Microwave-induced thermoacoustic imaging enhanced with a microwave contrast agent

Nanoparticles have drawn great attention as targeted imaging and therapeutic agents recently. However, so far, no research on radio-frequency absorption contrast agents in soft tissues has been reported. A microstructure agent, carbonyl iron, which has strong microwave absorption, is applied to microwave-induced thermoacoustic imaging. The size of this microsphere is approximately 2 μm that is safe for biological and medical application. The carbonyl iron emulsion is prepared by the Sol-Gel process which makes carbonyl iron particles more hydrophilic and with good electromagnetic properties. A 320-element linear transducer array with a centre frequency of 3.5 MHz was adopted to capture thermoacoustic signals within limited field around the sample. Using phase-controlled focus technique to collect thermoacoustic signals, the data need not be averaged because of a high signal-to-noise ratio, resulting in a data acquisition time of less than 5 s at each step. The collected data was utilized to reconstruct the geometrical details based on limited-field-filtered back-projection algorithm. The effect of solid particles for enhancing thermoacoustic grey scale images was successfully investigated in tissue phantoms, the image brightness, i.e., mean grey scale level, increased with particle concentration. This particle may have potential to improve sensitivity and specificity for structural and functional thermoacoustic imaging and broadly expand the thermoacoustic imaging capability as a powerful medical diagnostic tool. This paper provides the first demonstration of a microwave contrast enhancing agent for thermoacoustic imaging.