Module 4: Point of care ultrasound in first trimester abortion (POCUS)

4. Theory of ultrasound

What is ultrasound?

Ultrasound is a medical imaging technique that uses high frequency sound waves and their echoes. The technique is similar to the echolocation used by bats, whales and dolphins, as well as SONAR used by submarines. In ultrasound, the following events happen:

• The ultrasound machine transmits high-frequency (1 to 15 megahertz) sound pulses into the body using a probe
• The sound waves travel into your body and hit a boundary between tissues, e.g. between fluid and soft tissue, soft tissue and bone
• Some of the sound waves get reflected back to the probe, while some travel on further until they reach another boundary and get reflected
• The reflected waves are picked up by the probe and relayed to the machine
• The machine calculates the distance from the probe to the tissue or organ (boundaries) using the speed of sound in tissue (1,540 m/s) and the time of each echo's return (usually on the order of millionths of a second)
• The machine displays the distances and intensities of the echoes on the screen, forming a two-dimensional image
• In a typical ultrasound, millions of pulses and echoes are sent and received each second. The probe can be moved along the surface of the body and angled to obtain various views.

Ultrasound can produce different types of image on a screen

The ultrasound that you will be using for this module presents a two-dimensional image, or "slice," of a three-dimensional object (fetus, organ). In addition some point of care ultrasound machines offer a doppler mode. Doppler measures movement (usually of fluids) representing the movement as colours on the two-dimensional image. For early pregnancy ultrasound for the abortion provider, it can give extra information about the fetal heart and blood flow in potential retained products of conception.

Doppler is available on some but not all handheld devices. For early pregnancy ultrasound for the abortion provider, it can give extra information about the fetal heart and blood flow in potential clinical situation where retained products of conception are suspected.