Dependence of apparent resistance of four-electrode probes on insertion depth

The apparent resistance of a finite-thickness layer measured with a four-electrode plunge probe depends on the electrode insertion depth, electrode spacing, and layer thickness, as well as the resistivity ratio of an underlying layer. A physical model consisting of air, a saline solution layer, and an agar layer simulates the real situation of resistivity measurement. The saline layer represents the finite-thickness layer whose resistivity is to be measured by a plunge electrode probe, and the agar layer represents an underlying perturbing layer. A micropositioner controls the insertion depth of the four electrodes into the saline solution. With the apparent resistance measured on a semi-infinite-thickness layer of saline solution as standard, measurement results show decreasing apparent resistance and increasing error with increasing electrode insertion depth. This information is important for correct measurement of myocardial resistivity in vivo and in vitro.     

In vitro measurement of myocardial impedivity anisotropy with a miniature rectangular tube

Due to rapid change of fiber orientation, it is difficult to measure myocardial impedivity separately in a longitudinal or transverse fiber direction without mutual influence in the two directions. Previously published values of the longitudinal and the transverse myocardial impedivity were derived indirectly from measurements that mixed the impedivity in all directions. Those values are questionable because the derivations were based on a simplified uniform myocardial fiber model. In this paper, a miniature rectangular tube was devised to facilitate direct measurement of myocardial impedivity in a uniform fiber direction. The average transverse-to-longitudinal ratio of the measured in vitro swine myocardial impedivity was about 1.66 from 1 Hz to 1 kHz and dropped to 1.25 at 1 MHz. The result is important for accurate modeling of the electrical property of myocardium in biomedical research of radio-frequency cardiac catheter ablation.     

Some polishing experiments on copper in a hyperbolic cell

Some polishing experiments have been carried out on copper anodes in a hyperbolic cell designed by Gilmont and Walton, using orthophosphoric acid as the electrolyte. The results obtained have been compared to those obtained in similar experiments in a Hull cell. It has been found that very similar bands of different reflectivity and polishes are found to form in both the cells. These bands shift with time and a study of such displacements has been made. The results are briefly discussed.     

Synthesis of Medium Voltage dc-to-dc Converters From Low-Voltage, High-Frequency PWM Switching Converters

Low-voltage dc-to-dc power conversion is a very mature industry which uses high-frequency pulsewidth modulation (PWM) switching techniques. The passive and active components needed to build low-voltage dc-to-dc converters are highly available, affordable and constantly improving. In this paper, a very simple and systematic method of using a large number of low-voltage, high-frequency PWM converters to synthesize highly redundant, medium voltage (4-40 kV) dc-to-dc converters is presented. Theoretical and practical considerations are discussed in necessary detail and test results of an actual 10-kW, 10 kV-to-400 V, converter built from 48 low-voltage forward converters are presented. Real-time and average reduced circuit models are derived to predict the dynamical behavior of the converter and to design the feedback control loop.     

In-vivo measurement of swine myocardial resistivity

We used a four-terminal plunge probe to measure myocardial resistivity in two directions at three sites from the epicardial surface of eight open-chest pigs in-vivo at eight frequencies ranging from 1 Hz to 1 MHz. We calibrated the plunge probe to minimize the error due to stray capacitance between the measured subject and ground. We calibrated the probe in saline solutions contained in a metal cup situated near the heart that had an electrical connection to the pig's heart. The mean of the measured myocardial resistivity was 319 ohm x cm at 1 Hz down to 166 ohm x cm at 1 MHz. Statistical analysis showed the measured myocardial resistivity of two out of eight pigs was significantly different from that of other pigs. The myocardial resistivity measured with the resistivity probe oriented along and across the epicardial fiber direction was significantly different at only one out of the eight frequencies. There was no significant difference in the myocardial resistivity measured at different sites.     

A new catheter design using needle electrode for subendocardial RF ablation of ventricular muscles: finite element analysis and in vitro experiments

Radio-frequency (RF) cardiac ablation has been very successful for treating arrhythmias related with atrioventricular junction and accessory pathways with successful cure rates of more than 90%. Even though ventricular tachycardia (VT) is a more serious problem, it is known to be rather difficult to cure VT using RF ablation. In order to apply RF ablation to VT, we usually need to create a deeper and wider lesion. Conventional RF ablation electrodes often fail to produce such a lesion. We propose a catheter-electrode design including one or more needle electrodes with a diameter of 0.5-1.0 mm and length of 2.0-10 mm to create a lesion large enough to treat VT. One temperature sensor could be placed at the middle of the needle electrode for temperature-controlled RF ablation. From finite element analyses and in vitro experiments, we found that the depth of a lesion is 1-2 mm deeper than the insertion depth of the needle and the width increases as we increase the diameter of the needle and the time duration. We showed that a single needle electrode can produce a lesion with about 10-mm width and any required depth. If a wider lesion is required, more than one needle with suggested structures can be used. Or, repeated RF ablations around a certain area using one needle could produce a cluster of lesions. In some cases, a catheter with both conventional electrode and needle electrode at its tip may be beneficial to take advantage of both types of electrode.     

Multi-loop control for quasi-resonant converters

A multiloop control scheme for quasi-resonant converters (QRCs) is described. Like current-mode control for pulse width modulation (PWM) converters, this control offers excellent transient response and replaces the voltage-controlled oscillator (VCO) with a simple comparator. In this method, referred to as current-sense frequency modulation (CSFM), a signal proportional to the output-inductor current is compared with an error voltage signal to modulate the switching frequency. The control can be applied to either zero-voltage-switched (ZVS) or zero-current-switched (ZCS) QRCs. Computer simulation is method applied to a ZCS buck QRC. A circuit implementation is presented that allows multiloop control to be used on circuits switching up to 10 MHz. This circuit requires few components and produces clean control waveforms. Experimental results are presented for zero-current flyback and zero-voltage buck QRCs, operating at up to 7 MHz. Good small-signal characteristics have been obtained     

A simple scheme for unity power-factor rectification for highfrequency AC buses

A simple scheme is proposed for offline unity power factor rectification for high-frequency AC buses (20 kHz). A bandpass filter of the series-resonant type, centered at the line frequency, is inserted between the line and the full-wave rectified load. The Q=Z ₀/R_L formed by the load and the characteristic impedance of the tank circuit determines the power factor, the boundary between continuous and discontinuous conduction modes, the peak stresses, and the transient response of the rectifier. It is shown that for Q>2/π the rectifier operates in continuous conduction mode and the output voltage is independent of the load. Also, it is shown that for Q>2 the line current is nearly sinusoidal with less than 5% third-harmonic distortion and the power factor is essentially unity. An increase in Q causes an increase in the peak voltages of the tank circuit and a slower transient response of the rectifier circuit. The DC, small-signal, and transient analyses of the rectifier circuit are carried out, and the results are in good agreement with simulation and experimental results     

Size effect study in magnetoelectric BiFeO3 system

In this paper, we report for the first time finite size effects on Néel temperature (T _N) of magnetoelectric BiFeO₃ system. Novel wet chemical route has been developed to produce fine particles of BiFeO₃ with controlled size and size distribution. Unlike other oxide systems, lattice volume contraction has been observed with decrease in particle size. The decrease in T _N is co-related to unit cell volume contraction occurring with reduction in particle size.     

Development of vocal tract length during early childhood: a magnetic resonance imaging study

Speech development in children is predicated partly on the growth and anatomic restructuring of the vocal tract. This study examines the growth pattern of the various hard and soft tissue vocal tract structures as visualized by magnetic resonance imaging (MRI), and assesses their relational growth with vocal tract length (VTL). Measurements on lip thickness, hard- and soft-palate length, tongue length, naso-oro-pharyngeal length, mandibular length and depth, and distance of the hyoid bone and larynx from the posterior nasal spine were used from 63 pediatric cases (ages birth to 6 years and 9 months) and 12 adults. Results indicate (a) ongoing growth of all oral and pharyngeal vocal tract structures with no sexual dimorphism, and a period of accelerated growth between birth and 18 months; (b) vocal tract structure's region (oral/anterior versus pharyngeal/posterior) and orientation (horizontal versus vertical) determine its growth pattern; and (c) the relational growth of the different structures with VTL changes with development-while the increase in VTL throughout development is predominantly due to growth of pharyngeal/posterior structures, VTL is also substantially affected by the growth of oral/anterior structures during the first 18 months of life. Findings provide normative data that can be used for modeling the development of the vocal tract.