Basics of Ultrasound

In this section, we will discuss how the echo machine creates images in 2D and 3D, as well as color flow Doppler (CFD) and Spectral Doppler.

Ultrasound uses very-high-frequency sound waves, typically >1.5 MHz (the normal sound range for humans is around 20 Hz to 20,000). The ultrasound probe sends out a sound wave of a known frequency into tissue and "listens" for the returning sound frequency. Based on the change in frequency and the time it takes to return to the receiver, the machine determines how dense and how far the object of interest is relative to the probe. Sound propagates through different tissues at different speeds, with air having the slowest propagation (330 m/s) and bone having the greatest (4080 m/s); however, blood and most soft tissue have the same propagation speed of 1540 m/s. For this reason, the ultrasound machine assumes that the sound waves will travel at this speed. By knowing this, and the amount of time it takes for the wave to bounce back to the receiver, the machine can calculate how far an object is located from the probe. It assumes that half of the time travel was towards the object and half was back to the probe, therefore, it can plot the object at that distance from the probe on the display.

The strength of the sound wave depends on its amplitude, which can change as the sound wave travels through tissue. Some of the energy is absorbed by the tissue itself and some energy is scattered or reflected from surfaces of differing acoustic impedance. This causes a change in the amplitude of the returning sound wave, which the machine will use to assign a certain amount of brightness to the object.

As the frequency of the sound waves increases, the resolution increases at the expense of decreased tissue penetration. However, if imaging deeper structures, the frequency can be reduced to improve tissue penetration at the expense of the image resolution. The sound waves used to obtain cardiac images in echocardiography are ultrasound waves that range in frequency from 2-12 MHz. Resolution of the images improves with frequency, but the wavelength is shortened in the process, which decreases the distance from the ultrasound transducer that can be imaged. As a result, adults are usually imaged using a 2-4 MHz transducer, while pediatric patients are imaged using a 7-12 MHz transducer.