Remote wound monitoring of chronic ulcers.

Chronic wounds or ulcers are wounds that do not heal in the usual manner. This type of wound is most common in the elderly and in paraplegic patients with an estimated 1% of the population suffering from leg ulcers and the costs adding up to 4% of the annual National Health Service budget in the U.K. There is an identified need to develop a device capable of remote wound monitoring that enables patients to take charge of their wound management under clinical guidance. A new ¿wound mapping¿ device has been developed, which is based on electrical impedance spectroscopy and involves the multifrequency characterization of the electrical properties of wound tissue under an electrode array. A key feature of the prototype device is the anticipated incorporation of the measuring array into standard commercial occlusive dressings, thereby protecting the wound from interference and contamination, and thus, promoting wound healing, while monitoring the protected wound. Further development is planned including wireless transmission, thus enabling telewound monitoring as described earlier.

II.1. Elementary electrodynamics.

The paper describes the physical phenomena involved in the conduction of electricity, with particular reference to living tissue. The conduction of electricity depends on the flow of charge carriers in the material, while the dielectric properties are due to the rotation of dipoles that can align along an applied electric field. The relation between the electric variables in a conducting medium and their physical meanings are explained. The phenomena responsible for the electric and dielectric properties of living tissues are described. The presence of cells limits the flow of charge carriers, in particular at low frequency, and the membranes are responsible for dielectric relaxation. The passive response of cell membranes to weak applied signals enables bioelectrical tissue characterisation. Practical tools for electrical impedance spectroscopy are given with an overview of the most recent applications.

II.2. Electrical safety.

Correct use of medical equipment within the clinical environment is of prime importance. This includes awareness of the safety issues regarding equipment, particular when it is an electrically powered device. Incidents can occur in the clinic in which a medical device is suspected of contributing to patient or staff injury. It is important that one can identify in advance any potential hazards which may arise with electrical equipment due to technical or environmental factors. This paper gives an overview of electrical safety.

2. Electrical safety.

Correct use of medical equipment within the clinical environment is of prime importance. This includes awareness of the safety issues regarding equipment, particular when it is an electrically powered device. Incidents can occur in the clinic in which a medical device is suspected of contributing to patient or staff injury. It is important that one can identify in advance any potential hazards which may arise with electrical equipment due to technical or environmental factors. This paper gives an overview of electrical safety.

Elementary electrodynamics.

The paper describes the physical phenomena involved in the conduction of electricity, with particular reference to living tissue. The conduction of electricity depends on the flow of charge carriers in the material, while the dielectric properties are due to the rotation of dipoles that can align along an applied electric field. The relation between the electric variables in a conducting medium and their physical meanings are explained. The phenomena responsible for the electric and dielectric properties of living tissues are described. The presence of cells limits the flow of charge carriers, in particular at low frequency, and the membranes are responsible for dielectric relaxation. The passive response of cell membranes to weak applied signals enables bioelectrical tissue characterisation. Practical tools for electrical impedance spectroscopy are given with an overview of the most recent applications.

Characterisation of a multi-frequency wound impedance mapping instrument.

A new instrument has been developed enabling clinicians to map and study the healing process of a wound. Early assessment of the instrument demonstrated promising results both during bench testing as well as during initial in vivo measurements on a skin abrasion. Clear differences between healthy and wounded tissue were demonstrated and a pattern was observed, potentially indicative of the healing process for this particular wound.

Bioimpedance and p-Health.

Bio-impedance is the electrical impedance of living matter. Bio-impedance methods present a range of known advantages for medical and clinical applications including low-cost, non-invasiveness and harmlessness. The measured parameter reflects the physiological and pathological processes that take place within human body. The technological progress in instrumentation has significantly contributed to the progress that has been observed during the last past decades in impedance spectroscopy and electrical impedance tomography. Although bioimpedance is not a physiological parameter, the method enables tissue characterisation and functional monitoring and can contribute to the monitoring of the health status of a person. The association of this flexible and versatile method with micro-electronics and wireless telecommunication systems opens a new field of potential applications.

Assessment of 1-lead and 2-lead electrode patterns in electrical impedance endotomography.

Electrical impedance endotomography (EIE) is a modality of impedance imaging where the electrodes are located on an insulating core placed at the centre of the region of interest. The absence of a physical limit to the medium surrounding the probe enables the use of remote electrodes. The present study compares the features of 2-lead measurements, where the two pairs of electrodes are located on the probe, to 1-lead measurements, where one of the two injection electrodes and one of the two sensing electrodes are located at a distance far away from the probe. The methodology was the characterization of the sensitivity matrix under the influence of electrode pattern, reconstruction radius and mesh construction. Three mesh constructions, three values of the reconstruction radius and five electrode patterns were compared. The study was carried out in 2D using calculated data. Measurement noise was simulated by an addition of 5% Gaussian white noise. The images were reconstructed using the Tikhonov method and L-curve technique. The results show that the reconstruction mesh and the radius of the reconstruction domain have less influence on the conditioning of the sensitivity matrix than the electrode pattern. Both 1-lead and 2-lead configurations enabled the reconstruction of images of relatively similar quality. Additional selection criteria are expected from hardware considerations.

Impedance changes in liver tissue exposed in vitro to high-energy ultrasound.

The present study reports the impedance changes observed in bovine liver samples exposed in vitro to high-intensity ultrasound. The measurement frequency ranged from 80 kHz to 2 MHz. The treatment resulted in the average increase of 20% in impedance magnitude at low frequency and the average decrease of 30% at high frequency. The phase angle increased significantly by more than 15 degrees at all measurement frequencies. The slope of the log-modulus of impedance against log-frequency increased in treated tissue at frequencies above 500 kHz. This change was attributed to the alteration of the capacitive response of the tissue. The experimental observations are consistent with the known changes induced by high-energy ultrasound in liver tissue. This study confirmed that ultrasound energy produces measurable changes in a tissue's impedance and that indices can be derived to distinguish between original and treated tissues. The results obtained in liver tissue need confirmation in organs treatable with therapeutic ultrasound, such as breast and prostate.

Electrical impedance endotomography: sensitivity distribution against bipolar current patterns.

Electrical impedance endotomography (EIE) is a modality where the electrodes are located around an insulating core placed inside the region of interest. This approach results in significant differences with respect to conventional EIT. The paper examines the sensitivity distribution of bipolar current patterns and the influence of the spacing between the drive electrodes using a two-dimensional (2D) mathematical model. The number of pixels of sensitivity above a given sensitivity threshold decreases faster with the distance to the probe for diametric and adjacent drive than for other bipolar drive patterns. The reconstruction of images from datasets collected in vitro using a 16-electrode probe confirmed the feasibility of the method at least within a range extending to three times the radius of the probe, under the described experimental conditions. Reduction of system noise, multiple-current patterns and the use of remote current and voltage electrodes are potential methods to increase the sensitivity range. Further work includes the improvement of the model to account for finite length electrodes and the miniaturization of the probe.

Electrical impedance endotomography.

In electrical impedance endotomography (EIE), the impedance measuring electrodes are placed at the centre of the region of interest instead of encircling it, as in usual electrical impedance tomography. EIE has been developed for prostate imaging. The developed mathematical model enables the derivation of analytical equations for electric potential, electric field and sensitivity. This paper focuses on the selection of an optimal current injection method capable of sensing distant points. Experimental measurements were carried out in vitro using an enlarged size, 50 mm diameter, 16-electrode mock-up probe placed in tap water. The collected data enabled the production of images of a circular insulating target, 10 mm in diameter located 100 mm from the axis of the probe. Future work includes extension of sensitivity range, improvement of image reconstruction, design of a multiple frequency hardware system and construction of a real-size, biocompatible impedance probe.

Electrical impedance endo-tomography: imaging tissue from inside.

This paper describes a new method for scanning the conductivity of a tissue or an organ using a multielectrode impedance probe placed at the center of the region of interest. The long-term objective of the study is the evaluation, using an urethral impedance probe, of the lesion produced by ultrasound ablathermy of localized prostate cancer. The probe consists of electrodes placed at the surface of an insulating cylinder. The injected current passes around the cylinder and spreads in the medium surrounding the probe. This paper presents the theoretical bases of this method, the calculated sensitivity distributions of electrode configurations involving a pair of diametrically opposed electrodes and an application in vitro. The experimental set-up consisted of a water tank and a 16-electrode prototype probe 50 mm in diameter. Data sets were collected in the presence of conductivity perturbations produced by small size insulators or conductors and a 7.5% constant perturbation model. The presented images, although reconstructed using a simple retro-projection algorithm, demonstrate the feasibility of the method. Improvements in data collection and image reconstruction are possible.