Electrical Impedance Tomography - A Radiation-Free Diagnosis

What Is Electrical Impedance Tomography?

Electrical impedance tomography (EIT) is a method for generating images that reconstruct a particular area within the human body based on the electrical conductivity of the biological tissue. Compared to techniques like ultrasound or X-rays, EIT permits the assessment of the physiology and potential pathological evolutions, such as the evolution of tumors.

In EIT, the area to be explored is surrounded by electrodes; these electrodes are placed on belts, and each pair of consecutive electrodes is used alternately to administer current and measure the voltage. The recorded voltages are reconstructed into an image using different dedicated algorithms. This method can be applied in both static and dynamic scenarios, commonly known as absolute and functional EIT, respectively. Functional EIT is the most commonly used technique in clinical practice and is typically available at the patient's bedside. Different parts of the body have been explored using EIT, for example- lungs, heart, breast, cervix, brain, etc.

What Is the Principle for EIT Imaging?

The electrical characteristics of any substance, including biological tissue, can be generally categorized into two groups, conductive and insulating. In a conductor, electrical charges move freely in response to the application of the electrical field, and in an insulator (also known as dielectric), the electric charges are fixed and are not free to move. Significant variations are present in the electrical characteristics of biological tissues. To a great extent, these disparities are influenced by the fluid content of the material. For example, blood and the brain exhibit relatively good electrical conductivity while body organs are poor conductors like lungs, skin, fat, and bone. The liver, spleen, and muscle are intermediate in their electrical conductivities.

EIT uses these electrical properties, along with impedance (a comprehensive term for any form of opposition to electrical flow), to create a tomographic image of them. There are three types of EIT

1. Conventional EIT - The conventional EIT uses a single frequency, most often 50 kHz and the images will be formed by taking two measurements at distinct time points to monitor alterations in conductivity (which will happen if there is any underlying pathology).

2. Dual Frequency EIT - It applies at two different frequencies and performs measurements at two different frequencies that allow differentiation between two types of tissues, thus revealing abnormalities in their cellular structure.

3. Multiple Frequency EIT - This technique works on the fact that each biological tissue has a specific conductivity that varies with frequency.

The tomographic image produced by any of the techniques mentioned above will depend on four parameters:

• The position and shape of the electrodes.
• The intensity of the currents applied.
• The conductivity potential of the part of the body to be imaged.
• The shape of this part.

Two types of images can be obtained using EIT, a static image representing the distribution of conductivity in a given time and a dynamic image representing the variation of the conductivity distribution between two instances.

What Are the Uses of EIT in Medicine?

EIT presents different advantages; it is non-invasive, radiation-less, low-cost, safe, easy to use, and suitable for bedside measurement and intensive care monitoring. The various scenarios where these properties play a vital role are as follows:

1. Role of EIT in Preoperative Assessment- Pre-operative EIT imaging has the potential to give relevant information to anesthesiologists, which can influence their strategy during the peri-operative time. It has become an integral tool in conventional pulmonary preoperative testing in patients affected by chronic obstructive pulmonary disease and asthma.
2. Role of EIT in Anesthesia Induction- EIT imaging is an important tool for assessing anesthesia-induced aeration loss and intraoperative functional residual capacity (FRC). FRC can be defined as the volume of gas that remains in the lungs at the end of expiration during periods of rest. FRC tends to fluctuate during and after the induction of anesthesia, which should be monitored and adjusted accordingly. In recent times, EIT has emerged as a promising way of assessing the effectiveness of various pre-oxygenation strategies that can prevent FRC fluctuation during anesthesia induction.
3. Role of Intraoperative Imaging to Guide Mechanical Ventilation- EIT can evaluate lung function during general anesthesia by imaging breath-by-breath changes in ventilation distribution, thereby helping physicians in tailoring mechanical ventilation.
4. Intraoperative Use of EIT for Hemodynamic Monitoring- Improving monitoring accuracy while minimizing invasiveness are two conflicting goals of intra-operative hemodynamic management. The conflict arises because advanced monitoring modalities like the dynamic assessment of cardiac function and detection of fluid responsiveness are typically invasive but necessary. EIT is a non-invasive alternative for monitoring hemodynamics during surgery.
5. Role of EIT Monitoring During the Post-operative Period- EIT can measure early and sensitive signs of postoperative atelectasis. Atelectasis can be defined as the partial collapse of the lungs; enhancing monitoring precision while minimizing invasiveness are two contradictory objectives. Atelectasis can also be seen immediately after the end of the surgery and after extubation due to various causes like persistency of anesthesia, reduced inspiratory effort, impaired cough reflex, etc. It can be identified at an early stage with the help of EIT monitoring.
6. EIT During Diagnostic Procedures in the Perioperative Setting- Due to its simplicity and various other advantages, EIT can be used in various perioperative diagnostic procedures like ambulatory pulmonary function tests and non-bronchoscopic blind broncho-alveolar lavage (BAL).

When the above-mentioned applications are used in the field of gynecology, the technique is known as gynecological impedance tomography (GIT). GIT provides dynamic 3D visualization of the spatial distribution of the static electrical characteristics of the mammary gland and cervical tissue.

What Are the Limitations of EIT?

Despite being a promising technique, EIT has a few limitations; which are as follows:

• It has relatively low spatial resolution when compared to its counterparts, like ultrasound and computed tomographic (CT) scans.
• The image obtained is greatly influenced by the position of the electrodes.
• EIT technology currently available produces a 2-D image.
• It cannot be used in patients with pacemakers or implantable cardioverter-defibrillators.

Conclusion

Despite its limitations, EIT holds great promise as a tool for personalized perioperative care to the specific needs of surgical patients. It can improve preoperative assessment, intraoperative monitoring, and early recognition of postoperative complications.