Electrical impedance tomography, EIT for short, is a non-invasive, radiation-free monitoring tool, which allows real-time imaging of ventilation. Electrical impedance tomography has a medium spatial resolution and a very high temporal resolution.
Electrical impedance tomography may either be performed as:
- Relative or differential EIT, also known as functional EIT or f-EIT, is the most commonly used; or
- Absolute impedance EIT, also known as a-EIT.
In electrical impedance tomography, an electrical conductivity, permittivity, and impedance of a part of the body is inferred from the surface electrode measurements and used to create a tomographic image of that part. Electrical conductivity differs considerably among several biological tissues (absolute EIT), or in the movement of fluids and gases within tissues (difference EIT). Although, the majority of EIT systems use small alternating currents at a single frequency. Some, however, use multiple frequencies to better differentiate between the normal and suspected abnormal tissue within the same organ (multi-frequency-EIT or electrical impedance spectroscopy).
The use of electrical impedance tomography
Electrical impedance tomography can be used for a variety of analysis, such as;
- To describe the distribution of alveolar ventilation.
- To detect lung collapse.
- To assess lung hyper-expansion.
- To monitor lung recruitment.
- To diagnose pneumothorax.
- To detect endobronchial intubation.
- To estimate ventilation/perfusion distributions, using hypertonic saline as a contrast medium.
- To determine optimal PEEP.
Principles of electrical impedance tomography
Impedance is an abstract physical variable that describes the resistivity characteristics (or the reciprocal value: conductivity) of an electrical circuit in the presence of an alternating current (AC).
Different biological tissues have different distinct impedance characteristics (bio-impedance). An increase of air in the lungs causes increased impedance. Perfusion leads to a change in thoracic bio-impedance from diastole to systole in a range of 3%, due to:
- Lung displacement during systole.
- Red blood cell alignment during systole, and a random positioning during diastole.
- Localised changes in cardiac fossa due to cardiac contraction.
Cross-sectional images of the lungs are generated by the administration of high frequency (around 50-80 Hz), low amplitude alternating electrical currents, usually through 16 or 32 electrodes. The pathways followed by the administered currents, however, vary according to:
- The shape of chest wall, and
- The differences in thoracic impedance.
The resulting electric potentials on the surface of the chest wall are then measured.
Using a reconstruction algorithm, an electric impedance distribution within the thorax is created. An IV injection of hypertonic saline may be used as a contrast medium for EIT images due to its exceptionally low impeditivity. When this is combined with a breath hold manoeuvre this allows for measurement of lung perfusion.
The extent of a lung recruitment may be measured by:
- The centre of gravity of ventilation images moved dorsally during lung recruitment and ventrally during lung collapse.
- Variation in the time delay between the start of inspiration and the start of regional inflation (ventilation delay index).
Advantages of electrical impedance tomography
The following are some of the advantages of electrical impedance tomography:
- It is cheap and affordable.
- It’s a non-invasive technique.
- It allows ventilation to be continuously monitored at the bedside.
- It is radiation free.
- The relative or differential EIT terminates the errors caused by assumptions made about thoracic shape.
- It offers quality resolution, with increased number of electrodes.
- It offers high temporal resolution.
- The use of brief periods of apnea or by means of filtering out ventilation allows monitoring of perfusion.
Disadvantages of electrical impedance tomography
Although, electrical impedance tomography offers a number of benefits. There are, however, a couple of disadvantages of electrical impedance tomography.
- The availability is poor.
- It is not widely accepted.
- Slight error in voltage measurements may lead to the creation of markedly different impedance distributions, which will require the use of regularisations in the estimation algorithms, at the expense of spatial resolution and attenuate extreme perturbations.
- Differential EIT does not offer information about baseline impedance or absolute measurements. For instance, a change in lung impedance from 5 to 10 ohms produces the same image as a change from 10 to 20 ohms.
- Differential EIT images only show the regions of the thorax that have changed their impedance over time. So the pre-existing areas of lung consolidation, pleural effusions or large bullae are not signified.
- Absolute EIT is likely to have lower quality images.
- Spatial resolution varies between different EIT devices and within a single device depending on the settings applied.
- Low spatial resolution in the craniocaudal direction.
- There’s lesser spatial resolution than in CT or MRI.
- A baseline measurement is required for pneumothorax detection.