Technical points

Most obstetric ultrasonography utilizes sound of 3.5 MHz. This yields a maximal resolution of 1 mm.

There are four main approaches to ultrasonography:

• A-mode: ultrasound echoes are displayed as spikes along a baseline, allowing measurement of distances along a single axis.
  
  
• B-mode: a 2D-fan-shaped image is displayed on the cathode ray tube. This allows the operator to assess three dimensional structure.
  
  
• M-mode: in a similar approach to A mode, serial measurements are taken along a single axis. The intensity of a dot on the cathode ray tube is proportional to the echogenicity at the corresponding point along that axis. Serial linear images are displayed and simultaneously scrolled across the screen to provide a record of distance relationships over time.
  
  
• Doppler mode: high intensity ultrasound may be reflected off moving red blood cells. The change in frequency of the reflected ultrasound, as compared to the emitted frequency, is determined by the velocity of the cells relative to the detector (the Doppler effect).