Ultrasound is based on "echo location," similar to the way bats find food and submarines use sonar. There is one major "flaw" with ultrasound and that is that ultrasound waves can't travel through air and other gases. This is because the molecular density is too low to transfer the ultrasound waves. Because of this problem, sonographers (ultrasound technicians) apply a water based gel to the patient's scanning area.

Fig. 4: Ultrasound Gel

             The ultrasound scanner, which is also known as a transducer, sends out a high pitched sound wave into the patient being scanned. The sound waves bounce back at various intervals depending on the type of material they pass through. Sound travels through materials at various speeds depending on the density and compressibility of the material. (Air and gases are not dense and therefore ultrasound waves can't pass through them.) When the sound waves hit the various materials inside your body, they bounce back to the transducer. The amount of time it takes for the sound waves to reflect back to the tranducer varies on the density and compressibility of the material. Please note that all materials in the body reflect the sound waves. It's the variation in sound wave return that determines the picture which comes up on the screen.

Speed of Sound Through Various Tissues
Material	Speed (meters/second)
Air	0 331
Fat	1 450
Water	1 495
Soft Tissue	1 540
Kidney	1 561
Muscle	1 585
Bone	4 080

            Humans rely heavily on our sense of sound. We can hear a relatively small range of sound waves, from about 16 - 200000 Hz. Ultrasound waves are similar to the ones we can hear. However, the major difference is that they are higher pitched. They vary in frequency from 20 000 Hz to 15 MHz (15 000 000 Hz). As a result of the higher frequency (cycles per second) of the sound waves, humans cannot hear ultrasound waves, nor see them.

Thermal: When the sound waves pass through the tissue, not all of the waves pass through the tissue or get reflected, some are absorbed by the material. Only a very small amount gets absorbed, and often the heat generated by the energy is quickly dissipated.

Non-thermal: The main non-thermal effect is called cavitation. Body tissues and fluids contain dissolved gases. When some of the sound energy is absorbed by the tissue, the energy could create bubbles from the dissolved gases. The bubbles then vibrate due to more absorption and finally burst, causing very intense, localized effects.

Please Note: The above described risks take place if the patient is exposed to the ultrasound for very extensive periods of time, significantly longer than what is used in diagnostic ultrasound. There have been no risks recorded for diagnostic ultrasound, though many studies have been conducted and research is ongoing.

DID YOU KNOW THAT...? Before using water based gels to transfer the ultrasound waves to the patient, technicians used baby oil on the patients. Not only was baby oil a mess to clean up because it was so runny, it was often hard to get all the oil off the patient. WHAT A MESS!