Oscar Casas

Wireless Magnetic Sensor Node for Vehicle Detection With Optical Wake-Up

Vehicle detectors provide essential information about parking occupancy and traffic flow. To cover large areas that lack a suitable electrical infrastructure, wired sensors networks are impractical because of their high deployment and maintenance costs. Wireless sensor networks (WSNs) with autonomous sensor nodes can be more economical. Vehicle detectors intended for a WSN should be small, sturdy, low power, cost-effective, and easy to install and maintain. Currently available vehicle detectors based on inductive loops, ultrasound, infrared, or magnetic sensors do not fulfill the requirements above, which has led to the search for alternative solutions. This paper presents a vehicle detector which includes a magnetic and an optical sensor and is intended as sensor node for use with a WSN. Magnetic sensors based on magnetoresistors are very sensitive and can detect the magnetic anomaly in the Earths magnetic field that results from the presence of a car, but their continuous operation would drain more than 1.5 mA at 3 V, hence limiting the autonomy of a battery-supplied sensor node. Passive, low-power optical sensors can detect the shadow cast by car that covers them, but are prone to false detections. The use of optical triggering to wake-up a magnetic sensor, combined with power-efficient event-based software, yields a simple, compact, reliable, low-power sensor node for vehicle detection whose quiescent current drain is 5.5 μA. This approach of using a low-power sensor to trigger a second more specific sensor can be applied to other autonomous sensor nodes.

Transmural versus nontransmural in situ electrical impedance spectrum for healthy, ischemic, and healed myocardium

Electrical properties of myocardial tissue are anisotropic due to the complex structure of the myocardial fiber orientation and the distribution of gap junctions. For this reason, measured myocardial impedance may differ depending on the current distribution and direction with respect to myocardial fiber orientation and, consequently, according to the measurement method. The objective of this study is to compare the specific impedance spectra of the myocardium measured using two different methods. One method consisted of transmural measurements using an intracavitary catheter and the other method consisted of nontransmural measurements using a four-needle probe inserted into the epicardium. Using both methods, we provide the in situ specific impedance spectrum (magnitude and phase angle) of normal, ischemic, and infarcted pig myocardium tissue from 1 kHz to 1 MHz. Magnitude spectra showed no significant differences between the measurement techniques. However, the phase angle spectra showed significant differences for normal and ischemic tissues according to the measurement technique. The main difference is encountered after 60 min of acute ischemia in the phase angle spectrum. Healed myocardial tissue showed a small and flat phase angle spectrum in both methods due to the low content of cells in the transmural infarct scar. In conclusion, both transmural and nontransmural measurements of phase angle spectrum allow the differentiation among normal, ischemic, and infarcted tissue.

Heart Rate Detection From Plantar Bioimpedance Measurements

The heart rate is a basic health indicator, useful in both clinical measurements and home health care. Current home care systems often require the attachment of electrodes or other sensors to the body, which can be cumbersome to the patient. Moreover, some measurements are sensitive to movement artifacts, are not user-friendly and require a specialized supervision. In this paper, a novel technique for heart rate measurement for a standing subject is proposed, which is based on plantar bioimpedance measurements, such as those performed by some bathroom weighting scales for body composition analysis. Because of the low level of heart-related impedance variations, the measurement system has a gain of 1400. We have implemented a fully differential AC amplifier with a common-mode rejection ratio (CMRR) of 105 dB at 10 kHz. Coherent demodulation based on synchronous sampling yields a signal-to-noise ratio (SNR) of 55 dB. The system has a sensitivity of 1.9 V/Omega. The technique has been demonstrated on 18 volunteers, whose bioimpedance signal and ECG were simultaneously measured to validate the results. The average cross-correlation coefficient between the heart rates determined from these two signals was 0.998 (std. dev. 0.001)

Use of electrical impedance tomography (EIT) for the assessment of unilateral pulmonary function

We describe a fully automatable quantification process for the assessment of unilateral pulmonary function (UPF) by means of EIT and propose a measurement protocol for its clinical implementation. Measurements were performed at the fourth and sixth intercostal levels on a first group of ten healthy subjects (5M, 5F, ages 26-48 years) to define the proper protocol by evaluating the most common postures and ventilation modes. Several off-line processing tools were also evaluated, including the use of digital filters to extract the respiratory components from EIT time series. Comparative measures were then carried out on a second group consisting of five preoperatory patients with lung cancer (4M, IF, ages 25-77 years) scheduled for radionuclide scanning. Results show that measurements were best performed with the subject sitting down, holding his arms up and breathing spontaneously. As regards data processing, it is best to extract Fourier respiratory components. The mean of the healthy subject group leads to a left-right division of lung ventilation consistent with literature values (47% left lung, 53% right lung). The comparative study indicates a good correlation (r = 0.96) between the two techniques, with a mean difference of (-0.4+/-5.4)%, suggesting that the elimination of cardiac components from the thoracic transimpedance signal leads to a better estimation of UPF.

Passive transmission of ischemic ST segment changes in low electrical resistance myocardial infarct scar in the pig

OBJECTIVES To analyze the passive electrical properties of a healed infarction and assess their role on transmission of contiguous ischemic ST segment potential changes. METHODS We measured tissue resistivity (omega cm) at 1 kHz and the epicardial ST segment during 1 h of proximal reocclusion of the left anterior descending (LAD) coronary artery in 12 anesthetized pigs with one-month-old transmural infarction elicited by LAD ligature below the first branch. The impedance spectrum (1 to 1000 kHz) of normal and infarcted myocardium was measured in seven other pigs with similar infarctions. Electrical transmission of current pulses (30 microA) in infarcted tissue and in test solutions was also investigated. RESULTS The infarct scar has a lower than normal resistivity (110 +/- 30 omega cm vs. 235 +/- 60 omega cm, p < 0.0001) and, unlike the normal myocardium, resistivity and phase angle of the scar did not change at increasing current frequencies, reflecting no capacitative response. LAD reocclusion induced a resistivity rise (510 +/- 135 omega cm, p < 0.01) and a ST segment elevation (0.6 +/- 0.7 to 9.5 +/- 5.1 mV, p = 0.002) in the ischemic peri-infarction zone, whereas the infarcted area showed ST segment elevation (0.5 +/- 0.5 to 3.8 +/- 2.6 mV, p = 0.03) with no resistivity changes. Potential decay of both ST segment and current pulses in the scar and in 0.9% NaCl solution was less than 1 mV/mm. Transmural deposition of connective tissue was seen in the center of the infarction. CONCLUSIONS A one-month-old transmural infarction is a low resistance, noncapacitative medium that allows a good transmission of current pulses and of ST segment potential changes generated by contiguous peri-infarction ischemia.

Heart rate detection from an electronic weighing scale.

We propose a novel technique for beat-to-beat heart rate detection based on the ballistocardiographic (BCG) force signal from a subject standing on a common electronic weighing scale. The detection relies on sensing force variations related to the blood acceleration in the aorta, works even if wearing footwear and does not require any sensors attached to the body because it uses the load cells in the scale. We have devised an approach to estimate the sensitivity and frequency response of three commercial weighing scales to assess their capability to detect the BCG force signal. Static sensitivities ranged from 490 nV V(-1) N(-1) to 1670 nV V(-1) N(-1). The frequency response depended on the subjects mass but it was broad enough for heart rate estimation. We have designed an electronic pulse detection system based on off-the-shelf integrated circuits to sense heart-beat-related force variations of about 0.24 N. The signal-to-noise ratio of the main peaks of the force signal detected was higher than 30 dB. A Bland-Altman plot was used to compare the RR time intervals estimated from the ECG and BCG force signals for 17 volunteers. The error was +/-21 ms, which makes the proposed technique suitable for short-term monitoring of the heart rate.

A parallel broadband real-time system for electrical impedance tomography

This paper deals with the design, implementation and performance of TIE-4sys, an electrical impedance tomograph. This instrument is a parallel broad-band real-time system. It measures impedance using an array of 16 electrodes and reconstructs the images using a weighted back-projection technique. The objective of this development is to enable multifrequency EIT clinical studies to be undertaken. The system is capable of acquiring 25 frames/s and makes multifrequency cardiac-gated images. The frequency range is from 10 kHz to 250 kHz and the signal to noise ratio for the real component is better than 60 dB.

A microcontroller-based interface circuit for lossy capacitive sensors

This paper introduces and analyses a low-cost microcontroller-based interface circuit for lossy capacitive sensors, i.e. sensors whose parasitic conductance (Gx) is not negligible. Such a circuit relies on a previous circuit also proposed by the authors, in which the sensor is directly connected to a microcontroller without using either a signal conditioner or an analogue-to-digital converter in the signal path. The novel circuit uses the same hardware, but it performs an additional measurement and executes a new calibration technique. As a result, the sensitivity of the circuit to Gx decreases significantly (a factor higher than ten), but not completely due to the input capacitances of the port pins of the microcontroller. Experimental results show a relative error in the capacitance measurement below 1% for Gx < 200 nS, which is quite remarkable considering the simplicity of the circuit proposed. The measurement of a commercial capacitive humidity sensor subjected to condensation (in order to have a significant value of Gx) shows the effectiveness of the circuit.

Percutaneous electrocatheter technique for on-line detection of healed transmural myocardial infarction.

Healed myocardial infarction has been recognized by its particular tissue electrical impedance spectrum measured with intramural needle electrodes in animal models. The aim of this study was to develop a percutaneous approach for in vivo recognition of areas of healed myocardial infarction by measuring myocardial electrical impedance with an intracavitary contact electrocatheter. Electrical impedance (resistance and phase angle) of normal myocardium and of a 2‐month‐oid anterior transmural infarction were measured in nine chloralose anesthetized pigs by applying alternating currents from 1 kHz to 1 MHZ between a bipolar intracavitary catheter and a reference electrode placed on the epicardium (group I, n = 4) or on the precordium (group II, n = 5). Resistance of the infarcted myocardium was lower than that of healthy tissue at all current frequencies (ANOVA, P < 0.001) (i.e., at 1 kHz: 15 ± 4 Ω vs 50 ± 19 Ω in group I, and 64+ 13 Ω vs 76 ± 13 Ω in group II). Phase angle at 316 kHz best differentiated transmural infarction from normal tissue (group I: ‐2.5 ± 1.9 degrees vs ‐14.8 ± 4.6 degrees, P < 0.001; group II: +0.7 ±1.0 degrees vs ‐2.7 ± 1.4 degrees, P < 0.001). This study shows that analysis of myocardial impedance spectrum using a percutaneous intracavitary contact catheter approach permits on‐line recognition of areas of healed transmural myocardial infarction. This technique may be useful to optimize clinical application of energy sources (i.e., radiofrequency ablation, laser myocardial revascularization).

Characterisation of dynamic biologic systems using multisine based impedance spectroscopy

The characterization of biological materials and systems using electrical impedance spectroscopy has traditionally been performed using the frequency sweep technique. When applied to in-vivo measurements, the movement induced modulation has often a period shorter than the sweep time. This drawback can be overcome using broadband signal bursts. Given that the energy amount to be injected to the biological material is limited for safety reasons, the best choice is the use of multisine signals, which concentrate all that energy in the measurement frequencies, then achieving an optimal signal-to-noise ratio. The uniform distribution of frequencies is not adequate due to the system nonlinearities and to the need of covering a three-decade frequency range. This work is concerned with the design of a quasilogarithmic multisine with a similar number of frequencies at each decade and with a safety band around each measurement frequency. This band will be free of harmonics and quadratic intermodulation products. The system has been implemented using a virtual instrument based on an arbitrary waveform generator, a digital oscilloscope and an analog frontend. The system has been validated using passive RC networks and has been applied to the in-vivo characterization of infarcted myocardium in pigs.