Biomedical subjects

Andy Adler

Publications and source records attributed to Andy Adler.

6 recordsLinked to original sources

PubMed2006

Uses and abuses of EIDORS: an extensible software base for EIT.

EIDORS is an open source software suite for image reconstruction in electrical impedance tomography and diffuse optical tomography, designed to facilitate collaboration, testing and new research in these fields. This paper describes recent work to redesign the software structure in order to simplify its use and provide a uniform interface, permitting easier modification and customization. We describe the key features of this software, followed by examples of its use. One general issue with inverse problem software is the difficulty of correctly implementing algorithms and the consequent ease with which subtle numerical bugs can be inadvertently introduced. EIDORS helps with this issue, by allowing sharing and reuse of well-documented and debugged software. On the other hand, since EIDORS is designed to facilitate use by non-specialists, its use may inadvertently result in such numerical errors. In order to address this issue, we develop a list of ways in which such errors with inverse problems (which we refer to as 'cheats') may occur. Our hope is that such an overview may assist authors of software to avoid such implementation issues.

Algorithms↗

PubMed2006

Imaging of conductivity changes and electrode movement in EIT.

Electrical impedance tomography (EIT) attempts to reconstruct the internal impedance distribution in a medium from electrical measurements at electrodes on the medium surface. One key difficulty with EIT measurements is due to the position uncertainty of the electrodes, especially for medical applications, in which the body surface moves during breathing and posture change. In this paper, we develop a new approach which directly reconstructs both electrode movements and internal conductivity changes for difference EIT. The reconstruction problem is formulated in terms of a regularized inverse, using an augmented Jacobian, sensitive to impedance change and electrode movement. A reconstruction prior term is computed to impose a smoothness constraint on both the spatial distribution of impedance change and electrode movement. A one-step regularized imaging algorithm is then implemented based on the augmented Jacobian and smoothness constraint. Images were reconstructed using the algorithm of this paper with data from simulated 2D and 3D conductivity changes and electrode movements, and from saline phantom measurements. Results showed good reconstruction of the actual electrode movements, as well as a dramatic reduction in image artefacts compared to images from the standard algorithm, which did not account for electrode movement.

Algorithms↗

PubMed2005

Automatic detection of detached and erroneous electrodes in electrical impedance tomography.

One unfortunate occurrence in experimental measurements with electrical impedance tomography is electrodes which become detached or poorly connected, such that the measured data cannot be used. This paper presents an automatic approach to detect such erroneous electrodes. It is based on the assumption that all valid measurements are related by the image reconstruction model, while the measurements from erroneous electrodes are unrelated. The method estimates the data at an electrode based on the measurements from all other electrodes, and compares it to the measurements. If these data match adequately, the set of electrodes does not contain an erroneous electrode. In order to detect an erroneous electrode amongst N electrodes, all sets of N-1 electrodes are tested, and the set with the best match between measurements and estimate is identified as the one which excludes the erroneous electrode. The method was tested on simulated and experimental data and showed consistent identification of erroneous electrodes with those made by experts.

Algorithms↗

PubMed2004

Unrestrained plethysmography is an unreliable measure of airway responsiveness in BALB/c and C57BL/6 mice.

There has been significant utilization of the technique described by Hamelmann et al. (Am J Respir Crit Care Med 156: 766-775, 1997) in which a parameter, enhanced pause (Penh), related to airways responsiveness is noninvasively measured by unrestrained plethysmography (UP). Investigating this technique, we sought to answer these questions: 1). How do changes in Penh compare with changes in traditional plethysmographic and lung mechanical parameters? 2). How do UP parameters perform in two different mouse strains? Awake immunized and control BALB/c (n = 16) and C57BL/6 (n = 14) mice were placed in the UP chamber and exposed to doses of aerosolized methacholine while the following parameters were measured at each concentration: inspiratory time (Ti), expiratory time (Te), total time (Ttot), Ti/Ttot, peak inspiratory pressure, peak expiratory pressure, Pause, Penh, tidal volume (Vt), Vt/Ti, Vt/Te, and Vt/Ttot. The next day, lung resistance (Rl) and compliance (Cl) were invasively measured in the same animals. For the BALB/c, the parameters with the highest magnitude of correlation coefficient vs. Rl are (in order) 1). Cl, 2). Pause and Penh, 3). parameters of breathing frequency (Te, Ttot, Ti), and 4). parameters related to Vt (inspiratory pressure, expiratory pressure). Flow parameters (Vt/Ttot, Vt/Te, Vt/Ti) and duty cycle parameters (Ti/Ttot) had insignificant correlations. This ordering is significantly different in C57BL/6 mice, in which the parameters with the largest correlations are 1). Cl, 2). parameters of breathing frequency, and 3). flow parameters. Pause, Penh, Vt, and duty cycle parameters had insignificant correlations. These data show that Penh is problematic in the sense that it is strain specific; it behaves very differently in BALB/c and C57BL/6 mice. We suggest that UP parameters largely originate as part of reflex control of breathing processes, rather than in the lung mechanics and conclude that it is inappropriate to use UP parameters in general, and Penh specifically, as substitute variables for invasive mechanical indexes such as Rl.

Airway Resistance↗

PubMed2004

Accounting for erroneous electrode data in electrical impedance tomography.

An unfortunate occurrence in experimental measurements with electrical impedance tomography is electrodes which become detached or poorly connected, such that the measured data cannot be used. This paper develops an image reconstruction methodology which allows use of the remaining valid data. A finite element model of the EIT difference imaging forward problem is linearized as z = Hx, where z represents the change in measurements and x the element log conductivity changes. Image reconstruction is represented in terms of a maximum a posteriori (MAP) estimate as x = inv(Htinv(Rn) + inv(Rx))Htinv(Rn)z, where Rx and Rn represent the a priori estimates of image and measurement noise crosscorrelations, respectively. Using this formulation, missing electrode data can be naturally modelled as infinite noise on all measurements using the affected electrodes. Simulations indicate position error and resolution are close (+/- 10%) to the values calculated without missing electrode data as long as the target was further than 10% of the medium diameter from the affected electrode. Applications of this technique to experimental data show good results in terms of removing artefacts from images.

Artifacts↗

PubMed2002

A reevaluation of the validity of unrestrained plethysmography in mice.

Presently, unrestrained plethysmography is widely used to assess bronchial responsiveness in mice. An empirical quantity known as enhanced pause is derived from the plethysmographic box pressure [P(b)(t), where t is time] and assumed to be an index of bronchoconstriction. We show that P(b)(t) is determined largely by gas conditioning when normal mice breathe spontaneously inside a closed chamber in which the air is at ambient conditions. When the air in the chamber is heated and humidified to body conditions, the changes in P(b)(t) are reduced by about two-thirds. The remaining changes are thus due to gas compression and expansion within the lung and are amplified when the animals breathe through increased resistances. We show that the time integral of P(b)(t) over inspiration is accurately predicted by a term containing airway resistance, functional residual capacity, and tidal volume. We conclude that unrestrained plethysmography can be used to accurately characterize changes in airway resistance only if functional residual capacity and tidal volume are measured independently and the chamber gas is preconditioned to body temperature and humidity.

Airway Resistance↗