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A uniform energy field of microwave illumination on sample is assumed for microwave-induced thermoacoustic
tomography. However, microwave transmitting out of the waveguide surface is nonuniform due to microwave diffraction,
which would lead to uneven excitation of thermoacoustic pressure. Hence, the thermoacoustic images may be distorted
especially at the edge of microwave radiation. A fast thermoacoustic tomography system at 6 GHz was developed for
phantom study and in vivo animals imaging. The effects of microwave distribution inhomogeneity on nonuniform
excitation of acoustic pressure were theoretically studied and a corresponding calibration algorithm for image distortion
was also provided and experimentally verified. The distribution formulas of microwave field were derived using a
Huygens diffraction principle model. Then a point microwave absorber moved under the microwave waveguide to
measure the microwave field distribution. The measure data is in good agreement with the deduced result.
Once the calibration map was obtained via the theoretical calculation, the TAT (thermoacoustic tomography)
images could be calibrated by dividing the reconstructed image by the calibration map. Thermoacoustic images without
and with calibration were reconstructed for comparison. According to the statistical results, after calibration the
thermoacoustic contrast can be enhanced 2 times or more. Also it can be supposed that the farther the distance away
from the illumination centre, the greater signal-noise-ratio (SNR) could be enhanced by the calibration. The results of
experiment showed that this method could achieve even distribution of SNR and improve the reconstructed image
quality. Therefore, this calibration method has potential application in solving the problem of imaging distortion
especially at the edge of microwave illumination.
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