Electrical Impedance Tomography and Neurophysiology

Electrical Impedance Tomography is a recently developed imaging technique, with which images of the internal impedance of the subject can be rapidly collected with rings of external ECG-type electrodes. It is fast, inexpensive, portable and sensitive to physiological changes which affect electrical impedance properties. Since its development in the 1980’s, the main research interest internationally has been in imaging lung ventilation and cardiac output in the thorax. The work of our group at UCL has been to extend its use for imaging brain and nerve function. We have shown that it can produce accurate images of fast electrical activity over milliseconds in the brain and in peripheral nerves such as the vagus nerve in the neck.

Key advances

• Development of idea that EIT could provide tomographic images of fast electrical activity in the brain and so provide a uniquely new method to test theories in computational neuroscience (Holder, 1987)

Fig 1: fMRI-like EIT images during visual evoked activity in the anaesthetised rabbit with cortical electrodes (Holder et al, 1997).

• Demonstration that EIT with the Sheffield Mark 1 EIT system could produce reproducible EIT images of stroke (Holder, 1992), epileptic seizures, functional activity (Holder et al, 1996, Oh et al, 2011) and the phenomenon of spreading depression which is thought to underlie migraine (Boone et al, 1994) in anaesthetised experimental animals (rats or rabbits) ith a ring of electrodes on exposed brain.

• Development of image reconstruction software able to produce accurate EIT images of brain function in 3D with scalp electrodes, using anatomically realistic Finite Element Models of the brain (Bagshaw et al, 2003). Refinements include anatomically realistic Finite Element Model meshes (Vonach et al, 2012, Aristovich et al, 2014), adjustment for anisotropy (Abascal et al, 2008), improved regularization methods (Aristovich et al, 2014), a general method to improve signal-to-noise in EIT imaging (Mason et al, 2024), and novel methods for imaging at a single moment in time in acute stroke based on recording with multiple frequencies (Malone et al, 2013, Malone et al, 2014).

Fig. 3. EIT systems developed at UCL. Left :UCH Mk2.5 (single channel record and current drive, single/multifrequency),16 or 32 electrodes,  20Hz-256kHz, McEwan et al, 2006). Middle : UCL “ScouseTom” fast neural EIT (Single pair inject, parallel record, single frequency 100Hz-5kHz, 32-128 electrodes, Avery et al, 2017). Right : ScouseTom system packaged for human clinical trial use (Witkowska-Wrobel et al, 2021).

• Development of electronic hardware specially tailored for EIT imaging in acute stroke (Yerworth et al, 2003; McEwan et al, 2006, Oh et al, 2007), epileptic seizures (Yerworth et al 2002) and fast neural activity in the brain (Avery et al, 2017).
• First clinical studies in humans of EIT in stroke (Romsauerova et al, 2006, Goren et al, 2018), epileptic seizures (Fabrizi et al, 2006) and blood flow related changes over seconds in normal cortical evoked activity (Tidswell et al, 2001). Although these demonstrated the method, clinical safety and some proof of principle, it was not possible to produce clinically useful reliable images. However, fast neural EIT images with a millisecond and sub-millimetre resolution were obtained using subdural custom-made silicone rubber-platinum electrode mats during epileptic seizures or normal activity in anaesthetised rats (Aristovich et al, 2015, Hannan et al, 2020). EIT of fMRI-like slow impedance changes over seconds were also obtained in a pig model of epilepsy (Witkowska-Wrobel et al, 2021). 

Fig. 4. Images of the spread of an epileptic seizure at A) 350 B) 600 C) 1050 and D) 2000 milliseconds in a rat model of epilepsy.

Fig. 5. Fast neural EIT images of compound action potentials in fascicles in peripheral nerve. Left : Diagram of nerve with silicone rubber/platinum foil electrodes with 14 electrodes in rings around the nerve. The single ring is for EIT and the dual ring for selective stimulation between electrode pairs.  Middle and right – Illustration of EIT images of activated fascicles with validation by tracing of fascicles to tributary nerves and neural tracers. Upper – rat sciatic and Lower – Pig cervical vagus nerves (Ravagli et al, 2020; Thompson et al, 2023)

• Use of fast neural EIT with a nerve cuff for imaging activity in the fascicles inside peripheral nerves. This could be transformative in the new field in medicine – “Electroceuticals”. Autonomic nerves may stimulated electrically and improve treatment of endocrine, autoimmune and neurological diseases. Cervical vagus nerve stimulation is already well accepted for the treatment of resistant epilepsy. However, current approaches stimulate the entire vagus nerve in the neck, which produces unwanted side effects. 

  We have developed a flexible nerve cuff that enables imaging of activation of individual pathways inside nerves. The same cuffs which contain one or two rings of 14 electrodes can selectively stimulate identified organ fascicles and so offer a means  to mitigate the off-target effects associated with current Vagal Nerve Stimulation and so improve treatment outcomes. With this approach, our group shown that there are fascicles dedicated to cardiac efferent and afferent, pulmonary and laryngeal function in the cervical vagus nerve. (Thompson et al, 2023 and Thompson et al, 2024)

Current projects

Projects in progress in the group are:

Development of new EIT technology for imaging brain and nerve function: 

• Non-invasive imaging of human brain function over milliseconds with magnetic detection EIT with atomic magnetometers (Mason et al, 2023).
• Fast neural real-time EIT using parallel current injection and frequency division multiplexing.

Exploring the functional anatomy of the vagus nerve:

• Determining functional anatomy of cardiac and pulmonary fascicles in human subjects in vivo with UCL nerve cuff, selective stimulation and fast neural EIT (in epilepsy patients undergoing vagal nerve stimulator implants, UCL Hospital, Queen Square, London)
• Histological and uCT tracing of human cadaver nerves

Further reading on EIT of brain and nerve function

• Introduction to EIT
• Introduction to bioimpedance
• Overview of work on EIT of brain function at UCL
• Spatially-selective vagus nerve stimulation and fast neural EIT of the cervical vagus nerve

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