How Does Ultrasound Work?

Abstract

Ultrasound (US) is a relatively cheap, widely available imaging technology that uses sound waves to create images. No ionizing radiation is used. This chapter will focus primarily on fundamental concepts pertaining to B-mode (brightness mode) or 2D mode ultrasound, which is used in anatomic imaging. However, one should be aware of other ultrasound modes such as A-mode (amplitude mode) used in therapeutic ultrasound and M-mode (motion mode) ultrasound used in obstetric imaging to evaluate fetal cardiac.

What are ultrasonic sound waves?	High-frequency sound waves >20 kHz. Typical diagnostic frequencies are between 1 and 20 mHz [1, 2].
How do US create images?	High-frequency sound waves produced by the transducer reflect off objects in the body and return to the transducer at different time points (echoes) [1].
How are sound waves generated?	Electric energy is transformed into mechanical energy (sound wave) in piezoelectric crystals in the ultrasound transducer. When the sound waves echo back to the transducer, the mechanical energy is transformed back into a measurable electrical signal by the crystals [3].
What is the speed of sound in human tissue?	1540 m/s [1]
What is B-mode US (2D mode)?	B-mode US creates a two-dimensional image in real time. When you see an US image of the solid organs, you are seeing a B-mode US image [1].
How does wavelength influence tissue penetration depth in US?	Longer wavelengths have deeper tissue penetration, but have poorer axial resolution (decreased ability to delineate two objects in the same direction as the transducer) [1,2,3].
What is US gain?	Ultrasound gain is an adjustable setting that controls the amplification of echoes displayed. The higher the gain, the brighter the image [3].