The Dielectric Properties of Axillary Lymph Nodes

Knowledge of axillary lymph node dielectric properties is fundamental to assess microwave imaging as a potential technique to diagnose axillary lymph nodes in patients affected by breast cancer. The present chapter addresses the characterisation of axillary lymph node dielectric properties with different approaches, as this characterisation is challenged by the axillary lymph node heterogeneity and the clinical restrictions in measuring them. The complex permittivity of human and animal healthy lymph nodes was measured using the open-ended coaxial-probe technique, over the 0.5–8.5 GHz frequency band. It was found that their relative permittivity ranges between 30 and 50 at 4.5 GHz. As a parallel approach, a novel methodology to infer tissue dielectric properties based on magnetic resonance imaging data is proposed as a more accessible method to infer axillary lymph node permittivity. Our results confirm the high variability of healthy axillary lymph node dielectric properties, and suggest that metastasised axillary lymph node properties are 32% (average) higher and more consistent than those of healthy axillary lymph nodes, which motivates studying axillary lymph node microwave imaging. Finally, the effects of freezing and defrosting on the dielectric properties of biological tissues are studied, which is motivated by the possibility of measuring lymph nodes from dead animals, that were previously frozen and defrosted. The results suggest that the measurements of defrosted lymph nodes will not affect their dielectric properties by more than 9% at 4.5 GHz, paving the way for previously frozen lymph node measurements.