40 MHz annular-array in utero imaging of mouse embryos with chirp coded excitation

High-frequency ultrasound (HFU) imaging systems operating at 40-MHz provide fine-resolution for small-animal imaging applications. Commercial and research HFU systems are based on fixed-focus transducers, which suffer from a limited depth of field (DOF) because of the short wavelength and the low fixed F-number. At high frequencies, imaging is limited to shallow depths because of the significant attenuation in most tissues. Exciting transducers with coded excitations has been shown to increase in signal-to-noise ratio (SNR) and penetration depth. In the present study, we implemented and validated chirp-imaging methods with an annular array operating near a center frequency of 40-MHz. Following validation studies, chirp-imaging was applied to visualize mouse embryos. Chirp-based images of the mouse embryos showed increased sensitivity compared to a standard monocycle imaging method. Three-dimensional (3D) datasets of 12.5-day-old mouse-embryo heads were acquired in utero using chirp and conventional excitations. Images were formed and brain ventricles were segmented and reconstructed in 3D. The brain-ventricle volumes for the monocycle excitation exhibited artifacts that were visible with the chirp-based dataset reconstruction.