Abstract
Frequency-difference electrical impedance tomography (fdEIT) reconstructs frequency-dependent changes of a complex impedance distribution. It has a potential application in acute stroke detection because there are significant differences in impedance spectra between stroke lesions and normal brain tissues. However, fdEIT suffers from the influences of electrode-skin contact impedance since contact impedance varies greatly with frequency. When using fdEIT to detect stroke, it is critical to know the degree of measurement errors or image artifacts caused by contact impedance. To our knowledge, no study has systematically investigated the frequency spectral properties of electrode-skin contact impedance on human head and its frequency-dependent effects on fdEIT used in stroke detection within a wide frequency band (10 Hz-1 MHz). In this study, we first measured and analyzed the frequency spectral properties of electrode-skin contact impedance on 47 human subjects’ heads within 10 Hz-1 MHz. Then, we quantified the frequency-dependent effects of contact impedance on fdEIT in stroke detection in terms of the current distribution beneath the electrodes and the contact impedance imbalance between two measuring electrodes. The results showed that the contact impedance at high frequencies (>100 kHz) significantly changed the current distribution beneath the electrode, leading to nonnegligible errors in boundary voltages and artifacts in reconstructed images. The contact impedance imbalance at low frequencies (<1 kHz) also caused significant measurement errors. We conclude that the contact impedance has critical frequency-dependent influences on fdEIT and further studies on reducing such influences are necessary to improve the application of fdEIT in stroke detection.
Highlights
Stroke is a severe cerebrovascular disease which can be clinically divided into two categories: ischemic and hemorrhagic stroke [1]
When the current was injected through Electrode 1 and 9, the current distribution beneath Electrode 5 in six cases where the contact impedance decreased gradually are shown as Fig 7
The current distribution was hardly impacted by the large contact impedance (>10 kohms in the real part) below 1 kHz and the influence on the current distribution increased with decreasing contact impedance
Summary
Stroke is a severe cerebrovascular disease which can be clinically divided into two categories: ischemic and hemorrhagic stroke [1]. Ischemic and hemorrhagic stroke patients require completely different treatments. Ischemic patients need to be treated with thrombolytic (clot-dissolving) drugs within 4.5-6h after the onset of stroke while the hemorrhagic stroke patients require emergency surgery [3]. A prompt brain imaging for stroke detection should be performed before treatment. A portable and inexpensive medical imaging tool for rapid stroke detection is urgently needed. In such a case electrical impedance tomography (EIT) appears promising because the impedance of biological tissues differs for normal physiological and pathological states [5,6,7]
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