In sonography (ultrasound imaging), inaudible high frequency of sound waves in the range of $1-5 \mathrm{MHz}$ are used.
Related Theory
Sonography is a medical imaging technique that uses high-frequency sound waves to produce images of internal body structures. It is also known as ultrasound. After the $X$-ray test, ultrasound is the most used diagnostic imaging technique. It uses a device called a transducer to send out ultrasound waves and listen for the echo. A computer translates the ultrasound waves into an image. A trained technician can see, measure, and identify structures in the image.