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Medical Physics and Biomedical Engineering

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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 electrode

EIT group shot

It is fast, inexpensive, portable and sensitive to physiological changes which affect electrical impedance properties. For about two decades, satisfactory images have been obtained of changes over time related to gastric emptying and ventilation and cardiac output in the thorax. 

The work of our group at UCL has been to pioneer EIT for imaging brain function. It could be used to image in acute stroke or epileptic seizures, when its portability and low cost would give it unique practical advantages over existing methods such as fMRI. It could also provide images of fast neural activity in the brain over milliseconds which would constitute a revolutionary advance in neuroscience technology.

Innovations in hardware and image reconstruction algorithms enable accurate images to be collected in tanks and in experimental animals with electrodes on the brain; the next challenge is to see if recent technical improvements allow us to collect clinically useful images in human subjects with scalp electrodes.


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)
  • 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, 1996Oh 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) with a ring of electrodes on exposed brain

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

EIT equipment examples
UCL ScouseTom human EIT system

Fig 3: Examples of EIT systems developed at UCL. Left : UCH Mk2.5 (single channel record and current drive, multiplexed to 16 or 32 electrodes, switches serially, ~ 3 images per second (McEwan et al, 2006) Middle: KHU Mark 1 (16 parallel record single drive, ~ 1 image per second; Oh et al, 2007) Right : UCL ScouseTom human EIT system (2019)
 
  • First clinical studies in humans of EIT in stroke (Romsauerova et al, 2006), epileptic seizures (Fabrizi et al, 2006) and blood flow related changes over seconds in normal cortical evoked activity (Tidswell et al, 2001).
     
  • A recent breakthrough has been the production of the first tomographic images of fast neural activity during cortical evoked activity or epilepsy in the anaesthetized rat using epidural electrode mats (Aristovich et al, 2015, Hannan et al, 2020). 
    3D Fast Neural Epilepsy
  • Use of fast neural EIT with a nerve cuff for imaging activity in the fascicles inside peripheral nerves. There is a new field in medicine – “Electroceuticals”. Autonomic nerves are stimulated electrically with the aim of treating a broad range of endocrine, autoimmune and other disease. However, current approaches stimulate entire autonomic nerves such as the vagus nerve in the neck; this produces unwanted side effects. With Glaxo Smith Kline and Google Health (Verily) we have developed a flexible nerve cuff that enables imaging of activation of individual pathways inside nerves. Once identified, these can then be activated selectively using the same cuff; this has the potential to avoid the unwanted side effects. (Ravagli et al, 2020). 
    recording arrangement – 3 rings of 14 electrode nerve cuffs

    Left : recording arrangement – 3 rings of 14 electrode nerve cuffs. Upper : Proof of principle in rat sciatic nerve : fast neural EIT reliably can image individual fascicles. Lower : EIT in pig cervical vagus nerve reliably images fascicles for heart lung and laryngeal function; correlates with µCT tracing studies.  (Ravagli, E., Mastitskaya, S., Thompson, N. et al. Imaging fascicular organization of rat sciatic nerves with fast neural electrical impedance tomography. Nat Commun 11, 6241 (2020). https://doi.org/10.1038/s41467-020-20127-x).
     
  • Organotopic organization of the porcine mid-cervical vagus nerve (Thompson et al, 2023)

Current projects

Projects in progress in the group are :

  1. Imaging fast neural circuits in the brain in health and diseases with EIT with intracerebral depth probes
  2. Non-invasive imaging of human brain function over milliseconds with magnetic detection EIT with MEG or time-of-flight CHIRP EIT.  
  3. Selective vagal nerve stimulation using fast neural EIT in heart failure.

Further reading on EIT of brain function