The sensitivity of focused electrical impedance measurements

Regional Pulmonary Ventilation Assessment Method and System Based on Impedance Sensing Information from the Pentapulmonary Lobes

Regional lung ventilation assessment is a critical tool for the early detection of lung diseases and postoperative evaluation. Biosensor-based impedance measurements, known for their non-invasive nature, among other benefits, have garnered significant attention compared to traditional detection methods that utilize pressure sensors. However, solely utilizing overall thoracic impedance fails to accurately capture changes in regional lung air volume. This study introduces an assessment method for lung ventilation that utilizes impedance data from the five lobes, develops a nonlinear model correlating regional impedance with lung air volume, and formulates an approach to identify regional ventilation obstructions based on impedance variations in affected areas. The electrode configuration for the five lung lobes was established through numerical simulations, revealing a power-function nonlinear relationship between regional impedance and air volume changes. An analysis of 389 pulmonary function tests refined the equations for calculating pulmonary function parameters, taking into account individual differences. Validation tests on 30 cases indicated maximum relative errors of 0.82% for FVC and 0.98% for FEV1, all within the 95% confidence intervals. The index for assessing regional ventilation impairment was corroborated by CT scans in 50 critical care cases, with 10 validation trials showing agreement with CT lesion localization results.

Sensitivity study of a locally developed six electrode focused impedance method

The Focused Impedance Method (FIM) is a new technique of electrical bioimpedance measurements in the human body. The idea originated in Bangladesh and provides an opportunity for localized measurement of bioimpedance down to reasonable depths from the body surface using skin surface electrodes. This has potential applications for physiological studies of targeted organs in the body and in detecting or diagnosing diseases and disorders. FIM is based on the age-old Tetra-Polar Impedance Measurement (TPIM) but provides a few significant improvements. Technology must be developed indigenously to obtain long-term benefits, particularly in Low and Medium Income countries (LMIC). This paper presents an experimental sensitivity study of the six-electrode version of the Focused Impedance method (FIM-6) with the circuit and phantom indigenously designed in Nepal. The work involved sensitivity studies of both FIM-6 and TPIM with the necessary circuit blocks developed through experimental validation. The sensitivity studies were performed on a simple 2D phantom with different electrode arrangements for FIM-6 and linear TPIM. A cylindrical object was placed at different positions for this study. The FIM-6 gave a high sensitivity in the central part, which remained almost constant within a small region that may be termed as the focused region. On the other hand, TPIM results fell off sharply away from the central point, making it unsuitable for practical measurements on target organs. Besides, there were areas with large negative sensitivities in TPIM, which were much smaller in FIM. The results obtained through this work clearly show the improvement offered by FIM over TPIM.

A Multi-Frequency Focused Impedance Measurement System Based on Analogue Synchronous Peak Detection

Monitoring of anatomical structures and physiological processes by electrical impedance has attracted scientists as it is noninvasive, nonionizing and the instrumentation is relatively simple. Focused Impedance Method (FIM) is attractive in this context, as it has enhanced sensitivity at the central region directly beneath the electrode configuration minimizing contribution from neighboring regions. FIM essentially adds or averages two concentric and orthogonal combinations of conventional Tetrapolar Impedance Measurements (TPIM) and has three versions with 4, 6, and 8 electrodes. This paper describes the design and testing of a multi-frequency FIM (MFFIM) system capable of measuring all three versions of FIM at 8 frequencies in the range 10 kHz—1 MHz. A microcontroller based multi-frequency signal generator and a balanced Howland current source with high output impedance (476 kΩ at 10 kHz and 58.3 kΩ at 1 MHz) were implemented for driving currents into biological tissues with an error <1%. The measurements were carried out at each frequency sequentially. The peak values of the amplified voltage signals were measured using a novel analogue synchronous peak detection technique from which the transfer impedances were obtained. The developed system was tested using TPIM measurements on a passive RC Cole network placed between two RC networks, the latter representing skin-electrode contact impedances. Overall accuracy of the measurement was very good (error <4% at all frequencies except 1 MHz, with error 6%) and the resolution was 0.1 Ω. The designed MFFIM system had a sampling rate of >45 frames per second which was deemed adequate for noninvasive real-time impedance measurements on biological tissues.

Robustness of focused and global impedance estimates of bladder volumes against uncertainty of urine conductivity

Bioimpedance measurements are currently used to monitor various biological processes and are potentially useful for studies of urodynamics. Global impedance (GI) and focused impedance measurements (FIM) can be used to monitor bladder volumes, but these are subject to varying conductivity of urine. To address this, we emulated a human bladder using an agar phantom filled with saline solutions of varying conductivities and estimated volumes using a modified FIM-based approach. Using this novel strategy, electrical potentials did not change significantly with constant liquid volumes, even when the conductivity of the saline solutions was varied between 1.027 to 1.877 and 2.610 S/m. Conversely, GI and classic FIM measurements of constant liquid volumes varied with conductivity. These observations suggest that the proposed FIM approach is suitable for bladder volume estimation due to its robustness against uncertainties of conductivity. The bioimpedance hardware used in our experiments comprised 8 electrodes and a a small and low cost impedance measurement system based on an AFE4300 direct impedance measurement device.

A new six-electrode electrical impedance technique for probing deep organs in the human body

Electrical impedance measurements of biological tissue have many potential applications and tetrapolar impedance measurement (TPIM) with four electrodes is traditionally used which eliminates high skin contact impedance. A linear array of four electrodes for TPIM on the horizontal plane of a cylindrical volume conductor of diameter D, where the length of the array is πD/2 with potential electrodes near the centre of the array, will give a high sensitivity near the surface which reduces rapidly with depth. A recently proposed six-electrode variation of TPIM uses an additional pair of potential electrodes on the opposite side of the volume conductor in the same horizontal plane around the circumference, with the expectation that the sensitivity of the deeper regions will thereby be enhanced. The present work carries out a finite element simulation (using COMSOL) and an experimental phantom study (saline phantom) to quantitatively evaluate the improvement obtained by this new method. The new configuration doubled the sensitivity at the central region, which was reasonably uniform over a wider zone, gradually increasing towards the potential electrodes on both sides. This would be useful for a range of biological studies of deep body organs such as lungs, stomach, and bladder. where the respective external body shapes may be approximated by an oval cylinder and where electrical impedance techniques have shown promise.

Probing for Stomach using the Focused Impedance Method (FIM)

For probing deep organs of the body using electrical impedance, the conventional method is to use Electrical Impedance Tomography (EIT). However, this would be a sophisticated machine and will be very expensive when a full 3D EIT is developed in the future. Furthermore, for most low income countries such expensive devices may not deliver the benefits to a large number of people. Therefore, this paper suggests the use of simpler techniques like Tetrapolar Impedance Measurement (TPIM) or Focused Impedance Method (FIM) in probing deeper organs. Following a method suggested earlier by one of the authors, this paper studies the possibility of using TPIM and FIM for the stomach. Using a simplified model of the human trunk with an embedded stomach, a finite element simulation package, COMSOL, was used to obtain transfer impedance values and percentage contribution of the stomach region in the total impedance. For this work, judicious placement of electrodes through qualitative visualizations based on point sensitivity equations and equipotential concepts were made, which showed that reasonable contribution of the stomach region is possible through the use of TPIM and FIM. The contributions were a little over 20% which is of similar order of the cross-sectional area percentage of the stomach with respect to that of the trunk. For the case where the conductivity of the stomach region was assumed about 4 times higher, the contributions increased to about 38%. Through further studies this proposed methods may contribute greatly in the study of deeper organs of the body.

Use of a Conical Conducting Layer with an Electrical Impedance Probe to Enhance Sensitivity in Epithelial Tissues

Tetra-polar electrical impedance measurement (TPIM) with a square geometry of electrodes is useful in the characterization of epithelial tissues, especially in the detection of cervical cancer at precancerous stages. However, in TPIM, the peak planar sensitivity just below the electrode surface is almost zero and increases to a peak value at a depth of about one third to one half of the electrode separation. To get high sensitivity for the epithelial layer, having thicknesses of 200 μm to 300 μm, the electrode separation needed is less than 1 mm, which is difficult to achieve in practical probes. This work proposes a conical conducting layer in front of a pencil like probe with a square geometry of TPIM electrodes to create virtual electrodes with much smaller separation at the body surface, thus increasing the sensitivity of the epithelial tissues. To understand the improvements, if any, 3D sensitivity distribution and transfer impedance were simulated using COMSOL Multiphysics software for a simplified body tissue model containing a 300 μm epithelial layer. It has been shown that fractional contribution of an epithelial layer can be increased several times placing a cylindrical conducting layer in between the tissue surface and the electrodes, which can further be enhanced using a conical conducting layer. The results presented in this paper can be used to choose an appropriate electrode separation, conducting layer height and cone parameters for enhanced sensitivity in the epithelial layer.

Measurement and modelling the sensitivity of tetrapolar transfer impedance measurements

Finite element method (FEM) modelling of a small disk in a homogeneous saline medium showed that the sensitivity distribution for tetrapolar transfer impedance measurements was dependant on the ratio, σdisk/σsaline, and not absolute conductivity values. In addition, the amplitude of the negative sensitivity regions between the drive and receive electrodes decreased non-linearly with σdisk/σsaline for σdisk/σsaline < 1, eventually becoming zero. This non-linear behaviour determined the limit of the assumption of a small change in conductivity in Geselowitz's lead theorem with 0.5 <σdisk/σsaline <1.5 for the measurements reported. The modelling supported the design of a sensitivity measurement system using an insulating support and a metal disk in a saline filled tank. Measurements were shown to give good agreement with sensitivity predictions from Geselowitz's lead theorem. Replacing the homogeneous medium in the FEM model with layers of different conductivity parallel to the plane of the electrodes changed the sensitivity distribution when the thickness of the layers adjacent to the electrodes were less than ½ the electrode spacing. A layer of greater conductivity over a layer of lesser conductivity next to the electrodes gave a peak in the sensitivity distribution and extended regions of negative sensitivity further into the tissue.

FOCUSED IMPEDANCE METHOD TO DETECT LOCALIZED LUNG VENTILATION DISORDERS IN COMBINATION WITH CONVENTIONAL SPIROMETRY

Conventional spirometry gives information on the overall ventilation of a person's lung; it cannot detect localized disorders in ventilation as occurring in pulmonary edema, pneumonia, tumor, TB, etc. Here we propose a new technique involving the recently developed focused impedance method (FIM) in combination with conventional spirometry to detect localized lung ventilation disorders. Electrical impedance of lung tissue changes as a function of air content and FIM provides a measurement of localized electrical impedance with sensitivity down to reasonable depths inside the body using a few surface electrodes; here we used a six-electrode version. At least four quadrants of the lungs in the frontal plane can be separately measured using a hand-held probe with spring backed skin surface electrodes. Firstly, spatial sensitivity distribution of the six-electrode FIM was obtained using finite element simulation which verified the focusing effect and its depth sensitivity. Percent change in impedance between maximum inspiration and expiration were measured at four quadrants of the chest of a healthy male subject giving four different values; that at the lower right quadrant was found to be the maximum, as also expected based on anatomy. Changes in impedance at this quadrant of the same subject were found to vary proportionately with exhaled air volumes, measured using a bellows-type spirometer. Similar FIM measurements at lower right lung of seven healthy subjects were found to be almost proportional (R2 = 0.7) to the total exhaled air volumes (vital capacity). This was the basis of the new technique. For a healthy individual, the ratio of the local impedance change to vital capacity (VC) will fall within a certain range for each of the four lung quadrants. A lower value at any quadrant would indicate disorder within that quadrant, while a larger value would indicate disorder in a region outside the particular quadrant. The FIM electrode probe can then be moved to take measurements at the other quadrants to locate the region of disorder. This preliminary study indicates that FIM in combination with conventional spirometry could be used to detect localized ventilation defects.

Classification of breast tumour using electrical impedance and machine learning techniques

When a breast lump is detected through palpation, mammography or ultrasonography, the final test for characterization of the tumour, whether it is malignant or benign, is biopsy. This is invasive and carries hazards associated with any surgical procedures. The present work was undertaken to study the feasibility for such characterization using non-invasive electrical impedance measurements and machine learning techniques. Because of changes in cell morphology of malignant and benign tumours, changes are expected in impedance at a fixed frequency, and versus frequency of measurement. Tetrapolar impedance measurement (TPIM) using four electrodes at the corners of a square region of sides 4 cm was used for zone localization. Data of impedance in two orthogonal directions, measured at 5 and 200 kHz from 19 subjects, and their respective slopes with frequency were subjected to machine learning procedures through the use of feature plots. These patients had single or multiple tumours of various types in one or both breasts, and four of them had malignant tumours, as diagnosed by core biopsy. Although size and depth of the tumours are expected to affect the measurements, this preliminary work ignored these effects. Selecting 12 features from the above measurements, feature plots were drawn for the 19 patients, which displayed considerable overlap between malignant and benign cases. However, based on observed qualitative trend of the measured values, when all the feature values were divided by respective ages, the two types of tumours separated out reasonably well. Using K-NN classification method the results obtained are, positive prediction value: 60%, negative prediction value: 93%, sensitivity: 75%, specificity: 87% and efficacy: 84%, which are very good for such a test on a small sample size. Study on a larger sample is expected to give confidence in this technique, and further improvement of the technique may have the ability to replace biopsy.

Comparison of four different FIM configurations--a simulation study

Focused impedance measurements (FIM) are used in several fields, and address the problem of measuring the volume impedance of an object within a volume conductor. Several electrode configurations are possible, and these have different properties. Sensitivity fields of four configurations have been investigated. We present one new development of an existing FIM configuration, and we made finite element models of the configurations to analyse and compare them both graphically and numerically. The models developed have a variable-sized mesh that allows us to build complex models that fit easily in computer memory. We found that one configuration in particular, FIM4, was superior to the others in most aspects. We also analysed the effects of very high sensitivities in and under the electrodes. We found that even if the sensitivity is very high under the electrodes, the effects of inhomogeneities were not as high as one might expect.

The possible use of combined electrical impedance and ultrasound velocity measurements for the non-invasive measurement of temperature during mild hyperthermia

This paper explores the possibility of using combined measurements of electrical impedance and changes in ultrasound time of flight for determining deep body temperature during mild hyperthermia. Simultaneous electrical impedance spectra (1 kHz-1024 kHz) and ultrasound time-of-flight measurements were made on layered sheep liver and fat tissue samples as the temperature was increased from 30-50 °C. The change in propagation velocity for 100% fat and 100% liver samples was found to vary linearly with temperature and the temperature coefficient of the time-of-flight was shown to vary linearly with the % fat in the sample (0.009% °C-1%-1). Tetrapolar impedance measurements normalized to 8 kHz were shown to have a small sensitivity to temperature for both liver (0.001% °C-1 ≤ 45 °C) and fat (0.002% °C-1 ≤ 512 kHz) and the best linear correlation between the normalized impedance and the % fat in the sample was found at 256 kHz (gradient 0.026%-1, r2 = 0.65). A bootstrap analysis on 15 layered tissue samples evaluated using the normalized impedance at 256 kHz to determine the % fat in the sample and the temperature coefficient of the time of flight to determine the temperature. The results showed differences (including some large differences) between the predicted and measured temperatures and an error evaluation identified the possible origins of these.

Determination of abdominal fat thickness using dual electrode separation in the focused impedance method (FIM)

Subcutaneous fat layer thickness in the abdomen is a risk indicator of several diseases and disorders like diabetes and heart problems and could be used as a measure of fitness. Skinfold measurement using mechanical calipers is simple but prone to error. Ultrasound scanning techniques are yet to be established as accurate methods for this purpose. magnetic resonance imaging (MRI) and computed tomography (CT) scans can provide the answer but are expensive and not available widely. Some initiatives were made earlier to use electrical impedance to this end, but had inadequacies. In the first part of this paper, a 4-electrode focused impedance method (FIM) with different electrode separations has been studied for its possible use in the determination of abdominal fat thickness in a localized region. For this, a saline phantom was designed to provide different electrode separations and different layers of resistive materials adjacent to the electrodes. The background saline simulated the internal organs having low impedance while the resistive layers simulated the subcutaneous fat. The plot of the measured impedance with electrode separation had different 'slopes' for different thicknesses of resistive layers, which offered a method to obtain an unknown thickness of subcutaneous fat layer. In the second part, measurements were performed on seven human subjects using two electrode separations. Fat layer thickness was measured using mechanical calipers. A plot of the above 'slope' against fat thickness could be fitted using a straight line with an R(2) of 0.93. Then this could be used as a calibration curve for the determination of unknown fat thickness. Further work using more accurate CT and MRI measurements would give a better calibration curve for practical use of this non-invasive and low-cost technique in abdominal fat thickness measurement.