Expectant parents get their first fuzzy glimpse of their new child from an ultrasound image. Medical doctors also use ultrasound images to diagnose medical problems in the heart, gall bladder, kidneys, liver, and other internal organs. Ultrasound images can also help physicians locate and diagnose tumors.
What is ultrasound and how does it work?
What is Ultrasound?
Medical ultrasound imaging uses sound waves very similar to those people hear. However ultrasound uses very high frequency sound waves having a pitch too high for the human ear to hear.
The frequencies used in medical ultrasound imaging range from about 1 megahertz to about 15 megahertz. The highest frequencies the human ear can hear are about 20 kilohertz. A megahertz is a million complete cycles in a second and a kilohertz is a thousand cycles a second.
Why Ultrasound Images Are Fuzzy
Medical ultrasound imaging uses high frequency sound waves to produce more detail in the diagnostic images than lower frequency sound waves would produce. For sound, or any other type of wave, the frequency is inversely proportional to the wavelength. A higher frequency wave has a shorter wavelength. The frequencies used for medical ultrasound imaging correspond to wavelengths of the sound waves in human tissue of about 0.1 to 1.5 millimeters.
For any type of imaging the smallest detail that can be resolved is approximately the size of the wavelength of the sound or electromagnetic waves making the image. Medical ultrasound images can not show details smaller than 0.1 to 1.5 millimeters, so ultrasound images are usually a little fuzzy.
Using higher frequency ultrasound would produce more detailed diagnostic images, however the ability to image the interior of the human body would be reduced. The ultrasound waves can only penetrate to a depth of about 500 times their wavelength into the human body. Beyond that depth, body tissue absorbs the ultrasound waves. The shortest wavelength (0.1 millimeters) and highest resolution ultrasound waves can therefore only image to a depth of about 50 millimeters (=5 centimeters=2 inches). Deeper ultrasound imaging requires longer wavelengths which produce less resolution. Hence ultrasound images have poor resolution and look fuzzy. If physicians need higher resolution images, they must use other medical imaging techniques such as X-rays or MRI.
Safety of Medical Ultrasound Imaging
Ultrasound imaging is very safe. The patient being imaged by ultrasound is exposed only to sound waves.
This safety is the primary reason that physicians use ultrasound for fetal images. X-ray Medical imaging techniques expose the fetus to X-rays that are especially risky for a developing fetus. Ultrasound poses no such risk for a fetus.
Doppler Ultrasound
When sound or electromagnetic waves originate or are reflected from a moving object their wavelength and frequency changes. The change in pitch of a police siren as it passes is an example of this effect, which is called the Doppler effect. The amount the frequency changes depends of the speed of the moving object.
Using the Doppler effect, ultrasound can detect and measure movement, such as blood flow, within the body. For example an echocardiogram uses Doppler ultrasound to make a diagnostic ultrasound image of the heart as well as blood flow and valve movement within the heart.
Medical diagnostic ultrasound imaging is a very safe noninvasive way to image the interior of the human body that only exposes patient to high frequency sound waves.
Further Reading
Giambattista, A., Richardson, B.M, Richardson, R.C., College Physics, McGraw Hill, 2007.
Paul A. Heckert - I have a Ph.D. in astrophysics, over 30 years experience teaching physics and astronomy, and over 60 published research articles.
