Electrical Impedance Tomography for Cardio-Pulmonary Monitoring
Electrical Impedance Tomography (EIT) is a bedside monitor that can be used to visualize the local airflow and perhaps lung perfusion. The paper summarizes and analyzes the methodological and clinical aspects of the thoracic EIT. Initially, researchers addressed the validity of EIT to assess regional ventilation. Present research is focused on its clinical applications to determine the extent of lung collapse, Tidal Recruitment, and lung overdistension to measure positive end-expiratory pressure (PEEP) and the volume of tidal. In addition, EIT may help to detect pneumothorax. Recent studies examined EIT as a tool to measure regional lung perfusion. Indicator-free EIT measurements may be sufficient to continuously measure the heart stroke volume. A contrast agent like saline may be necessary for assessing regional lung perfusion. As a result, EIT-based assessment of regional respiratory and lung perfusion may reveal the local perfusion and ventilation, which can be helpful in treating patients suffering from acute respiratory distress syndrome (ARDS).
Keywords: electrical impedance tmography Bioimpedance; image reconstruction Thorax; regional vent, regional perfusion; monitoring
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 to allow clinical applications of this method, and the thoracic EIT has been successfully used in both adult and pediatric patients [ 1., 1 2.
2. Basics of Impedance Spectroscopy
Impedance Spectroscopy may be described as the biomaterial’s voltage response to externally applied alternating electron current (AC). It is commonly obtained using four electrodes, where two are used for AC injection, and the remaining two for voltage measurement 3.,3. Thoracic EIT measures the regional range of intra-thoracic bioimpedance. This can be seen by extending the principle of four electrodes into the image-plane spanned by an electrode belt [ 11. Dimensionally, electrical inductance (Z) is equivalent to resistance, as is its equivalent International System of Units (SI) unit is Ohm (O). It is often expressed as a complex number in which the real portion is resistance, and the imaginary component is called reactance. It quantifies effects resulting from resistance or capacitance. The amount of capacitance is determined by biomembranes’ particulars of the tissue such as ion channels and fatty acids as well as gap junctions. However, resistance is determined by the nature and amount of extracellular fluid [ 1, 22. When frequencies are below 5 kilohertz (kHz) an electrical current circulates through extracellular fluids and is primarily dependent upon its resistive properties of tissues. At higher frequencies of up to 50 kHz the electrical currents are slightly diverted at cells’ membranes which causes an increase in tissue capacitive properties. At frequencies above 100 kHz electric currents are able to travel through cell membranes and decrease the capacitive component [ 2[ 1, 2]. Thus, the factors that determine the tissue’s impedance depend on the utilized stimulation frequency. Impedance Spectroscopy is often described as conductivity or resistance. Both equalizes conductance and resistance to unit length and area. The SI units for the same comprise Ohm-meter (O*m) for resistivity, and Siemens per meter (S/m) on conductivity. The thoracic tissue’s resistance ranges from 150 O*cm in blood, to 700 O*cm for deflated lung tissue, as high as 2400 O*cm when dealing with tissues that have been inflated ( Table 1). In general, the tissue’s resistance or conductivity is dependent on levels of ion and fluid content. When it comes to respiratory lungs it is dependent on the quantity of air in the alveoli. Although most tissues exhibit isotropic behavior, heart and skeletal muscle behave anisotropic, meaning that resistivity strongly depends on the direction from which the measurement is made.
Table 1. The electrical resistivity of the thoracic tissues.
3. EIT Measurements and Image Reconstruction
In order to conduct EIT measurements electrodes are placed around the thorax in a transverse plan which typically occurs in the 4th to the 5th intercostal areas (ICS) at just below the parasternal line5. Subsequently, the changes of impedance can be assessed in the lower lobes of both the right and left lungs, and also in the heart region [ ,21 2. To position the electrodes above the 6th ICS is not easy as the diaphragm and abdominal contents frequently enter the measurement plane.
Electrodes can be self-adhesive or single electrodes (e.g. electrocardiogram ECG) that are positioned individually in a similar spacing between electrodes or are incorporated 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 and conductive wire sutures can hinder or greatly affect EIT measurements. Commercially available EIT devices typically utilize 16 electrodes, but EIT systems with 8 to 32 electrodes may be available (please consult Table 2 for more details) For more information, refer to Table 2. ,2[ 1,2].
Table 2. Available electrical impedance tomography (EIT) gadgets.
During an EIT test, low AC (e.g. 5, mA at a frequency of 100 kHz) are applied to different electrode pairs. The generated voltages are measured with the remaining other electrodes [ 6. The bioelectrical resistance between the injecting and the electrode pairs measuring the electrodes is calculated using the applied current and the measured voltages. The majority of the time adjacent electrode pairs are utilized for AC application within a 16-elektrode configuration however 32-elektrode systems usually utilize a skip-pattern (see Table 2.) in order to extend the space between the electrodes that inject the current. The resulting voltages are then measured with other electrodes. At present, there is an ongoing discussion about different kinds of current stimulation, as well as their particular advantages and disadvantages . In order to obtain an complete EIT data set that includes bioelectrical tests, the injecting and the electrode pairs that measure are continually rotationally positioned around the entire chest .
1. The measurements of voltage and current are made around the thorax by using an EIT device with 16 electrodes. Within milliseconds as well as the voltage and current electrodes as well as these active electrodes can be turned within the thorax.
The AC utilized during EIT measurements is 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.
This EIT data set that is recorded over a single cycle during AC programs is known as a frame . It is comprised of the voltage measurements needed to produce an unprocessed EIT image. Frame rate refers to the number of EIT frames captured per second. Frame rates that exceed 10 images/s are necessary to monitor ventilation and 25 images/s to check perfusion or cardiac function. Commercially accessible EIT equipment uses frames that have a frame rate of between 40 and 50 images/s (see Figure 2), as described in
To generate EIT images from recorded frames, the technique known as reconstructing of images is carried out. Reconstruction algorithms seek to solve the issue that causes EIT that is the recovery of the conductivity distribution in the thorax using the voltage measurements made at the electrodes of the thorax’s surface. At first, EIT reconstruction assumed that electrodes were placed on a circular or ellipsoid plane, but more recent algorithms incorporate information about how the anatomical shape of thorax. At present, the Sheffield back-projection algorithm as well as the finite-element method (FEM) using a linearized Newton–Raphson algorithm ], and the Graz consensus reconstruction algorithm for EIT (GREIT) [10is frequently employed.
The majority of EIT images can be compared to a two-dimensional computed-tomography (CT) image: these images are conventionally rendered so that the operator is looking across the entire cranial region when taking a look at the picture. In contrast to a CT image one can observe that an EIT image doesn’t show an actual “slice” but an “EIT sensitivity region” . The EIT sensitive region is a lens-shaped intrathoracic region where impedance fluctuations contribute to EIT imaging process [1111. The size and shape of EIT sensitivity region are dependent on the dimensions, the bioelectric characteristics, and the contour of the thorax as on the utilized current injection and voltage measurement pattern [12It is important to note that the shape of the thoracic thorax can.
Time-difference image is a technique that is used for EIT reconstruction in order to display changes in conductivity rather than total conductivity. It is a technique that uses time to show the change in conductivity. EIT image compares the changes in impedance to a base frame. This affords the opportunity to examine the effects of time on physiological events like lung ventilation and perfusion . Color-coding for EIT images is not unicoded but commonly displays the change in impedance in relation to a reference level (2). EIT images are usually colored using a rainbow color scheme with red representing the greatest value of relative imperf (e.g. in the time of inspiration) with green being a medium relative impedance and blue the smallest relative impedance (e.g. during expiration). For clinical purposes one option to consider is to employ color scales that vary from black (no impedance changes) up to blue (intermediate impedance change) as well as white (strong impedance changes) to code ventilation , or from black, to white, then up to mirror-perfusion.
2. Different color codings for EIT images when compared with the CT scan. The rainbow color scheme uses red to indicate the highest absolute impedance (e.g. in the time of inspiration) as well as green for a intermediate relative impedance, and blue to indicate the least relative imperceptibility (e.g. at expiration). A more recent color scale uses instead of black (which has no 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 performed using EIT waveforms which are created inside individual image pixels within the form of a sequence of raw EIT images that are scanned over time (Figure 3). In a region of focus (ROI) can be defined to represent activity within individual pixels in the image. Within each ROI, the waveform displays changes in the region’s conductivity over time resulting from breathing (ventilation-related signal, VRS) or heart activity (cardiac-related signal CRS). Additionally, electrically conductive contrast agents such as hypertonic saline can be used to generate an EIT form (indicator-based signal, IBS) and is linked to the perfusion of the lung. The CRS could be a result of both the heart and lung region and may also be associated with lung perfusion. Its exact origin and composition is not fully understood 1313. Frequency spectrum analysis is frequently used to discriminate between ventilationas well as cardiac-related changes in impedance. Impedance changes that aren’t periodic may be caused by changes in ventilator settings.
Figure 3. EIT form and function EIT (fEIT) photos are extracted from raw EIT images. EIT waveforms may be defined either pixel-wise or in a region or region of interest (ROI). Conductivity changes result naturally from breathing (VRS) as well as cardiac activity (CRS) however they could be caused artificially, e.g. with the injection of bolus (IBS) for perfusion measurements. FEIT images show the variables of regional physiological activity like perfusion (Q) and ventilation (V) and 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 a sequence of raw images and the corresponding EIT form . 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 assessed and displayed (Figure 3.). Information from EIT waveforms as well as simultaneously recorded airway pressure values can be utilized to calculate the lung’s compliance as well as lung closing and opening for each pixel using changes of impedance and pressure (volume). Similar EIT measurements of increments of inflation and deflation in the lungs enable the display of pressure-volume curves at scales of pixel. Based on the mathematical procedure, different kinds of fEIT photographs could reflect different functional characteristics from the cardio-pulmonary apparatus.