Sonography, also called ultrasound, uses sound waves in the inaudible ultrasound range to create images. No X-rays are involved. Ultrasound has proven to be so safe that it can even be used during early pregnancy. Sound waves in the range of 3.5 to 13 MHz are emitted by the device’s transducer, penetrate the body, and are reflected at tissue boundaries, for example in the gallbladder. A computer calculates the exact location based on the echo time of the reflected waves to generate an image. Air, such as in the lungs or intestines, strongly attenuates the sound, making imaging impossible in those areas. Bony structures also obstruct the examination. A gel is applied to the skin to eliminate the air layer between the transducer and the body. Ultrasound can examine almost all organs, such as the liver, spleen, and kidneys, as well as the thyroid, salivary glands, and breast (high-frequency breast sonography).