Applications for multifrequency ultrasound biomicroscopy in mice from implantation to adulthood

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Abstract
A new multifrequency (19–55 MHz) ultrasound biomicroscope with two-dimensional imaging and integrated Doppler ultrasound was evaluated using phantoms and isoflurane-anesthetized mice. Phantoms revealed the biomicroscope’s lateral resolution was between 50 and 100 μm, whereas that of a conventional 13 MHz ultrasound system was 200–500 μm. This difference was apparent in the markedly higher resolution images achieved using the biomicroscope in vivo. Transcutaneous images of embryos in pregnant mice from ∼2 days after implantation (7 days gestation) to near term (17.5 days) were obtained using frequencies from 25 to 40 MHz. The ectoplacental cone and early embryonic cavities were visible as were the placenta and embryonic organs throughout development to term. We also evaluated the ability of the biomicroscope to detect important features of heart development by examining embryos from 8.5 to 17.5 day gestation in exteriorized uteri using 55 MHz ultrasound. Cardiac looping, division of the outflow tract, and ventricular septation were visible. In postnatal imaging, we observed the heart and kidney of neonatal mice at 55 MHz, the carotid artery in juveniles (∼8 g body wt) and adults (∼25 g body wt) at 40 MHz, and the adult heart, aorta, and kidney at 19 MHz. The coefficient of variation of carotid and aortic diameter measurements was 1–3%. In addition, blisters in GRIP1 −/− embryos and aortic valvular stenosis in two adults were readily visualized. Using image-guided Doppler function, low blood velocities in vessels as small as 100 μm in diameter including the primitive heart tube at day 8.5 were measurable, but high blood velocities (>37.5 cm/s) such as in the heart and large arteries in late gestation and postnatal life were off-scale. Accurate cardiac dimension measurements were impeded by poor temporal resolution (4 frames/s). In summary, the multifrequency ultrasound biomicroscope is a versatile tool well suited to detailed study of the morphology of various organ systems throughout development in mice and for hemodynamic measurements in the low velocity range.