The mouse is a tremendously valuable animal model system in which t o explore both normal development and human disease models. Because humans share approximately 95% of their genes with the mouse a large number of diseases such as congenital heart disease, cancer and atherosclerosis are now being studied. The present study focuses on the formation of the embryonic mouse heart and in particular on changes in backscatter intensities from the developing blood system. Significant changes in brightness of the ultrasound images of the embryonic mouse heart at different ages of gestation have been observed. We hypothesize that these changes are due t o the early development of Red Blood Cells (RBCs). In order t o quantify these changes, measurements of the frequency dependence of the backscatter coefficient from blood within heart chambers were performed in utero and in vivo. For this calculation 100 radio-frequency lines were collected from the chambers of the embryonic mouse heart positioned at the focal zone of the probe, using a 40 MHz ultrasound biomicroscope (VisualSonics VS40). These signals were corrected for diffraction effects and the system transfer function in order t o estimate the backscatter coefficient. Measurements were performed on 23 embryos in 6 anesthetized pregnant CD1 mice at frequencies of 25 MHz and 38 MHz. At 25 MHz the backscatter coefflcient increased between days 12.5 and 13.5 of embryonic development. Then this coefficient decreased from 13.5 Embryonic Days (ED) t o 17.5 ED. At 38 MHz the backscatter coefficient behaved the same way. The observed variations are consistent with the stages of RBC development in which the nucleus is progressively lost between days 13.5 t o 17.5. Correlation with histology is shown.
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