Electrical Impedance Tomography for Cardio-Pulmonary Monitoring

Abstract

Electrical Impedance Tomography (EIT) is an instrument used to monitor the bed that is non-invasively able to assess local ventilation and arguably lung perfusion distribution. The present article reviews and analyzes both methodological and clinical aspects of thoracic EIT. Initially, researchers focused on the validity of EIT to determine regional ventilation. Current studies focus mainly on clinical applications of EIT to quantify lung collapse, TIDAL recruitment, as well as lung overdistension. The goal is to monitor positive end expiratory pressure (PEEP) and the volume of tidal. In addition, EIT may help to detect pneumothorax. Recent studies assessed EIT as a tool to determine regional lung perfusion. Indicator-free EIT measurements may be sufficient to continuously monitor cardiac stroke volume. A contrast agent such as saline might be required in order to determine regional lung perfusion. This is why EIT-based surveillance of regional airflow and lung perfusion may reveal the perfusion match and local ventilation that could prove useful in the treatment of patients suffering from chronic respiratory distress syndrome (ARDS).

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

1. Introduction

The electrical impedance imaging (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 introduced to allow clinical applications of this method, and thoracic EIT has been used safely in both pediatric and adult patients [ 1, 2.

2. Basics of Impedance Spectroscopy

Impedance Spectroscopy can be defined as the range of the biological tissue’s voltage to externally applied alternating voltage (AC). It is normally measured using four electrodes, where two are utilized for AC injection and the other two for voltage measurement 3,[ 3, 4]. Thoracic EIT measures the regional distribution of intra-thoracic Impedance Spectroscopyand is seen to extend the principle of four electrodes to the imaging plane, which is divided with the belt of electrodes [ 1]. Dimensionally, electrical resistance (Z) is exactly the same as resistance. its equivalent International System of Units (SI) unit is Ohm (O). It can be described as a complex number where the real part is resistance and the imaginary part is called reactance, which measures the effects of the inductance of capacitance. Capacitance is a function of biomembranes’ characteristics of the tissue such as ion channels and fatty acids as well as gap junctions. However, resistance is mostly determined by the composition and quantity of extracellular fluid [ 1., 2]. At frequencies below 5 kilohertz (kHz) electricity flows through extracellular fluid and is primarily dependent upon the characteristics of resistivity of tissues. When frequencies are higher, up to 50 kHz. electrical currents are slightly redirected at the cell membranes resulting in an increase of capacitive tissue properties. In frequencies that exceed 100 kHz the electrical current is able to pass through cell membranes and reduce the capacitive portion 2]. Therefore, the effects that determine the tissue’s impedance depend on the utilized stimulation frequency. Impedance Spectroscopy is often described as conductivity and resistivity. They is a measure of conductance or resistance to unit length and area. The SI units of equivalent are Ohm-meter (O*m) for resistivity and Siemens per meters (S/m) for conductivity. The resistance of the thoracic tissues ranges from 150 O*cm when blood is present and up to 700 o*cm for deflated lung tissue, up to 2400 O*cm in an inflated lung tissue ( Table 1). In general, the tissue’s resistance or conductivity will vary based on quantity of fluid in the tissue and the concentration of ions. Regarding lung function, this is dependent on the quantity of air that is present in the alveoli. While most tissues exhibit isotropic characteristic, the heart and the muscle fibers in the skeletal system exhibit anisotropic behavior, meaning that the resistance is strongly dependent on the direction that the measurement is made.

Table 1. Electrical resistivity of thoracic tissues.

3. EIT Measurements and Image Reconstruction

To perform EIT measurements electrodes are set around the Thorax in a transverse, usually in the 4th to the 5th intercostal spaces (ICS) near that line called parasternal [55. The changes in impedance can also be measured in the lower lobes of the left and right lungs and also in the heart region ,2]. It is possible to position the electrodes below the 6th ICS might be challenging as the diaphragm as well as abdominal content periodically enter the measurement plane.

Electrodes are self-adhesive electrodes (e.g., electrocardiogram ECG) that are placed individually with equal spacing between the electrodes or integrated into electrode belts [ ,21 2. Additionally, self-adhesive strips are offered for a more user-friendly application [ ,21. Chest tubes, chest wounds Non-conductive bandages and conductive wire sutures may preclude or significantly affect EIT measurements. Commercially available EIT devices usually use 16 electrodes. However, EIT devices that use 8 or 32 electrodes is also available (please refer to Table 2 for information) There are also 32 electrodes (please refer to Table ,21.

Table 2. Electronic impedance (EIT) gadgets.

During an EIT test, low AC (e.g. the smallest value of 5 million mA with a frequency of 100 kHz) are applied to several pairs of electrodes and the output voltages are analyzed using the remaining electrodes 6. Bioelectrical impedance between the injecting and electrodes that are measuring is calculated based on the applied current and measured voltages. Most commonly electrode pairs that are adjacent to each other are used for AC application in a 16-elektrode set-up and 32-elektrode systems typically employ a skip pattern (see Table 2) that increases the distance between the electrodes for current injection. The resultant voltages are measured with those remaining electrodes. Presently, there’s a constant debate regarding different types of current stimulation and their specific advantages and disadvantages [7]. To obtain a full EIT data set of bioelectrical tests in the injecting as well as the electrode pairs measuring are continuously rotated throughout the entire thorax .

1. Current application and voltage measurements within the thorax, using an EIT system that has 16 electrodes. Within milliseconds, as well as the voltage and current electrodes as well as the active voltage electrodes are turned around the thorax.

The AC utilized during EIT tests are safe for use on the body 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.

It is the EIT data set which is captured during a single cycle that is recorded during one cycle of AC applications is technically termed frame. It contains voltage measurements used to create this image. EIT image. The term “frame rate” reflects the amount of EIT frames that are recorded every second. Frame rates of at minimum 10 images/s are essential for monitoring ventilation and 25 images/s to check heart function or perfusion. Commercially accessible EIT devices have frames of between 40 and 50 images/s as is shown in

To generate EIT images from recorded frames, the technique known as image reconstruction is applied. Reconstruction algorithms attempt to solve the inverse problem of EIT that is the recovery of the conductivity distribution in the thorax by analyzing the voltage measurements acquired at the electrodes on the thorax surface. In the beginning, EIT reconstruction assumed that electrodes were placed on an ellipsoid or circular plane. However, newer techniques utilize information about the anatomical contour of the thorax. The current algorithms include an algorithm called the Sheffield back-projection algorithm along with the finite elements method (FEM) which is a linearized Newton-Raphson algorithm [ ], and the Graz consensus reconstruction algorithm for EIT (GREIT) [10is frequently employed.

A lot of the time, EIT photographs are similar to a two-dimensional computed (CT) image. These images are conventionally rendered so that the viewer is looking from cranial towards caudal when looking at the image. In contrast to CT images, unlike a CT image, an EIT image does not display a “slice” but an “EIT sensitivity region” [11]. The EIT sensitization region is a lens-shaped intrathoracic region from which impedance changes contribute to EIT picture generationIt is a lens-shaped intra-thoracic volume that contributes to the generation. The size and shape of EIT area of sensitivity are dependent upon the dimensions, the bioelectric propertiesas well as the contour of the thorax as well and the applied current injection and voltage measurement pattern [12].

Time-difference imaging can be described as a technique that is employed in EIT reconstruction, which displays changes in conductivity instead of absolute conductivity levels. It is a technique that uses time to show the change in conductivity. EIT image compares the changes in impedance to a baseline frame. This provides the chance to observe time-dependent physiological processes like lung ventilation and perfusion [22. Color coding of EIT images isn’t unified but commonly displays the change in impedance to the reference level (2). EIT images are generally created using a spectrum of colors with red representing the greatest absolute impedance (e.g. when inspiration occurs) while green is a moderate relative impedance, and blue the lowest relative impedance (e.g. for expiration). In clinical settings An interesting approach is to use color scales that range from black (no changes in impedance) or blue (intermediate impedance changes) and white (strong impedance changes) to code ventilation , or from black, to red, and white to mirror perfusion.

2. Different color codes that are available for EIT images in comparison to the CT scan. The rainbow-color scheme uses red for the highest percentage of the relative imperceptibility (e.g. during inspiration) while green is used for intermediate relative impedance, and blue to indicate the least relative imperceptibility (e.g. at expiration). A newer color scheme uses instead of black to avoid any impedance changes) while blue is used for an intermediate change in impedance, as well as white for the greatest impedance variation.

4. Functional Imaging and EIT Waveform Analysis

Analyzing Impedance Analyzers data is done using EIT waveforms that are formed in the individual pixels of a series of raw EIT images over length of (Figure 3.). An area of concern (ROI) is a term used to describe activity in the individual pixels in the image. Within each ROI, the waveform displays fluctuations in regional conductivity in time due to the process of ventilation (ventilation-related signal, also known as VRS) (or cardiac activity (cardiac-related signal, CRS). Additionally, electrically conductive contrast agents like hypertonic saline could be used in the production of the EIT waveform (indicator-based signal, IBS) and could be connected to lung perfusion. The CRS can originate from both the lung as well as the cardiac region, and is possibly associated with lung perfusion. Its exact origin and composition is not fully understood 1313. Frequency spectrum analysis can be utilized to differentiate between ventilationand cardiac-related impedance fluctuations. Impedance changes outside of the periodic cycle could result from changes in settings for the ventilator.

Figure 3. EIT waveforms and functional EIT (fEIT) image originate from the raw EIT images. EIT waves can be defined as pixel-wise, or by using a region or region of interest (ROI). Conductivity changes occur naturally as a result of the process of ventilation (VRS) (or cardiac activity (CRS) but may be artificially induced, e.g. or through the injection of bolus (IBS) for perfusion measurements. FEIT images depict local physiological parameters such as perfusion (Q) and ventilation (V) and blood flow (Q) and perfusion (Q) that are extracted from the raw EIT images by using a mathematical procedure over time.

Functional EIT (fEIT) images are generated by applying a mathematical function on a sequence of raw images and the corresponding EIT spectrums. Since the mathematical operation is applied to determine an appropriate physiological parameter for each pixel, physiological regional traits like regional airflow (V), respiratory system compliance as in addition to respiratory system compliance as well as regional perfusion (Q) can be measured and display (Figure 3.). The data generated from EIT waveforms along with simultaneously registered airway pressure values can be used to determine the lung’s compliance as also the rate of lung opening and closing for each pixel through changes of impedance and pressure (volume). Similar EIT measurements during the inflation and deflation steps of lung volumes allow for the display of volume-pressure curves at one pixel. The mathematical operations used to calculate different types of fEIT images could be used to analyze different functions that are associated with the cardiovascular system.