Probing Deep Lung Regions using a New 6-electrode Tetrapolar Impedance Method.

Probing deep regions of the lung using electrical impedance is very important considering the need for a low cost and simple technique, particularly for the low and medium income countries. Because of complexity and cost, Electrical Impedance Tomography is not suitable for this envisaged application. The simple Tetrapolar Impedance Measurement (TPIM) technique employing four electrodes is the age old technique for bioelectrical measurements. However, it has its limitations in respect of organ localisation and depth sensitivity using skin surface electrodes. Recently, a new 6-electrode TPIM with two current electrodes but two pairs of appropriately connected potential electrodes positioned on the front and back of the thorax, proposed by one of the authors, came with a promise. However, this work gave a qualitative proposal based on concepts of physics and lacked a quantitative evaluation. In order to evaluate the method quantitatively, the present work employed finite element method based COMSOL Multiphysics software and carried out simulation studies using this new 6-electrode TPIM and compared the results with those from 4-electrode TPIM, with electrodes applied either on the front or at the back of the thorax for the latter. Initially, it carried out a sensitivity distribution study using a simple rectangular volume conductor which showed that the 6-electrode TPIM gives better depth sensitivity throughout the lung region. Next it used a near life like thorax model developed by another of the authors earlier. Using this model, extensive studies were carried out to quantify the overall sensitivity over a target lung region, the contribution of the target lung to the total measured impedance, and several other parameters. Through these studies, the 6-electrode TPIM was established on a stronger footing for probing deep regions of the lungs.

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.

The sensitivity of focused electrical impedance measurements.

One of the problems with tetrapolar impedance measurements is the lack of spatial sensitivity within the measured volume. In this paper we compare the sensitivity of tetrapolar measurements and the focused impedance measurements (FIM) proposed by Rabbani et al (1999 Ann. New York Acad. Sci. 873 408-20), which give an improved sensitivity profile. Using a previously validated model of sensitivity based on Geselowitz's lead theory, the sensitivity of FIM using eight, six and four electrodes was investigated. All electrode configurations showed a maximum in the average sensitivity of a plane at a depth of one-third of the drive-receive electrode spacing. No difference was found in the sensitivity value of this maximum between electrode configurations having the same drive-receive electrode spacing. The six- and eight-electrode configurations showed negative sensitivity regions down to half of the drive-receive electrode spacing, whilst the four-electrode measurement showed negative sensitivity regions down to one-third of the drive-receive electrode spacing. The single peak in sensitivity beneath the centre of the electrode configuration became dominant at 0.56, 1.4 and 0.14 of the receive electrode spacing for the eight-, six- and four-electrode configurations respectively. Thus, the four-electrode FIM configuration gives a single peak closest to the surface.