Low Frequency Ultrasound Transcranial Imaging with Coherent Compounding of Diverging Chirp Waves

For transcranial imaging based on contrast microbubbles, the acoustic window is limited to the temporal bone with strong attenuation. Consequently, the images obtained with conventional compounded plane waves (CPW)transmission is often not good in contrast-to-noise ratio (CNR). This is partly attribute to that short pulse commonly has low ability to penetrate the temporal bone. And moreover, CPW will raise a shadow area in the relatively deep field where beams are not overlapped. Although this can be improved by increasing the numbers of transmitting angles, it will reduce the frame rate. Therefore, to realize improvement both in detective sensitivity of microbubbles and CNR in transcranial imaging, the compounding of diverging chirp waves (CDCW), which combined the low frequency signals and chirp excitation with compounded diverging waves imaging, was investigated in this study. After confirming that the deflection angle of 30° and the angle interval of 2° should provide satisfactory imaging result, a phantom experiment with the 3-D printed skull and blood vessel model was performed and compared with CPW method. The results proved that CDCW can improve 2.95 dB of CNR with larger perspective, which can be an effective method for the future study of transcranial ultrasound imaging.