Jesse Shriki, DO, MS, RDMSab,     aDepartment of Emergency Medicine, Scottsdale Emergency Associates, 7400 E Osborn Avenue, Scottsdale, AZ 85251, USA; bThe University of Arizona, Tucson, AZ 85721, USA

Bedside ultrasound has become an important modality for obtaining critical information in the acute care of patients. It is important to understand the physics of ultrasound in order to perform and interpret images at the bedside. The physics of both continuous wave and pulsed wave sound underlies diagnostic ultrasound. The instrumentation, including transducers and image processing, is important in the acquisition of appropriate sonographic images. Understanding how these concepts interplay with each other enables practitioners to obtain the best possible images.

Keywords

Ultrasound physics

Frequency

Period

Transducer

Instrumentation

Doppler shift

Key points

Introduction

Point-of-care emergency ultrasound has become the modern-day physician’s stethoscope equivalent. The concepts fundamental to ultrasound physics are critical in both understanding point-of-care ultrasound and obtaining the best possible images. Without knowledge of how the ultrasound system interprets and acquires sound waves, understanding the perceived anatomy can be difficult at best. Knowing how the images are created can help practitioners determine if an image produced is an accurate depiction of the anatomy or if artifacts are confounding (or contributing) to the acquired images.

Basic sound

In order to understand diagnostic ultrasound, sound should be thought of as more than just the familiar sense of hearing. Rather, sound should be thought of as the interaction of energy and matter. In contradistinction to electromagnetic energy, sound is mechanical energy transmitted by pressure waves in a medium,1 which means that sound exists in the form of particles moving in a medium. A sound source, such as a tuning fork, acts like a piston pushing waves of vibration longitudinally through tissue. The sound wave produced has areas of high pressure (or high density) and low pressure (or low density). The high-pressure areas (compression) are where the sound waves are compressed together and the low-pressure areas (rarefaction) are where the sound waves are spaced apart (Fig. 1).

Sound particles should be thought of as elements of transverse and longitudinal waveforms moving in a medium. In diagnostic ultrasound, the media can be air, blood, or soft tissue. In the absence of media (ie, a vacuum), sound cannot propagate. In a transverse wave, displacement of the medium is perpendicular to the direction of propagation of the wave, as in a ripple on a pond. In longitudinal waves, the displacement of the medium is parallel to the propagation of the wave, moving like a Slinky or caterpillar back and forth (Fig. 2). Only longitudinal waves effectively traverse distances and, therefore, only longitudinal waves are important in diagnostic ultrasound.

Sound source

The production of sound requires an oscillating or vibrating source. A tuning fork is a good example of how sound is produced by oscillation and vibration (see Fig. 1). When a tuning fork vibrates, it moves adjacent air molecules causing them, in turn, to vibrate. Sound spreads throughout the medium, air, as a wave in all directions.

In the ultrasound system, the sound source is a piezoelectric crystal, such as quartz. Modern transducers typically use a lead zirconate titanate (PZT) amalgam. The piezoelectric effect allows for these crystals to vibrate when an electrical voltage is applied across it and subsequently creates sound waves. Conversely, piezoelectric crystals also can convert sound waves back into electrical energy so that the sound waves can be converted into data that can be processed into anatomic images.

Waves

A single-frequency sound wave is commonly conceptualized as a single sine wave causing alternating pressure variations in the air (Fig. 4). Ultrasound waves are rarely, however, waves of a single frequency and are generally made up of multiple frequencies. Accordingly, these waves can interfere with each other either constructively or destructively (Fig. 3).1

Acoustic parameter and variables

To understand the basic physics of ultrasound, acoustic parameters and acoustic variables must be defined; they are the basis of describing waves. In physics nomenclature, sound waves are defined by acoustic variables and characterized by acoustic parameters. Acoustic variables are pressure, density, and distance and help define a sound wave. Once the sound wave is defined, it can be described by its frequency, amplitude, power, intensity, wavelength, and propagation speed.

Period and frequency

The most well-known acoustic variables are period and frequency. Period is the time to complete a single cycle. It can also be stated as the time from the start of 1 cycle to the start of the next cycle (see Fig. 4). In ultrasound, period is the time from the start of 1 peak, including 1 valley, to the next peak. Typical values in diagnostic ultrasound for period are expressed in microseconds. Frequency is the number of events that occur in a particular time frame. In ultrasound, the frequency of a wave is the number of cycles that occur in 1 second. Typical frequencies in diagnostic ultrasound are expressed in megahertz. Ultrasound transducer frequencies vary from 1 MHz to 15 MHz. Given this inverse relationship, period and frequency are the reciprocal of each other.1

Wavelength

The distance between 1 peak and the next represents 1 cycle; it is the distance between 2 similar points on corresponding waves and represents 1 wavelength. It is a simple but important concept to understand that wavelength is a distance, whereas period is a time (see Fig. 4). Typical diagnostic ultrasound wavelengths are in the millimeter range. Wavelength (λ) and frequency (f) are also inversely related to one another, and their product is the speed (v) of sound in a medium (Fig. 5). Sound is presumed to travel at 1540 m/s in soft tissue, which is approximately 1 mile per second. Therefore, sound of a 1-MHz frequency has a wavelength of 1.54 mm.

Parameters of magnitude

So far, the time parameters in which a wave occurs have been described. The next 3 parameters describe the magnitude and strength (or “bigness”) of the sound wave. These parameters are amplitude, power, and intensity. In conjunction with acoustic variables, these parameters are important in describing how the waves interact with the medium. Additionally, these parameters build on each other and are mathematically related to each other.

Amplitude is the size of the wave. Graphically, it is the difference between the maximum value and the average value of the wave. Amplitude is the strength of the wave measured from the zero line to the top of the wave (see Fig. 4). Power is the next parameter and is defined as the energy (joules) generated per unit of time. It can also be thought of as the rate of at which work is performed. Power...