RF testbed for thermoacoustic tomography.

Thermoacoustic signal excitation is a function of intrinsic tissue properties and illuminating electric field. De-ionized (DI) water is a preferred acoustic coupling medium for thermoacoustics because acoustic and electromagnetic waves propagate in DI water with very little loss. We have designed a water-filled testbed propagating a controlled electric field with respect to pulse shape, power, and polarization. Directional coupler line sections permit measurement of incident, reflected, and transmitted powers. Both S-parameters and E(y) measurement show that the electric-field distribution is relatively uniform in testbed. Comparing baseline power measurements to those taken with a test object in place yields power loss in the object, which should correlate to thermoacoustic signal strength. Moreover, power loss--and therefore thermoacoustic computerized tomography signal strength--is sensitive to the orientation of the object to the polarization of the electric field. This testbed will enable quantitative characterization of the thermoacoustic contrast mechanism in ex vivo tissue specimens.