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, while 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: the length (distance) of one complete cycle
Frequency: The number of cycles per second
Period: the time per wavelength (1/f)
Velocity: Speed at which the wave travels (v=fλ)
Power: 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.
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. The amount of this is specific for each type of tissue as below:
Note: this only applies to sound travelling in uniform tissues, and also depends on the frequency of the initial wave. In general, you can use this to determine through which tissues will cause attenuation of the ultrasound signal (high values have poorer penetration). Similarly, a initially high frequency will also have poorer 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.
For example: placing an ultrasound on a truly uniform substance would not yield an 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.
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.
These properties have several effects. 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 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.
Note improved temporal resolution with higher frame rate on the right (same patient)
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
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.
Shadow: Reduction in reflected echoes deep to a strongly reflecting or attenuating structure
Example of shadowing artifact from gallstones
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: 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.
Note lateral cystic shadow at periphery of gallbladder as well as posterior acoustic enhancement behind gallbladder
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’
Example of reverberation artifact from needle during attempted vessel cannulation
Comet tail: Subset of reverberation artifact caused by sound bouncing between small highly reflective interfaces.
Example of comet tail artifact from reverberation between visceral and parietal pleura on intracostal view of lung (also note shadow artifact behind ribs on lateral portions of image)
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
Example of mirroring artifact of liver on RUQ FAST view (artifactual mirror image of liver appears on left side of screen)
Written by: Aalap Shah MD
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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