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Notizen: Objectives To establish the relation between fetal abdominal circumference and birthweight in a large population of fetuses; to identify whether the error in estimating birthweight by abdominal circumference varied with the magnitude of abdominal circumference; and to establish whether adding femur length to abdominal circumference caused a clinically important reduction of error in predicting birthweight.Design A retrospective study.Setting The ultrasound department of a teaching maternity hospital offering a tertiary referral service.Sample From 3512 nondiabetic women with a normally formed singleton fetus, an abdominal circumference measurement of the infant was made within seven days of delivery; of these, 1213 had a femur length measurement performed at the same time.Results There was a linear relation between abdominal circumference and birthweight. There was a strong inverse correlation between the proportional error in predicting birthweight from the abdominal circumference and the magnitude of the abdominal circumference. Both the Campbell and Wilkin equation (abdominal circumference alone) and the Hadlock equation (abdominal circumference and femur length) were associated with systematic errors, especially with larger birthweight infants. The median absolute errors for the two equations were not significantly different overall (6.98% and 6.86% respectively), although the Hadlock equation was significantly more accurate in predicting birthweight in infants weighing greater than 4500 g. However, no threshold value of abdominal circumference or of estimated fetal weight using the Hadlock equation had a positive predictive value in estimating infants of 〉 4500 g of greater than 35%.Conclusions Prediction of birthweight should be by abdominal circumference alone. 〈link href="#t1"〉Table 1 presents robust estimates of the error of predicting birthweight using fetal abdominal circumference.〈tabular xml:id="t1"〉1〈title type="main"〉 The relation between fetal abdominal circumference (AC) and birthweight (BW). 〈table colsep="0" rowsep="0" frame="topbot" pgwide="0" orient="port"〉〈tgroup cols="5" align="left"〉〈colspec colnum="1" colname="col1" align="left"/〉〈colspec colnum="2" colname="col2" align="center"/〉〈colspec colnum="3" colname="col3" align="center"/〉〈colspec colnum="4" colname="col4" align="center"/〉〈colspec colnum="5" colname="col5" align="center"/〉〈thead valign="bottom"〉〈row rowsep="1"〉AC (mm)nMedian BW(g)10th-90th centile BW (g)Range BW(g)〈tbody valign="top"〉200-20913900750-1030740-1040210-219201040830-1370780-1400220–229201060750-1410650-1460230-239281255980-1470900-1860240-2493614351200-17901080-1950250-2593715801290-19251180-2260260-2695618351490-21901340-2400270-2798920001640-23201390-2620280-28913422651920-26601530-2910290-29921925302130-29001820-3100300-30935026852340-30802010-3420310-31938728502470-32902110-3650320-32948430602700-34702350-3770330-33943932602880-37002570-3980340-34942333803040-38602670-4240350-35931436153240-40402890-4460360-36924537503330-11903020-4610370-37911738403480-43603180-4790380-3896641403660-46403470-4820390-3993542903665-46753640-5000

Notizen: Some important characteristics are described of one- and two-dimensional gas proportional detectors with delay line position readout which are currently in use at beam line X12B of the National Synchrotron Light Source. The importance of careful choice of geometric design parameters is emphasized with respect to position linearity and electrical stability; position resolution of 100 μm FWHM and counting rates up to several times 105 s−1 are obtained in calibration studies. Results from some recent beam-line experiments further illustrate the excellent position resolution and linearity, and demonstrate the usefulness of these detector systems for small-angle scattering, particularly in dynamic studies. © 1995 American Institute of Physics.

Notizen: The use of primary electron counting techniques as an alternative to the more usual parallel plate avalanche chamber that has been employed in soft x-ray scattering experiments is being investigated at the National Synchrotron Light Source. The theoretical aspects of primary electron counting and motivation behind building a primary electron counting detector are described, as well as characteristics and future improvements of the device constructed at the NSLS. The detector consists of a low electric field drift region and a low pressure multistep avalanche region which can be operated with two or three stages of electron multiplication. The device has worked well in extensive tests as a simple parallel plate avalanche chamber, providing energy resolutions of 58% and 43% at 277 and 500 eV, respectively. Operated as a primary electron counter, preliminary results show an energy resolution of 38% at 500 eV. © 1995 American Institute of Physics.

Notizen: A brief description of the most common encoding methods, and their relative merits, used with position sensitive proportional detectors at synchrotron facilities will be made. While some of these systems are reaching maturity, continued development is resulting in further improvements, especially in terms of linearity and resolution. The use of detectors with special geometries for specific experiments will be described. The microstrip gas chamber will be addressed, with particular emphasis on a comparison of its capabilities with those of the multiwire proportional chamber. The development of proportional detectors intended for dynamic studies at second- and third-generation sources, with counting rate capabilities well in excess of these generally available at present, will be described. The use of gas based detectors, albeit with modest energy resolution, for photon detection in the energy regime of a few hundred electron volts will be mentioned. © 1995 American Institute of Physics.

Notizen: Measurements for axially incident electrons are reported for the energy and spatial dependencies of the detection efficiency of a Philips X919BL single channel electron multiplier (CEM) in both the pulse counting and analog amplification modes. The behavior may be understood in terms of the general theory reported earlier.1 Essentially there are three regions to consider: the horn, the channel and the edge of the channel. In the pulse counting mode the difference in efficiencies between the channel and horn may reach a factor of two whereas in the analog mode the difference between the channel edge and the horn may reach a factor of five. The same behavior occurs when the CEM is used as the output stage of an electron spectrometer. The above variations may lead to (i) loss of efficiency in spectrometers, (ii) spectral distortion arising from the associated presence of stray magnetic fields, and (iii) irreproducibility of spectra from different regions of the sample surface, unless specific measures, such as larger horns, off-axis channels or lens defocusing, are used to eliminate them.