|NAME||Dr. David S. Holder,
Senior Lecturer and Hon. Consultant in Clinical Neurophysiology
|COMPANY / INSTITUTION||University College
Electrical Impedance Tomography has a powerful potential for providing novel
images of brain function. Electrical impedance changes in the brain during activity,
either over tens of seconds due to increased blood flow or cell swelling, in
much the same way as fMRI, or over milliseconds, due to opening of ion channels
during depolaraization. It therefore has the potential to provide a benchtop
machine which can produce images like fMRI, but with images several times a
second and at low cost. It may, in time, provide a system which can image action
potentials or synaptic depolarization during evoked activity, which would be
a revolutionary advance; unique imaging of these changes is not presently possible
with any other method.
Unfortunately, EIT of brain function presents unique technical difficulties, as the impedance changes are small, and the skull has a high resistance and so provides a barrier to current injection. Our group at University College London, UK, has been addressing these problems over the past decade or so. We have demonstrated that reliable EIT images of blood flow and related changes over seconds can be produced in physiological experiments with electrodes on the brain during epilepsy, stroke, spreading depression and evoked activity. Recent studies in humans with scalp electrodes have shown, for the first time, that reproducible raw impedance changes can be recorded during evoked responses in adults and newborn infants but these do not yet reconstruct into reliable images. We have also developed EIT hardware optimized for imaging of brain function, and algorithms which can produce good quality images in realistic saline filled tanks. Using these new systems, we are conducting trials in humans during epilepsy and stroke. We have also just completed a reconstruction algorithm based on a finite element model of the head which permits incorporation of realistic geometry and tissue conductivity. It is being appraised in simulation and tank experiments.
EIT of brain function is not yet at a stage where it can be used routinely for clinical or psychophysiological imaging , but this may become possible with technical advances in the near future.