Effect of skin impedance on image quality and variability in electrical impedance tomography: a model study
TL;DR: It is concluded that the patient's skin should be abraded to reduce impedance, and measurements should be avoided in the first 10 min after electrode placement, to allow satisfactory images.
Abstract: A computer simulation is used to investigate the relationship between skin impedance and image artefacts in electrical impedance tomography. Sets of electrode impedance are generated with a pseudo-random distribution and used to introduce errors in boundary voltage measurements. To simplify the analysis, the non-idealities in the current injection circuit are replaced by a fixed common-mode error term. The boundary voltages are reconstructed into images and inspected. Where the simulated skin impedance remains constant between measurements, large impedances (> 2k omega) do not cause significant degradation of the image. Where the skin impedances 'drift' between measurements, a drift of 5% from a starting impedance of 100 omega is sufficient to cause significant image distortion. If the skin impedances vary randomly between measurements, they have to be less than 10 omega to allow satisfactory images. Skin impedances are typically 100-200 omega at 50 kHz on unprepared skin. These values are sufficient to cause image distortion if they drift over time. It is concluded that the patient's skin should be abraded to reduce impedance, and measurements should be avoided in the first 10 min after electrode placement.
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TL;DR: This paper reviews ultrasound segmentation methods, in a broad sense, focusing on techniques developed for medical B-mode ultrasound images, and presents a classification of methodology in terms of use of prior information.
Abstract: This paper reviews ultrasound segmentation methods, in a broad sense, focusing on techniques developed for medical B-mode ultrasound images. First, we present a review of articles by clinical application to highlight the approaches that have been investigated and degree of validation that has been done in different clinical domains. Then, we present a classification of methodology in terms of use of prior information. We conclude by selecting ten papers which have presented original ideas that have demonstrated particular clinical usefulness or potential specific to the ultrasound segmentation problem
1,150 citations
TL;DR: Results show that CAP can efficiently separate several steatosis grades and suggest that CAP is a noninvasive, immediate, objective and efficient method to detect and quantify Steatosis.
Abstract: There is a need for noninvasive methods to detect liver steatosis, which can be a factor of liver fibrosis progression. This work aims to evaluate a novel ultrasonic controlled attenuation parameter (CAP) devised to target, specifically, liver steatosis using a sophisticated process based on vibration control transient elastography (VCTE™). CAP was first validated as an estimate of ultrasonic attenuation at 3.5 MHz using Field II simulations and tissue-mimicking phantoms. Performance of the CAP was then appraised on 115 patients, taking the histological grade of steatosis as reference. CAP was significantly correlated to steatosis (Spearman ρ = 0.81, p < 10(-16)). Area under receiver operative characteristic (ROC) curve (AUC) was equal to 0.91 and 0.95 for the detection of more than 10% and 33% of steatosis, respectively. Furthermore, results show that CAP can efficiently separate several steatosis grades. These promising results suggest that CAP is a noninvasive, immediate, objective and efficient method to detect and quantify steatosis.
679 citations
TL;DR: Results on real images demonstrate that the proposed adaptation of the nonlocal (NL)-means filter for speckle reduction in ultrasound (US) images is able to preserve accurately edges and structural details of the image.
Abstract: In image processing, restoration is expected to improve the qualitative inspection of the image and the performance of quantitative image analysis techniques. In this paper, an adaptation of the nonlocal (NL)-means filter is proposed for speckle reduction in ultrasound (US) images. Originally developed for additive white Gaussian noise, we propose to use a Bayesian framework to derive a NL-means filter adapted to a relevant ultrasound noise model. Quantitative results on synthetic data show the performances of the proposed method compared to well-established and state-of-the-art methods. Results on real images demonstrate that the proposed method is able to preserve accurately edges and structural details of the image.
547 citations
Cites methods from "Effect of skin impedance on image q..."
...This framework is based on Field II simulation [ 53 ]....
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TL;DR: The minimum variance (MV) adaptive beamformer is applied to medical ultrasound imaging and shown significant improvement in image quality compared to delay-and-sum (DAS).
Abstract: We have applied the minimum variance (MV) adaptive beamformer to medical ultrasound imaging and shown significant improvement in image quality compared to delay-and-sum (DAS). We demonstrate reduced main-lobe width and suppression of sidelobes on both simulated and experimental RF data of closely spaced wire targets, which gives potential contrast and resolution enhancement in medical images. The method is applied to experimental RF data from a heart phantom, in which we show increased resolution and improved definition of the ventricular walls. A potential weakness of adaptive beamformers is sensitivity to errors in the assumed wavefield parameters. We look at two ways to increase robustness of the proposed method; spatial smoothing and diagonal loading. We show that both are controlled by a single parameter that can move the performance from that of a MV beamformer to that of a DAS beamformer. We evaluate the sensitivity to velocity errors and show that reliable amplitude estimates are achieved while the mainlobe width and sidelobe levels are still significantly lower than for the conventional beam-former.
535 citations
Cites methods from "Effect of skin impedance on image q..."
...We simulated an 18.5 mm, 96 element, 4 MHz transducer using Field II [ 14 ], imaging a number of pairwise reflectors located at depths 30–80 mm. The reflectors were separated by 2 mm. Transmit focus was 60 mm. We added white, Gaussian noise to each receiver channel before beamforming....
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TL;DR: This paper, the first from a series of three papers on the application of coded excitation signals in medical ultrasound, discusses the basic principles and ultrasound-related problems of pulse compression and the selection of coded waveforms suitable for ultrasound imaging.
Abstract: This paper, the first from a series of three papers on the application of coded excitation signals in medical ultrasound, discusses the basic principles and ultrasound-related problems of pulse compression. The concepts of signal modulation and matched filtering are given, and a simple model of attenuation relates the matched filter response with the ambiguity function, known from radar. Based on this analysis and the properties of the ambiguity function, the selection of coded waveforms suitable for ultrasound imaging is discussed. It is shown that linear frequency modulation (FM) signals have the best and most robust features for ultrasound imaging. Other coded signals such as nonlinear FM and binary complementary Golay codes also have been considered and characterized in terms of signal-to-noise ratio (SNR) and sensitivity to frequency shifts. Using the simulation program Field II, it is found that in the case of linear FM signals, a SNR improvement of 12 to 18 dB can be expected for large imaging depths in attenuating media, without any depth-dependent filter compensation. In contrast, nonlinear FM modulation and binary codes are shown to give a SNR improvement of only 4 to 9 dB when processed with a matched filter. Other issues, such as depth-dependent matched filtering and use of filters other than the matched filter (inverse and Wiener filters) also are addressed.
406 citations
Cites methods from "Effect of skin impedance on image q..."
...Fig. 1 shows simulation results of coded phased-array imaging using the simulation program Field II [ 27 ], [28]....
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...These results are demonstrated in this paper through computer simulations using Field II [ 27 ]....
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...This is done in this section with the simulating program Field II [ 27 ]....
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References
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TL;DR: Any improvement in spatial resolution can only be made at the expense of speed and sensitivity which may well be the more important factors in determining the clinical utility of APT.
Abstract: Because of the intrinsically low sensitivity of any surface potential measurement to resistivity changes within a volume conductor, any data collection system for impedance imaging must be sensitive to changes in the peripheral potential profile of the order of 0.1%. For example, whilst the resistivity changes associated with lung ventilation and the movement of blood during the cardiac cycle range from 3 to 100% the changes recorded at the surface are very much less than this. The Sheffield data collection system uses 16 electrodes which are addressed through 4 multiplexers. Overall system accuracy is largely determined by the front-end equivalent circuit which is considered in some detail. This equivalent circuit must take into account wiring and multiplexer capacitances. A current drive of 5 mA p-p at 5 kHz is multiplexed to adjacent pairs of electrodes and peripheral potential profiles are recorded by serially stepping around adjacent electrode pairs. The existing Sheffield system collects the 208 data points for one image in 79 ms and offers 10 image data sets per second to the microprocessor. For a homogeneous circular conductor the ratio of the maximum to minimum signals within each peripheral potential profile is 45:1. The temptation to increase the number of electrodes in order to improve resolution is great and an achievable performance for 128 electrodes is given. However, any improvement in spatial resolution can only be made at the expense of speed and sensitivity which may well be the more important factors in determining the clinical utility of APT.
392 citations
"Effect of skin impedance on image q..." refers methods in this paper
...This study only considers the imaging of resistivity differences, using linear back-projection reconstruction (BARBER and SEAGAR, 1987)....
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Book•
01 Jan 1972
329 citations
"Effect of skin impedance on image q..." refers background in this paper
...The latter two assumptions are derived from measurements of the electrode-saline interface (GEDDES, 1972)....
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TL;DR: A sensitivity study showed that the reconstructed resistivity distributions are less dependent on the unknown contact resistance values for a compound electrode than a conventional electrode and that the use of a compound electrodes results in improved images for the reconstruction algorithm.
Abstract: A compound electrode composed of a large outer electrode to inject current and a small inner electrode to sense voltage was developed and used to measure voltages from a physical phantom. The measured voltages were smaller in amplitude than those from conventional electrodes, demonstrating that the compound electrode can minimize contact impedance voltage drop from the measured data. A finite-element model was used for the compound electrode and incorporated into the regularized Newton-Raphson reconstruction algorithm. A sensitivity study showed that the reconstructed resistivity distributions are less dependent on the unknown contact resistance values for a compound electrode than a conventional electrode and that the use of a compound electrode results in improved images for the reconstruction algorithm. >
84 citations
TL;DR: To validate the method of data analysis, a theoretical model was developed, with the limiting case being the turn-on transient for a membrane covered spherical cathode, and the results agree with previously published data.
Abstract: To validate the method of data analysis, a theoretical model was developed, with the limiting case being the turn-on transient for a membrane covered spherical cathode. To better understand the performance of the shallowly recessed electrode used in the retinal measurement, it was investigated by computer simulation using the conformal mapping technique. The simulated transients for the electrode with various depth and recess materials were fitted to the previously chosen semiempirical equation to determine the relationship between the parameter tc, the characteristic time in the semiempirical equation, and D. To validate the experimental method, transients were measured in various glycerine solutions. The results agree with previously published data. Spatial variation of D in the cat retina was measured under normoxic darkand light-adapted conditions and hyperoxic dark-adapted conditions and in the cornea under normoxic conditions. The average D value in vitreous humor was found to be 1.99 _+ 0.30.10 -5 cm2/s, which was 71.8 +_ 10.9~ of that in isotonic saline at 37~ The local D in the cat retina was practically homogeneous with an average value of 1.97 ___ 0.11.10 -s cm2/s, 70.6 + 3.3~ of that in isotonic saline at 37~ In the cat corneal stroma at 33~ D was 1.19 + 0.2.10 -5 cm2/s, 42.8 _+ 7.3~ of that in saline at 37~
12 citations
01 Jan 1995
TL;DR: In this article, the mesures peuvent etre affectees d'artefacts dus au bruit et aux derives are discussed, e.g., les variations de l'impedance du fluide and celles des impedances d'electrode sont peu importantes.
Abstract: En Tomographie d'Impedance Electrique (TIE) les mesures peuvent etre affectees d'artefacts dus au bruit et aux derives. Parmi les sources possibles d'artefacts ont peut citer l'instrumentation, les impedances de contact et les variations de l'impedance de la peau. Nous avons mesure le bruit et la derive avec le systeme de Sheffield Mark I sur un reseau de resistances, une cuve de solution saline et sur la peau humaine. Le bruit et la derive mesures sur la cuve et le reseau de resistances etaient similaires, ce qui suggere que les variations de l'impedance du fluide et celles des impedances d'electrode sont peu importantes. Sur la peau humaine, le bruit et la derive etaient environ 10 fois superieurs, et, en general, bien plus eleves sur la peau non preparee. Nous estimons que la contribution de l'instrumentation au bruit et a la derive est d'environ 10% et celle de l'impedance de la peau d'environ 65%. Les autres 25% proviennent probablement des variations de l'impedance du tissu lui-meme. C'est pourquoi la preparation de la peau apparait importante pour des mesures a faible niveau de bruit. Cependant, l'abrasion de la peau sans mesure d'impedance n'a pas garanti une faible impedance de peau a 50kHz. L'abrasion de la peau doit donc etre accompagnee d'une mesure d'impedance a la frequence de mesure.
9 citations