Abstract
An Electrical Impedance Tomography (EIT) system has been developed for dynamic three-dimensional imaging of changes in conductivity distribution in the human head, using scalp-mounted electrodes. We attribute these images to changes in cerebral perfusion. At 100 frames per second (fps), voltage measurement is achieved with full-scale signal-to-noise ratio of 105 dB and common-mode rejection ratio > 90 dB. A novel nonlinear method is presented for 3-D imaging of the difference in conductivity distribution in the head, relative to a reference time. The method achieves much reduced modelling error. It successfully localizes conductivity inclusions in experimental and simulation tests, where previous methods fail. For > 50 human volunteers, the rheoencephalography (REG) waveform is observed in EIT voltage measurements for every volunteer, with peak-to-peak amplitudes up to approx. 50 μVrms. Images are presented of the change in conductivity distribution during the REG/cardiac cycle, at 50 fps, showing maximum local conductivity change of approx. 1% in grey/white matter. A total of 17 tests were performed during short (typically 5s) carotid artery occlusions on 5 volunteers, monitored by Transcranial Doppler ultrasound. From EIT measurements averaged over complete REG/cardiac cycles, 13 occlusion tests showed consistently decreased conductivity of cerebral regions on the occluded side, and increased conductivity on the opposite side. The maximum local conductivity change during occlusion was approx. 20%. The simplicity of the carotid artery intervention provides a striking validation of the scalp-mounted measurement system in imaging cerebral hemodynamics, and the REG images indicate its unique combination of sensitivity and temporal resolution.
Highlights
MEDICAL application of Electrical Impedance Tomography (EIT) is well established, e.g. for imaging of lung function [1] [2] [3] [4]
Successful imaging within the human head using fEITER with scalp-mounted electrodes is critically dependent upon two attributes of the methodology described in Section II: low-noise measurement, as discussed in sub-sections II.A and II.B; and the capability of the software algorithm and modelling package to produce images as discussed in sub-section II.D by using a geometric head model that is not derived from anatomical scans of each human subject, i.e. a non-subject model as described in sub-section II.C
A geometric model with 93,687 elements was created from a human MRI image, but with various dimensional differences compared with the tank, e.g. the nasion to the inion distance was 360mm for the tank, but it was 380mm for the human head
Summary
MEDICAL application of Electrical Impedance Tomography (EIT) is well established, e.g. for imaging of lung function [1] [2] [3] [4]. In the case of human head studies, convincing validation of EIT sensitivity to dynamic variation in cerebral perfusion status is an important step in its potential uptake for clinical use Such a demonstration is non-trivial in several respects, e.g. the wide variety of entirely normal phenomena that can influence the trans-impedance EIT measurement, and the safety of volunteer subjects. In developing the fast EIT system reported here, it was found that the trans-impedances recorded on the scalp undergo significant cyclical variation in synchronism with cardiac activity This bio-impedance phenomenon, referred to as rheoencephalography (REG), has been widely studied in non-imaging mode [9] [10] and has shown the capability to track cerebral blood flow (CBF) [11]. REG is explored further by EIT imaging in steady state
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