Bioimpedance spectroscopy has shown potential as a method for characterising biological tissue with the use of a tetrapolar electrode configuration. Brown et al. (2000) demonstrated that the configuration is capable of distinguishing between normal squamous epithelium and Cervical Intra-epithelial Neoplasia (CIN). However little has been done to identify the volumes of tissue that contribute to the measured impedance. Brown et al. employed a probe with a single tetrapolar electrode set thus analysing single points of tissue. The probe was required to be moved in order to "sample" other areas of tissue. This method provides no spatial information of the lesion boundaries. The overall objective of this research was to design and construct an impedance mapping system (IMS) for objective virtual biopsy of lesions by bioimpedance spectroscopy (BIS). Initially freshly excised cervical tissue was to be tested however as the study progressed this proved problematic and bovine blood was chosen as a suitable substitute. Specific aims were to;
- .Investigate the spatial sensitivity distribution of the tetrapolar electrode configuration via finite element analysis (FEA).
- Design a novel front end multiplexing system and multi-electrode array for mapping the impedance of the tissue of interest.
- .Experimentally confirm the efficacy of the approach to identify regions of different impedances and their boundaries using bioimpedance mapping.
The present study used finite element analysis (FEA) to investigate the spatial variation in sensitivity of the tetrapolar electrode configuration and identify which volumes of tissue were included in the measured impedance. An impedance mapping device was also designed and constructed utilising the tetrapolar electrode configuration in an expanded array of 25 electrodes. This array allowed the surface of an area of tissue to be mapped and lesion boundaries identified in an objective manner. FEA was also used to model lesions in healthy tissue and the sensitivity fields associated with the tetrapolar configuration. The FEA indicated that anomalous results would be obtained when a lesion was located between a drive and measurement electrode pair. In this case the lesion resulted in an increase in impedance with respect to the impedance of healthy tissue, whereas a lesion should result in a decrease in measured impedance relative to that of healthy tissue. The anomaly was found to produce false negative results for small lesions up to 0.4 mm and even a lesion with radius of approximately 0.75 mm could be undetected as the measured impedance spectrum for such a lesion is similar to that of healthy tissue. Modelling also provided insight into the sensitivity fields for an electrode array and its efficacy in accurately measuring the surface impedance of tissue and lesions of interest. The impedance mapping system (IMS) developed used an array of 25 (5x5) electrodes. The array allows 64 individual tetrapolar measurements to be obtained at 16 locations, providing an impedance map of 49 mm2 on the surface of a tissue sample. Multiple measurements at each location reduce the chance of anomalous results since these can be identified and excluded. Software was developed to display the measured impedance maps and regions of different impedance were easily identified Testing of the IMS using bovine blood showed separation of the measured impedance for a range of haematocrit between 0 - 80%. Introduced volumes of red blood cells (RBC) or clots (to mimic lesions) to the plasma (haematocrit 0%) were also clearly identified using the IMS. It was seen that measurements made at the boundary of 2 different haematocrits (ie 2 volumes of different impedance) resulted in an anomalous result as indicated by the FEA modelling. However it was demonstrated that these anomalies can be used to objectively identify the introduced RBC (lesion) boundaries. A more efficient electrode stepping sequence was also developed taking advantage of the reciprocal nature of the tetrapolar electrode configuration. This development allows for the electrode array to be doubled in size using the same components, and to sample twice the surface area in the same time taken using the initially developed system. In summary, an impedance mapping system has been modelled, designed and developed for tissue characterisation by bioimpedance measurements. The technique has been shown experimentally to be able to detect regions of differ- ent impedance and is in agreement with the finite element analysis performed. Further development of the IMS will allow progressive monitoring of suspect lesions in-vivo and better identification of their spatial distribution for biopsy.
| Identifer | oai:union.ndltd.org:ADTP/265745 |
| Date | January 2008 |
| Creators | Smith, Jye Geoffrey |
| Publisher | Queensland University of Technology |
| Source Sets | Australiasian Digital Theses Program |
| Detected Language | English |
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