In utero, high-frame-rate mouse embryo cardiovascular retrospective imaging using 40 MHz annular array and Doppler-derived synchronization signals

An annular-array-based retrospective imaging protocol was developed to acquire high-frame-rate cine loops of in vivo, in utero mouse embryo hearts. A continuous wave Doppler apparatus was made from two 20 MHz, single-element, 3-mm diameter, PZT transducers with a Doppler sample volume of 15 mm. The imaging transducer was a custom, 40 MHz, annular array. The central element was used in transmit and M-mode data were acquired on all five channels simultaneously at a series of lateral locations. A synthetic-focusing algorithm was used to improve depth-of-field and signal-to-noise ratio. Embryos aged from 12.5 to 14.5 days were imaged in utero and respiratory gating was used to suppress motion artifacts. An automated algorithm was developed to identify the cardiac cycle phases using the spectrogram of the Doppler waveforms, segment the co-registered M-mode data, and reconstruct B-mode image cine loops at effective frame rates of 1 kHz. The mean cardiac cycle length was 300 ms and the ultra-fine temporal resolution (1 ms) provided detailed visualization of the heart chamber dynamics, including fast events such as the opening and closing of the mitral valve. This approach allowed for quantifying the cardiac dynamics of the embryonic heart at each developmental stage in optimal conditions, without sacrificing the embryo or the mother.