Pulse inversion based multi-subharmonic composite cavitation imaging

Cavitation imaging is necessary for safety and efficiency during HIFU therapy. Unencapsulated cavitation bubbles have greater nonlinearity than encapsulated contrast microbubbles, which could make it easier to generate subharmonic. Since the generation of subharmonics is related to bubble size, which is in a wide range, it is difficult to detect cavitation bubbles with various sizes by using only 1/2 subharmonic. Therefore, a multi-subharmonic composite cavitation imaging method was proposed in this study for improvement of cavitation imaging sensitivity. Several components including 1/2, 1/3 and 1/4 subharmonic were extracted by bandpass filters with corresponding center frequency respectively. The pulse inversion (PI) technique was utilized to eliminate the fundamental component confused with subharmonics. To image all the subharmonic components in one image, different subharmonic components whose intensities were greater than a threshold were coded with different colors and displayed simultaneously. The dynamic cavitation images were obtained between HIFU exposures to observe the evolution of cavitation bubbles. Imaging pulses with different pressure (0.15∼1.2 MPa) and cycle number (1∼5 cycles) were used to investigate their effect on sensitivity of subharmonic generation. The proposed method was validated with the experiments of porcine muscle in vitro. It was found that the obtained composite cavitation images can greatly suppress the tissue scattering, and have higher sensitivity than PI-based second harmonic and 1/2 subharmonic images. Moreover, the composite cavitation images can reflect the distribution of cavitation with respect to bubble size. In general, all the subharmonic components increase with HIFU treatment time. The highest intensities of subharmonic components were obtained when cycle number is 4 or 5 and pressure is 0.75 MPa. The results imply that the subharmonics would not be generated with low acoustic energy and high acoustic energy could make cavitation bubble collapse causing decrease of subharmonics.