Medium
Speed of sound (m/s)
Ultrasound Physics
Ultrasound Physics
Ultrasound Physics
It is important to have a basic understanding of ultrasound physics to help with interpretation of different images and the use of ultrasound to aid diagnosis.
The basic physics should help you understand why different probes are used for different types of imaging, how B and M-mode is used, how to improve image acquisition and how artefact is used to help with image interpretation (particularly in lung ultrasound).
The key topics covered below are:
Properties of sound waves
Ultrasound travels as a longitudinal wave and consists of areas of compression and rarefaction. It is a sound wave with a frequency above that of the audible range in humans. Medical ultrasound commonly uses ranges from 2-15MHz.
Audible range roughly 20Hz-20kHz
Ultrasound >20kHz
Longitudinal sound wave representation using slinky coil by Evan Toh
Like all waves, US waves can be described in terms of their:
Amplitude (A)
- This is the height of the sound wave, is the maximum increase or decrease in sound pressure. Amplitude has no relationship to a sound’s wavelength or frequency.
Frequency (ƒ)
- This is the number of complete cycles per second and is expressed in hertz (Hz). One hertz equals one cycle per second. In the auditory range of sound and music, it is often referred to as “pitch.”
Wavelength (λ)
- This is the distance over which a wave’s shape repeats, determined by measuring the distance between two consecutive corresponding points of the same phase. It is expressed in units of distance (m, cm, mm, etc.)
Ultrasound wave representing frequency as the number of cycles completed per second.
Top: US probe with US wave propagating out with areas of compression and rarefaction.
Bottom: Waveform representing pressure differences over the distance of the wave. The amplitude is the height of this wave.
Propagation Velocity (c)
- Sound travels at different speeds through different medium. This is known as propagation velocity (c).
The propagation Velocity (c) of sound (m/s) is related to frequency (f in Hz) and wavelength (λ in m) by the following equation:
c = ƒλ
Ultrasound in the body
Propagation velocity in different media
Clinical ultrasound is transmitted through the soft tissues of the body at an average velocity of 1540 m/s.
The propagation velocity of sound waves through tissues is affected by the density and elasticity of the medium through which it is travelling. Higher propagation velocities are seen with tissues that have increased stiffness and density.
Air
331 (quite slow)
Fat
1430
Blood
1570
Muscle
1575
Liver
1580
Bone
3000 to 5000 (very fast)
Frequency and attenuation
Frequency and attenuation are closely correlated, and a higher frequency or shorter wavelength will result in greater image resolution. However, this will be at the expense of greater attenuation and poorer penetration or depth of field.
Higher frequency = Shorter wavelength (λ) = Higher resolution
- Penetration through medium poor (Greater attenuation)
- Near structures visualised in greater detail
- Far / Distant structures poorly imaged
Low frequency = Longer wavelength (λ) = Lower resolution
- Good penetration through medium, but less detail seen
- Good for imaging deep / distant structures
Attenuation
The culminating effect of tissue on sound as it travels through the body is termed attenuation. Attenuation is the loss of strength or decreasing energy of an US wave as it travels through a medium. This loss of sound energy is due to tissue absorption, as well as sound waves being reflected and scattered at the boundaries of tissues with different densities and acoustic impedances.
The attenuation coefficient of tissues is the relation of attenuation to distance and depends on the tissues traversed and the frequency of the US wave. Attenuation is high in muscle and skin and low in fluid-filled structures. Higher frequency US waves are subject to greater attenuation compared to lower frequency US waves. To compensate for tissue attenuation, returning echoes can be amplified by the US system, by increasing the gain.
Medium
Attenuation coefficient
(dB/cm at 1MHz)
Water
0.002
Blood
0.18
Fat
0.63
Liver
0.5-0.94
Kidney
1.0
Muscle
1.3-3.3
Bone
5
Sound and interfaces
Sound waves can be absorbed/attenuated, reflected, refracted and/or scattered.
Absorption accounts for the main form of attenuation. As an US wave is transmitted through a medium, particles vibrate and friction converts this energy into heat, reducing the US wave energy. This loss of energy is referred to as attenuation.
Reflection occurs when a sound wave transfers from one medium to another i.e. between two tissues. At the moment an US wave hits this interface, part of the wave is reflected back (echo) towards the original medium, reducing the strength of the US wave moving forward. The greater the acoustic impedance between the two tissue surfaces, the greater the reflection and the brighter the returning echo will appear on US, as a greater amount of energy is reflected back towards the transducer.
Reflection can be categorised as either specular or diffuse/scatter.
- Specular: When US waves hit a smooth, large surface, such as bone, the sound wave is reflected back in a singular uniform direction, this is referred to as a specular reflection. Often specular reflectors due to the significant difference in acoustic impedance between the two surfaces, appear very bright on the US image.
- Diffuse/scatter: When US waves hit an irregular surface between two tissues, such as in soft tissue, the reflections return to the transducer in a chaotic and disorganised way relative to the initial US beam. This is termed diffuse reflection and is often seen on US image as various shades of grey located within the structure imaged.
Specular vs. diffuse reflection
Examples of reflected, refracted and attenuated ultrasound waves
Refraction describes reflection where sound strikes the boundary of two tissues with different impedance, at an oblique angle.
As sound passes the border between media, depending upon the relative refractive indices of the two media, the light will either be refracted to a lesser angle or a greater one. These angles are measured with respect to the normal line, represented perpendicular to the boundary.
The angle of refraction is dependent on the angle the sound wave strikes the boundary between the two tissues and the difference in their propagation velocities.
If the propagation velocity is greater in the first medium and the sound waves are slowed down, refraction occurs towards the centre/normal line, or perpendicular.
If the propagation velocity is greater in the second medium, the sounds waves speed up, and refraction occurs away from the originating beam / normal line.
The largest possible angle of incidence which results in a refracted beam is called the critical angle and results in the refracted beam travelling along the boundary between two media.
Incidence angles greater than the ‘critical angle’ result in the sound waves being completely reflected by the boundary, a phenomenon known as total internal reflection.
With refraction and reflection, the US image produced may not be clear or be altered due to the US waves not being reflected back directly to the transducer.
Acoustic impedance
Acoustic impedance is the resistance to the propagation of ultrasound waves through tissues. Each type of tissue offers a unique resistance or “acoustic impedance”. Acoustic impedance (Z) is expressed in the following equation, where p= the density of the medium ultrasound is being transmitted through and v= the speed of sound through that medium.
Z=pv
Descriptive terms: Appearances of different tissue on ultrasound
Anechoic = tissues that produce no echo (seen with Rayleigh Scattering within blood vessels. As no echoes returning to the transducer, the structures on image (B-mode) appear black).
Hyperechoic = tissues that produce a strong reflection back to the transducer, with only a small amount of the US beam continuing through are termed hyperechoic. Hyperechoic tissues often appear bright on B-mode. Fascia and bone are examples of hyperechoic structures.
Hypoechoic = tissues that produce weak echoes are termed hypoechoic and appear as varying shades of grey on B-mode image. Fat is an example of a hypoechoic tissue. Deep structures may appear hypoechoic as well, due to the incident US beam being weak from attenuation, resulting in diminished returning echoes.
Tissues may be described as heterogenous (e.g. muscle) which are comprised of multiple structures of different acoustic impedance causing varying degrees of brightness on the B-mode image, or homogenous (e.g. liver) where tissue acoustic impedance is similar appearing uniformly on image.
Original
Labelled
Lung US - showing hyperechoic ribs with strong reflection from the bone surface and the casting of the rib shadow where no US beam is able to pass deeper.
US right internal jugular vein and carotid artery. The blood in the vessels appears anechoic and dark. The muscles appear hypoechoic with some hyperechoic areas due to the surrounding fascia and perimysium.
Biological effects - heat generation and safety
No adverse biological effects through diagnostic ultrasound in humans have been demonstrated, however there has also been little experimentation in the field. The risk of pulmonary capillary haemorrhage has been demonstrated in numerous animal models though has yet to be demonstrated in humans. Generally speaking, it is recommended that the ALARA (As low as reasonably achievable) principle is applied when performing POCUS in eye, foetus and lung.
Miller DL, Abo A, Abramowicz JS, Bigelow TA, Dalecki D, Dickman E, Donlon J, Harris G, Nomura J. Diagnostic Ultrasound Safety Review for Point‐of‐Care Ultrasound Practitioners. Journal of Ultrasound in Medicine. 2020 Jun;39(6):1069-84.
Infection control
A far more pressing safety consideration in the clinical application of ultrasound is ultrasound gel and its properties as a bacterial culture medium, which is of particular concern during US guided procedures such as drains and lines. Aseptic gel should be used during these procedures along with effective pre-, intra- and post-procedure infection control techniques.
Solaimalai D, Ragupathi NK, Ranjini K, Paul H, Verghese VP, Michael JS, Veeraraghavan B, James EJ. Ultrasound gel as a source of hospital outbreaks: Indian experience and literature review. Indian journal of medical microbiology. 2019 Nov 1;37(2):263-7.
Rooker KT. Does ultrasound gel used during needle guided procedures put patients at risk for nosocomial infection?. Ultrasound in Medicine and Biology. 2019 Jan 1;45:S115-6.
Mullins, K., Burnham, K., Henricson, E.K., Cohen, S., Fair, J. and Ray, J.W., 2020. Identification and analysis of bacterial contamination of ultrasound transducers and multiuse ultrasound transmission gel bottle tips before and after the aseptic cleansing technique. Journal of Ultrasound in Medicine, 39(10), pp.1957-1963.
Sound generation
Piezo-electric effect and transducer design
Sound is used to create images of structures within the body using a pulse-echo technique. The Ultrasound transducer emits brief sound impulses at a fixed rate and then ‘listens’ in between pulses for returning echoes. US waves are created and transmitted by numerous crystals (elements) aligned in succession along the face of the transducer. Typically, these crystals are piezoelectric crystals, but some newer devices use silicon chip technology (e.g. Butterfly IQ).
When a voltage is applied across piezoelectric crystals, the crystals oscillate / change shape, producing an US wave and converting electrical energy into sound energy. This US wave then travels away from the transducer through different medium, where it is attenuated and/or reflected, refracted, absorbed depending on properties of the medium. Different medium and tissues attenuate US waves differently.
Reflected US waves or echoes, return to the transducer, where during the ‘listening phase’, the piezoelectric crystals work in reverse, where returning sound waves are absorbed, transferring sound energy to the crystals and causing a voltage change which can be measured. Based on the time and strength of these echoes, a microprocessor within the US system assigns each one a position and colour in grey scale, ultimately forming a B-mode (Brightness Mode) image.
Modern US machines are referred to as ‘real-time’ imaging devices because they have the ability to rapidly display B-Mode images so that any motion that occurs is visualised as it happens.
How Ultrasound Works by National Institute of Biomedical Imaging and Bioengineering
Types of transducers/probes
Types of transducers/probes
Name
Linear “Vascular probe”
Probe
Field
Frequency
High (5-10MHz)
Good/bad
High resolution/image quality
Low penetration/field depth
Big footprint
Bad for movement
Use
Vascular, optic nerve, venous access
Name
Curvilinear “Abdominal probe”
Probe
Field
Frequency
Low (2-5MHz)
Good/bad
Low resolution/image quality
High penetration/field depth
Big footprint
Bad for movement
Use
Abdominal, FAST, lung, pleura, Obs-Gynae
Name
Phased array “Cardiac probe”
Probe
Field
Frequency
Low (2-5MHz)
Good/bad
Low resolution/image quality
Higher penetration/field depth
Small footprint
Good for movement
Use
Cardiac, lung, FAST, transcranial doppler
B-mode (brightness mode)
This is the most commonly used image modality. Multiple beam positions and a series of reflected echoes produces a 2D black/grey and white image, similar to that of an anatomical slice.
B-mode ultrasound clip giving a 2D image of the heart and an underlying pleural effusion in the deep PLAX view
M-mode (motion mode)
M-mode is defined as time motion display wave along a chosen US line. This is where one particular line is recorded repeatedly to examine a moving structure (e.g. a heart valve or lung pleura). This is plotted against time to demonstrate how the structures move over a particular time period.
M-mode ultrasound clip showing a B-mode image at the top with the chosen line by which the time motion display is being recorded at the bottom. This is a MAPSE (mitral annular plane systolic excursion) demonstrating longitudinal contraction of the LV over time.
Image quality
Frame rate
Frame rate is the number of images generated per second, expressed as frames per second (FPS) and contributes to the “smoothness” of motion capture.
Temporal resolution
Temporal resolution, expressed in Hertz (Hz) is directly related to the frame rate. It describes the ability of the ultrasound to demonstrate movement. It is most relevant in echocardiography where the assessment of rapidly moving valve leaflets can be of significant importance.
Spatial resolution
Spatial resolution describes an ultrasound machine’s ability at discerning between smaller targets within a tissue. Low spatial resolution may display targets as one larger target rather than the two smaller targets they actually are. This can be expressed in terms of lateral (left vs right) and axial (near vs far)
The higher the frequency, the shorter the spatial pulse length and the higher the axial resolution. As previously discussed, lower frequencies are required to penetrate deeper into tissues and produce deeper visual fields. This therefore means that increased field depth comes with the cost of poorer axial resolution. It is important to bear this in mind when scanning and to use the lowest field of depth possible for the task at hand to ensure higher resolution images.
Lateral resolution depends on the distance between the individual crystals in the probe being used, rather than the distance between the objects being viewed. Resolution diminishes past the focal zone, termed the ‘far zone’ as the US beams begin to diverge and are attenuated by tissue. This is a particular risk when viewing deeper structures as the ultrasound signal undergoes significant attenuation, reflection and refraction.
Focus
Much like a lens, focus position adjusts the focused part of the ultrasound beam which gives the best resolution for structures of interest. Ideally focus should be set at the level of the point of interest.
Example of the focus point from an US probe
Gain
Gain (’brightness’): compensates for attenuation of the US waves that occur due to tissue interaction. Gain amplifies only returning echoes, increasing or decreasing from all imaging depths and is similar to turning up the volume on a radio. Gain should be adjusted so that the sound/tissue interaction produces optimal images of the structure of interest. Too little gain results in a dark image where it may be impossible to identify any structures. Too much gain results in a very bright image (‘snow-storm’), which again can make it impossible to identify structures correctly.
Example US button layout and controls
Artefacts
An artefact is any phenomenon that affects the acquisition or interpretation of an US image. artefacts can occur because of properties within the tissue itself or be created by the US operator. The most commonly seen artefacts are air, shadow, reverberation and acoustic enhancement. The interpretation of artefacts in point of care ultrasound (POCUS) e.g. in Lung US can be clinically useful and help distinguish between different disease processes and pathology.
Air artefact occurs when the transducer does not fully contact the skin, with part of the image appearing black. There is a large difference in acoustic impedance between air and tissue, so US cannot penetrate the skin and is reflected off. US gel applied to the transducer and even pressure on the skin, should prevent or correct air artefact.
Shadow artefact occurs when an US wave comes into contact with tissues that have a high attenuation coefficient, creating a shadow beyond the tissue. This acoustic shadowing appears behind structures that either highly reflect or absorb the sound beam. The result is that there is a strong loss of US signal behind said structure and therefore a loss or complete lack of image, resulting in a shadow. Commonly encountered structures where acoustic shadowing may be seen include ribs, mechanical prosthetic valves and air. The acoustic shadow created behind structures, also reduces the ability of the user to visualise structures in the shadow behind. Increasing the gain or adjusting US beam approach may reduce shadow artefact and allow visualisation of structures previously unseen in the shadow.
Acoustic Enhancement occurs when sound passes through an area of very low acoustic impedance into tissue whose impedance is much higher. This can cause tissue of a structure to appear more echogenic (brighter/thicker) than other parts and may be misinterpreted as pathology.
Reverberation artefact can be thought of as an “echo”, where the soundwave is trapped between two reflective surfaces. This could be calcified tissue or metallic structures such as prosthetic heart valves or a needle. The signal transmitted to the transducer appears as many echogenic surfaces rather than two.
Mirror artefact is where there is a highly reflective surface (such as the diaphragm) will reflect the primary ultrasound beam back towards the US probe then the US beam reflects from another structure back to the diaphragm again and then this reflects the beam back to the US probe. This delay means the image is displayed deeper than it should be and because the other structure is also captured by the US the structure appears to form a mirror image – with the highly reflective surface appearing as the mirror.
Mirror artefact in deep PLAX view: mitral valve motion can be seen above and below the pericardium where the pericardium is acting like a mirror.
Clinically useful Artefacts:
A-Lines
In lung ultrasound, A-lines are horizontal bright reverberation artefacts created by the pleural line. The same distance between the skin and the pleural line, is repeated between the pleural line and an A-line. A-line presence can be useful when interpreted with other Lung US signs. A-lines are seen in normal lungs, but also in pneumothorax.
Original
Labelled
Lung US demonstrating A-lines: the pleural line reflects US beam back to the probe which reflects back to the pleural line and this is repeated. The lines deep to the pleural line are just artefact of the pleural line reflections.
B-Lines
B-lines are an example of a “comet tail” reverberation artefact arising from the pleural line found in lung ultrasound. B lines are hyperechoic, well defined and extend indefinitely, erasing A-lines. They often move with respiration if lung sliding present. These are generally absent in normal conditions, with their presence suggesting alveolar-interstitial pathology. However, a small number of B-line (<3 per field of view) can be normal around the lung bases, especially in the elderly.
Lung US example of B-lines extending from the pleural line
Doppler
The Doppler Effect is a change in sound wave frequency during the relative motion between a wave source and its observer. When an object producing sound moves towards you, the frequency of the sound wave increases, leading to a higher pitch. If an object producing sound moves away from you, the frequency of the sound wave decreases, leading to a lower pitch.
In diagnostic ultrasound, Doppler is used to detect and measure blood flow. This is accomplished using Colour Flow and Spectral Analysis. When an US transducer is placed over a blood vessel, if the blood flow is towards the transducer, the reflected US wave will have a higher frequency than the initial US signal. This is a positive Doppler shift. Conversely, if blood flow is away from the US probe, the reflected US wave will have a lower frequency than the initial US signal, a negative Doppler shift.
The angle between the receiver and the transmitter determines the amount of Doppler shift. Doppler shifts are calculated by using the cosine of the angle between the axis of the US beam and the direction of blood flow. Maximum Doppler shift occurs when flow is directly towards or away from the probe at 0o. When flow is perpendicular at 90o, no shift is detected.
Ultrasound systems have the ability to determine both direction and velocity of blood flow using Colour Flow and Spectral Analysis. The colour Red often denotes blood flow in a vessel moving towards the US probe (positive shift), whereas the colour Blue often denotes blood flow moving away from the probe (negative shift). Both the red and blue colour have been assigned to represent blood flow in a vessel in relation to the US beam and does not mean red is an artery or blue is a vein!
Clinically Colour Doppler combined with pulsatility and compressibility of a vessel, can help distinguish between an artery and a vein.
Example of colour flow doppler over the right popliteal artery and vein
Pulse wave doppler allows you to measure the velocity of blood flow at a particular point, set by the user at a sample gate. Doppler shifts are recorded from the sample gate and plots the flow over time, with positive deflections coming towards the gate, and negative travelling away.
Continuous wave doppler utilises two crystals, one transmitting, the other receiving, continuously. This technique is used to assess the maximum velocity of blood in a vessel. However, it is unable to accurately locate exactly where the maximal velocity is located.
Probe manipulation nomenclature
Pressure
Increasing the force you are pushing the probe on to the skin/tissue
Sliding
Moving the probe along the skin scanning and area which is superior, inferior, distal, proximal, lateral, or medial to previous
Fanning/Tilting
Changing the angle of the probe so that it is no longer perpendicular to the skin, thus “fanning” through or scanning the tissue directly below the probe, producing multiple cross sectional images of a structure
Rotating
Turning the probe around the midline, enabling imaging of the focus of interest to be view through a different plane
Rocking
Changing the direction in which the ultrasound beam is transmitted by altering the angle of the probe left and right. Achieves “in-plane” motion
Illustration of different ways to manipulate the US probe - view from 3 different planes
Content created by Tom Hawthorne & Ben Stoney
Design by Max Broadbent
The ultrasound images and clips used on this website have be reproduced following the local clinical governance guidance.
