Written by: Aalap Shah MD
Peer-Reviewed by: Lori Stolz, MD, RDMS
All images obtained by Aalap Shah MD and Lori Stolz, MD, RDMS except as otherwise noted 

HOW DOES IT WORK?

Ultrasound describes a sound wave with frequency above what humans can hear. The human ear operates in a frequency range of 20Hz to 20kHz. Anything above that range can be classified as ultrasound. The frequency range of diagnostic ultrasound varies from 1MHz to 20MHz. Similar to all sound waves, ultrasound requires a medium through which to transmit energy. The way in which this occurs depends on several acoustic variables:

Wavelength (λ) : the length (distance) of one complete cycle

Frequency ( f ) : The number of cycles per second

Period (T) : the time per wavelength (1/f)

Velocity (v): Speed at which the wave travels (v=fλ). AKA Propogation Speed

Power (P) : strength of the sound wave. Proportional to amplitude²  (measured in watts)

The most important concept to understand in terms of medical ultrasound is velocity, as this varies dependent on the material through which sound is propagating. Ultrasound machines use this property to produce an image by using pulsed ultrasound. This means that the probe intermittently produces sound waves and has a ‘listening period’, during which it listens to the echo that returns from an object and uses the time difference and change in energy to make a judgment on the object's distance and composition. It can use the change in propagation speed (v) in different tissues to create a visual representation of the underlying material.

TISSUE INTERACTIONS

The ultrasound probe contains a piezoelectric crystal that vibrates at a specified frequency when current is applied to it. This causes a sound wave of that frequency to be produced from that probe. When this current is pulsed at specific intervals, it propagates sound waves through a given tissue. As this sound wave travels through the tissue, waves are either:

Absorbed: energy absorbed to tissue, usually through heat.

Reflected: as the sound wave approaches a boundary between tissue of 2 different densities, some waves bounce back toward the source

Scattered: sound waves which bounce back in direction not directly toward source

Refracted: path of sound wave is ‘bent’ as it travels between interface of two different densities

All of these cause attenuation, or weakening of sound energy as the wave passes through tissue. Each type of tissue has a theoretical amount of attenuation associated with it.

Note: this only applies to sound travelling in uniform tissues, and also depends on the frequency of the initial wave.

For our purposes in medical ultrasound, penetration is a more relevant measurement. This refers to the distance a wave travels through a tissue before attenuation causes enough loss of energy to prevent image formation. In general, the higher the frequency of the the initial wave, the lower the penetration.

Different tissues in the body also have different impedance values. This refers to the tissue’s resistance to sound propagating through it. It is directly related to density. This is important and essential to the use of ultrasound because the reflection of sound waves is dependent on the relative impedance between the two materials at an interface.

Generally, when the impedance (density) is very different between two tissues more sound is reflected.

For example: placing an ultrasound on a truly uniform substance yields a uniform appearing image. However, in areas with multiple different impedances (i.e: biological tissue) the reflections of sound waves at these interfaces allow you to ‘see’ these deep tissues as interfaces. In areas where the difference in impedance is very high (i.e: lung/tissue) almost all of the sound waves are reflected

Solids such as bone have high impedance, whereas liquids (fluid filled structures, as well as ‘solid’ organs such as liver/kidney, effectively function as liquids for US purposes) have low impedance values. Air has the lowest impedance, but propagation in air is complicated by high attenuation.

 Importantly, the sound waves which are reflected to the source (in this case, the probe) are absorbed by the piezoelectric crystal, during the ‘listening period’. The crystal then vibrates, producing an electric current which is relayed back to the machine. This data is interpolated into an image. Since the reflections of sound waves from deeper structures take a longer time to travel to and return to the probe, we are able to determine the relative depth of different structures. These are displayed as a cross sectional image, with structures closest to the probe towards the top of the screen, and structures further from the probe lower on the screen.

RESOLUTION

Resolution refers to the ability to determine two closely spaced objects as two distinct points. There are multiple types of resolution in ultrasound:

Axial Resolution: The ability to differentiate objects close together depending on depth. This is better with higher frequency.

Lateral Resolution: ability to differentiate objects which are close together ‘side by side’. This is dependent on the width of the ultrasound beam, as well as the number of crystals in the ultrasound probe

Temporal resolution-ability to track moving objects over time. Related to number of frames/sec. Higher frame rate allows for smoother image aquisition

It is important to balance these properties in choosing the appropriate probe for a particular exam. A high frequency probe will have better axial resolution but poorer penetration. A probe with smaller footprint will generally have worse lateral resolution.

Note improved temporal resolution with higher frame rate on the right (same patient) 

ECHO CHARACTERISTICS

Ultrasound has specific terms used to describe the images in terms of relative appearance, or echogenicity

Echogenic/Hyperechoic: refers to tissue which appears brighter than surroundings

Hypoechoic: refers to tissue which is darker than surroundings

Anechoic: Appears black (such as in a simple fluid)

Solid/Reflective: no visible echogenicity behind object, casts a shadow (such as bone, or calcified material)

Homogenous: uniform in echogencity

Heterogenic: not uniform in echogenicity

Artifacts

In a perfect world, using ultrasound would produce a beautiful image every time. However, sometimes, the way information is gathered by the probe and interpreted by the ultrasound machine is not perfect. This leads to imaging artifacts.

This occurs when there is a mismatch between assumptions made by the ultrasound machine algorithm and the behavior of sound waves which return to the probe. Knowledge of these artifacts is useful in interpreting what type of structure is potentially producing them.

Shadowing: Reduction in reflected echoes deep to a strongly reflecting or attenuating structure

Posterior acoustic enhancement: Occurs when a fluid structure is juxtaposed near normal tissue. The sound is less attenuated through fluid than through surrounding tissue, causing the tissue behind (or deep to) the fluid structure to appear brighter, or more echogenic

Lateral Cystic shadowing (edge artifact): The reflection angle at the edges of a spherical structure do not allow reflection back towards the probe. Therefore this information is lost and appears as shadowing posterior to the lateral edges of these structures.

Reverberation: caused by sound bouncing between tissue boundaries before returning to the receiver. The increased time to return to the probe is displayed as increased depth in the image. Also known as ‘ring down’

Comet tail: Subset of reverberation artifact caused by sound bouncing between small highly reflective interfaces. Thought to be from reverberations of very close, non-parallel surfaces. This coalesces to form vertical hyperechoic lines.

Mirroring: Occurs when sound bounces off a strong, smooth reflector (i.e diaphragm). The reflected sound takes moments longer to return to the probe, interacting with tissue along the way. The time is interpreted as depth, and a mirror image is displayed on the opposite side of the reflector

References:

Edelman, Sidney K. Understanding Ultrasound Physics. Woodlands, TX: ESP, 2012. Print. 

Hoffman, Beatrice. "Ultrasound Physics." Ultrasound Physics. ACEP, n.d. Web. 

Noble, Vicki E., and Bret Nelson. Manual of Emergency and Critical Care Ultrasound. Cambridge: Cambridge UP, 2011. Print. 

Smith, Pattie. Bedside Ultrasound Guide for Emergency Medicine. Cincinnati, OH. Print