How Does an Ultrasound Scan Work?

Transducer

An ultrasound starts with a hand held device called a transducer. When held directly against skin tissue, the transducer emits sound waves. When trying to scan deeper tissues, these sound waves reside around the 2 to 6 MHz level. When scanning more superficial tissues, higher frequencies are used between 7 and 18 MHz.

Gel

For the sound waves to transmit easier from the transducer into the skin of the patient, a medium is used to facilitate the transfer. This medium is usually a water based gel that is liberally applied to the area that is going to be scanned.

The Process

Once the transducer is placed on the gel covered skin, sound waves are emitted into the body. These sound waves travel until they reach different types of tissues which reflect the sound back. For example, when the sound passes from fluid to bone, some of the sound waves bounce off the bone and are reflected back to the transducer. This process continues each time the sound waves pass through a different medium within the body, whether it be organ tissue, bone or other tissues in the body.

The Results

As each sound wave is reflected back to the transducer, the ultrasound machine calculates the distance that the sound wave traveled between the transducer and the tissue or other medium. It does this using a mathematical equation using the average speed of sound in tissue (about 5,005 feet per second) and the time it took for the reflection to return to the transducer.

Image

Using the sound data, the ultrasound scanner is able to chart the different results and convert them into a 2D image. This image can then be examined by a health professional and used for diagnosis, or in cases like pregnancy ultrasounds it can be used to check on the progress of the baby.