The influence of ultrasound scanner beam width on femur length measurements

Systematic differences have been found in measurements of femur lengths made with (1) a Hitachi EUB 25, representative of older machines in use when most of the femur length-gestation curves were produced; and (2) a modern high resolution machine (Acuson 128). It is proposed that these differences may be accounted for by the different beam widths of the two systems. Six operators measured the femur lengths of several normal fetuses using both machines. Analysis of the results revealed a highly significant (p < 0.005) difference between machines, that did not correlate with femur length (r = -0.022). The mean difference was 1.02 +/- 0.33 mm (Hitachi larger). The mean femur length was 46.44 mm (n = 57). Pulse-echo beam plots showed that, for the probes used, the Hitachi 6 dB beam width is greater than the Acuson beam width by typically 2 mm, depending on the range and Acuson focus setting. This beam width difference can account for the difference in femur length measurements, if allowance is made for the variable angle between femurs and ultrasound beams. The results suggest that femur length-gestation curves for use with modern machines should be based on measurements made with modern machines.