How an Ultrasound Works

One week from today, we have our next doctor's appointment and (if the baby is cooperative) we'll find out if we will be having a little boy or a little girl.

How do they find out this useful information? Is it how high/low Jill is carrying the baby? Do they use trained dogs? Nope, for this, they bring in an ultrasound machine.

So for all of the inquisitive minds out there, I did a little research to find out just how, exactly, an ultrasound works. The best place for this kind of information? HowStuffWorks.com

And here is what I found...

Ultrasound or ultrasonography 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:

1. The ultrasound machine transmits high-frequency (1 to 5 megahertz) sound pulses into your body using a probe.
2. The sound waves travel into your body and hit a boundary between tissues (e.g. between fluid and soft tissue, soft tissue and bone).
3. Some of the sound waves get reflected back to the probe, while some travel on further until they reach another boundary and get reflected.
4. The reflected waves are picked up by the probe and relayed to the machine.
5. The machine calculates the distance from the probe to the tissue or organ (boundaries) using the speed of sound in tissue (5,005 ft/s or1,540 m/s) and the time of the each echo's return (usually on the order of millionths of a second).
6. 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.

A basic ultrasound machine has the following parts:

• transducer probe - probe that sends and receives the sound waves
• central processing unit (CPU) - computer that does all of the calculations and contains the electrical power supplies for itself and the transducer probe
• transducer pulse controls - changes the amplitude, frequency and duration of the pulses emitted from the transducer probe
• display - displays the image from the ultrasound data processed by the CPU
• keyboard/cursor - inputs data and takes measurements from the display
• disk storage device (hard, floppy, CD) - stores the acquired images
• printer - prints the image from the displayed data

In the past two years, ultrasound machines capable of three-dimensional imaging have been developed. In these machines, several two-dimensional images are acquired by moving the probes across the body surface or rotating inserted probes. The two-dimensional scans are then combined by specialized computer software to form 3D images:


For an ultrasound exam, you go into a room with a technician and the ultrasound machine. The following happens:

1. You remove your clothes (all of your clothes or only those over the area of interest).
2. The ultrasonographer drapes a cloth over any exposed areas that are not needed for the exam.
3. The ultrasonographer applies a mineral oil-based jelly to your skin -- this jelly eliminates air between the probe and your skin to help pass the sound waves into your body.
4. The ultrasonographer covers the probe with a plastic cover.
5. He/she passes the probe over your skin to obtain the required images.
6. You may be asked to change positions to get better looks at the area of interest.
7. After the images have been acquired and measurements taken, the data is stored on disk. You may get a hard copy of the images.
8. You are given a towelette to clean up.
9. You get dressed.

Sound simple enough, huh? We'll see first hand next week!