HIGH-FREQUENCY PULSE-COMPRESSION ULTRASOUND IMAGING WITH AN ANNULAR ARRAY

High-frequency ultrasound (HFU) allows fine-resolution imaging at the expense of limited depth-of-field (DOF) and shallow acoustic penetration depth. Coded-excitation imaging permits a significant increase in the signal-to-noise ratio (SNR) and therefore, the acoustic penetration depth. A 17-MHz, five-element annular array with a focal length of 31 mm and a total aperture of 10 mm was fabricated using a 25-mu m thick piezopolymer membrane. An optimized 8-mu s linear chirp spanning 6.5-32 MHz was used to excite the transducer. After data acquisition, the received signals were linearly filtered by a compression filter and synthetically focused. To compare the chirp-array imaging method with conventional impulse imaging in terms of resolution, a 25-mu m wire was scanned and the -6-dB axial and lateral resolutions were computed at depths ranging from 20.5 to 40.5 mm. A tissue-mimicking phantom containing 10-mu m glass beads was scanned, and backscattered signals were analyzed to evaluate SNR and penetration depth. Finally, ex-vivo ophthalmic images were formed and chirp-coded images showed features that were not visible in conventional impulse images.