5B-6 Chirp Pulse-Compression Imaging Using a 40-MHz Annular Array

Very-high-frequency (>30 MHz) ultrasound allows fine-resolution imaging at the expense of depth-of-field and acoustic penetration depth. Mechanically scanned annular arrays have demonstrated that they can extend depth-of-field significantly. This study investigates chirp-coded excitation to increase penetration depth. A 40-MHz, five-element annular array with a focal length of 12 mm and a total aperture of 6 mm was excited with a 4-mus linear chirp spanning 15 to 65 MHz. The chirp was optimized based on ex vivo experiments and computer simulations. The chirp was tapered in order to limit pulse- compression axial side-lobe artifacts. To form images, all 25 transmit/receive signal combinations were digitized and post- processed for compression and synthetic focusing. Compression consisted of linearly filtering the signals with the time-reversed excitation chirp modulated by a Dolph-Chebyshev window. A 12-mum wire phantom was scanned to quantify lateral and axial resolutions; results showed that the chirp did not significantly degrade resolution. Experiments with scattering phantoms showed that penetration depth was improved by 1.8 mm (i.e., 25%). Images of mouse embryos, human eye-bank eyes, and low- contrast phantoms also showed marked improvements in image- quality.