The use of electrical impedance spectroscopy in the detection of cervical intraepithelial neoplasia.

The objective of this study was to assess the performance of cervical impedance spectroscopy in the detection of cervical intraepithelial neoplasia (CIN) using the new MKIII impedance probe. A prospective observational study recruited women referred to colposcopy with an abnormal Papanicolaou smear. A pencil probe incorporating four gold electrodes was used to measure electrical impedance spectra from cervical epithelium. Colposcopy examinations, including probe positioning, were video recorded to allow for correlation between results obtained from colposcopic impression, histopathologic examination of colposcopic punch biopsies, and impedance measurements. Cervical impedance-derived parameters R, S, R/S, C, and Fc were assessed to see if significant difference in values obtained in CIN and normal epithelium existed. The performance of the probe in identifying women with CIN was also assessed. One hundred seventy-six women were recruited and 1168 points analyzed. Parameters R, S, and Fc showed significant separation of CIN or squamous intraepithelial lesion (SIL) from squamous, mature metaplastic, and columnar epithelium. Sensitivities of 74% and specificity of 53% can be achieved in identifying CIN 2/3 (High-grade SIL) in screened women. We conclude that cervical impedance spectrometry provides a potentially promising real-time screening tool for CIN with similar sensitivity and specificity to currently used screening tests. Further research is ongoing to develop the probe for potential clinical use.

Study of the optimum level of electrode placement for the evaluation of absolute lung resistivity with the Mk3.5 EIT system.

Inter-subject variability has caused the majority of previous electrical impedance tomography (EIT) techniques to focus on the derivation of relative or difference measures of in vivo tissue resistivity. Implicit in these techniques is the requirement for a reference or previously defined data set. This study assesses the accuracy and optimum electrode placement strategy for a recently developed method which estimates an absolute value of organ resistivity without recourse to a reference data set. Since this measurement of tissue resistivity is absolute, in Ohm metres, it should be possible to use EIT measurements for the objective diagnosis of lung diseases such as pulmonary oedema and emphysema. However, the stability and reproducibility of the method have not yet been investigated fully. To investigate these problems, this study used a Sheffield Mk3.5 system which was configured to operate with eight measurement electrodes. As a result of this study, the absolute resistivity measurement was found to be insensitive to the electrode level between 4 and 5 cm above the xiphoid process. The level of the electrode plane was varied between 2 cm and 7 cm above the xiphoid process. Absolute lung resistivity in 18 normal subjects (age 22.6 +/- 4.9, height 169.1 +/- 5.7 cm, weight 60.6 +/- 4.5 kg, body mass index 21.2 +/- 1.6: mean +/- standard deviation) was measured during both normal and deep breathing for 1 min. Three sets of measurements were made over a period of several days on each of nine of the normal male subjects. No significant differences in absolute lung resistivity were found, either during normal tidal breathing between the electrode levels of 4 and 5 cm (9.3 +/- 2.4 Omega m, 9.6 +/- 1.9 Omega m at 4 and 5 cm, respectively: mean +/- standard deviation) or during deep breathing between the electrode levels of 4 and 5 cm (10.9 +/- 2.9 Omega m and 11.1 +/- 2.3 Omega m, respectively: mean +/- standard deviation). However, the differences in absolute lung resistivity between normal and deep tidal breathing at the same electrode level are significant. No significant difference was found in the coefficient of variation between the electrode levels of 4 and 5 cm (9.5 +/- 3.6%, 8.5 +/- 3.2% at 4 and 5 cm, respectively: mean +/- standard deviation in individual subjects). Therefore, the electrode levels of 4 and 5 cm above the xiphoid process showed reasonable reliability in the measurement of absolute lung resistivity both among individuals and over time.

Detection of emboli in vessels using electrical impedance measurements--phantom and electrodes.

A phantom was constructed to simulate the electrical properties of the neck. A range of possible electrode configurations was then examined in order to improve the sensitivity of the impedance measurement method for the in vivo detection of air emboli. The neck phantom consisted of simulated skin, fat and muscle layers made of agar and a conductive rubber tube mimicking the common carotid artery. The ring-shaped electrodes with a guard electrode showed the highest sensitivity to emboli at short distances.

Estimation of the size of air emboli detectable by electrical impedance measurement.

Non-invasive detection of air emboli in blood is investigated in vitro using a tetrapolar electrical impedance measurement. A cubic tank with a linear array of four electrodes, spaced approximately 1 cm apart down one side, was filled with 0.2 Sm(-1) saline. Bubbles were generated by carbon dioxide gas. Electrical transfer impedance was measured every 8.2 ms at 1.25 MHz. The movement of bubbles was recorded by a video camera, and their sizes and depths from the middle of the array were measured using captured video images. Changes in transfer impedance caused by passage of bubbles were clearly observed and almost identical with those calculated theoretically. Using lead field theory and experimental results, the fundamental limit on the detectable size of bubbles was estimated at the carotid artery, the great saphenous vein and the cephalic vein. The theoretical results showed that a 0.5 mm diameter bubble is detectable at a depth of 5.3 mm, similar to the depth of the great saphenous and the cephalic veins, and a 2.3 mm diameter bubble is detectable at a depth of 21 mm, similar to the depth of the common carotid artery.

Electrical impedance tomography spectroscopy (EITS) for human head imaging.

Electrical impedance tomography (EIT) is a recently developed medical imaging method which has practical advantages for imaging brain function as it is inexpensive, rapid and portable. Its principal use in validated human studies to date has been to image changes in impedance at a single excitation frequency over time, but there are potential applications where it is desirable to obtain images from a single point in time, which could be achieved by imaging over multiple frequencies. We describe a novel multifrequency EIT design which provides up to 64 electrodes for imaging in the head. This was achieved by adding a multiplexer to a single channel of an existing system, the Sheffield Mark 3.5. This provides a flexible protocol for addressing up to 64 electrodes but CMRR decreases from 90 dB to 80 dB and analogue amplifier bandwidth from > 1.6 MHz to 0.8 MHz. This did not significantly affect performance, as cylinders of banana, 10% of the diameter of a saline filled spherical tank, could be visualized with frequency referenced imaging. The design appears to have been an acceptable compromise between practicality and performance and will now be employed in clinical trials of multifrequency EIT in stroke, epilepsy and neonatal brain injury.

Neonatal lungs: maturational changes in lung resistivity spectra.

The electrical resistivity of lung tissue can be related to the structure and composition of the tissue and also to the air content. Electrical impedance tomographic measurements have been used on 155 normal children over the first three years of life and 25 pre-term infants, to determine the absolute resistivity of lung tissue as a function of frequency. The results show consistent changes with increasing age in both lung tissue resistivity (5.8 ohm m at birth to 20.9 ohm m at 3 years of age) and in the changes of resistivity with frequency (Cole parameter ratio R/S=0.41 at birth and 0.84 at 3 years of age). Comparison with a lung model showed that the measurements are consistent with maturational changes in the number and size of alveoli, the extracapillary blood volume and the size of the extracapillary vessels. However, the results show that the process of maturation is not complete at the age of three years.

Neonatal lungs--can absolute lung resistivity be determined non-invasively?

The electrical resistivity of lung tissue can be related to the structure and composition of the tissue and also to the air content. Conditions such as pulmonary oedema and emphysema have been shown to change lung resistivity. However, direct access to the lungs to enable resistivity to be measured is very difficult. We have developed a new method of using electrical impedance tomographic (EIT) measurements on a group of 142 normal neonates to determine the absolute resistivity of lung tissue. The methodology involves comparing the measured EIT data with that from a finite difference model of the thorax in which lung tissue resistivity can be changed. A mean value of 5.7 +/- 1.7 omega(m) was found over the frequency range 4 kHz to 813 kHz. This value is lower than that usually given for adult lung tissue but consistent with the literature on the composition of the neonatal lung and with structural modelling.

Accuracy of an optically isolated tetra-polar impedance measurement system.

Accurate electrical transfer impedance measurement at the high frequencies (> 1 MHz) required to characterise blood and intracellular structures is very difficult, owing to stray capacitances between lead wires. To solve this problem, an optically isolated measurement system has been developed using a phase-locked-loop technique for synchronisation between current injection (drive) and voltage measurement (receive) circuits. The synchronisation error between drive and receive circuits was less than 1 ns. The accuracy and reproducibility of the developed system was examined using a tissue equivalent Cole model consisting of two resistors and one capacitor. The absolute value Z and phase shift theta in impedance of the Cole model was measured at 1.25 MHz by both an LCR meter and the isolated measurement system. The difference between the values measured by the isolated measurement system and those measured by the LCR meter was less than 0.27omega (2.9%) in Z and 0.79 degree in theta. The standard deviation was less than 0.09 omega in Z and 0.60 degree in theta.

Mk3.5: a modular, multi-frequency successor to the Mk3a EIS/EIT system.

This paper describes the Sheffield Mk3.5 EIT/EIS system which measures both the real and imaginary part of impedance at 30 frequencies between 2 kHz and 1.6 MHz. The system uses eight electrodes with an adjacent drive/receive electrode data acquisition protocol. The system is modular, containing eight identical data acquisition boards, which contain DSPs to generate the drive frequencies and to perform the FFT used for demodulation. The current drive is in three sequentially applied packets, where each packet contains ten summed sine waves. The data acquisition system is interfaced to a host PC through an optically isolated high speed serial link (RS485) running at 2 Mbaud (2 Mbits s(-1)). Measurements on a saline filled tank show that the average signal to noise performance of the system is 40 dB measured across all frequencies and that this figure is independent of frequency of measurement. These results suggest that the current system is 10 dB better in absolute terms than the previous Sheffield (Mk3a) system.

Cole equation modelling to measurements made using an impulse driven transfer impedance system.

Electrical impedance measurements are used to obtain information about a subject, tissue sample or tissue model under test. There are several ways of obtaining these impedance data and thereafter analysing the data to obtain relevant parameters. This paper shows how a completely isolated drive and receive system using current pulses, as opposed to sine waves, achieves good fitted results with resistor-capacitor Cole phantoms.

Diuretic induced change in lung water assessed by electrical impedance tomography.

Monitoring patients with left ventricular failure can be difficult. Electrical impedance tomography (EIT) produces cross-sectional images of changes in the impedance of the thorax. We measured changes in the electrical impedance of the lung in nine volunteers following a diuretic challenge. The hypothesis was that lung impedance would increase with diuretic induced fluid loss. Heart rate, blood pressure and urine output were also recorded. After diuretic the mean urine output was 1220 ml compared with 187 ml after placebo. Following diuretic administration, mean thoracic impedance increased by 13.6% (p < 0.01) and lung impedance increased by 7.8% (p < 0.05). Taken as a group there was a correlation between overall impedance change and total urine output. However, for each individual, the time course of change in impedance and urine output did not correlate significantly. Our findings show that EIT may offer a better guide to the response of the lung to diuretic treatment than simply measuring urine output. The urine output is neither specific nor sensitive in the assessment of lung water. Mean lung impedance, however, is largely determined by lung water. The study showed that lung impedance can be recorded at supra-normal values. EIT may help in the management of patients with excess lung water.

Monitoring patients with left ventricular failure by electrical impedance tomography.

Acute left ventricular failure (LVF) is a common medical emergency but detection and monitoring of pulmonary oedema remains problematic. Fluid is an important determinant of tissue impedance. Electrical impedance tomography (EIT) is a non-invasive technique allowing localisation of impedance changes within tissue. We have investigated the relationship between LVF and the electrical impedance of lung tissue. Twenty patients with a clinical diagnosis of acute left ventricular failure were compared with 30 normal subjects. Patients were monitored using serial chest radiographs and electrical impedance tomography measurements of lung impedance during hospital admission. Radiographs were graded according to the severity of pulmonary oedema by two independent radiologists. Lung impedance was significantly (P<0.0001) lower than normal in patients with left ventricular failure. Values returned towards the normal range as LVF resolved. There was a similar improvement in the score of the chest radiographs. The electrical impedance of the lung is low in left ventricular failure and increases following treatment.

Information from time-varying vibrotactile stimuli.

Experiments have been carried out to investigate the information transfer available via a single vibrator on the fingertip. In a first experiment, for stimuli with durations 80 to 320 ms, discrimination of a one-octave step change in frequency at the halfway point was investigated. Results were similar for three stimulus types--sinewave, monophasic pulse and tetraphasic pulse--suggesting temporal cues are more important than spectral cues in this task. In a second experiment, subjects were required to perceive changes in a sequence of stimulus elements. A presentation rate of 6.25 elements s-1 was found to give better results than a rate of 12.5 elements s-1. In the former case, the potential information transfer per element was estimated to be approximately 1.0 bits, corresponding to an information transfer rate of around 6 bits s-1. Implications for the design of a tactile aid to lipreading are discussed.

Coding of acoustic features for a single-channel tactile aid.

Measurements have been made on the discrimination of speech contrasts on the basis of single-channel vibrotactile presentation of a variety of speech-derived signals, coded as amplitude- and frequency-modulated pulse trains. Stimulation was at the index fingertip. The signals chosen for tactile presentation were the speech amplitude envelope, the voice fundamental frequency FO and the zero-crossing frequency in the 1.3-6.6 kHz band. "Two-feature' codings, which present two of these signals simultaneously (one coded as stimulus frequency and one coded as stimulus amplitude), were found to be no more effective than "single feature' codings which present only one signal (coded as both amplitude and frequency). Scores for consonant discrimination were highest for the single-feature coding of zero-crossing frequency, although differences between the codings were not, in general, significant. Scores for emphatic-stress discrimination were highest for the single-feature coding of F0, and this coding produced best results overall. A practical wrist-worn device, whose design is influenced by these experimental results is briefly described.

Using an assessment centre for the selection of a unit general manager.

Selecting a general manager is a key decision with long-term consequences. The traditional interview has led to many successful appointments, but is there not a better way? Peter Milnes describes how the regional assessment centre was used to select a UGM for community and mental handicap services.