Electrical Impedance Tomography for Cardio-Pulmonary Monitoring
Abstract
Electrical Impedance Tomography (EIT) is a bedside monitor that does not require any surgery to see the local airflow and possibly lung perfusion distribution. This paper reviews and analyzes both methodological and clinical aspects of the thoracic EIT. Initially, investigators addressed the validation of EIT to determine regional ventilation. Current studies focus mainly on its clinical applications to measure lung collapse TIDAL recruitment, as well as lung overdistension in order to regulate positive end-expiratory pressure (PEEP) and tidal volume. In addition, EIT may help to detect pneumothorax. Recent research has evaluated EIT as a method to gauge regional lung perfusion. The absence of indicators in EIT measurements might be sufficient for continuous measurement of cardiac stroke volume. The use of a contrast agent such as saline may be required to measure the regional lung perfusion. This is why EIT-based monitoring of respiratory ventilation as well as lung perfusion can be used to assess local ventilation and perfusion matching which could be beneficial in the treatment of patients with acute respiratory distress syndrome (ARDS).
Keywords: Electrical impedance tomography bioimpedance; image reconstruction Thorax; regional ventilation; regional perfusion; monitoring
1. Introduction
EI tomography (EIT) is an non-radiation functional imaging method that permits non-invasive bedside monitoring of regional lung ventilation as well as arguably perfusion. Commercially available EIT devices were introduced to allow clinical application of this technique, and the thoracic EIT can be used with safety for both pediatric and adult patients 1, 1 2.
2. Basics of Impedance Spectroscopy
Impedance Spectroscopy is the voltage response of biological tissue to an externally applied electrical current (AC). It is usually achieved using four electrodes, where two are employed to inject AC injection, and the remaining two for voltage measurement 3,,]. Thoracic EIT measures the regional Impedance Spectroscopy distribution in the thoracic region. It is seen like an extension of four electrode principle onto the image plane which is defined by an electrode belt 11. In terms of dimension, electrical impedance (Z) is equivalent to resistance, as is the corresponding International System of Units (SI) unit is Ohm (O). It is easily expressed in a complex form, where the real component is resistance and the imaginary part is known as 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. In contrast, resistance is determined by the composition of the tissue and the quantity of extracellular fluid 1, 2[ 1, 2]. Below 5 kilohertz (kHz) an electrical current is carried by extracellular fluid and is primarily dependent upon the characteristics of resistivity of tissues. Higher frequencies, as high as 50 kHz. electrical currents are slightly deflected at cells’ membranes which causes an increase in capacitive tissue properties. At frequencies above 100 kHz electrical current can flow through cell membranes and lower the capacitive component [ 2[ 1, 2]. So, the results that determine the impedance of tissue depend on the used stimulation frequency. Impedance Spectroscopy is often described as resistivity or conductivity, which equalizes conductance and resistance to units’ area and length. The SI equivalent units can be described as Ohm-meter (O*m) for resistivity, and Siemens per meters (S/m) in the case of conductivity. The resistance of lung tissue can range from 150 o*cm for blood, to 700 O*cm for air-filled lung tissue, and between 2400 and 2400 O*cm of ballooned lung tissue ( Table 1). In general, the tissue’s resistance or conductivity varies based on level of fluids and ions. When it comes to respiratory lungs it is dependent on the quantity of air present in the alveoli. While most tissues exhibit anisotropic behavior, the heart as well as muscle in particular exhibit anisotropic properties, this means that resistivity is heavily 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 Thorax in a horizontal plane which is typically located within the 4th to 5th intercostal areas (ICS) at just below the parasternal line]. Subsequently, the changes of impedance can also be measured in the lower lobes in the left and right lungs as well as in the heart area ,2[ 1,2]. It is possible to position the electrodes below the 6th ICS could be difficult since the diaphragm as well as abdominal content often enter the measurement plane.
Electrodes can be self-adhesive or single electrodes (e.g. electrocardiogram, ECG) which are placed with equal spacing between electrodes or are embedded in electrode belts ,21 2. Also, self-adhesive electrodes are available for a more user-friendly application [ ,2]. Chest wounds, chest tubes and non-conductive bandages as well as conductive wire sutures can hinder or significantly impact EIT measurements. Commercially available EIT systems typically employ 16 electrodes. However, EIT systems with eight to 32 electrodes may be available (please check Table 2 for specifics) It is recommended to consult Table 2 for more details. ,21.
Table 2. Available electrical impedance tomography (EIT) instruments.
During an EIT measurement sequence, small AC (e.g. five million mA with a frequency of 100 kHz) is applied through various electrode pairs. The resultant voltages are recorded using the remaining electrodes ]. The bioelectrical impedance of the injecting and measuring electrode pairs can be calculated by using the applied current and measured voltages. Most commonly, adjacent electrode pairs are utilized for AC application within a 16-elektrode configuration as opposed to 32-elektrode systems, which typically apply a skip pattern (see Table 2.) for increasing the spacing between electrodes for current injection. The resultant voltages are measured using one of the other electrodes. There is currently an ongoing discussion on different current stimulation patterns , and their unique advantages and disadvantages [7]. For a complete EIT data set that includes bioelectrical measurements both the injecting and electrode pairs used for measuring are constantly rotating around the entire thorax .
1. Current measurement and voltage measurements around the thorax with an EIT system featuring 16 electrodes. Within milliseconds as well as the voltage and current electrodes as well as these active electrodes can be turned across the upper thorax.
The AC that is used in EIT measurements is safe to apply to the body that is undetectable by the 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 that is recorded during one cycle of AC applications is technically known as an image frame. It includes voltage measurements required to create EIT’s Raw EIT image. The term frame rate refers to the amount of EIT frames recorded each second. Frame rates that are at least 10 images/s are needed to monitor ventilation and 25 images/s to monitor heart function or perfusion. Commercially available EIT devices employ frame rates between 40 and 50 images/s as illustrated in
To produce EIT images from the captured frames, the technique known as image reconstruction is applied. Reconstruction algorithms attempt to solve the reverse problem of EIT which is the recuperation of the conductivity distribution inside the thorax based on the voltage measurements that have been made at the electrodes of the thorax surface. At first, EIT reconstruction assumed that electrodes were placed in an ellipsoid or circular plane. However, newer techniques use information about the anatomical contour of the thorax. Today, we use the Sheffield back-projection algorithm [ and the finite element algorithm (FEM) based linearized Newton-Raphson algorithm [ ], and the Graz consensus reconstruction algorithm for EIT (GREIT) [10are often employed.
A lot of the time, EIT images are comparable to a 2-dimensional computed (CT) image. These images are rendered conventionally so that the operator is looking from caudal to cranial when looking at the image. Contrary to the CT image An EIT image doesn’t show the form of a “slice” but an “EIT sensitivity region” [1111. The EIT sensitivity region is a lens-shaped intra-thoracic area where impedance fluctuations contribute to EIT production of the image [11It is a lens-shaped intra-thoracic volume that contributes to the generation. The shape and the thickness of the EIT sensitization region is determined by the dimensions, bioelectric propertiesand shape of the thorax, as well as on the utilized voltage measurement and current injection pattern [1212.
Time-difference image is a technique that is used for EIT reconstruction to show variations in conductivity, rather than relative conductivity of the levels. An time-difference EIT image shows the difference in impedance to a base frame. This provides the chance to examine the effects of time on physiological events like lung ventilation and perfusion [22. The color code of EIT images isn’t uniform, but commonly displays the change in impedance to the reference level (2). EIT images are generally coded using a rainbow-colored scheme with red indicating the highest value of relative imperf (e.g. in the time of inspiration) while green is a moderate relative impedance, and blue being the lowest relative impedance (e.g., during expiration). For clinical applications one option to consider is to use color scales that range from black (no change in impedance) or blue (intermediate impedance change), and white (strong impedance shift) to code ventilation or between black and red, and white for mirror perfusion.
2. Different color codes are available for EIT images in comparison to CT scan. The rainbow-color scheme employs red for the highest relative impedance (e.g. when inspiration occurs) while green is used for intermediate relative impedance, and blue when the relative resistance is lowest (e.g. when expiration is in progress). A newer color scales use instead of black, which has no impedance change) while blue is used for an intermediate change in impedance, and white for the most powerful impedance changes.
4. Functional Imaging and EIT Waveform Analysis
Analyzing Impedance Analyzers data is done using EIT waveforms which are created in individual image pixels in the form of a sequence of raw EIT images that are scanned over time (Figure 3). A region of interest (ROI) can be defined to describe activity in the individual pixels in the image. Within every ROI, the image shows variations in the conductivity of the region over time resulting from the process of ventilation (ventilation-related signal, VRS) and cardiac activities (cardiac-related signal CRS). Additionally, electrically conductive contrast agents such as hypertonic sodium can be used in the production of the EIT signal (indicator-based signal, IBS) and may be linked to the perfusion of the lung. The CRS could be a result of both the heart and lung region and may be partly due to lung perfusion. The precise origins and components are not well understood. 1313. Frequency spectrum analysis is frequently utilized to differentiate between ventilationand cardiac-related impedance variations. Impedance changes that do not occur regularly could be caused by adjustments in the ventilation settings.
Figure 3. EIT Waveforms as well as functional EIT (fEIT) pictures are created from raw EIT images. EIT waves can be defined by pixel or on a particular region of interest (ROI). Conductivity changes result naturally from ventilatory (VRS) or the activity of cardiac muscles (CRS) but they may be artificially induced, e.g. with the injection of bolus (IBS) for perfusion measurement. FEIT images are a visual representation of specific physiological parameters of the region, such as perfusion (Q) and ventilation (V) as well as perfusion (Q) as extracted from the raw EIT images by using an algorithmic process over time.
Functional EIT (fEIT) images are produced through the application of a mathematical algorithm on a sequence of raw images and the corresponding pixel EIT Waveforms. Since the mathematical procedure is used to calculate a physiologically relevant parameter for each pixel. The regional physiological features like regional ventilation (V), respiratory system compliance as in addition to regional perfusion (Q) can be determined and visualized (Figure 3). The information derived drawn from EIT waveforms as well as simultaneously recorded airway pressure measurements can be utilized to calculate the lung’s compliance, as well as lung closing and opening times in each pixel using the changes in pressure and impedance (volume). The comparable EIT measurements taken during gradual inflation and deflation of the lungs enable the display of pressure-volume curves at scales of pixel. Based on the mathematical process, various types of fEIT pictures can be used to examine different functional aspects within the cardio-pulmonary systems.