An HP 8510 automatic network analyser and a six-port reflectometer have been used with a purpose-built calculable open-ended coaxial line sensor to measure the reflection coefficients of various materials, including dielectric reference liquids, in the frequency range 50 MHz-2.0 GHz. Factors crucial for calculable measurements have been identified including associated measurement uncertainties. The reference material measurements have been used in critical studies of (i) a commonly employed lumped equivalent circuit model of the fringing fields of the sensor, and (ii) a numerical point-matching theory of the propagating and evanescent modes at the termination of the sensor. The equivalent circuit model is shown, both theoretically and experimentally, to be generally inadequate for the full range of complex permittivities and frequencies which the sensor could otherwise cover. The point-matching theory has been used to predict the reference material reflection coefficients and the agreement with the expected behaviour measured for deionised water is to within 0.007 and 0.7" for the magnitude and phase of the reflection coefficient, respectively. An inverse solution based on the point-matching theory has been developed to derive complex permittivity from reflection coefficients and it has been used to measure various polar liquids. We conclude that the inverse point-matching theory should enable a more widespread and accurate exploitation of the sensor technique for various applications including those in biomedicine and industrial quality control.

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An investigation has been undertaken of the effect of flow on NMR images. We term the new technique of flow analysis examined here, NMR rheotomography. Images were acquired through use of a spin-echo technique. They were of doped water passing through a glass U-tube, simulating a small vein, which fitted into the magnetic field gradient and radiofrequency coils of an NMR imaging apparatus. For the case of the inlet arm of the U-tube, the flow images displayed an intensity distribution which compared favorably to a simple physical model based on laminar flow. The intensity distribution observed for the outlet arm was explained through consideration of the perturbation to flow introduced by the U-bend. Generally, the results of the feasibility study indicate that (a) the NMR spin-echo image is modified by flow in a predictable manner, (b) display of the velocity profile is possible, (c) through use of a U-tube some separation can be demonstrated within a single image of the twin factors, proton incursion into the image "slice" and proton dephasing, which generally combine to modify the flow image intensity, and (d) discrimination between different modes of flow may be possible by NMR CT. The clinical potential suggested by the results is discussed; NMR rheotomography may prove to be particularly useful for the noninvasive diagnosis of structurally-originating cardiovascular defects.

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