Microwave radiometric thermometry in layered tissue structures.

Non-invasive thermal imaging and temperature measurement by microwave radiometry is investigated for medical diagnostic applications and monitoring hyperthermia treatment of cancer, in the context of the heterogeneous body structure. The temperature measured by a radiometer is a function of the emission and propagation of microwaves in tissue and the receiving characteristics of the radiometric probe. Propagation of microwaves in lossy media is analysed by a spectral diffraction approach. Extension of this technique via a cascade transmission line model provides an efficient algorithm for predicting the field patterns of aperture antennas contacting multi-layered tissue. Comparisons of computer simulations with field measurements in homogeneous and bi-layered tissue-equivalent media confirm the validity of the algorithm. A coherent radiative transfer analysis is used to relate the field pattern of a radiating antenna to its receiving characteristics when used as a radiometer probe, leading to a method for simulating radiometric data. The design and construction of a 4.6 GHz radiometer is described and good agreement is found between computer simulations and radiometer measurements in tissue equivalent phantoms. Measurements and simulations are used to assess the effect of overlying fat layers upon radiometer response to temperature hot-spots in muscle-type media. It is shown that dielectric layering in tissue greatly influences measured temperatures and should be accounted for in the interpretation of radiometric data. The feasibility of employing microwave radiometry for tomographic mapping of differential temperature distributions induced by hyperthermia is examined. A suitable reconstruction algorithm is proposed; however the limited 'depth of view' in lossy tissue is shown to restrict the volume which can be imaged and thus its use for monitoring deep hyperthermia is doubtful. Alternative applications of this technique in medical diagnostics are proposed.