Annular‐array 35‐MHz chirp imaging applied to biomedical studies.

High‐frequency ultrasound (HFU) is an attractive means of obtaining fine‐resolution images of biological tissues for ophthalmologic, dermatological, and small‐animal applications. Nevertheless, even with current improvements in circuit designs and high‐frequency equipment, HFU suffers from two limitations. First, HFU images have a limited depth‐of‐field (DOF) because of the short wavelength and the low‐fixed F‐number of HFU transducers. Second, HFU is usually limited to shallow imaging because of significant attenuation in tissues. Previously, a 17‐MHz five‐element annular array was excited using chirp‐coded signals and a synthetic‐focusing algorithm was used to form images. Results demonstrated significant increase in DOF and penetration depth. In the present study, the same approach was implemented with 35‐MHz five‐element annular arrays. Vitreous‐hemorrhage‐mimicking phantom was imaged. Chirp images of a phantom showed increased sensitivity compared to conventional monocycle images and blood droplets into the phantom could be visualized 4 mm deeper. Three‐dimensional datasets of 12.5‐day‐old‐mouse‐embryo heads were also acquired in utero using chirp and conventional excitations. Synthetically focused images were formed and the brain ventricles were segmented and reconstructed in three dimensions. Significant artifacts for the ventricle volumes were observed with the conventional imaging technique but were not apparent on the chirp‐based dataset reconstruction. [Work supported by NIH Grant No. EB006509.]