40-MHz ultrasound imaging with chirps and annular arrays.

High-frequency ultrasound (HFU) shows promise for fine-resolution imaging. However, the depth of field (DOF) and penetration depth of HFU waves limit clinical significance. In a previous study using a 17-MHz annular array, we established that chirp coded excitation and synthetic focusing can improve penetration depth and DOF simultaneously. In this study, we evaluated the same approach using two five-element annular arrays with focal lengths of 12 mm, total apertures of 6 mm. The annular arrays had center frequencies of approximately 35 MHz and were excited by a 4-micros chirp signal spanning the frequency range 15-65 MHz. Results demonstrated that DOF could be increased by a factor of about 3, SNR could be increased by more than 10 dB, and penetration depth into an attenuating phantom could be increased by 1.8 mm. The chirp imaging method was then evaluated on low-contrast phantoms and mouse embryos.