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Tarotin I, Mastitskaya S, Ravagli E, Perkins JD, Holder D, Aristovich K. Overcoming temporal dispersion for measurement of activity-related impedance changes in unmyelinated nerves. J Neural Eng 2022; 19. [PMID: 35413701 DOI: 10.1088/1741-2552/ac669a] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/19/2022] [Accepted: 04/11/2022] [Indexed: 11/11/2022]
Abstract
Objective.Fast neural electrical impedance tomography is an imaging technique that has been successful in visualising electrically evoked activity of myelinated fibres in peripheral nerves by measurement of the impedance changes (dZ) accompanying excitation. However, imaging of unmyelinated fibres is challenging due to temporal dispersion (TP) which occurs due to variability in conduction velocities of the fibres and leads to a decrease of the signal below the noise with distance from the stimulus. To overcome TP and allow electrical impedance tomography imaging in unmyelinated nerves, a new experimental and signal processing paradigm is required allowing dZ measurement further from the site of stimulation than compound neural activity is visible. The development of such a paradigm was the main objective of this study.Approach.A finite element-based statistical model of TP in porcine subdiaphragmatic nerve was developed and experimentally validatedex-vivo. Two paradigms for nerve stimulation and processing of the resulting data-continuous stimulation and trains of stimuli, were implemented; the optimal paradigm for recording dispersed dZ in unmyelinated nerves was determined.Main results.While continuous stimulation and coherent spikes averaging led to higher signal-to-noise ratios (SNRs) at close distances from the stimulus, stimulation by trains was more consistent across distances and allowed dZ measurement at up to 15 cm from the stimulus (SNR = 1.8 ± 0.8) if averaged for 30 min.Significance.The study develops a method that for the first time allows measurement of dZ in unmyelinated nerves in simulation and experiment, at the distances where compound action potentials are fully dispersed.
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Affiliation(s)
- Ilya Tarotin
- Department of Medical Physics and Biomedical Engineering, University College London, Gower Street, London WC1E 6BT, United Kingdom
| | - Svetlana Mastitskaya
- Department of Medical Physics and Biomedical Engineering, University College London, Gower Street, London WC1E 6BT, United Kingdom
| | - Enrico Ravagli
- Department of Medical Physics and Biomedical Engineering, University College London, Gower Street, London WC1E 6BT, United Kingdom
| | - Justin D Perkins
- Clinical Science and Services, Royal Veterinary College, Hawkshead Lane, Hatfield, United Kingdom
| | - David Holder
- Department of Medical Physics and Biomedical Engineering, University College London, Gower Street, London WC1E 6BT, United Kingdom
| | - Kirill Aristovich
- Department of Medical Physics and Biomedical Engineering, University College London, Gower Street, London WC1E 6BT, United Kingdom
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Ravagli E, Mastitskaya S, Thompson N, Aristovich K, Holder D. Optimization of the electrode drive pattern for imaging fascicular compound action potentials in peripheral nerve with fast neural electrical impedance tomography. Physiol Meas 2019; 40:115007. [PMID: 31694004 PMCID: PMC7214787 DOI: 10.1088/1361-6579/ab54eb] [Citation(s) in RCA: 12] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/06/2023]
Abstract
OBJECTIVE The main objective of this study was to investigate which injection pattern led to the best imaging of fascicular compound activity in fast neural EIT of peripheral nerve using an external cylindrical 2 × 14-electrodes cuff. Specifically, the study addressed the identification of the optimal injection pattern and of the optimal region of the reconstructed volume to image fascicles. APPROACH The effect of three different measurement protocol features (transversal/longitudinal injection, drive electrode spacing, referencing configuration) over imaging was investigated in simulation with the use of realistic impedance changes and noise levels. Image-based metrics were employed to evaluate the quality of the reconstructions over the reconstruction domain. The optimal electrode addressing protocol suggested by the simulations was validated in vivo on the tibial and peroneal fascicles of rat sciatic peripheral nerves (N = 3) against MicroCT reference images. MAIN RESULTS Injecting current transversally, with spacing of ⩾4 electrodes apart (⩾100°) and single-ring referencing of measurements, led to the best overall localization when reconstructing on the edge of the electrode array closest to the reference. Longitudinal injection protocols led to a higher SNR of the reconstructed image but poorer localization. All in vivo EIT recordings had statistically significant impedance variations (p < 0.05). Overall, fascicle center-of-mass (CoM) localization error was estimated at 141 ± 56 µm (-26 ± 94 µm and 5 ± 29° in radial coordinates). Significant difference was found (p < 0.05) between mean angular location of the tibial and peroneal CoMs. SIGNIFICANCE This study gives the reader recommendations for performing fast neural EIT of fascicular compound activity using the most effective protocol features.
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Affiliation(s)
- Enrico Ravagli
- Medical Physics and Biomedical Engineering, University College London, London, United Kingdom
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Hope J, Aqrawe Z, Lim M, Vanholsbeeck F, McDaid A. Increasing signal amplitude in electrical impedance tomography of neural activity using a parallel resistor inductor capacitor (RLC) circuit. J Neural Eng 2019; 16:066041. [PMID: 31536974 DOI: 10.1088/1741-2552/ab462b] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
Abstract
OBJECTIVE To increase the impedance signal amplitude produced during neural activity using a novel approach of implementing a parallel resistor inductor capacitor (RLC) circuit across the current source used in electrical impedance tomography (EIT) of peripheral nerve. APPROACH The frequency response of the impedance signal was characterized in the range 4-18 kHz, then a frequency range with significant capacitive charge transfer was selected for experiment with the RLC circuit. Design of the RLC circuit was aided by in vitro impedance measurements on nerve and nerve cuff in the range 5 Hz to 50 kHz. MAIN RESULTS The frequency response of the impedance signal across 4-18 kHz showed maximum amplitude at 6-8 kHz, and steady decline in amplitude between 8 and 18 kHz with -6 dB reduction at 14 kHz. The frequency range 17 ± 1 kHz was selected for the RLC experiment. The RLC experiment was performed on four subjects using an RLC circuit designed to produce a resonant frequency of 17 kHz with a bandwidth of 3.6 kHz, and containing a 22 mH inductive element and a 3.45 nF capacitive element with +0.8/- 3.45 nF manual tuning range. With the RLC circuit connected, relative increases in the impedance signal (±3σ noise) of 44% (±15%), 33% (±30%), 37% (±8.6%), and 16% (±19%) were produced. SIGNIFICANCE The increase in impedance signal amplitude at high frequencies, generated by the novel implementation of a parallel RLC circuit across the drive current, improves spatial resolution by increasing the number of parallel drive currents which can be implemented in a frequency division multiplexed (FDM) EIT system, and aids the long term goal of a real-time FDM EIT system by reducing the need for ensemble averaging.
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Affiliation(s)
- J Hope
- The Department of Mechanical Engineering, The University of Auckland, Auckland, New Zealand. Dodd Walls Centre for Photonic and Quantum Technologies, Dunedin, New Zealand
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Tarotin I, Aristovich K, Holder D. Simulation of impedance changes with a FEM model of a myelinated nerve fibre. J Neural Eng 2019; 16:056026. [DOI: 10.1088/1741-2552/ab2d1c] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/25/2022]
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Hope J, Aristovich K, Chapman CAR, Volschenk A, Vanholsbeeck F, McDaid A. Extracting impedance changes from a frequency multiplexed signal during neural activity in sciatic nerve of rat: preliminary study in vitro. Physiol Meas 2019; 40:034006. [DOI: 10.1088/1361-6579/ab0c24] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022]
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Tarotin I, Aristovich K, Holder D. Effect of dispersion in nerve on compound action potential and impedance change: a modelling study. Physiol Meas 2019; 40:034001. [DOI: 10.1088/1361-6579/ab08ce] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/14/2022]
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Chapman CAR, Aristovich K, Donega M, Fjordbakk CT, Stathopoulou TR, Viscasillas J, Avery J, Perkins JD, Holder D. Electrode fabrication and interface optimization for imaging of evoked peripheral nervous system activity with electrical impedance tomography (EIT). J Neural Eng 2018; 16:016001. [PMID: 30444215 DOI: 10.1088/1741-2552/aae868] [Citation(s) in RCA: 18] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/21/2022]
Abstract
OBJECTIVE Non-invasive imaging techniques are undoubtedly the ideal methods for continuous monitoring of neural activity. One such method, fast neural electrical impedance tomography (EIT) has been developed over the past decade in order to image neural action potentials with non-penetrating electrode arrays. APPROACH The goal of this study is two-fold. First, we present a detailed fabrication method for silicone-based multiple electrode arrays which can be used for epicortical or neural cuff applications. Secondly, we optimize electrode material coatings in order to achieve the best accuracy in EIT reconstructions. MAIN RESULTS The testing of nanostructured electrode interface materials consisting of platinum, iridium oxide, and PEDOT:pTS in saline tank experiments demonstrated that the PEDOT:pTS coating used in this study leads to more accurate reconstruction dimensions along with reduced phase separation between recording channels. The PEDOT:pTS electrodes were then used in vivo to successfully image and localize the evoked activity of the recurrent laryngeal fascicle from within the cervical vagus nerve. SIGNIFICANCE These results alongside the simple fabrication method presented here position EIT as an effective method to image neural activity.
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Affiliation(s)
- Christopher A R Chapman
- Department of Medical Physics and Biomedical Engineering, University College London, Gower Street, London WC1E 6BT, United Kingdom
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Hope J, Vanholsbeeck F, McDaid A. Drive and measurement electrode patterns for electrode impedance tomography (EIT) imaging of neural activity in peripheral nerve. Biomed Phys Eng Express 2018; 4. [DOI: 10.1088/2057-1976/aadff3] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/10/2018] [Accepted: 09/10/2018] [Indexed: 11/12/2022]
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Aristovich K, Donegá M, Blochet C, Avery J, Hannan S, Chew DJ, Holder D. Imaging fast neural traffic at fascicular level with electrical impedance tomography: proof of principle in rat sciatic nerve. J Neural Eng 2018; 15:056025. [DOI: 10.1088/1741-2552/aad78e] [Citation(s) in RCA: 28] [Impact Index Per Article: 4.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022]
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Tarotin I, Aristovich K, Holder D. Model of Impedance Changes in Unmyelinated Nerve Fibers. IEEE Trans Biomed Eng 2018; 66:471-484. [PMID: 29993457 DOI: 10.1109/tbme.2018.2849220] [Citation(s) in RCA: 15] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
Abstract
OBJECTIVE Currently, there is no imaging method that is able to distinguish the functional activity inside nerves. Such a method would be essential for understanding peripheral nerve physiology and would allow precise neuromodulation of organs these nerves supply. Electrical impedance tomography (EIT) is a method that produces images of electrical impedance change (dZ) of an object by injecting alternating current and recording surface voltages. It has been shown to be able to image fast activity in the brain and large peripheral nerves. To image inside small autonomic nerves, mostly containing unmyelinated fibers, it is necessary to maximize SNR and optimize the EIT parameters. An accurate model of the nerve is required to identify these optimal parameters as well as to validate data obtained in the experiments. METHODS In this study, we developed two three-dimensional models of unmyelinated fibers: Hodgkin-Huxley (HH) squid giant axon (single and multiple) and mammalian C-nociceptor. A coupling feedback system was incorporated into the models to simulate direct and alternating current application and simultaneously record external field during action potential propagation. RESULTS Parameters of the developed models were varied to study their influence on the recorded impedance changes; the optimal parameters were identified. The negative dZ was found to monotonically decrease with frequency for both HH and C fiber models, in accordance with the experimental data. CONCLUSION AND SIGNIFICANCE The accurate realistic model of unmyelinated nerve allows the optimization of EIT parameters and matches literature and experimental results.
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Hope J, Vanholsbeeck F, McDaid A. A model of electrical impedance tomography implemented in nerve-cuff for neural-prosthetics control. Physiol Meas 2018; 39:044002. [DOI: 10.1088/1361-6579/aab73a] [Citation(s) in RCA: 13] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022]
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Faulkner M, Hannan S, Aristovich K, Avery J, Holder D. Characterising the frequency response of impedance changes during evoked physiological activity in the rat brain. Physiol Meas 2018; 39:034007. [DOI: 10.1088/1361-6579/aab01f] [Citation(s) in RCA: 12] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022]
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Fouchard A, Coizet V, Sinniger V, Clarençon D, Pernet-Gallay K, Bonnet S, David O. Functional monitoring of peripheral nerves from electrical impedance measurements. ACTA ACUST UNITED AC 2017; 110:361-371. [PMID: 28564589 DOI: 10.1016/j.jphysparis.2017.05.003] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/30/2016] [Revised: 03/05/2017] [Accepted: 05/26/2017] [Indexed: 12/13/2022]
Abstract
Medical electrical stimulators adapted to peripheral nerves use multicontact cuff electrodes (MCC) to provide selective neural interfaces. However, neuroprostheses are currently limited by their inability to locate the regions of interest to focus. Intended until now either for stimulation or recording, MCC can also be used as a means of transduction to characterize the nerve by impedancemetry. In this study, we investigate the feasibility of using electrical impedance (EI) measurements as an in vivo functional nerve monitoring technique. The monitoring paradigm includes the synchronized recording of both the evoked endogenous activity as compound action potentials (CAP) and the superimposed sine signal from the EI probe. Measurements were conducted on the sciatic nerve of rodents, chosen for its branchings towards the peroneal and tibial nerves, with both mono- and multi-contact per section electrodes. During stimulation phases, recordings showed CAP with consistent fiber conduction velocities. During coupled phases of both stimulation and sine perturbation, impedance variations were extracted using the mono-contact electrode type for certain frequencies, e.g. 2.941kHz, and were temporally coherent with the previous recorded CAP. Using a MCC, localized evoked CAP were also recorded but the signal to noise ratio (SNR) was too low to distinguish the expected associated impedance variation and deduce an image of impedance spatial changes within the nerve. The conducted in vivo measurements allowed to distinguish both evoked CAP and associated impedance variations with a strong temporal correlation. This indicates the feasibility of functional EI monitoring, aiming at detecting the impedance variations in relation to neural activity. Further work is needed to improve the in vivo system, namely in terms of SNR, and to integrate new multicontact devices in order to move towards EI tomography with the detection of spatially-localized impedance variations. Eventually, regions that are interesting to be targeted by stimulation could be identified through these means.
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Affiliation(s)
- Alexandre Fouchard
- CEA-LETI, MINATEC Campus, F-38000 Grenoble, France; Univ Grenoble Alpes, Grenoble Institut des Neurosciences, GIN, F-38000 Grenoble, France; Inserm, U1216, F-38000 Grenoble, France
| | - Véronique Coizet
- Univ Grenoble Alpes, Grenoble Institut des Neurosciences, GIN, F-38000 Grenoble, France; Inserm, U1216, F-38000 Grenoble, France
| | - Valérie Sinniger
- Univ Grenoble Alpes, Grenoble Institut des Neurosciences, GIN, F-38000 Grenoble, France; Inserm, U1216, F-38000 Grenoble, France
| | - Didier Clarençon
- Univ Grenoble Alpes, Grenoble Institut des Neurosciences, GIN, F-38000 Grenoble, France; Inserm, U1216, F-38000 Grenoble, France; Institut de Recherche Biomédicale des Armées, F-91223 Brétigny-sur-Orge, France
| | - Karin Pernet-Gallay
- Univ Grenoble Alpes, Grenoble Institut des Neurosciences, GIN, F-38000 Grenoble, France; Inserm, U1216, F-38000 Grenoble, France
| | | | - Olivier David
- Univ Grenoble Alpes, Grenoble Institut des Neurosciences, GIN, F-38000 Grenoble, France; Inserm, U1216, F-38000 Grenoble, France.
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Avery J, Dowrick T, Faulkner M, Goren N, Holder D. A Versatile and Reproducible Multi-Frequency Electrical Impedance Tomography System. SENSORS (BASEL, SWITZERLAND) 2017; 17:E280. [PMID: 28146122 PMCID: PMC5336119 DOI: 10.3390/s17020280] [Citation(s) in RCA: 53] [Impact Index Per Article: 7.6] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 12/05/2016] [Accepted: 01/25/2017] [Indexed: 11/16/2022]
Abstract
A highly versatile Electrical Impedance Tomography (EIT) system, nicknamed the ScouseTom, has been developed. The system allows control over current amplitude, frequency, number of electrodes, injection protocol and data processing. Current is injected using a Keithley 6221 current source, and voltages are recorded with a 24-bit EEG system with minimum bandwidth of 3.2 kHz. Custom PCBs interface with a PC to control the measurement process, electrode addressing and triggering of external stimuli. The performance of the system was characterised using resistor phantoms to represent human scalp recordings, with an SNR of 77.5 dB, stable across a four hour recording and 20 Hz to 20 kHz. In studies of both haeomorrhage using scalp electrodes, and evoked activity using epicortical electrode mats in rats, it was possible to reconstruct images matching established literature at known areas of onset. Data collected using scalp electrode in humans matched known tissue impedance spectra and was stable over frequency. The experimental procedure is software controlled and is readily adaptable to new paradigms. Where possible, commercial or open-source components were used, to minimise the complexity in reproduction. The hardware designs and software for the system have been released under an open source licence, encouraging contributions and allowing for rapid replication.
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Affiliation(s)
- James Avery
- Department Medical Physics and Biomedical Engineering, University College London, London WC1E 6BT, UK.
| | - Thomas Dowrick
- Department Medical Physics and Biomedical Engineering, University College London, London WC1E 6BT, UK.
| | - Mayo Faulkner
- Department Medical Physics and Biomedical Engineering, University College London, London WC1E 6BT, UK.
| | - Nir Goren
- Department Medical Physics and Biomedical Engineering, University College London, London WC1E 6BT, UK.
| | - David Holder
- Department Medical Physics and Biomedical Engineering, University College London, London WC1E 6BT, UK.
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