their advantages and disadvantages [7]. For a complete EIT data set of bioelectrical measurements The injecting and electrode pairs that measure are continually moved around the entire thorax .

1. Measurements of voltage and current in the thorax using an EIT system consisting of 16 electrodes. Within milliseconds simultaneously, the current electrode and those with active voltage electrodes are repeatedly rotating in the area of the thorax.

The AC employed during EIT measurements are safe for use on the body and will not be detected 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.

This EIT data set which is recorded during one cycle within AC software is termed a frame . It is comprised of the voltage measurements used to create this Raw EIT image. Frame rate refers to the number of EIT frames recorded each second. Frame rates of no less than 10 frames/s are required in order to monitor ventilation and 25 images/s to check perfusion or cardiac function. Commercially accessible EIT devices use frame rates of 40 to 50 images/s [2], as described in

To create EIT images from the recorded frames, a process known as reconstructing of images is carried out. Reconstruction algorithms are designed to address the problem that is the reverse of EIT that is reconstruction of the conductivity distribution inside the thorax on the basis of the voltage measurements obtained at the electrodes located on the thorax’s surface. At first, EIT reconstruction assumed that electrodes were placed on an ellipsoid plane, whereas newer algorithms employ information on the anatomical structure of the thorax. The current algorithms include using the Sheffield back-projection algorithm as well as the finite element technique (FEM) built on a linearized Newton and Raffson algorithm [ ], and the Graz consensus reconstruction algorithm for EIT (GREIT) [10is frequently employed.

The majority of EIT pictures are similar to a two-dimensional computed (CT) image. These images are traditionally rendered so that the viewer looks from cranial towards caudal when studying the image. In contrast to an CT image An EIT image does not show an actual “slice” but an “EIT sensitivity region” [11]. The EIT sensitive region is a lens-shaped intrathoracic region where impedance fluctuations contribute to EIT imaging process [11(11, 11). The dimensions and shape of the EIT sensitization region is determined by the dimensions, bioelectric properties, and the structure of the chest as well with the type of current injection and voltage measurement pattern [12(13, 14).

Time-difference imaging is a method which is employed for EIT reconstruction to display changes in conductivity and not the relative conductivity of the levels. The time-difference EIT image compares changes in impedance to a baseline frame. This gives the possibility to trace time-varying physiological phenomena such as lung breathing and perfusion [22. The color code of EIT images may not be uniform but commonly displays the change in the impedance of the patient to a standard (2). EIT images are typically colored using a rainbow color scheme with red indicating the most significant value of relative imperf (e.g. in the time of inspiration) and green for a middle relative impedance, and blue the smallest relative impedance (e.g. during expiration). For clinical applications it is possible to utilize color scales that range from black (no impedance change) or blue (intermediate impedance changes), and white (strong impedance change) for coded ventilation. from black to white and red for mirror perfusion.

2. Different color codes that are available for EIT images as compared to CT scan. The rainbow-color scheme employs red for the greatest ratio of resistance (e.g. when inspiration occurs), green for a moderate relative impedance, blue as the one with the lowest impedance (e.g., during expiration). A newer color scheme uses instead of black to avoid any impedance changes) and blue for an intermediate impedance variation, while white is the one with the strongest impedance change.

4. Functional Imaging and EIT Waveform Analysis

Analysis of Impedance Analyzers data is done using EIT waveforms that are formed in the individual pixels of the raw EIT images over duration (Figure 3.). A “region of study” (ROI) can be defined as a summary of activity in specific pixels in the image. Within each ROI, the waveform displays changes in the region’s conductivity over time as a result of either ventilation (ventilation-related signal, VRS) and cardiac activities (cardiac-related signal, CRS). Additionally, electrically conducting contrast agents such as hypertonic Saline can be used to obtain the EIT pattern (indicator-based signal IBS) and can be linked to perfusion in the lung. The CRS could come from both the lungs and the cardiac region and may also be associated with lung perfusion. Its precise source and composition are incompletely understood [ 1313. Frequency Spectrum Analysis is typically used to distinguish between ventilator- and cardiac-related Impedance Analyzers changes. Impedance changes that are not periodic could result from changes in settings for the ventilator.

Figure 3. EIT waveforms , as well as the functional EIT (fEIT) pictures originate from the Raw EIT images. EIT waveforms can be identified in a pixel-wise manner or based on a specific region to be studied (ROI). Conductivity changes occur naturally as a result of ventilation (VRS) or the activity of cardiac muscles (CRS) but they may be produced artificially e.g. through injection of bolus (IBS) to determine perfusion. fEIT images display some of the regional physiological parameters including ventilation (V) along with perfusion (Q) taken from the raw EIT images by using a mathematical operation over time.

Functional EIT (fEIT) images are generated by applying a mathematical function on a sequence of raw images as well as the corresponding pixel EIT signal waveforms. Since the mathematical procedure is used to determine an appropriate physiological parameter for each pixel, physiological regional parameters like regional respiration (V) and respiratory system compliance, as in addition to regions perfusion (Q) can be assessed to be displayed (Figure 3.). Information drawn from EIT waveforms and simultaneously registered airway pressure values can be used to calculate lung’s compliance as also the opening and closing of the lungs in each pixel using the changes in pressure and impedance (volume). Similar EIT measurements of the inflation and deflation steps of the lungs can be used to display of pressure-volume curves on a pixel level. Depending on the mathematical operation different types of fEIT scans could reflect different functional characteristics for the cardio-pulmonary system.

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