Imaging calibration of thermoacoustic tomography distortion caused by microwave diffraction

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.