What is a Medical Ultrasound?

Ultrasound refers to any sounds that are above the average range of human hearing, about 20 kilohertz. And like all sounds, it is a pressure wave. Ultrasound has many applications, from helping animals sense objects in their environment to imaging the human body.

4-month-old fetus
A fetus at 4 months seen via ultrasound.
Credit: CDC

Obstetric ultrasound is a technique used during pregnancy to create images of a human fetus. The standard mode is 2D ultrasound, in which sound waves are beamed straight at the fetus and the echoes are reflected back. The technique was developed by obstetrician Ian Donald and engineer Tom Brown, who first used it clinically in 1956 in Glasgow, Scotland. It wasn't until the 1970s that British and American hospitals started using it, but it has now become routine throughout the developed world. The technique is believed to be safe, but limits exist on the energy that can be used.

3D ultrasound produces images with depth, by beaming sound waves at an angle to the fetus. A computer program processes the echoes and produces a non-moving 3D reconstruction of the fetus. This technique was developed by Olaf von Ramm and Stephen Smith at Duke University in 1987. While primarily used to reveal fetal anatomy, 3D ultrasound is also used to promote maternal bonding.

4D ultrasound is similar to 3D ultrasound, except it produces 3D images in real time.

Ultrasound scans are also done on different organs, such as the abdomen, kidneys, breast, thyroid, liver, or carotid artery.

Doppler ultrasound is a noninvasive method of measuring blood flow and blood pressure by bouncing ultrasound waves off red blood cells. It can be used to diagnose a variety of conditions, including blood clots, defective heart valves or blocked arteries. 

Ultrasound scans

Medical ultrasound scanners use a device called a piezoelectric transducer to produce sound waves at specific frequencies between 2 and 18 megahertz. The shape of the transducer, a lens or a controlled set of pulses help focus the sound. A water-based gel is applied to the patient's skin to enable sounds to transmit more efficiently.

The sound waves produced by the transmitter are reflected off different layers of body tissue, and some of the echoes make it back to the transducer. Here, the waves are converted into electrical signals that are used to create an image. The scanner measures the time it took for echoes to return, the image's focal length, and the strength of the echo.

An ultrasound technician, also known as a diagnostic medical sonographer, uses ultrasound scanners to diagnose medical problems. Technicians can specialize in fields such as obstetric and gynecologic sonography, neurosonography or cardiac sonography. The average annual salary of ultrasound technicians was $65,210 in 2011, and the average hourly salary was $31.35.

Fetal ultrasounds

Fetal ultrasounds are performed throughout pregnancy to determine a baby's due date, gender and health. They provide different information depending on the part of the pregnancy:

  • Weeks 4-5: Used to detect ovulation, diagnose pregnancy, determine due date, or check for problems.
  • Weeks 6-7: Can see twins and fetal heart beats 
  • Weeks 8-9: May see arm and leg buds
  • Weeks 10-18: See more detail
  • Weeks 19-21: Find out baby's gender  
  • Week 31: Baby is so big can only see parts of it
  • Week 35-37: Check baby's position (head down or breech) or perform non-stress test

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Tanya Lewis, LiveScience Staff Writer

Tanya Lewis

Tanya has been writing for Live Science since 2013. She covers a wide array of topics, including neuroscience, biotech, robotics, astronomy and strange or cute animals. She received a graduate certificate in science communication from the University of California, Santa Cruz, and a bachelor of science with honors in biomedical engineering from Brown University, with postgraduate research experience in a neuroscience. She has previously written for Science News, Wired, The Santa Cruz Sentinel, the radio show Big Picture Science and other places. Tanya has lived on a tropical island, witnessed volcanic eruptions and flown in zero gravity (without losing her lunch!). To find out what her latest project is, you can visit her website or follow Tanya on twitter or .
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