Measurement and simulation of acoustic radiation force on a planar reflector.

The accurate calculation of the acoustic radiation force is important for ultrasonic application techniques. Usually, the acoustic radiation force can be divided into the near-field and the far-field force according to the ratio of the emitter-reflector distance to the wavelength. In this study, appropriate theories and methods are explored to simulate the far-field and the near-field acoustic radiation force exerted on a planar reflector. The comparison between simulation and experiment indicates that the far-field force is not sensitive to the boundary shape and size while the near-field force is highly sensitive to the boundary size. Only the acoustic model with the minimized boundary size could yield the near-field force consistent with the experiment. Further calculations reveal that the far-field force first increases and then decreases with the rise of the reflector radius, and that the near-field force fluctuates with the acoustic frequency, especially when the emitter-reflector distance is very small. The near-field repulsive force can be changed into the attractive force when the acoustic frequency is lowered.

Umbilical cord blood lead levels in Shanghai, China.

This study was designed to determine the cord blood lead (BPb) levels of babies born in one urban area of Shanghai, and to preliminarily identify the demographic, social environment and prenatal factors which have an effect on the cord BPb concentrations. From August to November 1993, umbilical cord blood samples were obtained from 605 live newborns in the Yangpu Maternal and Child Hospital. 257 samples were excluded from measurement because of clotting. In 348 cord samples, the geometric mean of cord BPb levels was 9.2 micrograms/dl, with a 95% confidence interval of the mean 8.86-9.54 (micrograms/dl). 142 babies (40.8%) had cord BPb levels of 10 micrograms/dl or greater. As a result of this high percentage of newborns with BPb levels equal to or greater than 10 micrograms/dl, we estimate that each year in the Shanghai City about 60,000 newborns are at risk for developing neuropsychological deficiencies caused by maternal lead exposure during pregnancy. To investigate the factors affecting cord blood levels, the subjects with levels greater than the 70th percentile (10.7 micrograms/dl) (n = 104) and less than the 30th percentile (7.4 micrograms/dl) (n = 104) were selected to compare the demographic, environment and prenatal medical history. Increased BPb levels at birth were associated with maternal passive smoking, a family member being occupationally exposed to lead, proximity to major traffic way, household coal combustion, neighborhood coal combustion, low level of maternal occupations, and the increasing occurrence of having the high lead foodstuff pidan (preserved duck egg) during pregnancy. We conclude that prenatal lead exposure has become an important health issue for young children in Shanghai.