Recommended Ultrasound Terminology
A
aberration
The failure of transmitted or received waves to align in phase and amplitude at an intended point, curve, or plane, which may be due to different sound speeds in tissues or materials in the acoustic paths.
aberration correction
A method of realignment or compensation to bring aberrated waves into synchronous convergence at an intended point, curve, or plane.
absolute maximum
The largest possible value of a specified quantity, either for an individual instrument or for all instruments of a given generic type. This value shall include effects of inaccuracies and imprecision of the measurement process(es) used to determine it.
absolute minimum
The smallest possible value of a specified quantity, either for an individual instrument or for all instruments of a given generic type. This value shall include effects of inaccuracies and imprecision of the measurement process(es) used to determine it.
absorbed dose
The thermal energy imparted to matter by absorption of acoustic radiation of irradiated material at the site of interest. The unit is joule (J).
absorption
The process by which energy is deposited in the medium through which it propagates. The absorbed energy is ultimately converted to thermal energy (“heat”).
absorption coefficient
Fractional decrease of the amplitude of sound waves solely due to absorption per unit distance. Also called the amplitude absorption coefficient. The unit is neper per centimeter (Np/cm) or decibel per centimeter (dB/cm). The unit Neper per centimeter is sometimes given as cm–1. Note: Because absorption varies with frequency, the frequency at which the absorption coefficient is measured should be specified. Sometimes, absorption is expressed as a power law of frequency, α = α0 ·fy, where ƒ is frequency in megahertz, α0 is the absorption coefficient at 1.0 MHz, and y is the power exponent. For typical tissues, y ~ 1; for water, y = 2. Then the absorption coefficient, α, is described by the unit Np/(cm-MHzy ) or dB/(cm-MHzy ).
Related terms: specific absorption rate (SAR), absorption, absorption loss
absorption dose rate
The time rate of deposition of absorbed dose. The unit is watt (W).
absorption loss
That part of the transmission loss due to the conversion of sound energy to thermal energy, within the medium. The unit is neper or decibel (Np or dB). That part of the transmission loss of amplitude solely due to absorption within the medium. The unit is neper or decibel (Np or dB).
Related terms: absorption, acoustic signal loss, attenuation, attenuation coefficient, attenuation loss
absorption per wavelength
Absorption of the ultrasound beam intensity over a distance equal to the wavelength.
acceleration
The rate of change of velocity. Acceleration is a vector quantity whose magnitude is measured in meters per second squared (m/s2) in a specified direction.
acoustic absorption
See absorption.
acoustic, acoustical
The qualifying adjectives “acoustic” and “acoustical” mean containing, producing, arising from, actuated by, related to, or associated with sound. Acoustic is used when the term being qualified designates something that has the properties, dimensions, or physical characteristics associated with sound waves; acoustical is used when the term being qualified does not designate explicitly something that has such properties, dimensions, or physical characteristics. Note 1: The following examples qualify as having the properties or physical characteristics associated with waves and hence would take “acoustic”: impedance, output (sound power), energy, wave, medium, signal, and transducer. Note 2: The following examples do not have the requisite physical characteristics and therefore take “acoustical”: method, engineer, symbol, problem, and measurement. Note 3: As illustrated in the preceding notes, usually the generic term is modified by “acoustical,” whereas the specific technical implication calls for “acoustic.”
acoustical system
System capable of receiving, transmitting, or generating acoustic signals.
acoustical window
See scanning window.
acoustic angiography
An ultrasound imaging technique used to visualize the inside (or lumen) of blood vessels of the body. This is typically performed by injecting and imaging a microbubble intravascular contrast agent.
acoustic attenuation
The reduction of the amplitude or intensity of an acoustic signal as it propagates through a material. It includes the effects of absorption, scattering, and beam divergence.
Related term: attenuation
acoustic axis
See beam axis.
acoustic cavitation
Cavitation caused by acoustic waves.
Related term: cavitation
acoustic crest factor
The ratio of the spatial-peak temporal-peak acoustic pressure to the root mean square (RMS) acoustic pressure at the position in an acoustic field corresponding to the spatial-peak temporal-peak acoustic pressure. The RMS acoustic pressure is taken over the pulse-peak cycle at the point in the acoustic field corresponding to the spatial- peak temporal-peak acoustic pressure (see Figure 31).
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Archaic term
acoustic emission (AE)
The phenomenon of radiation of acoustic (elastic) waves in solids that occurs when a material undergoes irreversible changes in its internal structure, for example as a result of crack formation or plastic deformation due to aging, temperature gradients, or external mechanical forces.
acoustic energy
Mechanical energy transported by an acoustic wave. The unit is joule (J).
acoustic enhancement
A manifestation of an increased acoustic signal amplitude returning from regions lying beyond an object that causes little or no attenuation of the sound beam. Contrasts with acoustic shadow. Acoustic enhancement is one type of sonographic artifact (see Figure 15).
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acoustic field
The distribution of acoustic energy in space or space and time.
acoustic holography
See holography.
acoustic impedance
The ratio of the instantaneous acoustic pressure at a point to the instantaneous particle velocity (specific acoustic impedance). A property of a medium computed as the product of the density and sound propagation speed (characteristic acoustic impedance). In general, the acoustic impedance is a complex number: the resistance is the real part, and the reactance is the imaginary part.
Related terms: acoustic impedance mismatch, acoustic reactance, acoustic resistance, complex number, complex parameter, imaginary, real
acoustic impedance match
The condition in which two contiguous media have equal or nearly equal characteristic acoustic impedances, thereby minimizing reflection of acoustic energy at the interface.
Related terms: impedance ratio, reflection coefficient
acoustic impedance mismatch
The condition in which two contiguous media have different characteristic acoustic impedances, thereby providing a mechanism for the production of acoustic reflections. Waves incident on discontinuities in acoustic impedance are reflected. These reflections are the main mechanism for producing pulse echoes on which ultrasound imaging and color flow Doppler detection are based. Reflection causes a reduction in transmission of ultrasound.
Related terms: impedance ratio, reflection coefficient, transmission coefficient
acoustic initialization fraction
See acoustic power-up fraction.
acoustic insulation material
Material used to prevent the transmission of sound.
acoustic intensity
See intensity.
acoustic lens
See lens.
acoustic microscope
A device for the generation of micron level resolution acoustic images. Acoustic microscopes usually operate at frequencies greater than 100 MHz, and their resolution is comparable with that of optical microscopes.
acoustic oscillation
Movement of particles in an elastic medium about an equilibrium position.
Related term: sound
acoustic output
The total energy emitted by a source such as an ultrasound transducer. Acoustic output may be quantified by one or more experimentally measurable physical quantities such as particle velocity, pressure, or power or a parameter mathematically derived from these variables such as ISPTA (spatial-peak temporal-average intensity).
acoustic output freeze
A condition of a system for which the acoustic output is disabled when there is no active updating of ultrasonic echo information.
Related term: freeze frame
acoustic phase coefficient
See wave number.
acoustic power
Acoustic energy transported per unit time (usually a temporal average is quoted). The unit is watt (W).
Related term: average acoustic power
acoustic power-up fraction
Ratio of the peak negative pressure when the system is in power-up of a specified mode to the maximum value for any system settings for that mode. This ratio is determined from measurements made at the position that yields the maximum pulse-pressure-squared integral (or maximum mean square acoustic pressure for continuous wave systems). The ratio is usually expressed as a percentage (see Figures 12, 31, and 32).
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acoustic pressure
The value of the total (absolute) pressure minus the ambient pressure. The unit is pascal (Pa).
acoustic pulse waveform
The temporal waveform of the acoustic pressure at a specified position in an acoustic field, displayed over a period sufficiently long to include all significant acoustic information from a single pulse or tone burst or from 1 or more cycles in a continuous wave (see Figures 12, 31, and 32).
acoustic radiation force (ARF)
A small force field that rides along an acoustic transient perturbation in a medium. If a boundary, target, small particles, tissues, or bubbles are in the path of this force field, the forces can be measured or visualized. Alternatively, streaming acoustic field patterns can be observed in fluids by adding a visualizing agent. Though small, acoustic radiation forces can be put to use and are being studied and utilized in a wide variety of applications from levitation, neuromodulation, therapy, drug delivery to elastography.
Related terms: acoustic radiation pressure, acoustic radiation force imaging, radiation force, radiation force balance, shear wave elastography
acoustic radiation force imaging (ARFI)
An imaging technique whereby the radiation force due to an applied intense sound field generates a shear wave in a medium, whose speed of propagation can be measured and related to the shear elastic properties of the medium.
acoustic radiation pressure
The pressure exerted by an acoustic wave on a surface due to the transfer of wave momentum. Acoustic radiation force per unit area. This pressure plays a role in acoustic radiation force imaging (ARFI) and other ultrasound-based force applications.
Related term: acoustic radiation force
acoustic radiometer
Instrument for measuring acoustic radiation pressure.
acoustic reactance
Imaginary part of acoustic impedance. Acoustic impedance is a complex quantity consisting of a real part (resistance) and an imaginary part (reactance).
Related terms: imaginary part, complex number
acoustic resistance
Real part of acoustic impedance.
Related term: complex number
acoustics
The science of sound, including its production, propagation, and effects.
acoustic scan line
A direction along which echo information is both transmitted and acquired (See Figures 29, 30, and 34).
Related terms: scan line, synthetic scan line
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acoustic shadow
Sonographic appearance of reduced echo amplitude from regions lying beyond an attenuating object. It is important to distinguish between acoustic shadows and regions of low reflectivity. An acoustic shadow is one type of sonographic artifact. The opposite of acoustic shadow is acoustic enhancement (see Figures 16 and 22).
acoustic signal loss
The decrease in amplitude or intensity as sound travels through a material caused by attenuation, reflection, and beam variations.
acoustic stiffness
The reciprocal of acoustic compressibility. The ratio of the change in pressure to the fractional change in volume. The unit is pascal (Pa).
acoustic streaming
An acoustically generated transport of fluid within the body of the insonified fluid or tissue.
acoustic vibration
See sound.
acoustic wave
A mechanical vibration that propagates in a medium. An acoustic wave cannot travel in a vacuum. Liquids and gases support longitudinal (compressional) waves. Solids support other vibrational modes in addition to longitudinal waves. Soft tissues support primarily longitudinal waves. Compact bone supports both longitudinal and other vibrational modes.
Related terms: longitudinal wave, shear wave
acoustic wave field
See acoustic field.
acoustic waveform
See waveform.
acoustic wavefront
A contiguous surface of equal phase in a propagating wave.
acoustic wavelength
The distance traveled by a wave during the period of one cycle. The distance between any two adjacent points at which the phase, at the same instant, differs by 2π radians; λ = c/ƒ, where λ is the wavelength, c is the speed of sound, and ƒ is the frequency. For water or tissue at 1 MHz, the wavelength is approximately 1.5 mm.
acoustic working frequency
The mean of the frequencies ƒ1 and ƒ2 at which the amplitude of the spectrum of the acoustic signal; ie, the output of a hydrophone placed in an acoustic field at a specified position, first becomes 3 dB lower than the peak amplitude. The mean may be either arithmetic (ƒ1+ ƒ2)/2 or geometric √(ƒ1· ƒ2). The unit is megahertz (MHz).
acousto-optical converter
Any device that permits the visualization of a sound wave by converting the sound energy into a corresponding light pattern.
active element
The region of a single transducer that is capable of being energized.
active transducer aperture
The active transducer aperture is the part of the surface of the transducer or transducer assembly that emits ultrasonic radiation.
Related terms: transducer, transducer aperture
adaptive beamforming
A beamforming algorithm that alters beam characteristics in response to changing data inputs. Focuses on signal processing during receive. It is a computationally intensive process that happens prior to image formation.
adaptive gain adjustment
The automatic adjustment of an ultrasound system’s gain settings based on echo information obtained during the examination of a patient. Both the time-gain compensation (TGC) and overall gain of a system may be adjusted in this form of image optimization. Also known as automatic gain adjustment or automatic time-gain compensation adjustment.
adaptive image optimization
An image optimization technique in which an ultrasound system’s gain and control settings are automatically determined by the echo information obtained from the patient being examined. Control adjustments may include overall gain, time-gain compensation, gray scale, color, frequency filtering, focusing adjustments, and Doppler settings.
adiabatic bulk modulus
The elastic modulus associated with the volume elasticity of a fluid medium when heat is not exchanged with the external environment. The adiabatic bulk modulus (K) is given by K = 1/b, where b is the adiabatic compressibility. The unit is pascal (Pa).
adiabatic compressibility
The fractional change in volume V per unit change in pressure for fluid media when heat is not exchanged with the external environment. Adiabatic compressibility (b) is the reciprocal of the adiabatic bulk modulus (K). The unit is inverse pascal (Pa–1).
admittance
Reciprocal of impedance.
after-ringing
Undesired damped oscillation of a transducer occurring after the main pulse (see Figure 20).
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aliasing
The introduction of artifactual frequency components into a recovered signal as a result of sampling an original signal at a rate that is lower than twice its highest frequency component. Aliasing of a Doppler signal will result in an erroneous representation of the Doppler shift frequency.
Related term: Nyquist limit
A-line
See A-mode.
ALARA
Acronym for “as low as reasonably achievable.” It means making every reasonable effort to use the lowest possible acoustic output and transducer dwell time to obtain ultrasound images and measurements with quality sufficient for clinical diagnosis.
AM
Amplitude modulation or amplitude-modulated.
ambient
Refers to the local environment surrounding an object.
ambient pressure
The absolute pressure at a point in the absence of acoustic waves.
A-mode (amplitude mode)
A method of displaying echoes received from a specific direction in which time is represented along a vertical axis and echo amplitudes are displayed along a horizontal axis. Since time is proportional to reflector distance, this trace also represents echo signal amplitude versus reflector distance (see Figure 28).
Related term: A-line
amorphous tissue
In the acoustical sense, tissue lacking echo-producing structures. These tissues include fluid-filled structures such as cysts and other acoustically homogeneous regions.
amplifier
A device that magnifies the amplitude or power of its input signal.
amplitude modulation (AM)
A process by which the amplitude of a higher-frequency (“carrier”) wave is multiplied by a lower-frequency (“signal”) wave as a means of encoding ultrasound signals. Examples: Vibro-acoustography, amplitude modulation contrast-enhanced ultrasound.
Amplitude modulation (AM)-based contrast-enhanced ultrasound imaging
AM-based contrast-enhanced ultrasound imaging takes advantage of the nonlinear acoustic behavior of contrast microbubbles to isolate the nonlinear signals generated by microbubbles while suppressing signals from surrounding tissue. AM-based contrast-enhanced imaging sequence involves multiple transmissions of acoustic pulses with different amplitudes (e.g., a high-amplitude pulse with full acoustic pressure, and a low-amplitude pulse with half acoustic pressure) and processing of the backscattered signals from these pulses (e.g., by analyzing the difference between the low- and high-amplitude echoes) to extract the microbubble signals from tissue.
amplitude-modulated display (A-mode)
See A-mode.
amplitude modulation factor
The value of the expression 100 (A – B )/{A+B), expressed as a percentage, where A and B are the respective absolute maximum and minimum amplitudes of the envelope of an amplitude-modulated acoustical or electrical carrier (first-order quantity).
analog signal
A signal that can have all values between its minimum and maximum values and that may change continuously as a function of time.
analytic signal
A complex signal, the real part of which is the original signal, and the imaginary part, also called the quadrature signal, is calculated by taking the Hilbert transform of the real signal (see Figure 26).
Related terms: complex parameter, Hilbert transform, imaginary, real
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analytic signal envelope
An envelope formed by taking the square root of the sum of the real original signal squared plus the quadrature signal squared (see Figure 26).
Related term: Contrast with rectification
anechoic
The property of appearing echo free or without echoes in an ultrasound image. A clear cyst appears anechoic in the absence of any artifact (see Figure 17).
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angle of attack
See beam-to-vessel angle.
angle of incidence
The angle between the axis of an acoustic beam encountering an interface and the vector normal (perpendicular) to the interface (see Figure 6).
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Related terms: angle of reflection, angle of refraction, Snell’s law
angle of reflection
The angle between the axis of the reflected beam and the axis of the normal of a reflecting surface. The magnitude of the angle of reflection equals that of the angle of incidence (see Figure 6).
Related terms: angle of incidence, angle of refraction, Snell’s law
angle of refraction
The angle between the axis of the refracted beam in medium two and the axis of the normal of a reflecting surface. (see Figure 6).
Related terms: angle of incidence, angle of reflection, Snell’s law
angular deviation loss
Sensitivity level on the principal axis minus the sensitivity level of the transducer for a specified direction. The unit is decibel (dB).
angular frequency
Frequency expressed as radian/second, symbol: ω = 2πƒ, (ƒ = frequency in hertz [Hz]).
anisotropic
A property of tissues that exhibit different characteristics depending on the direction of measurement. In ultrasound imaging, anisotropy manifests as an angle-generated artifact that occurs when the ultrasound beam is not perpendicular to the structure being examined. This can result in a hypoechoic (darker) appearance of structures like tendons, ligaments, or muscles. Antonym: isotropic.
annular array
A multiple-element transducer array in which the individual transducer elements are arranged in concentric rings around a central circular element (see Figure 18).
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annular enhancement (arterial)
See rim enhancement.
antinode
Point, line, or surface in a standing wave where some specified characteristic of the wave field has a local maximum amplitude. Note: The appropriate modifier should be used before the word “antinode” to signify the type that is intended: eg, displacement antinode, particle velocity antinode, or sound pressure antinode.
antiradial scanning (breast)
A method of orienting the plane of view in breast sonography such that the scan plane is perpendicular to that in radial scanning.
Related term: radial scanning (breast)
antiresonance
Phenomenon of a system in forced oscillation such that any change in the frequency of excitation, however small, results in an increase in a response of the system. Antiresonance is a condition for which the impedance of an electrical or acoustical system is very high. Note: The quantity that is observed for the measurement of response must be indicated: eg, velocity antiresonance.
aperture path difference
The difference in acoustic path lengths to a specified geometric focus from 2 points in the transducer aperture: one at the intersection with the beam axis and the other at the intersection with the geometric beam boundary.
aperture width
The full width of the transducer aperture measured orthogonal to the beam axis (see Figures 2 and 7).
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apodization
The electronic, mechanical, or computational amplitude weighting or shading of the elements in a transducer array aperture. The purpose of apodization is to shape the beam and suppress side lobes.
Related term: side lobe
apparent backscatter
The measured backscattered ultrasonic signal uncompensated for the effects of attenuation and beam diffraction.
arc scan
A scanning technique in which the transducer is swept through an arc with its beam directed toward a fixed point.
array
A spatial arrangement of 2 or more individually addressable transducers or transducer elements. The array may be a linear array (ie, elements arranged in a line) or a rectangular array (ie, transducers arranged in a rectangular pattern) or may be formed in other patterns such as hexagonal, annular, circular, etc. (See Figures 19, 27, 29, 30, 33, 34, and 35.)
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See transducer element.
array elements
Group of transducer elements (see Figures 19, 27, and 35).
See subelement.
arterial phase of enhancement
The phase of contrast medium circulation when contrast-enhanced blood reaches an organ through its arterial supply. It starts a few seconds after the enhancement of large arteries. In this phase, parenchymal and lesion enhancement results exclusively from arterial flow (although in the liver, there is a rapid overlap with the portal venous induced enhancement). In the liver, this vascular phase starts 10 to 20 seconds after contrast medium injection in a peripheral vein and ends as soon as 30 to 40 seconds after contrast medium injection, depending on the agent used. The synonymous term “early phase” should be avoided because it may be erroneously intended as the whole vascular phase (in opposition to the late or postvascular phase).
artifact
An anomaly in an ultrasound image that does not correspond to an actual anatomical structure or target. Artifacts can manifest as additional echoes, distortions, or missing information due to the interaction of ultrasound waves with tissues or structures. Reverberations and shadowing are examples of such artifacts (see Figures 15, 16, 21, 22, 23, and 24).
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Artificial intelligence in ultrasound
Adaptive computer processing of data to enhance or interpret ultrasound images and their information content.
Related term:
A-scan
A scanning procedure in which echo information is displayed using the A-mode (see Figure 28).
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Related term: A-mode.
attack, angle of
See beam-to-vessel angle.
attenuation
The decrease in amplitude or intensity as sound travels through a material. Attenuation is due to absorption and scattering, which are properties of the medium.
Related terms: absorption, acoustic signal loss, attenuation coefficient, attenuation loss
attenuation coefficient
The quantitative measure of attenuation due to the medium and not to beam divergence. The relative change in the acoustic wave amplitude per unit path length in a medium. Commonly used units are decibels per centimeter (dB/cm) and nepers per centimeter (Np/cm). The attenuation coefficient is specified for a given frequency. Note: To be distinguished from absorption coefficient.
Related terms: absorption, acoustic signal loss, attenuation, attenuation loss
attenuation-compensated flowmeter
A device based on a technique that does not require knowledge of the vessel cross-sectional area or Doppler angle to estimate blood flow rate. The specialized ultrasound beam geometries required at present limit this technique to large vessels.
attenuation compensation
See time-gain compensation (TGC).
attenuation loss
The transmission loss due to attenuation. The unit is neper (Np) or decibel (dB).
Related terms: absorption, acoustic signal loss, attenuation, attenuation coefficient
attenuator
A device or material that reduces the amplitude of a signal.
atypical hemangioma
A hemangioma without the typical contrast-enhanced ultrasound image enhancement pattern (ie, peripheral globular enhancement with progressive fill-in). An atypical hemangioma may show very fast and homogeneous enhancement or very slow enhancement with incomplete fill-in.
autocorrelation
A mathematical procedure in which a waveform is multiplied by successively time-shifted versions of itself, and the results are summed. The autocorrelation method can be used to quantify periodicity or the time variation in an ultrasound echo signal.
Related term: correlation
auto-scanning systems
Historical term describing ultrasonic devices in which the beam was steered mechanically or electronically but not manually.
automated breast ultrasound (ABUS)
A diagnostic imaging modality used to automatically acquire a 3-dimensional volume set of ultrasound images of the entire breast, either by pulse-echo or through-transmission methods.
avascular lesion
Lesion appearing anechoic, with lack of contrast enhancement (markedly negative lesion-to-parenchyma contrast gradient). There is no subjective or measured difference in lesion echogenicity before and after contrast medium injection.
average acoustic power
The power output from an acoustic transducer averaged over time.
average intensity
See intensity.
axial direction
See beam axis (see Figure 27).
axial maximum
The point on the beam axis where an acoustic variable has its largest magnitude. For measurement purposes, this is usually where the pulse-pressure-squared integral has its largest value (see Figure 2). Other variables can be pressure and intensity. In the case of several maxima, the one farthest from the transducer or acoustic source is usually selected.
Related terms: beam axis, pulse-pressure-squared integral, spatial peak
axial resolution
The minimum detectable separation between two targets along the axis of sound travel. In general, the axial resolution will depend on the control setting, range, and nature of the targets.
Related terms: azimuthal resolution, depth resolution, longitudinal resolution, range resolution
axial resolving power
Maximum axial resolution achieved by an ultrasound instrument.
axial sensitivity
Of a hydrophone, for a specified frequency, free-field sensitivity to sound waves whose direction of propagation is toward the hydrophone and along the principal axis.
axial strain
A component of the strain tensor in the direction of the main axis of compression. The axis of compression is usually coincident with the axis of the ultrasonic beam and is thus in the imaging plane.
axial strain elastogram
An elastogram that depicts the distribution of the axial strain.
azimuthal axis
An axis in the azimuthal plane perpendicular to the beam axis.
azimuthal plane
The scan plane or principal longitudinal plane (see Figure 27).
Related term: elevation plane
azimuthal resolution
The minimum angular separation between 2 equally reflecting point targets at the same range such that each can be separately distinguished on the display. The resolution is limited by the beamwidth of the transducer at that range of the targets.
Related terms: axial resolution, depth resolution, lateral resolution, longitudinal resolution, range resolution
