Electrical Impedance Tomography for Cardio-Pulmonary Monitoring
Abstract
Electrical Impedance Tomography (EIT) is an instrument for monitoring bedside that visually examines the local environment and , possibly, lung perfusion distribution. In this article, we review and discusses both methodological and clinical aspects of thoracic EIT. Initially, investigators addressed the possibility of using EIT to measure regional ventilation. Recent studies concentrate on clinical applications of EIT to measure lung collapse, the tidal response, and lung overdistension, in order to determine 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 measure regional lung perfusion. The absence of indicators in EIT measurements might be sufficient to continuously measure cardiac stroke volume. Utilizing a contrast agent like saline could be necessary to check regional perfusion of the lungs. Thus, EIT-based monitoring of respiratory ventilation as well as lung perfusion can be used to assess the perfusion match and local ventilation that could prove useful in the treatment of patients with acute respiratory distress syndrome (ARDS).
Keywords: electrical impedance imaging; bioimpedance; image reconstruction; thorax; regional ventilation; regional perfusion; monitoring
1. Introduction
Electrical impedance tomography (EIT) is one of the non-radiation functional imaging modality that provides an uninvasive monitoring of regional lung ventilation , and possibly perfusion. Commercially-available EIT devices were developed for the clinical use of this technique, and the thoracic EIT has been used safely in both adult and pediatric patients 1., 2.
2. Basics of Impedance Spectroscopy
Impedance Spectroscopy can be defined as the biomaterial’s voltage response to an externally applied voltage (AC). It is commonly obtained using four electrodes. Two are employed for AC injection, and the remaining two are used to measure voltage 3,3. 4. Thoracic EIT measures the regional Impedance Spectroscopy of the thoracic region and can be considered to extend the principle of four electrodes to the image plane that is spanned by the electrode belt [ 11. Dimensionally, electrical impedance (Z) is the same as resistance and the corresponding International System of Units (SI) unit is Ohm (O). It is easily expressed in a complex form, where the real part is resistance and the imaginary is called reactance, which is the measurement of effects caused by the inductance of capacitance. Capacitance varies based on biomembranes’ features of the tissue , which includes ion channels and fatty acids as well as gap junctions. While resistance is mostly determined by the composition of the tissue and the quantity of extracellular fluid [ 1., 2]. In frequencies that are less than 5 kilohertz (kHz) an electrical current runs through extracellular fluid and is predominantly dependent on the characteristics of resistivity of tissues. In higher frequencies above 50 kHz, electrical currents are slightly slowed down at cells’ membranes which causes an increase in capacitive tissues properties. When frequencies exceed 100 kHz, electrical current can pass through cell membranes and decrease the capacitive portion 2]. Therefore, the effects that determine tissue impedance strongly depend on the stimulation frequency. Impedance Spectroscopy typically refers to conductivity or resistivity. These normalize resistance or conductance to the area of the unit and the length. The SI units of equivalent is Ohm-meter (O*m) for resistivity and Siemens per meter (S/m) (S/m) for conductivity. The tissue’s resistance varies between 150 O*cm of blood up to 700 O*cm for lung tissue that is deflated, all the way as high as 2400 O*cm when dealing with tissues that have been inflated ( Table 1). In general, tissue resistance or conductivity is dependent on amount of fluid and the ion concentration. In the case of breathing, it also depends on the volume of air present in the alveoli. Though most tissues exhibit an isotropic response, heart and muscle in particular exhibit anisotropic properties, meaning that the resistance is strongly dependent on the direction in which it’s measured.
Table 1. The electrical resistance of the thoracic tissue.
3. EIT Measurements and Image Reconstruction
To conduct EIT measurements electrodes are placed on the Thorax in a horizontal plane typically in the 4th-5th intercostal areas (ICS) in the line between parasternal and lateral [5]. The changes in impedance are measured in those lobes that are lower in the right and left lungs, as well as in the heart area ,22. The placement of the electrodes below the 6th ICS might be difficult as the abdominal contents and diaphragm regularly enter the measurement plan.
Electrodes are either single self-adhesive electrodes (e.g., electrocardiogram ECG,) that are placed with equal spacing between electrodes or are incorporated into electrode belts ,2[ 1,2]. Also, self-adhesive stripes are offered for a more user-friendly application ,21. Chest wounds, chest tubes (non-conductive) bandages or wire sutures could block or negatively impact EIT measurements. Commercially available EIT devices typically utilize 16 electrodes, but EIT systems with 8 or 32 electrodes is also available (please see Table 2 for details) It is recommended to consult Table 2 for more details. ,2[ 1,2].
Table 2. Commercially available electrical impedance tomography (EIT) technology.
In an EIT test, low AC (e.g. approximately 5 microamps at 100 kHz) are applied to several electrode pairs. The results are then measured using the other electrodes ]. The bioelectrical resistance between the injecting and electrode pairs used to measure the voltage is calculated from the known applied current as well as the measured voltages. Most often nearby electrode pairs are utilized to allow AC application in a 16-elektrode setup, while 32-elektrode systems often use a skip pattern (see the table 2) to increase the distance between current injecting electrodes. The resulting voltages are then measured by using one of the other electrodes. Currently, there is an ongoing debate about the various kinds of current stimulation, as well as their particular advantages and disadvantages [7]. To acquire a complete EIT data set that includes bioelectrical measurements, the injecting and the electrodes used to measure the electrodes are continuously rotated around the entire thorax .
1. Current measurements and voltage measurements around the thorax with an EIT system with 16 electrodes. Within milliseconds, as well as the voltage and current electrodes and those with active voltage electrodes are turned about the chest.
The AC utilized during EIT measurements are safe for use on body surfaces 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 that is recorded over a single cycle during AC programs is termed frames and includes the voltage measurements needed to produce this initial EIT image. The term “frame rate” reflects the amount of EIT frames recorded each second. Frame rates that are at least 10 images/s are essential to monitor ventilation and 25 images/s to track the perfusion or cardiac function. Commercially accessible EIT devices employ frame rates ranging from 40 to 50 images/s (see Figure 2), as depicted in
To produce EIT images from the recorded frames, the so-called reconstructing of images is carried out. Reconstruction algorithms aim to solve the reverse problem of EIT that is reconstruction of the conductivity distribution in the thorax using the voltage measurements that have been collected at electrodes on the thorax surface. In the beginning, EIT reconstruction assumed that electrodes were placed on an ellipsoid or circular plane. However, newer techniques employ information on the anatomical contour of the thorax. At present, an algorithm called the Sheffield back-projection algorithm [ and the finite element method (FEM) that is a linearized Newton-Raphson algorithm [ ] as well as the Graz consensus reconstruction algorithm for EIT (GREIT) [10is frequently employed.
A lot of the time, EIT pictures are similar with a two-dimensional computed (CT) image: these images are conventionally rendered so that the user is able to look from cranial towards caudal when analysing the image. In contrast to a CT image however, an EIT image doesn’t display a “slice” but an “EIT sensitivity region” [11]. The EIT sensitivity region is a lens-shaped intrathoracic region from which impedance changes contribute to EIT imaging process [11]. The dimensions and shape of the EIT sensitive region are determined by the dimensions, bioelectrical properties, and also the form of the thorax and the applied voltage measurement and current injection pattern [12(13, 14).
Time-difference-based imaging is a process that is used in EIT reconstruction in order to display changes in conductivity, not actual conductivity level. The time-difference EIT image compares the change in impedance to a base frame. This affords the opportunity to monitor the changes in physiological activity over time like lung ventilation or perfusion [2]. The color-coding used in EIT images is not uniform but generally displays the change in intensity to a baseline level (2). EIT images are generally created using a spectrum of colors with red indicating the greatest in relative intensity (e.g., during inspiration), green a medium relative impedance, and blue being the lowest relative impedance (e.g. during expiration). For clinical purposes it is possible to utilize color scales that range from black (no changes in impedance) and blue (intermediate impedance change) as well as white (strong impedance change) to code ventilation . from black to red, and white for mirror perfusion.
2. Different color codes are available for EIT images in comparison to the CT scan. The rainbow-color scheme makes use of red for the greatest relative impedance (e.g. in the time of inspiration) Green for a moderate relative impedance, blue for the lowest relative impedance (e.g. when expiration is in progress). The newer color scales employ instead black for no impedance changes), blue for an intermediate impedance change, and white for the highest impedance change.
4. Functional Imaging and EIT Waveform Analysis
Analysis of Impedance Analyzers data is done using EIT waveforms , which are generated within individual image pixels of a series of raw EIT images that are scanned over time (Figure 3). In a region of focus (ROI) is a term used to describe activity in the individual pixels in the image. In all ROIs, the image shows changes in the region’s conductivity over time , resulting from the process of ventilation (ventilation-related signal, VRS) or cardiac activity (cardiac-related signal CRS). Additionally, electrically conducting contrast agents like hypertonic saline could be used to obtain an EIT Waveform (indicator-based signal, IBS) which may be related to the perfusion of the lung. The CRS may originate from both the lung and the cardiac region, and is possibly due to lung perfusion. The exact source and composition isn’t fully understood 13]. Frequency Spectrum Analysis is typically used to discriminate between ventilationand cardiac-related impedance fluctuations. Impedance fluctuations that are not frequent can result from changes in the settings of the ventilator.
Figure 3. EIT Waveforms as well as functional EIT (fEIT) pictures are derived from EIT raw EIT images. EIT waveforms can be defined by pixel or on a particular region that is of particular interest (ROI). Changes in conductivity are naturally triggered by ventilatory (VRS) or heart activity (CRS) but they can also be created artificially, e.g. with the injection of bolus (IBS) to measure perfusion. FEIT images show the specific physiological parameters of the region like ventilation (V) or perfusion (Q) which are extracted from raw EIT images by using a mathematical operation over time.
Functional EIT (fEIT) images are produced by applying a mathematical operation on the raw images along with the associated pixel EIT form [14]. Because the mathematical process is used to calculate the physiologically relevant parameters for each pixel. Regional physiological characteristics like regional ventilation (V), respiratory system compliance, as well as the regional flow (Q) can be measured and display (Figure 3). The information derived generated from EIT waveforms and simultaneously registered airway pressure measurements can be utilized to calculate lung’s compliance, as well as lung closing and opening for each pixel by calculating changes in pressure and impedance (volume). The comparable EIT measurements taken during inflating and deflating the lungs allow the displaying of curves representing volume and pressure at scales of pixel. Depending on the mathematical method used, different kinds of fEIT photographs could reflect different functional characteristics within the cardio-pulmonary systems.
