Developing Tissue Phantom Materials with Required Electric Conductivities
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TALLINN UNIVERSITY OF TECHNOLOGY Faculty of Information Technology Thomas Johann Seebeck Department of Electronics Hip Kõiv 132707 IVEM Developing tissue phantom materials with required electric conductivities Master thesis Supervisor: Senior research scientist Rauno Gordon Tallinn 2015 Author’s declaration I declare that I carried out this master thesis independently, and only with the cited sources, literature and other professional sources. I certify that this research thesis or any part of it has not been previously submitted for a degree or any other qualification at the Tallinn University of Technology or any other institution. Date: 04.06.2015 Author: Hip Kõiv ………………………. /signature/ 2 Thesis Assignment Name of topic: Developing tissue phantom materials with required electric conductivities Kudedele sarnaste elektrijuhtivustega materjalide disainimine ja valmistamine Origin of topic: Tallinn University of Technology, Thomas Johann Seebeck Department of Electronics Thesis goals: Give an overview of bioimpedance, phantoms used to mimic tissues, and van der Pauw resistivity measurement method. Develop a fixed recipe for gelatine phantoms with desired electric conductivities. Manipulate the conductivities of the phantoms with sodium chloride and accurately measure and calculate resistivity with van der Pauw method. Expected results: Design tissue phantom material and manipulate its conductivities easily with NaCl. Use van der Pauw method and prove its reliability measuring resistivity of developed phantoms. To gain knowledge of bioimpedance, tissue phantoms and van der Pauw method. Solved problems: It is proposed how to prepare low-cost and simple tissue similar gelatine phantoms and how to measure their resistivity with van der Pauw method. Date: ……………………………… Student: …………………………… ………..…………………….. /given name(s) and surname/ /signature/ Supervisor: ...…………………………. …..………………………….. /given name(s) and surname/ /signature/ 3 Hip Kõiv Developing tissue phantom materials with required electric conductivities Master thesis Abstract This thesis is the final work for fulfilment of the Master of Science degree at the Thomas Johann Seebeck Department of electronics in Tallinn University of Technology. The overall aim of this research is to develop inexpensive tissue similar phantoms with required electric conductivities and find a reliable measurement technique for said phantoms. The master project includes two main parts: review of the state of art and phantom construction and testing. First part introduces different phantoms used in biomedical fields and potentially suitable materials for mimicking tissues. It also reviews the main concepts of bioimpedance and van der Pauw method for determining conductivity. This section is mainly based on scientific publications analysed to give theoretical basis for assumptions made in the study. Second part includes development process of phantoms and their measurement results with implemented measurement technique. After analysing subject matter literature and considering possibilities, gelatine material is used to make homogeneous phantoms with tissue similar conductivities because other discussed materials were either too expensive or too problematic to prepare. Van der Pauw method is applied for conductivity measurements due the interest to investigate its accuracy and reliability measuring thick biological samples. Biological tissues have different electric conductivities, which means it should be also possible to manipulate with phantom conductivities. 4 Gelatine phantoms developed in current study are manipulated with NaCl to increase resistivity of samples and get desirable electric properties. NaCl dependence of conductivity is confirmed to be linear. Van der Pauw method for gelatine phantoms between 100 Hz to 1 MHz was proved to be applicable and gave satisfying results. In addition, graphite powder was added to the gelatine mixture to analyse its influence to electrical conductivity on low frequencies. Although, the results did not show reliable trend in conductivities measured, it is worth developing and researching further in future studies. Presented gelatine phantoms and van der Pauw measurement system used, proved to meet the goal of the thesis. Experiments showed relatively good agreement between theory and implementation part and phantoms with desired electric conductivities were prepared for development of bioimpedance technologies. The thesis is in English and contains 66 pages of text, 6 chapters, 30 figures and 6 tables. 5 Abbreviations vdP – van der Pauw CT – computed tomography EIT – electrical impedance tomography PVA – polyvinyl alcohol MRI – magnetic resonance imaging AC – alternating current BLM – bilayer lipid membrane 2D – two dimensional CNC – computer numerical control PCB – printed circuit board Wt/vol% - weight/volume percentage NaCl – sodium chloride 6 List of figures and tables Figure 1. Complex plane ................................................................................................................. 17 Figure 2. Suspension of spherical cells, each of radius a is showed (a). Electric circuit equivalent of the effective conductivity of the suspension. (b) [33] .................................................................. 18 Figure 3. Overview of how alternating current passes through the cells in biological tissues on different frequencies. ....................................................................................................................... 19 Figure 4. α-, β-, and γ-dispersions of permittivity (ε) and conductance (σ) showed in relation to frequency (f). [Bourne 1996, 24] ..................................................................................................... 20 Figure 5. Flat lamella of arbitrary shape, with four small contacts M, N, O and P on the periphery. [43] ................................................................................................................................................... 22 Figure 6. Schematic illustration of four contact configurations of Van der Pauw method [2]. ...... 24 Figure 7. Very thin sample geometry. Contacts are at the perimeter (a). The contacts are placed across the entire edge to produce uniform current distribution (b). D is the diameter and d is the thickness. [51] .................................................................................................................................. 26 Figure 8. Designed conductance cell for four-electrode measurement, where h is the height of the cell. Electrodes are named A, B, C, D and voltage and current is applied. [45] .............................. 28 Figure 9. Plexiglas enclosure with PCB-s inside it. ........................................................................ 29 Figure 10. Examples of planar coils with electrodes. On the left there is a side PCB with two electrodes for van der Pauw measurement and coil with 19 turns, diameter of 2 cm, and trace width of 0.1 mm. On the right there is a planar coil that goes on top of the gelatine cube with the same coil parameters. ................................................................................................................................ 30 Figure 11. Heating the gelatine mixture on water bath in the fume hood. ...................................... 33 Figure 12. When temperature rises resistance decreases and the relation to temperature is fairly linear. ............................................................................................................................................... 34 Figure 13. Wayne Kerr impedance analyser connected to gelatine sample with four electrodes. .. 36 Figure 14. Van der Pauw measurement method set up used in the research. Blue colour indicates one pair of measurement (1 and 3) and green colour other pair (2 and 4). ...................................... 37 Figure 15. 0.7 wt/vol% of salt is added is added to the gelatine mixture and four measurements are made. ................................................................................................................................................ 38 Figure 16. Syringe with gelatine mixture for resistivity measurement and for checking the correctness of our interpretation of van der Pauw method. .............................................................. 40 Figure 17. 3 samples with different salt concentrations were measured and conductivities were calculated with syringe and vdP method. Light blue, light red and light green lines represent conductivities found with syringe method and blue, red and green lines are conductivities calculated with vdP method. ............................................................................................................ 42 Figure 18. Conductivity of 18 different gelatine samples calculated. ............................................. 43 Figure 19. Conductivity dependence on NaCl. ............................................................................... 44 Figure 20. Graphite powder added to the gelatine phantom. .......................................................... 46 Figure 21. Sample 2 - 1 wt/vol% of NaCl and 44 g of graphite powder. Graphite flakes have settled in the bottom. ................................................................................................................................... 47 Figure 22. Sample 3 – 4 wt/vol% NaCl, 0 wt/vol% NaCl + graphite, 0 wt/vol% NaCl. ................ 48 Figure 23. 3 different samples with different layers constructed.