Magneto-acousto-electrical tomography : a potential imaging method for current density and electrical impedance

This thesis is about investigating a potential imaging modality, magneto-acousto-electrical tomography (MAET), to provide high-spatial-resolution images of lead field current density and electrical impedance of biological tissues. A lead field current density distribution is the one obtained when a current/voltage source is applied to a sample via a pair of electrodes. The lead field current density distribution can potentially be used to obtain electrical impedance distribution which is helpful in differentiating normal and cancerous tissues. To image lead filed current density, instead of directly applying a current/voltage source to the sample, the sample is placed in a static magnetic field and an ultrasound is focused on it to simulate a point like current dipole source in the focal zone. Electrodes are used to detect the voltage/current generated by the ultrasound in the sample, which according to the reciprocity theorem is proportional to a component of the lead field current density.

Magneto-acousto-electrical tomography : a potential imaging method for current density and electrical impedance

This thesis is about investigating a potential imaging modality, magneto-acousto-electrical tomography (MAET), to provide high-spatial-resolution images of lead field current density and electrical impedance of biological tissues. A lead field current density distribution is the one obtained when a current/voltage source is applied to a sample via a pair of electrodes. The lead field current density distribution can potentially be used to obtain electrical impedance distribution which is helpful in differentiating normal and cancerous tissues. To image lead filed current density, instead of directly applying a current/voltage source to the sample, the sample is placed in a static magnetic field and an ultrasound is focused on it to simulate a point like current dipole source in the focal zone. Electrodes are used to detect the voltage/current generated by the ultrasound in the sample, which according to the reciprocity theorem is proportional to a component of the lead field current density.

A Lead Field Two-Domain Model for Longitudinal Neural Tracts—Analytical Framework and Implications for Signal Bandwidth

Somatosensory evoked potentials are a well-established tool for assessing volley conduction in afferent neural pathways. However, from a clinical perspective, recording of spinal signals is still a demanding task due to the low amplitudes compared to relevant noise sources. Computer modeling is a powerful tool for gaining insight into signal genesis and, thus, for promoting future innovations in signal extraction. However, due to the complex structure of neural pathways, modeling is computationally demanding. We present a theoretical framework which allows computing the electric potential generated by a single axon in a body surface lead by the convolution of the neural lead field function with a propagating action potential term. The signal generated by a large cohort of axons was obtained by convoluting a single axonal signal with the statistical distribution of temporal dispersion of individual axonal signals. For establishing the framework, analysis was based on an analytical model. Our approach was further adopted for a numerical computation of body surface neuropotentials employing the lead field theory. Double convolution allowed straightforward analysis in the frequency domain. The highest frequency components occurred at the cellular membrane. A bandpass type spectral shape and a peak frequency of 1800 Hz was observed. The volume conductor transmitting the signal to the recording lead acted as an additional bandpass reducing the axonal peak frequency from 200 Hz to 500 Hz. The superposition of temporally dispersed axonal signals acted as an additional low-pass filter further reducing the compound action potential peak frequency from 90 Hz to 170 Hz. Our results suggest that the bandwidth of spinal evoked potentials might be narrower than the bandwidth requested by current clinical guidelines. The present findings will allow the optimization of noise suppression. Furthermore, our theoretical framework allows the adaptation in numerical methods and application in anatomically realistic geometries in future studies.