A comparison study of electrodes for neonate electrical impedance tomography

Performance evaluation of electrode design and material for a large animal electrical impedance tomography belt

BACKGROUND

Electrical impedance tomography (EIT) produces lung ventilation images via a thoracic electrode belt. Robust electrode design and material, providing low electrode skin contact impedance (SCI), is needed in veterinary medicine. The aim of this study was to compare three EIT electrode designs and materials.

METHODS

Simulations of cylindrical, rectangular and spiked electrode designs were used to evaluate electrode SCI as a function of electrode size, where skin contact was uneven. Gold-plated washers (E_GW ), zinc-plated rivets (E_ZR ) and zinc-galvanised spikes (E_ZS ) were assigned randomly on two interconnected EIT belts. Gel was applied to the cranial or caudal belt and placed on 17 standing cattle. SCI was recorded at baseline and 3, 5, 7, 9 and 11 minutes later.

RESULTS

Simulations that involved electrodes with a greater skin contact area had lower and more uniform SCI. In cattle, SCI decreased with all electrodes over time (p < 0.01). Without gel, no difference was found between E_GW and E_ZS , while SCI was higher for E_ZR (p < 0.03). With gel, SCI was lower in E_GW and E_ZR (p < 0.026), with the SCI in E_GW being the lowest (p < 0.01).

LIMITATIONS

Low numbers of animals and static electrode position may affect SCI.

CONCLUSIONS

Electrode design is important for EIT measurement, with larger electrode designs able to compensate for the use of less conductive materials. Gel is not necessary to achieve acceptable SCI in large animals.

Advances in electrical impedance tomography-based brain imaging

Novel advances in the field of brain imaging have enabled the unprecedented clinical application of various imaging modalities to facilitate disease diagnosis and treatment. Electrical impedance tomography (EIT) is a functional imaging technique that measures the transfer impedances between electrodes on the body surface to estimate the spatial distribution of electrical properties of tissues. EIT offers many advantages over other neuroimaging technologies, which has led to its potential clinical use. This qualitative review provides an overview of the basic principles, algorithms, and system composition of EIT. Recent advances in the field of EIT are discussed in the context of epilepsy, stroke, brain injuries and edema, and other brain diseases. Further, we summarize factors limiting the development of brain EIT and highlight prospects for the field. In epilepsy imaging, there have been advances in EIT imaging depth, from cortical to subcortical regions. In stroke research, a bedside EIT stroke monitoring system has been developed for clinical practice, and data support the role of EIT in multi-modal imaging for diagnosing stroke. Additionally, EIT has been applied to monitor the changes in brain water content associated with cerebral edema, enabling the early identification of brain edema and the evaluation of mannitol dehydration. However, anatomically realistic geometry, inhomogeneity, cranium completeness, anisotropy and skull type, etc., must be considered to improve the accuracy of EIT modeling. Thus, the further establishment of EIT as a mature and routine diagnostic technique will necessitate the accumulation of more supporting evidence.

Amplitude-Integrated Electroencephalography: A Primer for Neonatologists and Practitioners in the NICU

Amplitude-integrated electroencephalography (aEEG) is an essential tool used in the NICU to monitor infants with central nervous system pathology and encephalopathy. This review provides a summary of aEEG, including clinical indications, interpretation of different tracing patterns, and seizure identification, which are essential skills for teams caring for sick newborns. We also discuss the limitations of the clinical application of aEEG in this population.

Optimal combination of electrodes and conductive gels for brain electrical impedance tomography

BACKGROUND

Electrical impedance tomography (EIT) is an emerging imaging technology that has been used to monitor brain injury and detect acute stroke. The time and frequency properties of electrode-skin contact impedance are important for brain EIT because brain EIT measurement is performed over a long period when used to monitor brain injury, and is carried out across a wide range of frequencies when used to detect stroke. To our knowledge, no study has simultaneously investigated the time and frequency properties of both electrode and conductive gel for brain EIT.

METHODS

In this study, the contact impedance of 16 combinations consisting of 4 kinds of clinical electrode and five types of commonly used conductive gel was measured on ten volunteers' scalp for a period of 1 h at frequencies from 100 Hz to 1 MHz using the two-electrode method. And then the performance of each combination was systematically evaluated in terms of the magnitude of contact impedance, and changes in contact impedance with time and frequency.

RESULTS

Results showed that combination of Ag⁺/Ag⁺Cl^- powder electrode and low viscosity conductive gel performed best overall (Ten 20^® in this study); it had a relatively low magnitude of contact impedance and superior performance regarding contact impedance with time (p < 0.05) and frequency (p < 0.05).

CONCLUSIONS

Experimental results indicates that the combination of Ag⁺/Ag⁺Cl^- powder electrode and low viscosity conductive gel may be the best choice for brain EIT.

High-power CMOS current driver with accurate transconductance for electrical impedance tomography

Current drivers are fundamental circuits in bioimpedance measurements including electrical impedance tomography (EIT). In the case of EIT, the current driver is required to have a large output impedance to guarantee high current accuracy over a wide range of load impedance values. This paper presents an integrated current driver which meets these requirements and is capable of delivering large sinusoidal currents to the load. The current driver employs a differential architecture and negative feedback, the latter allowing the output current to be accurately set by the ratio of the input voltage to a resistor value. The circuit was fabricated in a 0.6- μm high-voltage CMOS process technology and its core occupies a silicon area of 0.64 mm (2) . It operates from a ± 9 V power supply and can deliver output currents up to 5 mA p-p. The accuracy of the maximum output current is within 0.41% up to 500 kHz, reducing to 0.47% at 1 MHz with a total harmonic distortion of 0.69%. The output impedance is 665 k Ω at 100 kHz and 372 k Ω at 500 kHz.

Electrical impedance tomography to evaluate air distribution prior to extubation in very-low-birth-weight infants: a feasibility study

OBJECTIVES

Nasal continuous positive airway pressure is used as a standard of care after extubation in very-low-birth-weight infants. A pressure of 5 cmH2O is usually applied regardless of individual differences in lung compliance. Current methods for evaluation of lung compliance and air distribution in the lungs are thus imprecise for preterm infants. This study used electrical impedance tomography to determine the feasibility of evaluating the positive end-expiratory pressure level associated with a more homogeneous air distribution within the lungs before extubation.

METHODS

Ventilation homogeneity was defined by electrical impedance tomography as the ratio of ventilation between dependent and non-dependent lung areas. The best ventilation homogeneity was achieved when this ratio was equal to 1. Just before extubation, decremental expiratory pressure levels were applied (8, 7, 6 and 5 cmH(2)0; 3 minutes each step), and the pressure that determined the best ventilation homogeneity was defined as the best positive end-expiratory pressure.

RESULTS

The best positive end-expiratory pressure value was 6.3 ± 1.1 cmH(2)0, and the mean continuous positive airway pressure applied after extubation was 5.2 ± 0.4 cmH(2)0 (p = 0.002). The extubation failure rate was 21.4%. X-Ray and blood gases after extubation were also checked.

CONCLUSION

This study demonstrates that electrical impedance tomography can be safely and successfully used in patients ready for extubation to suggest the best ventilation homogeneity, which is influenced by the level of expiratory pressure applied. In this feasibility study, the best lung compliance was found with pressure levels higher than the continuous positive airway pressure levels that are usually applied for routine extubation.

Electrode challenges in amplitude-integrated electroencephalography (aEEG): research application of a novel noninvasive measure of brain function in preterm infants

Continuous real-time brain function monitoring of preterm infants offers a novel way to evaluate neurological development in neonatal intensive care. Direct measurement of brain function is difficult and complicated by vulnerabilities of the preterm infant population. This study illustrates the feasibility of using noninvasive hydrogel electrodes with amplitude-integrated electroencephalography (aEEG) as a simplified brain monitor in preterm infants. This article presents a systematic exploration of factors influencing the accuracy of aEEG measurement, especially skin preparation procedures and skin condition after electrode placement. The authors conducted aEEG recordings on 16 medically stable preterm infants at 31-36 weeks postmenstrual age in the neonatal intensive care unit between feedings and caregiving for approximately 3 hr. The authors systematically performed several strategies to improve electrode placement procedures and reduce skin impedance, including (a) examination of possible influences of environmental electrical equipment, (b) comparison of different hydrogel electrode types, (c) modification of skin preparation procedures, and (d) assessment of impacts of different skin conditions. The authors achieved improvements in the impedance value, length of uninterrupted recording, and percentage of the recording duration with measured impedance <20 kΩ (recommended acceptable limit). There was no report of skin irritation during or after the recording. The aEEG measurement at the bedside using hydrogel electrodes is noninvasive and feasible for reliable brain monitoring in preterm infants. This study demonstrated the importance of establishing systematic methods to ensure the accuracy and feasibility of physiologic measurements for nurse researchers.

Performance evaluation of five types of Ag/AgCl bio-electrodes for cerebral electrical impedance tomography

Electrical impedance tomography (EIT) is an emerging medical imaging technique, which has already been investigated in several clinical applications due to its low-cost, non-invasiveness, non-radioactivity, high temporal resolution, and great sensitivity to impedance changes. One potential use of EIT is to perform long-term continuous imaging monitoring of brain for patients who suffer from severe cerebral diseases. However, this application requires a demanding performance of electrodes because of the characteristics of cerebral EIT measurements. Although Ag/AgCl bio-electrodes are widely used for clinical practices or EIT research at the moment, influences of different types of Ag/AgCl electrodes on cerebral EIT measurements have not been investigated. In this study, five common types of Ag/AgCl bio-electrodes were put into comparison by measuring the forearm and the brain of 10 healthy adult volunteers and evaluating those data in frequency or time domain in terms of contact impedance, uniformity, signal-to-noise ratio, and stability. Results show that Ag/AgCl powder electrode has an overall best performance with as low contact impedance as commercial ECG electrodes (p > 0.05), high SNR (60.3 ± 4.5 dB), better uniformity (coefficient of correlation 0.95 ± 0.03), and greater stability (slope 0.68 ± 0.03). After further improvement in design and instrumentation, Ag/AgCl powder electrode is likely to become the optimal choice for cerebral EIT measurements and provide feasible technical support for further research or application in cerebral EIT.

Comparison of a new integrated current source with the modified Howland circuit for EIT applications

Multi-frequency electrical impedance tomography (MF-EIT) systems require current sources that are accurate over a wide frequency range (1 MHz) and with large load impedance variations. The most commonly employed current source design in EIT systems is the modified Howland circuit (MHC). The MHC requires tight matching of resistors to achieve high output impedance and may suffer from instability over a wide frequency range in an integrated solution. In this paper, we introduce a new integrated current source design in CMOS technology and compare its performance with the MHC. The new integrated design has advantages over the MHC in terms of power consumption and area. The output current and the output impedance of both circuits were determined through simulations and measurements over the frequency range of 10 kHz to 1 MHz. For frequencies up to 1 MHz, the measured maximum variation of the output current for the integrated current source is 0.8% whereas for the MHC the corresponding value is 1.5%. Although the integrated current source has an output impedance greater than 1 MOmega up to 1 MHz in simulations, in practice, the impedance is greater than 160 kOmega up to 1 MHz due to the presence of stray capacitance.