The term "ultrasound" refers to sound waves of a frequency greater than that which the human ear can appreciate, namely frequencies greater than 20,000 cycles per second or Hertz (Hz). To obtain images of the pregnant or nonpregnant pelvis, frequencies of 2 to 10 million Hertz (2 to 10 megahertz [MHz]) are typically required.
Real-time imaging is the most common sonographic technique used in obstetrics and gynecology. Multiple individual B-mode gray-scale images are obtained and rapidly displayed in succession, thereby creating a video of the area of interest over time that can be used to evaluate its structure and some aspects of its function. Real-time ultrasound is especially useful for imaging mobile subjects, such as the fetus or heart, and for quickly viewing an organ from different orientations.
The physical principles of ultrasound imaging are discussed separately. (See "Basic principles and safety of diagnostic ultrasound in obstetrics and gynecology".)
BASIC PROCEDURE AND EQUIPMENT
Sonography, like surgery, is an operator-dependent technology. A high level of competence can only be achieved by supervised experience with a large variety of normal and abnormal examinations.
Preprocedure issues — The sonographer should know the reason for the ultrasound examination and results of other evaluations related to the patient's problem. All of this information is critical for targeting specific structures, choosing whether to use a transvaginal and/or transabdominal technique, and deciding whether additional studies may be helpful (eg, saline infusion sonohysterography, Doppler velocimetry).