ABSTRACT
CONTENTS 12.1 Introduction ..................................................................................................................... 411 12.2 Development of MWI .................................................................................................... 413
12.2.1 Frequency Dependence.................................................................................... 413 12.2.2 Scattering Parameters....................................................................................... 414 12.2.3 Scattering-Parameter Imaging of Tissue Permittivity................................. 415 12.2.4 Power, Signal Attenuation, and Signal-to-Noise Ratio .............................. 417
12.3 Three-Dimensional Formulation .................................................................................. 418 12.4 Wave Equation and VIE ................................................................................................ 419
12.4.1 Microwave Imaging.......................................................................................... 421 12.4.2 Electrical Impedance Tomography ................................................................ 422
12.5 Three-Dimensional Images Reconstructed from Simulated Three-Dimensional MWI Data ..................................................................................... 423
12.6 Two-Dimensional Images Reconstructed from Measured Two-Dimensional EIT Data........................................................................................... 423 12.6.1 Case 1: One Insulator Object Inside the Container ..................................... 423 12.6.2 Case 2: Two Insulator Objects Inside the Container................................... 425 12.6.3 Case 3: One Conductive and One Resistive Object Inside
the Container..................................................................................................... 426 12.7 Summary and Conclusions ........................................................................................... 428 Acknowledgment....................................................................................................................... 428 References ................................................................................................................................... 428
Two emerging types of electromagnetic imaging are presented: electrical impedance imaging (or tomography, EIT) and microwave imaging (MWI). Both techniques rely on the contrast in electromagnetic properties (complex permittivity) of the tissues to be imaged with that of the neighboring tissue region. Both techniques use an array of sensors
For to a known voltage is passed between all electrodes within an array to determine the electrical impedance (or admittance) of the imaged tissue. For MWI, a microwave signal is sequentially transmitted through the imaged tissue to all antennas within an array to determine the scattering parameters (ratio of reflected and transmitted signals to the incident signal) of the imaged region. EIT uses a lower frequency (in the kHz or MHz range) so that the imaged region is small compared with the signal wavelength. MWI uses a higher frequency (in the GHz range) so that the imaged region is comparable to the signal wavelength. The MWI frequency is chosen to be low enough to yield an adequate depth of penetration into the imaged tissue, but high enough to allow the use of a number of small, closely spaced antennas in the array. Since antenna size is also comparable to signal wavelength, microstrip patches or waveguide apertures on high-permittivity substrates are often used to reduce the wavelength at the antenna. Once MWI and electrical impedance imaging are fully developed for clinical use, they
have great potential for the early detection of breast cancer. Other imaging methods are now available, but they have certain disadvantages that limit their acceptance as the method of choice for breast cancer screening. Magnetic resonance imaging (MRI) relies on large electrical currents in cryogenically cooled conductors to produce a strong magnetic field. Hardware and safety requirements may continue to make MRI too expensive for widespread use in breast cancer screening. X-ray imaging is now used fairly extensively, but many women dread the discomfort or pain associated with having their breast sandwiched between two hard, flat surfaces in preparation for the screening, and their aversion to this test may prevent them from participating. For testing patients in the clinic, both MWI and EIT will most likely use the same
arrangement as far as the patient is concerned. The patient will lie face down in a comfortable position on a cot or gurney, as illustrated in Figure 12.1. The breast to be screened extends into an opening containing body-temperature liquid that has approximately the same electrical properties as the normal female breast. This liquid is held within a cylindrical or rectangular thin-walled plastic container that is attached beneath the cot. For MWI, numerous small antennas are attached to the entire outer surface of the plastic container. For EIT, numerous small metal electrodes are attached on the inside surface of the plastic container so that the electrodes can flow current through the liquid and the breast tissue under test. These are the techniques, along with numerical modeling,
reported herein. Although we describe how we obtain the breast cancer images using phantom (artificial) tissues, we have not yet moved our imaging systems into the clinic to image breast cancer in real patients. The research and development we are now doing are necessary steps that must be taken before successful clinical applications can occur.
