Peripheral nerve stimulation characteristics of an asymmetric head-only gradient coil compatible with a high-channel-count receiver array.

PURPOSE: To characterize peripheral nerve stimulation (PNS) of an asymmetric head-only gradient coil that is compatible with a commercial high-channel-count receive-only array. METHODS: Two prototypes of an asymmetric head-only gradient coil set with a 42-cm inner diameter were constructed for brain imaging at 3T with maximum performance specifications of up to 85 mT/m and 708 T/m/s. Tests were performed in 24 volunteers to measure PNS thresholds with the transverse (x = left-right; y = anterior-posterior [A/P]) gradient coils of both prototypes. Fourteen of these 24 volunteers were also tested for the z-gradient PNS in the second prototype and were scanned with high-slew-rate echo planar imaging (EPI) immediately after the PNS tests. RESULTS: For both prototypes, the y-gradient PNS threshold was markedly higher than the x-gradient threshold. The z-gradient threshold was intermediate between those for the x- and y-coils. Of the 24 volunteers, only two experienced y-gradient PNS at 80 mT/m and 500 T/m/s. All volunteers underwent the EPI scan without PNS when the readout direction was set to A/P. CONCLUSION: Measured PNS characteristics of asymmetric head-only gradient coil prototypes indicate that such coils, especially in the A/P direction, can be used for fast EPI readout in high-performance neuroimaging scans with substantially reduced PNS concerns compared with conventional whole body gradient coils. Magn Reson Med 76:1939-1950, 2016. © 2015 International Society for Magnetic Resonance in Medicine.

Pulmonary Ventilation and Pulsatile Perfusion Imaging on Premature Neonates using Simultaneous Multi-Source EIT.

We applied our Simultaneous Multi-Source Electrical Impedance Tomography (SMS-EIT) system to detect pulmonary ventilation and pulsatile perfusion on 5 preterm newborns with respiratory distress syndrome under the nasal continuous positive airway pressure (CPAP) treatment. The results show that derived impedance changes have a potential for clinical application to evaluate effects in spontaneously breathing preterm infants with and without CPAP.

Efficient Simultaneous Reconstruction of Time-Varying Images and Electrode Contact Impedances in Electrical Impedance Tomography.

OBJECTIVE: In electrical impedance tomography (EIT), we apply patterns of currents on a set of electrodes at the external boundary of an object, measure the resulting potentials at the electrodes, and, given the aggregate dataset, reconstruct the complex conductivity and permittivity within the object. It is possible to maximize sensitivity to internal conductivity changes by simultaneously applying currents and measuring potentials on all electrodes but this approach also maximizes sensitivity to changes in impedance at the interface. METHODS: We have, therefore, developed algorithms to assess contact impedance changes at the interface as well as to efficiently and simultaneously reconstruct internal conductivity/permittivity changes within the body. We use simple linear algebraic manipulations, the generalized singular value decomposition, and a dual-mesh finite-element-based framework to reconstruct images in real time. We are also able to efficiently compute the linearized reconstruction for a wide range of regularization parameters and to compute both the generalized cross-validation parameter as well as the L-curve, objective approaches to determining the optimal regularization parameter, in a similarly efficient manner. RESULTS: Results are shown using data from a normal subject and from a clinical intensive care unit patient, both acquired with the GE GENESIS prototype EIT system, demonstrating significantly reduced boundary artifacts due to electrode drift and motion artifact.

Detection of small bleeds in the brain with electrical impedance tomography.

In this paper, we describe and assess feasibility of instrumentation and algorithms for detecting bleeding due to hemorrhagic strokes and traumatic brain injury using electrical impedance tomography, a novel biomedical diagnostic modality in which the body is probed noninvasively with generally imperceptible alternating currents applied in patterns to a set of electrodes placed in contact with the skin. We focus on the GENESIS instrument developed by GE Global Research and on the achievability of our goal to detect a bleed in the center of the head with a volume of several ml. Our main topic is compensation for the large changes in voltages that tend to occur when the electrodes are in contact with biological media, specifically either human subjects or with vegetable matter proxies which seem to exhibit the same 'drift' phenomenon. We show that these changes in voltages can be modeled by assuming that each electrode is attached to the body via a discrete complex impedance whose value is time-varying and describe how this discrete component value can be estimated and largely compensated-for. We compare this discrete model with changes in contact impedances estimated using the complete electrode model showing that the two models are roughly comparable in their ability to explain the data from a single human subject experiment with electrodes attached to the head. In a simulation study, we demonstrate that it is possible to detect a small bleed in the center of the head even in the case of large changes in electrode impedances, which can be treated as nuisance parameters.

Comparison of impedance measurements near the skin of newborns and adults.

Electrical impedance tomography (EIT) is a non-invasive imaging technology that has been extensively studied for monitoring lung function of neonatal and adult subjects, especially in neonatal intensive care unit (NICU) and intensive care unit (ICU) environments. The sources of the total impedance in these applications include internal organs, near-boundary tissues, electrode-skin impedance, electrodes and conducting wires. This total impedance must be considered for system design and setting voltage gain since it will contribute to the measured voltage. To adapt a single instrument for use on infants and adults, we studied the difference between the impedance near the skin in both classes of patients. We used a simultaneous multi-source EIT (SMS-EIT) system to make impedance measurements. Characteristic resistance was calculated for two different current patterns: one that is more sensitive to boundary region impedance and another that is more sensitive to interior changes. We present ratios of these resistances to assess the relative contribution of near-skin effects to the overall impedance. Twenty adult ICU subjects (10 male, 10 female, age: 49.05 ± 16.32 years (mean ± standard deviation)) and 45 neonates (23 male, 22 female, gestational age: 37.67 ± 2.11 weeks, postnatal age, 2.56 ± 2.67 d) were studied at Columbia University Medical Center. Impedance measurements at 10 kHz were collected for approximately one hour from each subject. The characteristic resistance ratio for each subject was computed and analyzed. The result shows the impedance at or near the skin of newborns is significantly higher than in adult subjects.

Real-time 3D electrical impedance imaging for ventilation and perfusion of the lung in lateral decubitus position.

We report a prototype Electrical Impedance Imaging System. It is able to detect the gravity-induced changes in the distributions of perfusion and ventilation in the lung between supine and lateral decubitus positions. Impedance data were collected on healthy volunteer subjects and 3D reconstructed images were produced in real-time, 20 frames per second on site, without using averaging or a contrast agent. Imaging data also can be reconstructed offline for further analysis.

Real-time 3D electrical impedance imaging for ventilation monitoring of the lung: Pilot study.

We report an Electrical Impedance Tomography device capable of detecting gravity-induced regional ventilation changes in real-time without averaging or using a contrast medium. Changes in lung ventilation are demonstrated in right and left lateral decubitus position and compared to those seen in an upright and supine normal subject.