Electrical Impedance Tomography for Cardio-Pulmonary Monitoring
Electrical Impedance Tomography (EIT) is an instrument for monitoring bedside that is non-invasively able to assess local ventilation as well as conceivably lung perfusion distribution. This paper reviews and discusses the methodological and clinical aspects of thoracic EIT. Initially, researchers focused on the validation of EIT to determine regional ventilation. Present research is focused on its clinical applications to quantify lung collapse, tidal recruitment, and lung overdistension to titrate positive end-expiratory pressure (PEEP) and the volume of tidal. In addition, EIT may help to detect pneumothorax. Recent studies evaluated EIT as a means to measure regional lung perfusion. The absence of indicators in EIT measurements might be sufficient to continuously measure cardiac stroke volume. A contrast agent like saline could be necessary to check the regional lung perfusion. Thus, EIT-based surveillance of regional airflow and lung perfusion might reveal the local perfusion and ventilation that could prove useful in the treatment of patients suffering from acute respiratory distress syndrome (ARDS).
Keywords: electrical impedance imaging bioimpedance; image reconstruction; thorax; regional ventilation as well as monitoring regional perfusion.
Electric impedance tomography (EIT) is an radiation-free functional imaging modality that permits non-invasive monitoring of bedside regional lung ventilation and arguably perfusion. Commercially-available EIT devices were first introduced for clinical applications of this method and the thoracic EIT is widely used in both pediatric and adult patients 1, 2.
2. Basics of Impedance Spectroscopy
Impedance Spectroscopy is the variation in the voltage of biological tissue to externally applied alternating electron current (AC). It is typically measured using four electrodes, where two are utilized to inject AC injection and the other two electrodes are used to measure voltage 3.,3. Thoracic EIT measures the regional range of intra-thoracic bioimpedance. This is seen as an extension of the four electrode principle into the image-plane spanned through the electro belt [ 1]. Dimensionallyspeaking, electrical impedance (Z) is similar to resistance, and the corresponding International System of Units (SI) unit is Ohm (O). It can be easily expressed as a complex number where the real component is resistance while the imaginary part is called reactance. This measures the effects of resistance or capacitance. The capacitance of a cell is determined by the biomembranes’ particulars of the tissue such as ion channels and fatty acids as well as gap junctions. Resistance is mostly determined by nature and amount of extracellular fluid [ 1, 22. At frequencies less than 5 Kilohertz (kHz) an electrical current travels through extracellular fluids and is mostly dependent on its resistive properties of tissues. Higher frequencies, as high as 50 kHz, electrical impulses are slightly deflected at cells’ membranes which causes an increase in capacitive tissue properties. If frequencies are higher than 100 kHz electrical currents can travel through cell membranes and lower the capacitive portion 22. Therefore, the effects that determine the impedance of tissue depend on the stimulation frequency. Impedance Spectroscopy usually refers to conductivity or resistance. Both equalizes conductance and resistance to unit length and area. The SI equivalent units is Ohm-meter (O*m) for resistivity and Siemens per meters (S/m) on conductivity. The thoracic tissue’s resistance ranges from 150 o*cm for blood as high as 700 O*cm with tissues that have been deflated and inflated, to all the way to 2400O*cm for inflated lung tissue ( Table 1). In general, the tissue’s resistance or conductivity is a function of fluid content and ion concentration. Regarding respiratory lungs it also depends on the volume of air inside the alveoli. While most tissues exhibit isotropic behavior, the heart as well as skeletal muscle behave anisotropic, meaning that resistivity strongly depends 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 positioned around the chest in a transverse plane which is typically located in the 4th through 5th intercostal areas (ICS) near Parasternal Line . In turn, the variations in impedance can be assessed in the lower lobes in the right and left lungs, and also in the area of the heart ,22. The placement of the electrodes below the 6th ICS might be challenging as abdominal content and the diaphragm periodically enter the measurement plane.
Electrodes can be self-adhesive or single electrodes (e.g. electrocardiogram ECG) that are placed individually with equal spacing in-between the electrodes or are integrated in electrode belts ,22. Additionally, self-adhesive strips are designed to be more comfortable for application ,2[ 1,2]. Chest tubes, chest wounds Non-conductive bandages and conductive sutures for wires can negatively impact EIT measurements. Commercially available EIT equipment typically uses 16 electrodes, but EIT devices that use 8 or 32 electrodes is available (please look at Table 2 for details) [ ,2[ 1,2.
Table 2. The commercially-available electrical impedance tomography (EIT) equipment.
During an EIT measurements, small AC (e.g. the smallest value of 5 milliamps at a frequency of 100 kHz) are applied to several pairs of electrodes and the generated voltages are measured with the other electrodes ]. Bioelectrical impedance between the injecting and electrode pairs measuring the electrodes can be calculated by using the applied current as well as the measured voltages. Most commonly adjacent electrode pairs are used to allow AC application in a 16-elektrode device and 32-elektrode systems typically utilize a skip-pattern (see the table 2.) which increases the distance of electrodes used for injecting current. The voltages that result are then measured with other electrodes. There is currently an ongoing debate on the different types of current stimulation and their unique advantages and disadvantages . For a complete EIT data set of bioelectrical tests The injecting and electrode pairs used for measuring are constantly rotating around the entire thorax .
1. Current measurement and voltage measurements around the thorax by using an EIT system that includes 16 electrodes. Within a few milliseconds, all the active voltage electrodes and an active voltage electrode will be repeatedly moved within the thorax.
The AC used during EIT tests is safe to use for a body surface application and will not be detected 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 during one cycle in AC apps is referred to as a frame and contains the voltage measurements needed to produce the raw EIT image. The term frame rate refers to the amount of EIT frames captured per second. Frame rates that are at least 10 images/s are necessary for monitoring ventilation and 25 images/s in order to monitor perfusion or cardiac function. Commercially available EIT devices run frames that have a frame rate of between 40 and 50 images/s as demonstrated in
In order to create EIT images using recorded frames, so-called image reconstruction technique is used. Reconstruction algorithms are designed to address the opposite problem of EIT, which is the determination of the conductivity distribution in the thorax by analyzing the voltage measurements obtained at the electrodes located on the thorax surface. Initially, EIT reconstruction assumed that electrodes were placed in an ellipsoid or circular plane, however, more modern algorithms incorporate information about anatomy of the thorax. Today, we use an algorithm called the Sheffield back-projection algorithm [ and the finite element method (FEM) using a linearized Newton–Raphson algorithm ], and the Graz consensus reconstruction algorithm for EIT (GREIT) [10is frequently employed.
A lot of the time, EIT pictures are similar to a computed two-dimensional (CT) image: these images are usually rendered so that the viewer looks from cranial to caudal while taking a look at the picture. Contrary to CT images, unlike a CT image, an EIT image does not show the form of a “slice” but an “EIT sensitivity region” [1111. The EIT sensitization region is a lens-shaped intra-thoracic area with impedance-related changes that contribute to EIT imaging process [11(11, 11). The shape and size of the EIT area of sensitivity are dependent upon the dimensions, bioelectrical properties, and also the anatomy of the Thorax with the type of voltage measurement and current injection pattern [1212.
Time-difference Imaging is a method that is used for EIT reconstruction to display changes in conductivity, not total conductivity. It is a technique that uses time to show the change in conductivity. EIT image compares the variation in impedance with the baseline frame. This is a great way to observe time-dependent physiological processes such as lung ventilation and perfusion . The color code of EIT images may not be uniform but typically shows the change in impedance to a reference level (2). EIT images are usually encoded using a color scheme that is rainbow-like with red representing the high proportional impedance (e.g. when inspiration occurs) as well as green, which is a medium relative impedance and blue being the lowest relative impedance (e.g. when expiration is in progress). For clinical applications, an interesting option is using color scales which range from black (no changes in impedance) and blue (intermediate impedance changes) as well as white (strong impedance changes) to code ventilation . between black and white, then to mirror perfusion.
2. There are a variety of color codes available for EIT images when compared with the CT scan. The rainbow color scheme uses red for the most powerful relative impedance (e.g. in the time of inspiration) and green for a low relative impedance and blue to indicate the least relative imperceptibility (e.g. when expiration is in progress). A more recent color scale uses instead black (no impedance changes) or blue to indicate the intermediate impedance change and white for the most powerful changing of the impedance.
4. Functional Imaging and EIT Waveform Analysis
Analyzing Impedance Analyzers data is based on EIT waveforms created by individual image pixels within an array of raw EIT images that are scanned over period of (Figure 3). A “region of study” (ROI) can be defined to show the activity of individual pixels in the image. Within each ROI, the waveform shows the changes in conductivity of the region over time due to breathing (ventilation-related signal, also known as VRS) (or cardiac activity (cardiac-related signal, CRS). Additionally, electrically conducting contrast agents like hypertonic saline could be used in the production of an EIT signal (indicator-based signal, IBS) and can be connected to perfusion in the lung. The CRS could come from both the heart and lung region and may be partly associated with lung perfusion. The exact cause and the composition are incompletely understood [ 13]. Frequency spectrum analysis is frequently employed to distinguish between ventilationand cardiac-related changes in impedance. Impedance fluctuations that are not frequent can result from adjustments in the ventilation settings.
Figure 3. EIT Waveforms as well as functional EIT (fEIT) Images are derived from the original EIT images. EIT waveforms can be defined either pixel-wise or in a region to be studied (ROI). Conductivity changes occur naturally as a result of the process of ventilation (VRS) or cardiac activity (CRS) however they could be artificially induced, e.g. using the injection of bolus (IBS) for measuring perfusion. FEIT images show the variables of regional physiological activity such as ventilation (V) and blood flow (Q) taken from raw EIT images by using an algorithmic process over time.
Functional EIT (fEIT) images are produced by applying a mathematical procedure on the sequence of raw pictures and the corresponding EIT waves . Since the mathematical procedure is used to calculate an appropriate physiological parameter for each pixel. Regional physiological aspects like regional ventilation (V) and respiratory system compliance as along with regional perfusion (Q) can be measured as well as displayed (Figure 3.). Data drawn from EIT waveforms along with simultaneously registered airway pressure values can be utilized to determine the lung’s compliance as well as lung closing and opening times for each pixel using changes of impedance and pressure (volume). Similar EIT measurements of stepwise inflation and deflation of the lungs can be used to display of curves representing volume and pressure at scales of pixel. Based on the mathematical operation, different types of fEIT photos could reflect different functional characteristics for the cardio-pulmonary system.