BackgroundLittle is known about cerebral artery resistive index values in infants born extremely preterm.ObjectiveTo report resistive index values in various cerebral arteries in a prospective cohort of preterm infants born at <29 weeks’ gestation, and to compare resistive index in these arteries and assess the relationship between resistive index and hemodynamically significant patent ductus arteriosus.Materials and methodsUsing Doppler imaging, we obtained resistive index values of internal carotid arteries, basilar artery, anterior cerebral artery, and pial and striatal arteries in the first 3 days of age and weekly thereafter until discharge or death. We analyzed paired observations using the Wilcoxon signed-rank test, between-group comparisons with the Mann–Whitney test.ResultsWe performed 771 examinations in 235 infants. Resistive indices differed among arteries: vessels with larger diameters showed significantly higher resistive indices. Resistive index in infants without patent ductus arteriosus was lower than that in infants with hemodynamically significant patent ductus arteriosus (median in anterior cerebral artery: 0.75 and 0.82, respectively; P<0.001), though this was not statistically significant in all arteries. There was no difference in pre- and post-ligation resistive indices in infants who underwent patent ductus arteriosus ligation.ConclusionFor accurate follow-up and comparison of cerebral artery resistive index, the same artery should be examined on each occasion.

Acoustically sensitive emulsion droplets composed of a liquid perfluorocarbon have the potential to be a highly efficient system for local drug delivery, embolotherapy or for tumor imaging. The physical mechanisms underlying the acoustic activation of these phase-change emulsions into a bubbly dispersion, termed acoustic droplet vaporization, have not been well understood. The droplets have a very high activation threshold, its frequency dependence does not comply with homogeneous nucleation theory and focusing spots have been observed. We showed that acoustic droplet vaporization is initiated by a combination of two phenomena: highly nonlinear distortion of the acoustic wave before it hits the droplet, and focusing of the distorted wave by the droplet itself. At high excitation pressures, nonlinear distortion causes significant superharmonics with wavelengths below the diameter of the droplet. Because these superharmonics strongly contribute to the focusing effect, the mechanism also explains pressure thresholding effects. In an accompanying paper, mathematical modeling aspects are presented. A proposed model is validated with experimental data captured with an ultra high-speed camera on the positions of the nucleation spots. Moreover, the presented mechanism explains the hitherto counterintuitive dependence of the nucleation threshold on the ultrasound frequency.