Electrical impedance measurements: an objective measure of prelabor cervical change.

OBJECTIVE: To investigate the use of electrical impedance measurements of the pregnant cervix as an objective measure of cervical favorability (Bishop score > or = 5). METHODS: A prospective study of 86 women, investigating electrical impedance measurements of the pregnant cervix at the time of induction of labor. Transfer electrical impedance measurements were made by placing a tetrapolar pencil probe of 8 mm in diameter on the surface of the cervix. A Bishop score was determined simultaneously. RESULTS: A mean resistivity (standard error of the mean) of 7.03 (6.01-8.04) omega(m) was measured for the unfavorable group and 5.34 (4.61-6.07) omega(m) for the favorable group. This was a statistically significant difference (p = 0.016). CONCLUSION: We highlight the ability of this safe, painless technique to differentiate the favorable from the unfavorable cervix at induction of labor.

Incorporating a priori information into the Sheffield filtered backprojection algorithm.

The Sheffield image reconstruction algorithm is based on the assumption that the initial conductivity distribution of the body being imaged is uniform. In many situations this assumption is violated. Previous attempts at incorporating a priori information by modifications to the backprojection matrices were ineffective. An alternative method of including a priori information became apparent following the recent reformulation of the Sheffield image reconstruction algorithm. Applying matrix algebra to the image reconstruction equations reveals that the incorporation of a priori information can be considered as either a modification to the original backprojection matrix or a component of a composite data filter. A simple two-dimensional model of the adult head was chosen to test this approach. The results of imaging studies using synthetic data derived from this model show that a priori information has been successfully incorporated into the image reconstruction process. The ability to include a priori information into the image reconstruction process may have significant implications for the more challenging applications of EIT such as imaging the adult head.

Image reconstruction using non-adjacent drive configurations.

The standard 'back projection' image reconstruction algorithm developed at Sheffield uses data obtained by passing current between adjacent pairs of electrodes and measuring voltage differences between the remaining pairs. Previously it has been argued that this configuration gives the best resolution compared to all other bipolar drive configurations. However, it also gives the worst signal to noise ratio data and it is possible that under conditions of poor signal to noise ratio it may be advantageous to use an alternative drive configuration, even at the expense of resolution. A general image reconstruction algorithm for any bipolar drive configuration has been produced. Reconstruction algorithms for the adjacent, cross (drive electrodes 90 degrees apart) and polar drive (drive electrodes 180 degrees apart) have been examined and compared in terms of their resolution and noise performance.

A real-time volumetric visualization system for electrical impedance tomography.

Three dimensional (3D) electrical impedance tomography (EIT) presents many additional challenges over and above those associated with two dimensional EIT systems. With present two dimensional (2D) systems, tomographs can be reconstructed and displayed on a PC with a standard computer monitor. In addition, using appropriate data acquisition hardware and simple image reconstruction algorithms, it is possible to collect, reconstruct and display volumetric EIT images in real time using parallel processing architectures. The advantages of this 'real-time' capability are many and include the ability to immediately assess the correct functioning of the system and the ability to track patient events and the effect of procedures in real time. Whilst 3D EIT boundary datasets can be collected in real time, their real-time image reconstruction and display presents some computational challenges. This explains why, to date, no real-time solutions have been presented. In addition the use of a standard computer monitor to display 3D volumes is unsatisfactory since not all depth cues are preserved when using this type of 2D display device. We present a system which is capable of displaying 3D EIT datasets in real time and allows interactive modification of the user's viewpoint. This allows the user to fly around (and through) the EIT volumetric dataset.

In vitro multifrequency electrical impedance measurements and modelling of the cervix in late pregnancy.

Idiopathic preterm labour is the greatest single perinatal problem occurring in an unpredictable 6-8% of all pregnancies and accounting for 75% of all perinatal deaths. Preterm cervical softening is used clinically as an important indicator of cervical dysfunction but the subjective nature of present clinical assessment methods prevents reliable prediction of preterm labour. This paper reports the finding of a pilot investigation concerned with obtaining quantitative measurements of the in vitro electrical impedance of the cervix using a four-electrode multifrequency impedance measurement system. Impedance measurement obtained from six samples of cervical tissue taken from different subjects of caesarean section were fitted to the Cole equation and parameters derived to describe the ratio of extra- versus intracellular impedance and the characteristic frequency. Subjects at term display a lower extra- versus intracellular impedance ratio than the preterm subjects. This appears consistent with the expected increase in the hydration of the cervix approaching term and the resulting decrease in the extracellular impedance. Further studies are required to determine if multifrequency electrical impedance tomography could be used as a non-invasive screening test for preterm labour.

Analytical solution to the three-dimensional electrical forward problem for an elliptical cylinder.

In electrical impedance tomography, the shape of the object being imaged (such as the human thorax) is often complex. For this reason, numerical techniques, such as finite element method, are often used for solving the forward problem in 3D rather than analytical solutions which can only model simple geometrical shapes. However, an analytical solution to the 3D forward problem can often be useful. This paper will present an analytical solution to the forward problem for an elliptical cylinder whose eccentricity can be easily modified to approximate the shape of the human thorax.

Three-dimensional image reconstruction for electrical impedance tomography.

Very little work has been conducted on three-dimensional aspects of electrical impedance tomography (EIT), partly due to the increased computational complexity over the two-dimensional aspects of EIT. Nevertheless, extending EIT to three-dimensional data acquisition and image reconstruction may afford significant advantages such as an increase in the size of the independent data set and improved spatial resolution. However, considerable challenges are associated with the software aspects of three-dimensional EIT systems due to the requirement for accurate three-dimensional forward problem modelling and the derivation of three-dimensional image reconstruction algorithms. This paper outlines the work performed to date to derive a three-dimensional image reconstruction algorithm for EIT based on the inversion of the sensitivity matrix approach for a finite right circular cylinder. A comparison in terms of the singular-value spectra and the singular vectors between the sensitivity matrices for a three-dimensional cylinder and a two-dimensional disc has been performed. This comparison shows that the three-dimensional image reconstruction algorithm recruits more central information at lower condition numbers than the two-dimensional image reconstruction algorithm.

Resistivity changes in conductive silicone sheets under stretching.

This paper reports a preliminary finding associated with an investigation of how tissues respond to mechanical stress. The stress distribution within the tissue may be the result of normal function, for example, joint forces, or it may result from interventions such as tissue suturing during or after surgery. We sought to combine electrical and mechanical computational models in order to better understand the interaction between the two. For example, if mechanical stress is applied to tissue this may change the cell arrangements within the tissue matrix and hence change the electrical properties. If this interaction could be determined, then it should be possible to use electrical impedance tomography measurements to identify stress patterns in tissues. Measurements of resistivity changes have been made in conductive silicone rubber sheets when subject to a uniaxial stress of up to 10%. Relatively large changes in resistivity are produced (up to 200%). These changes are far larger than those predicted arising from topological changes alone. It is suggested that under stress the conductive islands of carbon within the silicone rubber sheet undergo a reversible disassociation from their neighbours and that the material's electrical properties change under load. If similar stress-resistivity relationships occur within biological materials it may be possible to recover the stress fields within tissues from transfer impedance measurements and thereby predict if actions such as inappropriate suture tension will compromise tissue viability.

Anatomical and physiological models for surgical simulation.

A considerable amount of effort has been aimed towards developing real-time deformable objects for surgical simulation, but very little work has been aimed towards including physiology within the soft tissue models. A simulator that links the structural and functional aspects of the human body would allow the user to develop a better understanding of the intrinsic link between anatomy and physiology. This positional paper discusses the challenges facing the creation of and the development of an integrated physiological and anatomical soft tissue model for use in surgical simulators. It explores the artificial dichotomy between anatomy and physiology and the issues it raises, by considering a suturing simulator capable of modelling ischaemia.

Back-projection distortions in applied potential tomography images due to non-uniform reference conductivity distributions.

An investigation was conducted to determine and quantify the distortions in applied potential tomography (APT) images reconstructed from data originating from bodies of non-uniform reference conductivity distributions. The results show that the distortions in the images are dependent on the reference conductivity distribution and on whether the images are formed by back projection along the assumed equipotentials of a uniform reference conductivity distribution or along the equipotentials of the true conductivity distribution. We believe that this last finding is significant since our previously held expectation, similar to that of Yorkey and Webster (1987), that back projection along the true equipotentials of the reference conductivity distribution should result in an accurate reconstruction, is shown to be incorrect.

An in vivo comparative study of the pregnant and nonpregnant cervix using electrical impedance measurements.

Electrical impedance may be measured using electrodes on the surface of the cervix and recording the potential that results when an electrical current is passed. Increased hydration of the cervix has been described throughout pregnancy and occurs most dramatically before labour. This study compared tissue impedance measurements of the pregnant and non-pregnant cervix and found a statistically significant lower value (P < 0.001) in pregnancy. Further work may show that such measurements alter in relation to labour onset.