Complex permittivities of in vitro diseased and undiseased human female breast tissues have been measured at 3.2 GHz using a resonant cavity technique. Ranges of dielectric properties and water contents of these tissues are presented. Experimental data are compared with models predicted from mixture equations. Measured permittivity data lie within limits set by two-phase mixture theory, but some conductivity data are in excess of those expected for a mixture of saline and protein. At any particular microwave frequency in all tissue of a given type, the relationship between permittivity and conductivity may be parametrized using the Debye relaxation equations. For each breast tissue type a characteristic relaxation frequency was calculated and found to be lower than that of physiological saline at the same temperature. It is concluded that the dielectric relaxation of tissue water is not the only dispersive process occurring at this frequency: dielectric relaxation of bound water and the tail end of a beta-dispersion may also contribute to the dielectric properties. The similarity of the dielectric properties of benign and malignant breast tumours measured in this work suggest that in vivo dielectric imaging methods will not be capable of distinguishing them.

Post-market modification of valves from AMBU bags (AMBU Inc, Columbia, MD, USA) may be more susceptible to failure during use compared with our use of commercial pressure regulators (produced under ISO standards). Our data do not cover the full range of clinical parameters. For our studies, inspiratory times were kept fixed, although in actual patients, inspiratory times may be intermittently adjusted. Furthermore, this scheme is not intended as a permanent solution for ventilating multiple patients, and should be used only with hospital administration approval and acknowledgement of the unique ethical considerations during a crisis (such as the COVID-19 pandemic). 11,12 Although the COVID-19 pandemic inspired our designs, it may have utility in other mass casualty scenarios such as natural disasters, terrorist attacks, and battlefield medicine. Future versions should aim to extend to more than two patients per ventilator. Declarations of interest GWF is a consultant for and on the speaker's bureau of Edwards LifeSciences. All other authors have no conflicts to declare. 4. Wang D, Hu B, Hu C, et al. Clinical characteristics of 138 hospitalized patients with 2019 novel coronavirusinfected pneumonia in Wuhan, China. JAMA 2020; 323: 1061e9 5. Neyman G, Irvin CB. A single ventilator for multiple simulated patients to meet disaster surge.

The use of the Allan deviation for the analysis of signal noise and drift components is considered in the context of microwave radiometry. The noise behaviour of two types of microwave radiometer is modelled and compared with measurements of the performance of these radiometers analysed using the Allan deviation method.

A phantom target with a known linear temperature gradient has been developed for validating microwave radiometry for measuring internal temperature profiles within human tissue. The purpose of the phantom target is to simulate the temperature gradient found within the surface layers of a baby's brain during hypothermal neuroprotection therapy, in which the outer surface of the phantom represents the skin surface and the inner surface the brain core. The target comprises a volume of phantom tissue material with similar dielectric properties to high water-content human tissue, contained between two copper plates at known temperatures. The antenna of a microwave radiometer is in contact with one surface of the phantom material. We have measured the microwave temperature of the phantom with microwave radiometry in a frequency band of 3.0–3.5 GHz. Our microwave temperature measurements have small 0.05 °C (type A) uncertainties associated with random effects and provide temperatures consistent with values determined using theoretical models of the antenna–target system within uncertainties. The measurements are in good agreement with the major signal contribution being formed over a near plane-wave response within the material with a much smaller contribution from close to the antenna face.

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