RESUMO
Traditionally, image reconstruction in electrical impedance tomography (EIT) has been based on Laplace's equation. However, at high frequencies the coupling between electric and magnetic fields requires solution of the full Maxwell equations. In this paper, a formulation is presented in terms of the Maxwell equations expressed in scalar and vector potentials. The approach leads to boundary conditions that naturally align with the quantities measured by EIT instrumentation. A two-dimensional implementation for image reconstruction from EIT data is realized. The effect of frequency on the field distribution is illustrated using the high-frequency model and is compared with Laplace solutions. Numerical simulations and experimental results are also presented to illustrate image reconstruction over a range of frequencies using the new implementation. The results show that scalar/vector potential reconstruction produces images which are essentially indistinguishable from a Laplace algorithm for frequencies below 1 MHz but superior at frequencies reaching 10 MHz.
Assuntos
Algoritmos , Impedância Elétrica , Aumento da Imagem/métodos , Interpretação de Imagem Assistida por Computador/métodos , Imageamento Tridimensional/métodos , Modelos Biológicos , Tomografia/métodos , Simulação por Computador , Análise de Elementos Finitos , Armazenamento e Recuperação da Informação/métodos , Imagens de Fantasmas , Tomografia/instrumentaçãoRESUMO
We conducted a short study on 8 volunteer subjects to establish whether physiological changes occurring as a result of the menstrual cycle affect tissue electrical properties. For this study subjects submitted to electrical impedance tomographic breast measurement four times, over two cycles at two different points in the cycle. Statistical analysis based on reconstructed values of conductivity and permittivity were conducted using the t-test for difference of means. The results were inconsistent, with some subjects showing a difference between the two phases and in all tests, while others showed differences only in some of the tests. At this time we can only conclude that a difference is more likely than not, although it could be a phenomenon only measurable in some individuals and not others. It seems that a larger study may be in order to establish this fact definitively.
Assuntos
Mama , Impedância Elétrica , Ciclo Menstrual , Tomografia/métodos , Adulto , Feminino , Humanos , Pessoa de Meia-Idade , Pré-MenopausaRESUMO
Electrical impedance tomography (EIT) has been used in the recent past for a number of clinical applications. In this work we present recent tomographic and spectroscopic findings for breast imaging from clinical exams completed at Dartmouth. The results presented here are based on 18 normal and 24 abnormal subjects. The participants were classified as normal or abnormal using the American College of Radiology (ACR) indexing system for mammograms. The EIT data were collected for ten discrete frequencies in the range 10 kHz-1 MHz using a single array of 16 electrodes. The finite element method was used to reconstruct the images. The images were examined visually and were compared with mammograms. The results were also analyzed based on zonal averages of property values and breast tissue radiodensities. Statistical analysis showed a significance difference between the mean conductivity and permittivity values of normal and abnormal subjects for various zones defined on the reconstructed images. Tissues with high radiodensity also had increased conductivity and permittivity.
Assuntos
Neoplasias da Mama/diagnóstico , Mama , Impedância Elétrica , Tomografia/métodos , Feminino , Humanos , Programas de Rastreamento/métodos , Modelos BiológicosRESUMO
In electrical impedance tomography surface measurements of voltages and currents are recorded and the image reconstruction algorithm uses this set of boundary data to estimate internal electrical properties of the region under investigation. Therefore correct and accurate modelling of the current and voltage distributions (forward model) is an essential part of any reconstruction method. In this paper, we explored the root cause of a boundary layer effect in the reconstructed conductivity map and found it to be an artefact arising from 2D to 3D data-model mismatch within the imaging algorithm. We propose a data calibration scheme that improves the reconstruction results by removing these boundary or edge effects. We present both two-dimensional and three-dimensional images for agar phantoms using this data calibration scheme which are markedly better than their counterparts recovered when the measurement data are not calibrated with the procedure outlined herein.
