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Rapid microwave imaging of living tissues

Microwave imaging is a relatively new modality to perform non- invasive diagnostic of biological tissues. The potential of these techniques results from the dependence of the dielectric properties of these tissues with respect to quantities of practical and clinical relevance such as water content, blood flow rate, temperature, etc. Initiated in the 80s, these techniques have suffered from the complexity of the interaction mechanisms between microwave beams and biological structures. The compensation of strong diffraction effects by high dielectric contrast structures requires efficient reconstruction algorithms. Recently, major advances have been achieved for improving the time resolution and the quantification of microwave images. This paper is focused on the first aspect. It reports some unique results obtained, for the first time, with a microwave camera providing qualitative images of biological targets at the rate of 15 images per second. Image reconstruction is performed by means of spectral diffraction tomography algorithms. The camera consists of a 2D array of 32 X 32 sensors, covering a square area of approximately 22 cm X 22 cm. The camera is operated at 2.45 GHz, according to the Modulated Scattering Technology (MST). The biological target is immersed in water (or can be inserted between bolus) and illuminated by a lens-compensated horn antenna. From the amplitude/phase measurement of the field scattered by the target, microwave images can be reconstructed, thanks to numerical focusing, in any plane located between the transmitting antenna and the camera. Typically, the investigation depth is 25 cm, and the spatial resolution is about 5 mm. The capabilities of this microwave camera will be illustrated by means of a short VHS video tape showing quick motions of living structures. Expected improvements of the camera performances are discussed and possible clinical applications are analyzed.