Noninvasive phenotypic analysis of cardiovascular structure and function in fetal mice using ultrasound

We established methods for noninvasive mouse fetal heart imaging using an Acuson/Siemens Sequoia ultrasound scanner equipped with a single-pulse CHIRP Coded Excitation program, and a highfrequency linear array transducer. Mouse fetuses spanning gestation day 12.5 to 18.5 (E12.5-E18.5) were studied.

Controlled anesthetic and constant body temperature were found to be essential for hemodynamic stability of the mother and fetuses. Fetal heart rates increased from 160 to 220 beats/min as development progressed. These heart rates were lower than those of newborn mice, so that frame rates above 100 Hz adequately resolved structural details in 2D without misregistration. Analysis of 2D images showed a doubling in crown-torump length (8-19 mm), and rapid growth of the heart from 1 to 3 mm in diameter as fetuses developed from E12.5 to E18.5. A cumulative increase in scanning modalities was achieved with increasing developmental age, with the optimal stage for scanning being E16.5. At E16.5 right and left could be distinguished, and it was possible to obtain diagnostic 2D color flow Doppler in the four-chamber, apical long axis 3/5-chamber and short axis views. In addition, M-mode images of high quality were obtainable from E15.5 to E18.5, whereas spectral Doppler signals could be obtained readily from E12.5 onwards.

These studies show that ultrasound imaging can be used for structural and functional analysis of the developing mammalian heart, even at early stages of development. Such noninvasive cardiovascular ultrasonic evaluation should be ideally suited for high throughput screening of mutagenized mice.