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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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Ouypornkochagorn T, Terzija N, Wright P, Davidson JL, Polydorides N, McCann H. Scalp-Mounted Electrical Impedance Tomography of Cerebral Hemodynamics. IEEE SENSORS JOURNAL 2022; 22:4569-4580. [PMID: 35673527 PMCID: PMC9093315 DOI: 10.1109/jsen.2022.3145587] [Citation(s) in RCA: 2] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 10/29/2021] [Revised: 01/10/2022] [Accepted: 01/11/2022] [Indexed: 06/15/2023]
Abstract
An Electrical Impedance Tomography (EIT) system has been developed for dynamic three-dimensional imaging of changes in conductivity distribution in the human head, using scalp-mounted electrodes. We attribute these images to changes in cerebral perfusion. At 100 frames per second (fps), voltage measurement is achieved with full-scale signal-to-noise ratio of 105 dB and common-mode rejection ratio > 90 dB. A novel nonlinear method is presented for 3-D imaging of the difference in conductivity distribution in the head, relative to a reference time. The method achieves much reduced modelling error. It successfully localizes conductivity inclusions in experimental and simulation tests, where previous methods fail. For > 50 human volunteers, the rheoencephalography (REG) waveform is observed in EIT voltage measurements for every volunteer, with peak-to-peak amplitudes up to approx. 50 μVrms. Images are presented of the change in conductivity distribution during the REG/cardiac cycle, at 50 fps, showing maximum local conductivity change of approx. 1% in grey/white matter. A total of 17 tests were performed during short (typically 5s) carotid artery occlusions on 5 volunteers, monitored by Transcranial Doppler ultrasound. From EIT measurements averaged over complete REG/cardiac cycles, 13 occlusion tests showed consistently decreased conductivity of cerebral regions on the occluded side, and increased conductivity on the opposite side. The maximum local conductivity change during occlusion was approx. 20%. The simplicity of the carotid artery intervention provides a striking validation of the scalp-mounted measurement system in imaging cerebral hemodynamics, and the REG images indicate its unique combination of sensitivity and temporal resolution.
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Affiliation(s)
| | | | - Paul Wright
- Department of Electrical and Electronic EngineeringThe University of ManchesterManchesterM13 9PLU.K.
| | - John L. Davidson
- Department of Electrical and Electronic EngineeringThe University of ManchesterManchesterM13 9PLU.K.
| | - Nick Polydorides
- School of EngineeringThe University of EdinburghEdinburghEH9 3JLU.K.
| | - Hugh McCann
- School of EngineeringThe University of EdinburghEdinburghEH9 3JLU.K.
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Hahn G, Niewenhuys J, Just A, Tonetti T, Behnemann T, Rapetti F, Collino F, Vasques F, Maiolo G, Romitti F, Gattinoni L, Quintel M, Moerer O. Monitoring lung impedance changes during long-term ventilator-induced lung injury ventilation using electrical impedance tomography. Physiol Meas 2020; 41:095011. [PMID: 33035199 DOI: 10.1088/1361-6579/abb1fb] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
Abstract
OBJECTIVE The target of this methodological evaluation was the feasibility of long-term monitoring of changes in lung conditions by time-difference electrical impedance tomography (tdEIT). In contrast to ventilation monitoring by tdEIT, the monitoring of end-expiratory (EELIC) or end-inspiratory (EILIC) lung impedance change always requires a reference measurement. APPROACH To determine the stability of the used Pulmovista 500® EIT system, as a prerequisite it was initially secured on a resistive phantom for 50 h. By comparing the slopes of EELIC for the whole lung area up to 48 h from 36 pigs ventilated at six positive end-expiratory pressure (PEEP) levels from 0 to 18 cmH2O we found a good agreement (range of r 2 = 0.93-1.0) between absolute EIT (aEIT) and tdEIT values. This justified the usage of tdEIT with its superior local resolution compared to aEIT for long-term determination of EELIC. MAIN RESULTS The EELIC was between -0.07 Ωm day-1 at PEEP 4 and -1.04 Ωm day-1 at PEEP 18 cmH2O. The complex local time pattern for EELIC was roughly quantified by the new parameter, centre of end-expiratory change (CoEEC), in equivalence to the established centre of ventilation (CoV). The ventrally located mean of the CoV was fairly constant in the range of 42%-46% of thorax diameter; however, on the contrary, the CoEEC shifted from about 40% to about 75% in the dorsal direction for PEEP levels of 14 and 18 cmH2O. SIGNIFICANCE The observed shifts started earlier for higher PEEP levels. Changes of EELI could be precisely monitored over a period of 48 h by tdEIT on pigs.
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Affiliation(s)
- G Hahn
- Department of Anaesthesiology, University Medical Center Göttingen (UMG), Robert-Koch-Str. 40, D-37075, Göttingen, Germany
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Ouypornkochagorn T, Ouypornkochagorn S. In Vivo Estimation of Head Tissue Conductivities Using Bound Constrained Optimization. Ann Biomed Eng 2019; 47:1575-1583. [PMID: 30927169 DOI: 10.1007/s10439-019-02254-9] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/22/2019] [Accepted: 03/23/2019] [Indexed: 11/28/2022]
Abstract
The conductivity of head tissues was noninvasively estimated using electrical impedance tomography technique. Instead of using conventional unconstrained optimization method to estimate the conductivities, a constrained method with the scaled-logistic function was employed to improve the very high sensitivity of the skull region resulting in accuracy and robustness improvement. Estimation of five conductivities i.e. scalp, skull, cerebrospinal fluid (CSF), grey matter (GM), and white matter (WM) conductivity was investigated by simulation on random and low-value initial guesses. Simulation results showed that the performance of the unconstrained method depended directly to the difference between the exact skull conductivity value and the initial guess value of the skull conductivity. However, the approached constrained method was independent of the guess selection. It can reduce the sensitivity of the skull region by 126 times and reduce the condition number of the sensitivity matrix by 13-17 times. The estimation resulted in only positive and in-range of reported conductivity values. The estimation error of the skull conductivity decreased by 15% and the robustness increased by 2 times. However, the estimation of the CSF, the WM, and the GM may be not reliable due to the very low sensitivity of these regions in both methods.
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Abstract
The presence of multiple or diffuse lesions on imaging is a contraindication to surgery for patients with intractable epilepsy. Theoretically, as a functional imaging technique, electrical impedance tomography (EIT) can accurately image epileptic foci. However, most current studies are limited to examining epileptic spikes and few studies use EIT for real-time imaging of seizure activity. Moreover, little is known about changes in electrical impedance during seizures. In this study, we used EIT to monitor seizure progression in real time and analyzed changes in electrical impedance during seizures. EIT and electroencephalography data were recorded simultaneously in rats. Sixty-three seizures were recorded from the cortices of eight rats. During 54 seizures, the average impedance decreased by between 4.86 and 9.17% compared with the baseline. Compared with the control group, the average impedance of the experimental group decreased significantly (P=0.004). Our results indicate that EIT can be used to detect and image electrical impedance reduction within lesions during epileptic seizures.
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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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Hersek S, Toreyin H, Teague CN, Millard-Stafford ML, Jeong HK, Bavare MM, Wolkoff P, Sawka MN, Inan OT. Wearable Vector Electrical Bioimpedance System to Assess Knee Joint Health. IEEE Trans Biomed Eng 2016; 64:2353-2360. [PMID: 28026745 DOI: 10.1109/tbme.2016.2641958] [Citation(s) in RCA: 44] [Impact Index Per Article: 5.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022]
Abstract
OBJECTIVE We designed and validated a portable electrical bioimpedance (EBI) system to quantify knee joint health. METHODS Five separate experiments were performed to demonstrate the: 1) ability of the EBI system to assess knee injury and recovery; 2) interday variability of knee EBI measurements; 3) sensitivity of the system to small changes in interstitial fluid volume; 4) reducing the error of EBI measurements using acceleration signals; and 5) use of the system with dry electrodes integrated to a wearable knee wrap. RESULTS 1) The absolute difference in resistance ( R) and reactance (X) from the left to the right knee was able to distinguish injured and healthy knees (p < 0.05); the absolute difference in R decreased significantly (p < 0.05) in injured subjects following rehabilitation. 2) The average interday variability (standard deviation) of the absolute difference in knee R was 2.5 Ω and for X was 1.2 Ω. 3) Local heating/cooling resulted in a significant decrease/increase in knee R (p < 0.01). 4) The proposed subject position detection algorithm achieved 97.4% leave-one subject out cross-validated accuracy and 98.2% precision in detecting when the subject is in the correct position to take measurements. 5) Linear regression between the knee R and X measured using the wet electrodes and the designed wearable knee wrap were highly correlated ( R2 = 0.8 and 0.9, respectively). CONCLUSION This study demonstrates the use of wearable EBI measurements in monitoring knee joint health. SIGNIFICANCE The proposed wearable system has the potential for assessing knee joint health outside the clinic/lab and help guide rehabilitation.
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Mahara A, Khan S, Murphy EK, Schned AR, Hyams ES, Halter RJ. 3D Microendoscopic Electrical Impedance Tomography for Margin Assessment During Robot-Assisted Laparoscopic Prostatectomy. IEEE TRANSACTIONS ON MEDICAL IMAGING 2015; 34:1590-1601. [PMID: 25730825 DOI: 10.1109/tmi.2015.2407833] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/04/2023]
Abstract
Radially configured microendoscopic electrical impedance probes intended for intraoperative surgical margin assessment during robot-assisted laparoscopic prostatectomy (RALP) were examined through simulation, bench-top experimentation, and ex vivo tissue studies. Three probe designs with 8, 9, and 17 electrodes, respectively, were analyzed through finite element method based simulations. One mm diameter spherical inclusions ( σinclusion = 1 S/m) are positioned at various locations within a hemispherical background ( σbackground = 0.1 S/m) of radius 5 mm. An 8-electrode configuration is not able to localize the inclusion at these positions while 9 and 17-electrode configurations are able to accurately reconstruct the inclusion at maximum depth of 1 mm and 3 mm, respectively. All three probe designs were constructed and evaluated using saline phantoms and ex vivo porcine and human prostate tissues. The 17-electrode probe performed best in saline phantom studies, accurately reconstructing high contrast, 1-mm-diameter metal cylindrical inclusions in a saline bath ( σsaline = 0.1 S/m) with a position and area error of 0.46 mm and 0.84 mm2, respectively. Additionally, the 17-electrode probe was able to adequately distinguish cancerous from benign tissues in three ex vivo human prostates. Simulations, bench-top saline experiments, and ex vivo tissue sampling suggest that for intraoperative surgical margin assessment during RALP, the 17-electrode probe (as compared to an 8 and 9 electrode probe) will be necessary to provide sufficient accuracy and sensitivity.
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McCann H, Ahsan ST, Davidson JL, Robinson RL, Wright P, Pomfrett CJD. A portable instrument for high-speed brain function imaging: FEITER. ANNUAL INTERNATIONAL CONFERENCE OF THE IEEE ENGINEERING IN MEDICINE AND BIOLOGY SOCIETY. IEEE ENGINEERING IN MEDICINE AND BIOLOGY SOCIETY. ANNUAL INTERNATIONAL CONFERENCE 2012; 2011:7029-32. [PMID: 22255957 DOI: 10.1109/iembs.2011.6091777] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
Abstract
Electrical Impedance Tomography (EIT) can resolve dynamic physiological information deep within human subjects [1], but its sensitivity is challenged in the case of imaging the head [2]. Here, we report a new system called fEITER that has been designed and built to enable functional imaging of the human brain using EIT via scalp-mounted electrodes, integrated with stimulation of evoked responses. Using Field-Programmable Gate Array (FPGA) technology, it provides excellent flexibility in terms of current-pattern excitation and signal processing. The instrument operates at 100 frames/second (fps) with noise of 1 μV on the rms voltage measurements. Clinical trials have been authorized by the UK MHRA and example data from human subjects are presented.
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Affiliation(s)
- Hugh McCann
- School of Electrical and Electronic Engineering, University of Manchester, Manchester, UK.
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Oh TI, Wi H, Kim DY, Yoo PJ, Woo EJ. A fully parallel multi-frequency EIT system with flexible electrode configuration: KHU Mark2. Physiol Meas 2011; 32:835-49. [DOI: 10.1088/0967-3334/32/7/s08] [Citation(s) in RCA: 78] [Impact Index Per Article: 6.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
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Hahn G, Dittmar J, Just A, Quintel M, Hellige G. Different approaches for quantifying ventilation distribution and lung tissue properties by functional EIT. Physiol Meas 2010; 31:S73-84. [DOI: 10.1088/0967-3334/31/8/s06] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
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Fabrizi L, McEwan A, Oh T, Woo EJ, Holder DS. A comparison of two EIT systems suitable for imaging impedance changes in epilepsy. Physiol Meas 2009; 30:S103-20. [PMID: 19491447 DOI: 10.1088/0967-3334/30/6/s07] [Citation(s) in RCA: 23] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
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Fabrizi L, McEwan A, Oh T, Woo EJ, Holder DS. An electrode addressing protocol for imaging brain function with electrical impedance tomography using a 16-channel semi-parallel system. Physiol Meas 2009; 30:S85-101. [PMID: 19491446 DOI: 10.1088/0967-3334/30/6/s06] [Citation(s) in RCA: 23] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
Abstract
Electrical impedance tomography of brain function poses special problems because applied current is diverted by the resistive skull. In the past, image resolution was maximized with the use of an electrode addressing protocol with widely spaced drive electrode pairs and use of a multiplexer so that many electrode pairs could be flexibly addressed. The purpose of this study was to develop and test an electrode protocol for a 16-channel semi-parallel system which uses parallel recording channels with fixed wiring, the Kyung Hee University (KHU) Mk1. Ten protocols were tested, all addressing pairs of electrodes for recording or current drive, based on recording with a spiral, spiral with suboccipital electrodes (spiral s-o) and zig-zag configurations, and combinations of current injection from electrode pairs at 180 degrees , 120 degrees and 60 degrees . These were compared by assessing the image reconstruction quality of five simulated perturbations in a homogenous model of the human head and of four epileptic foci in an anatomically realistic model in the presence of realistic noise, in terms of localization error, resolution, image distortion and sensitivity in the region of interest. The spiral s-o with current injection at 180 degrees + 120 degrees + 60 degrees gave the best image quality and permitted reconstruction with a localization error of less than 10% of the head diameter. This encourages the view that it might be possible to obtain satisfactory images of focal abnormalities in the human brain with 16 scalp electrodes and improved instrumentation avoiding multiplexers on recording circuits.
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Affiliation(s)
- L Fabrizi
- Department of Medical Physics and Bioengineering, Malet Place Engineering Building, Gower Street, University College London, London WC1E 6BT, UK.
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Abascal JFP, Arridge SR, Atkinson D, Horesh R, Fabrizi L, De Lucia M, Horesh L, Bayford RH, Holder DS. Use of anisotropic modelling in electrical impedance tomography; Description of method and preliminary assessment of utility in imaging brain function in the adult human head. Neuroimage 2008; 43:258-68. [DOI: 10.1016/j.neuroimage.2008.07.023] [Citation(s) in RCA: 87] [Impact Index Per Article: 5.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/23/2008] [Revised: 06/26/2008] [Accepted: 07/16/2008] [Indexed: 11/15/2022] Open
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Abascal JFPJ, Arridge SR, Bayford RH, Holder DS. Comparison of methods for optimal choice of the regularization parameter for linear electrical impedance tomography of brain function. Physiol Meas 2008; 29:1319-34. [PMID: 18854604 DOI: 10.1088/0967-3334/29/11/007] [Citation(s) in RCA: 23] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
Abstract
Electrical impedance tomography has the potential to provide a portable non-invasive method for imaging brain function. Clinical data collection has largely been undertaken with time difference data and linear image reconstruction methods. The purpose of this work was to determine the best method for selecting the regularization parameter of the inverse procedure, using the specific application of evoked brain activity in neonatal babies as an exemplar. The solution error norm and image SNR for the L-curve (LC), discrepancy principle (DP), generalized cross validation (GCV) and unbiased predictive risk estimator (UPRE) selection methods were evaluated in simulated data using an anatomically accurate finite element method (FEM) of the neonatal head and impedance changes due to blood flow in the visual cortex recorded in vivo. For simulated data, LC, GCV and UPRE were equally best. In human data in four neonatal infants, no significant differences were found among selection methods. We recommend that GCV or LC be employed for reconstruction of human neonatal images, as UPRE requires an empirical estimate of the noise variance.
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