A study of contrast-enhanced functional microwave imaging

Microwave imaging (MWI) continues to develop as a low-cost portable complementary soft-tissue imaging modality, particularly in the context of breast cancer detection. Despite dramatic advancements in algorithmic development and signal acquisition, even the most recent imaging studies have shown that challenges still remain to improve this emerging technology's spatial and contrast resolution for anatomical imaging. However, MWI's ability to detect unique properties dependent on the physiological state of a tissue of interest at non-ionizing frequency ranges suggest it may be suitable for safer, cheaper functional imaging studies using non-radioactive contrast agents. This study explores applications in niches traditionally filled by nuclear medicine, where contrast-enhanced MWI could achieve resolutions comparable to existing imaging procedures but with no associated radiation dose. Non-toxic compounds that exhibit strong microwave-band responses, notably transition metal nanoparticles (S. Semenov et al., IFMBE Proc. 25/8, 311-313, 2009) and free radicals, may have promise in such contrast-enhanced imaging to provide metabolic rather than strictly anatomic data. To fully exploit the information available from these agents, the addition of an external weak polarizing magnetic field (PMF) across the imaging domain is necessary, which primarily influences ferromagnetic or strongly paramagnetic contrast agents within that domain (O.M. Bucci et al., IEEE Trans. Biomed. Eng., 58, 9, 2528-2536, 2011). Along with the traditionally measured changes in permittivity and conductivity, the PMF allows variations in magnetic susceptibility to contribute to the relevant microwave image data through resonance phenomena (P.C. Fannin, J. Mol. Liquids, 114, 79-87, 2004).