A detailed extended obstetric ultrasound examination can take considerably longer than a level II study. What are the safety elements of obstetric Ultrasound?
Though ultrasound is not ionizing radiation and is generally regarded as safe, use of ultrasound does mean exposing tissues to mechanical energy. The mechanical energy of ultrasound has two basic effects in tissue: heating, and mechanical compressive and shear forces. Heating, or thermal, effects result from the sound energy causing tissues to vibrate. This vibration results in a rise in temperature. Compressive and shear forces can disrupt some tissues and have been shown to cause hemorrhages in bowel and lung (in experimental animal studies). The mechanical forces are much intensified if gas is present (which should not be so in a fetus) or near the edges or surfaces of bone because the reflection from bone can be so strong. The shear forces can also cause cavitation, resulting in the formation and collapse of small bubbles. This exerts tremendous local forces and can disrupt normal tissues. In nonmedical situations, cavitation exerts a powerful enough force to erode metal.
The United States Food and Drug Administration (FDA) has imposed output limits on ultrasound machines because of these thermal and mechanical effects. The maximum output is 720 milliwatts (mW) per square centimeter (cm 2 ). However, for fetal US, the FDA limits output to 94 mW/cm 2 . This power limit is the reason that an obstetric ultrasound study is performed using the ultrasound machine’s “OB” setting. Machines set up the output power limits based on examination type. In agreeing to higher output limits for US machines overall, the FDA required manufacturers to display indices of the output power being used. This is known as the output display standard (ODS) and is the reason for the display on each image of two indices: the thermal index (TI) and the mechanical index (MI)