Electrode boundary conditions and experimental validation for BEM-based EIT forward and inverse solutions

In this paper, we present theoretical developments and experimental results for the problem of estimating the conductivity map inside a volume using electrical impedance tomography (EIT) when the boundary locations of any internal inhomogeneities are known. We describe boundary element method (BEM) implementations of advanced electrode models for the forward problem of EIT. We then use them in the inverse problem with known internal boundaries and derive the associated Jacobians. We report on the results of two EIT phantom studies, one using a homogeneous cubical tank, and one using a cylindrical tank with agar conductivity inhomogeneities. We test both the accuracy of our BEM forward model, including the electrode models, as well as our inverse solution, against the measured data. Results show good agreement between measured values and both forward-computed tank voltages and inverse-computed conductivities; for instance, in a phantom experiment, we reconstructed the conductivities of three agar objects inside a cylindrical tank with an error less than 2% of their true value.     

Including anharmonicity in the calculation of rate constants. 1. The HCN/HNC isomerization reaction

A method for calculating anharmonic vibrational energy levels in asymmetric top and linear systems that is based on second-order perturbation theory in curvilinear coordinates is extended to the bound generalized normal modes at nonstationary points along a reaction path. Explicit formulas for the anharmonicity coefficients, x(ij), and the constant term, E0, are presented, and the necessary modifications for resonance cases are considered. The method is combined with variational transition state theory with semiclassical multidimensional tunneling approximations to calculate thermal rate constants for the HCN/HNC isomerization reaction. Although the results for this system are not very sensitive to the choice of coordinates, we find that the inclusion of anharmonicity leads to a substantial improvement in the vibrational energy levels. We also present detailed comparisons of rate constants computed with and without anharmonicity, with various approximations for incorporating tunneling along the reaction path, and with a more practical approach to calculating the vibrational partition functions needed for larger systems.     

An Ugi Reaction in the Total Synthesis of (−)‐Dysibetaine

(?)‐Dysibetaine has been synthesized in 11 steps from readily available L ‐malic acid (see scheme). The key step is a unique Ugi 4‐center‐3‐component cyclization reaction, where an ester group acts as the carboxylic acid component. The use of 1,1,1,3,3,3‐hexamethyldisilazane as an ammonia equivalent and a specially designed isocyanide leads to an expeditious synthesis.

     

EPR and ENDOR Investigation of Rhodosemiquinone in Bacterial Reaction Centers Formed by B-Branch Electron Transfer

In photosynthetic bacteria, light-induced electron transfer takes place in a protein called the reaction center (RC) leading to the reduction of a bound ubiquinone molecule, Q_B, coupled with proton binding from solution. We used electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) to study the magnetic properties of the protonated semiquinone, an intermediate proposed to play a role in proton coupled electron transfer to Q_B. To stabilize the protonated semiquinone state, we used a ubiquinone derivative, rhodoquinone, which as a semiquinone is more easily protonated than ubisemiquinone. To reduce this low-potential quinone we used mutant RCs modified to directly reduce the quinone in the Q_B site via B-branch electron transfer (Paddock et al. in Biochemistry 44:6920–6928, 2005). EPR and ENDOR signals were observed upon illumination of mutant RCs in the presence of rhodoquinone. The EPR signals had g values characteristic of rhodosemiquinone (g _x  = 2.0057, g _y  = 2.0048, g _z  ~ 2.0018) at pH 9.5 and were changed at pH 4.5. The ENDOR spectrum showed couplings due to solvent exchangeable protons typical of hydrogen bonds similar to, but different from, those found for ubisemiquinone. This approach should be useful in future magnetic resonance studies of the protonated semiquinone.     

Reconstructions of chest phantoms by the D-bar method for electrical impedance tomography

The problem this paper addresses is how to use the two-dimensional D-bar method for electrical impedance tomography with experimental data collected on finitely many electrodes covering a portion of the boundary of a body. This requires an approximation of the Dirichlet-to-Neumann, or voltage-to-current density map, defined on the entire boundary of the region, from a finite number of matrix elements of the current-to-voltage map. Reconstructions from experimental data collected on a saline filled tank containing agar heart and lung phantoms are presented, and the results are compared to reconstructions by the NOSER algorithm on the same data.     

Disentangling the effects of phonation and articulation: Hemispheric asymmetries in the auditory N1m response of the human brain

Background  

The cortical activity underlying the perception of vowel identity has typically been addressed by manipulating the first and second formant frequency (F1 & F2) of the speech stimuli. These two values, originating from articulation, are already sufficient for the phonetic characterization of vowel category. In the present study, we investigated how the spectral cues caused by articulation are reflected in cortical speech processing when combined with phonation, the other major part of speech production manifested as the fundamental frequency (F0) and its harmonic integer multiples. To study the combined effects of articulation and phonation we presented vowels with either high (/a/) or low (/u/) formant frequencies which were driven by three different types of excitation: a natural periodic pulseform reflecting the vibration of the vocal folds, an aperiodic noise excitation, or a tonal waveform. The auditory N1m response was recorded with whole-head magnetoencephalography (MEG) from ten human subjects in order to resolve whether brain events reflecting articulation and phonation are specific to the left or right hemisphere of the human brain.     

A direct reconstruction algorithm for electrical impedance tomography

A direct (noniterative) reconstruction algorithm for electrical impedance tomography in the two-dimensional (2-D), cross-sectional geometry is reviewed. New results of a reconstruction of a numerically simulated phantom chest are presented. The algorithm is based on the mathematical uniqueness proof by A. I. Nachman [1996] for the 2-D inverse conductivity problem. In this geometry, several of the clinical applications include monitoring heart and lung function, diagnosis of pulmonary embolus, diagnosis of pulmonary edema, monitoring for internal bleeding, and the early detection of breast cancer.     

Rapid assessment of electrode characteristics for impedance imaging

Electrical impedance imaging is the technique for producing images of the resistivity of internal body structures based on measurements of voltage and current from electrodes applied to the body's surface. When a multiplicity of electrodes are applied in one or more rows around a body structure such as the thorax or limb, it is useful to be able to rapidly assess the general status of the electrode-body interface to determine if the skin has been suitably prepared, and that electrode and skin impedance are suitably low. In addition, assessment of the impedance of individual electrodes should precede acquisition of data for image formation. This communication presents techniques for assessing the overall skin and electrode impedances relative to the impedance of the body interior, and for assessing the integrity of each electrode's contact impedance.     

Electrode models for electric current computed tomography

This paper develops a mathematical model for the physical properties of electrodes suitable for use in electric current computed tomography (ECCT). The model includes the effects of discretization, shunt, and contact impedance. The complete model was validated by experiment. Bath resistivities of 284.0, 139.7, 62.3, 29.5 omega.cm were studied. Values of "effective" contact impedance zeta used in the numerical approximations were 58.0, 35.0, 15.0, and 7.5 omega.cm2, respectively. Agreement between the calculated and experimentally measured values was excellent throughout the range of bath conductivities studied. It is desirable in electrical impedance imaging systems to model the observed voltages to the same precision as they are measured in order to be able to make the highest resolution reconstructions of the internal conductivity that the measurement precision allows. The complete electrode model, which includes the effects of discretization of the current pattern, the shunt effect due to the highly conductive electrode material, and the effect of an "effective" contact impedance, allows calculation of the voltages due to any current pattern applied to a homogeneous resistivity field.