Unlike X-ray imaging,
In an ultrasound exam, a transducer (probe) is placed directly on the skin or inside a body opening. A thin layer of gel is applied to the skin to increase the ultrasound waves transmittion from the transducer through the gel into the body.
The ultrasound image is generated based on the reflection of the waves off from the body structures. Using the strength of the sound signal and the time taken for the wave to travel through the body and back to the transducer, the image can be produced.