Electrical Impedance Tomography for Cardio-Pulmonary Monitoring

Abstract

Electrical Impedance Tomography (EIT) is an instrument for monitoring bedside that is non-invasively able to assess local ventilation and , possibly, lung perfusion distribution. The paper summarizes and discusses both methodological and clinical aspects of thoracic EIT. Initially, researchers addressed the validity of EIT to determine regional ventilation. Recent studies concentrate on its clinical applications to measure lung collapse, the tidal response, 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 evaluated EIT as a method for measuring regional lung perfusion. Indicator-free EIT measurements may be sufficient to continuously measure the heart stroke volume. The use of a contrast agent like saline could be necessary to evaluate regional perfusion of the lungs. Therefore, EIT-based surveillance of regional airflow and lung perfusion can reveal the perfusion match and local ventilation that can prove beneficial in treating patients with chronic respiratory distress syndrome (ARDS).

Keywords: electrical impedance imaging bioimpedance, image reconstruction Thorax; regional vent Monitoring regional perfusion

1. Introduction

EI tomography (EIT) can be described as a non-radiation functional imaging modality that allows the non-invasive monitoring of bedside regional lung ventilation as well as arguably perfusion. Commercially available EIT devices were first introduced for clinical application of this technique and the thoracic EIT is safe for both pediatric and adult patients 1, 2.

2. Basics of Impedance Spectroscopy

Impedance Spectroscopy can be defined as the resistance of biological tissues to an externally applied electronic current (AC). It is typically measured using four electrodes. Two are utilized to inject AC injection and the other two electrodes are used to measure voltage 3,[ 3, 4]. Thoracic EIT measures the regional variation of the intra-thoracic bioimpedance. It is seen like an extension of four electrode principle to the imaging plane, which is divided with the belt of electrodes 11. Dimensionally, electrical inductance (Z) is the same as resistance and the corresponding International System of Units (SI) unit is Ohm (O). It is easily expressed as a complex number where the actual part is resistance while the imaginary part is called reaction, which quantifies effects resulting from capacitance or inductance. Capacitance depends on the biomembranes’ specifics of the tissue , including ion channels, fatty acids, and gap junctions. However, resistance is determined by the composition and the amount of extracellular fluid 1, 22. For frequencies lower than 5 kilohertz (kHz) that is, electrical energy travels through extracellular fluids and is heavily dependent on its resistive properties of tissues. For higher frequencies that exceed 50 kHz the electrical currents are slightly deflected at cell membranes . This leads to an increase in tissue capacitive properties. When frequencies exceed 100kHz the electrical current is able to pass through cell membranes and reduce the capacitive component [ 22. So, the results that determine the amount of tissue impedance depend on the used stimulation frequency. Impedance Spectroscopy is typically described as conductivity and resistivity. They compares conductance or resistance unit area and length. The SI units of equivalent comprise Ohm-meter (O*m) for resistivity and Siemens per meter (S/m) for conductivity. The resistance of lung tissue can range from 150 o*cm for blood up to 700 O*cm for tissues that have been deflated and inflated, to between 2400 and 2400 O*cm of inflated lung tissue ( Table 1). In general, the tissue’s resistance or conductivity is dependent on quantity of fluid in the tissue and the concentration of ions. In the case of respiratory lungs it also depends on the amount of air inside the alveoli. While most tissues exhibit isotropic response, heart and muscle in particular exhibit anisotropic properties, meaning that resistance is highly dependent on the direction from which they are measured.

Table 1. The electrical resistance of the thoracic tissue.

3. EIT Measurements and Image Reconstruction

To perform EIT measurements electrodes are placed on the chest in a transverse plane that is usually located in the 4th to the 5th intercostal spaces (ICS) at just below the parasternal line]. In turn, the variations in impedance can be assessed in areas of the lower part of the right and left lungs, as well as in the heart area ,2[ 1,2]. To place the electrodes below the 6th ICS could be difficult since the abdominal contents and diaphragm are frequently inserted into the measurement plane.

Electrodes are either single self-adhesive electrodes (e.g., electrocardiogram, ECG) that are positioned individually with equal spacing in-between the electrodes or are incorporated into electrode belts ,2]. Additionally, self-adhesive stripe are available for a more user-friendly application [ ,21. Chest wounds, chest tubes (non-conductive) bandages or wire sutures can block or substantially affect EIT measurements. Commercially available EIT devices usually use 16 electrodes. However, EIT systems that have 8 to 32 electrodes may be also available (please check Table 2 for specifics) There are also 32 electrodes (please refer to Table ,21 2.

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

During an EIT measurement , small AC (e.g. 5, million mA with a frequency of 100 kHz) are applied to several electrode pairs and the resultant voltages are recorded using the other electrodes ]. Bioelectrical impedance that is measured between the injecting and electrodes that are measuring is calculated from the known applied current and the measured voltages. The majority of the time, adjacent electrode pairs are used to allow AC application within a 16-elektrode configuration as opposed to 32-elektrode systems, which typically utilize a skip-pattern (see Table 2.) for increasing the spacing between electrodes used for injecting current. The voltages that result are then measured with all the electrodes. At present, there is an ongoing debate about the various current stimulation patterns and their distinct advantages and disadvantages [7]. To get a complete EIT data set of bioelectrical measurements both the injecting and electrode pairs that measure are continually moved around the entire thorax .

1. Current application and voltage measurements in the thorax using an EIT system that has 16 electrodes. In just a few milliseconds simultaneously, the current electrode as well as their active voltage electrodes are continuously rotating over the entire thorax.

The AC used during EIT tests is safe to use for use on the body and remains 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 captured during a single cycle within AC software is called frames and includes the voltage measurements used to create the original EIT image. The term frame rate refers to the amount of EIT frames that are recorded every second. Frame rates that are at least 10 images/s are required for monitoring ventilation and 25 images/s to monitor cardiac function or perfusion. Commercially available EIT devices run frames of between 40 and 50 images/s (see Figure 2), as shown in

To create EIT images from recorded frames, the process of image reconstruction is applied. Reconstruction algorithms try to solve the other aspect of EIT that is the reconstruction of the conductivity distribution in the thorax using the voltage measurements taken at the electrodes on the thorax’s surface. At first, EIT reconstruction assumed that electrodes were placed on an ellipsoid, circular or circular plane, however, more modern algorithms take into account anatomy of the thorax. Currently, an algorithm called the Sheffield back-projection algorithm [ as well as the finite-element method (FEM) which is a linearized Newton-Raphson algorithm ] and the Graz consensus reconstruction algorithm for EIT (GREIT) [10typically used.

The majority of EIT images have a similarity to a 2-dimensional computed (CT) image. These images are typically rendered in a way that the operator looks from cranial to caudal when looking at the image. In contrast to CT images, unlike a CT image the EIT image doesn’t display the appearance of a “slice” but an “EIT sensitivity region” [1111. The EIT sensitization region is a tubular intra-thoracic structure where impedance fluctuations contribute to EIT production of the image [11(11, 11). Shape and thickness of the EIT sensitization region is determined by the dimensions, bioelectric propertiesas well as the shape of the thorax as as on the utilized current injection and voltage measurement pattern [12].

Time-difference Imaging is a method that is used in EIT reconstruction in order to display changes in conductivity rather than total conductivity. Time-difference EIT image displays the change in impedance with a baseline frame. It is an opportunity to observe time-dependent physiological processes such as lung ventilation and perfusion [22. The color-coding used in EIT images isn’t unified but generally displays the change in the impedance of the patient to a standard (2). EIT images are generally encoded with a rainbow-color scheme with red representing the most significant in relative intensity (e.g. during inspiration) while green is a moderate relative impedance and blue the least relative impedance (e.g. for expiration). For clinical purposes there is a good option to utilize color scales that range from black (no changes in impedance) through blue (intermediate impedance changes), and white (strong impedance changes) for coded ventilation. from black, to white and red to mirror perfusion.

2. Different color codes are available for EIT images in comparison to CT scan. The rainbow-color scheme makes use of red to indicate the highest relative impedance (e.g. during inspiration), green for a medium relative impedance, and blue for the lowest relative impedance (e.g. when expiration is in progress). The newer color scales employ instead of black (which has no impedance change) while blue is used for an intermediate impedance variation, and white for the strongest impedance changes.

4. Functional Imaging and EIT Waveform Analysis

Analyzing Impedance Analyzers data is performed using EIT waveforms that form in each image’s pixels the raw EIT images over length of (Figure 3.). The term “region of interest” (ROI) can be defined as a summary of activity in specific pixels in the image. Within all ROIs, the waveform displays changes in the region’s conductivity over time , resulting from breathing (ventilation-related signal, also known as VRS) or activity in the heart (cardiac-related signal CRS). Additionally, electrically conductive contrast agents like hypertonic saline could be used to generate the EIT Waveform (indicator-based signal, IBS) and could be connected to perfusion in the lung. The CRS could come from both the lung and the cardiac region, and could be partially due to lung perfusion. The exact source and composition are incompletely understood [ 13]. Frequency spectrum analysis can be used to distinguish between ventilation- and cardiac-related Impedance Analyzers changes. Impedance changes that do not occur regularly could be caused by modifications in the settings of the ventilator.

Figure 3. EIT waveforms , as well as the functional EIT (fEIT) photographs are derived from raw EIT images. EIT waveforms are defined as pixel-wise, or by using a region or region of interest (ROI). Conductivity fluctuations are the result of ventilation (VRS) or the activity of cardiac muscles (CRS) but can also be created artificially, e.g. using injection of bolus (IBS) for perfusion measurement. FEIT images present local physiological parameters, such as ventilation (V) and perfusion (Q), extracted from the raw EIT images by using an equation over time.

Functional EIT (fEIT) images are created by applying a mathematical function on the sequence of raw images and the corresponding EIT signal waveforms. Because the mathematical process is applied to calculate an appropriate physiological parameter for each pixel, regional physiological characteristics like regional ventilation (V) and respiratory system compliance as in addition to the regional flow (Q) can be assessed and visualized (Figure 3.). The information derived collected from EIT waveforms along with simultaneously registered airway pressure values can be utilized to determine the lung compliance as well as lung opening and closing for each pixel by calculating changes of impedance and pressure (volume). Similar EIT measurements of the inflation and deflation steps of lung volume allow for the display of pressure-volume curves at the pixel level. The mathematical operations used to calculate different kinds of fEIT photographs may address different functional characteristics within the cardio-pulmonary systems.