High-intensity focused ultrasound (HIFU) has proved to be an effective minimally invasive surgical technology. In this study, we focus on the visualization of HIFU-induced lesions using microwave-induced thermoacoustic tomography (TAT). TAT has high spatial resolution, comparable with ultrasound imaging, and high contrast, which is induced by differences in the microwave absorption rates between tumor tissue and normal tissue. TAT can, in addition, differentiate tumors before and after treatment. A single, spherically focused transducer operating at a center frequency of approximately 4 MHZ was used to generate the focused field. The lesion was generated in porcine muscle. A local-tomography-type reconstruction algorithm was applied to reconstruct the TAT image of the lesions. The lesion shown by gross pathology confirms the corresponding region measured by TAT.
Reconstruction for thermoacoustic tomography in an arbitrary
detection geometry is proposed by time-reversing the measured field back to the time when the thermoacoustic sources are excited. Time reversal of the field can be implemented efficiently by applying the delay-and-sum algorithm. The theoretical conclusions are supported by a numerical simulation of three-dimensional thermoacoustic tomography.
The effects of wavefront distortions induced by acoustic heterogeneities in breast thermoacoustic tomography (TAT) are studied. First, amplitude distortions are shown to be insignificant for different scales of acoustic heterogeneities. Next, the effects of phase distortions (errors in time-of-flight) in our numerical studies are investigated, and the spreads of point sources and boundaries caused by the phase distortions are studied. After that, a demonstration showing that the blurring of images can be compensated for by using the distribution of acoustic velocity in the tissues in the reconstructions is presented. Last, the differences in the effects of acoustic heterogeneity and the generation of speckles in breast TAT and breast ultrasound imaging are discussed.
Microwave-induced thermoacoustic tomography was explored to image biological tissues. Short microwave pulses irradiated tissues to generate acoustic waves by thermoelastic expansion. The microwave-induced thermoacoustic waves were detected with a focused ultrasonic transducer to obtain two-dimensional tomographic images of biological tissues. The dependence of the axial and the lateral resolutions on the spectra of the signals was studied. A self-adaptive filter was applied to the temporal piezoelectric signals from the transducer to increase the weight of the high-frequency components, which improved the lateral resolution, and to broaden the spectrum of the signal, which enhanced the axial resolution.