Recommended Ultrasound Terminology
S
sagittal image plane
The anteroposterior plane parallel to the long axis of the body and passing through the midline. Anteroposterior planes not passing through the center of the body are frequently called parasagittal planes (see Figure 1).
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sample volume
The region within the ultrasound beam from which Doppler signals are acquired. In pulsed Doppler ultrasound, the sample volume is user-defined and determines the specific location of flow measurement. In continuous wave Doppler, the sample volume encompasses all overlapping beam regions along the Doppler line of sight. Also used for a region imaged by a 3D imaging system.
sampling angle
The acute angle (<90°) between the image ultrasound beam axis and the selected direction of flow within the imaged plane. This angle may be estimated during the Doppler examination. Often called the Doppler angle. Note: The Doppler angle is not necessarily the same as the angle of incidence. For example, in a vessel, the Doppler angle = 90° minus the angle of incidence relative to the direction of flow. Also the angle selected for a sample volume for color flow imaging, for example.
Related term: beam axis, color flow imaging, Doppler angle
sampling rate
The rate at which an analog (continuously varying) signal is sampled. It is the reciprocal of the time interval between digitization samples of an analog-to-digital conversion. The unit is hertz (Hz).
sampling theorem
Any band-limited signal can be specified completely by its sampled values, taken at regular intervals, provided that the sampling rate is more than twice the maximum frequency present in the frequency spectrum of the signal.
saturation
Limiting and distorting process that alters the linear relationship between the output and input of a device or a medium. The output is limited even with increasing input amplitude.
Related terms: saturation (acoustic), saturation (electronic)
saturation (acoustic)
An amplitude-limiting process due to the distortion of an acoustic wave signal caused by the nonlinear properties of the propagation medium. This severe distortion is a consequence of a relatively large-amplitude signal, with most of the energy going to harmonics or subharmonics. Further increases in a source signal amplitude do not result in an appreciable increase in an amplitude of the acoustic signal.
Related terms: saturation, saturation (electronic)
saturation (electronic)
Electronic limiting of the amplification that occurs if the input signal exceeds certain levels.
Related terms: saturation, saturation (acoustic)
sawtooth signal
A signal that increases linearly with time and is periodically reset to zero.
scale factor
A constant of proportionality used to compare sizes of objects, graphs, images, etc. Usually the objects are similar. In ultrasound diagnosis, it commonly refers to the ratio of the displayed size of an object to the real size of the object.
scan
The process of directing an ultrasound beam to acquire image data. This may be achieved electronically, mechanically, or manually. Electronic scanning uses an array transducer to steer and focus beams, while mechanical scanning involves transducer movement. Freehand scanning requires external tracking for spatial registration.
Related terms: acoustic scan area, freehand scanning, freehand scanning (3-dimensional), mechanical sector scanner, mechanically swept scanning (3-dimensional, scan format, scan line), scan line separation, scanning system, scanning window, scan plane, scan repetition period, scan repetition rate, scan surface, scan volume, synthetic scan line
scan area
For scanning systems, the area on a surface, consisting of all points within the beam area of any beam passing through the surface, during a scan repetition period. The unit is square meter (m2).
scan converter
An algorithm or instrument that transforms pulse echo or Doppler imaging data into an image format. Examples include interpolation of scan line data into a rectangular M x N image format or receive beamformed data at image pixel or voxel locations within a selected image format. sampling of information from one format into another. In diagnostic ultrasound instrumentation, the scan converter receives scan lines of echo information and changes this information into the desired rate and spatial arrangement for display.
Related terms: beamforming, Doppler imaging, image format, pixel, pulse echo, scan line, voxel
scan cross-sectional area (for auto-scanning systems)
See scan area.
scan format
The 2- or 3-dimensional spatial pattern of a sonographic image display. Two-dimensional scan formats include rectangular, sector, and trapezoidal types.
scanhead (also scan head)
See transducer array or transducer assembly.
scan line
A representation of the echoes transmitted along a specified direction. Ultrasound images are composed of many discrete scan lines. In general, the more scan lines an image has, the more spatial detail that can be portrayed, (see Figures 29 and 30).
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Related terms: acoustic scan line, synthetic scan line
scan line separation
The distance or angle between two specified scan lines of the same type within a specified plane. The unit is meter (m) or degree.
scanner
A device that moves an acoustic scan line relative to a target. This is accomplished mechanically or electronically by using a multielement transducer array.
Related term: medical diagnostic ultrasound system
scanning system
A system comprising transducers, beamformers, signal processors, and display units that acquire and reconstruct ultrasound images.
scanning window
The region on the skin surface through which an internal structure can be most readily imaged.
scan plane
A 2D plane consisting of multiple ultrasound scan lines that form an image. The orientation of the scan plane determines the image content and it can be sagittal, transverse, coronal, or oblique (see Figure 27).
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Related terms: coronal image plane, oblique image plane, sagittal image plane, scan line, transverse image plane
scan repetition period
The time interval between identical points on two successive frames, sectors, or scans. This applies to automatic scanning systems only. The unit is second.
Related term: scan repetition rate
scan repetition rate
The inverse of the scan repetition period. The repetition rate of a complete frame, sector, or scan. The term applies to auto-scanning systems only. The unit is hertz (Hz).
See frame rate.
Related terms: azimuthal plane, scan repetition period
scan surface
A scan surface (or plane) contains all the scan lines in a repetitive sequence often used to form an image frame. Usually for 2-dimensional imaging, the scan plane is the azimuth plane.
scan volume
A scan volume contains all the scan surfaces (planes) in a repetitive sequence used to form an image.
scatter
The redirection of ultrasound waves in multiple directions due to tissue inhomogeneities or structural boundaries. Scatter contributes to speckle in ultrasound images.
scatterer
An inhomogeneity or heterogeneity within the medium, which reflects ultrasound. Any structure within a medium that causes ultrasound wave redirection. Scatterers range from microscopic particles (red blood cells) to macroscopic tissue interfaces.
Related terms: diffractive scattering, diffusive scattering, intermediate scattering, Rayleigh scattering, specular reflection, specular scattering
scattering
The redistribution of ultrasound energy due to interactions with structures smaller than the ultrasound wavelength, leading to diffuse reflection and speckle formation.
Related term: scatter
scattering cross section of a surface
Area of a plane progressive wave intercepting an amount of sound power equal to that scattered from the stated surface over a hemisphere.
scattering cross section of an object or volume
Equivalent area intercepting an amount of sound power of a plane progressive wave equal to that scattered in all directions by the object or by the scatterers in the stated volume.
s/d ratio
An index of pulsatility defined as the ratio of the maximum (usually the peak systolic) to the minimum (usually the end-diastolic) estimated velocities. It is used in vascular studies to assess resistance in blood flow.
second harmonic
The acoustic signal at twice the fundamental frequency of the transmitted acoustic wave.
second-harmonic imaging
See harmonic imaging.
sector
A wedge-shaped region of an ultrasound image, typically produced by phased array transducers where scan lines diverge from a single point.
sector scan
A scanning method in which ultrasound beams are electronically or mechanically swept through an angular range to form a sector-shaped image. Common in phased array transducers. (see Figures 29, 30, and 35).
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sector scan format
The pie- or sector-shaped image pattern resulting from a sector scan (see Figure 29).
segmentation
An image-processing technique that divides a volume of information into recognizable and separate components.
self-heating
See transducer self-heating.
sensitivity, Doppler
A measure of the minimum detectable signal level at a specified Doppler shift frequency and output power level. This is one component of Doppler detectability.
Related terms: detectability, Doppler; dynamic range, Doppler
sequential access storage
An information storage process that requires the sequential interrogation of information in a memory. A magnetic tape is such a device or optical storage.
Related term: Compare with random-access memory
shadowing
Reduction in echo amplitudes distal to a strongly attenuating or reflecting structure (see Figures 16 and 22).
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Related term: acoustic shadow
shear modulus
The ratio of shear stress to shear strain. Also called the modulus of rigidity or torsion modulus. The unit is pascal (Pa).
shear strain
The fractional lateral (tangential) displacement of a material when the surface of that material is subjected to a shear stress. It has no unit.
shear stress
The force tangent to a material surface divided by the area on which the force acts. The unit is pascal (Pa).
shear wave
Wave motion in which the particle displacement in the medium is perpendicular to the direction of wave propagation. A vertical shear wave has a vertical displacement perpendicular to the propagation direction. A horizontal shear wave has a horizontal displacement perpendicular to the propagation direction.
Related term: Contrast with longitudinal wave
shear wave elastography (SWE)
Imaging of tissue mechanical properties derived from shear wave propagation parameters measured in tissue. Shear waves induced within tissue by external mechanical vibrations, intrinsic physiologic motions, or the radiation force of a focused ultrasound beam can be detected. Typically, the shear modulus can be determined from the measured shear wave propagation speed, cs, and ρ, the density of tissue through the formula µ = ρc2 , which assumes a homogeneous, isotropic, perfectly elastic medium.
Related term: shear wave elasticity imaging (SWEI)
shear wave interference pattern
The interference pattern produced by multiple propagating shear waves.
Related term: crawling wave elastography
shear wave speed
The rate at which shear waves propagate through a material (m/s), which is quantitatively related to the shear modulus and density of the medium. For viscoelastic media, the shear wave speed is also affected by the viscous properties of the medium.
shock parameter
A parameter characterizing the degree of nonlinearity in the propagation of an ultrasound beam. A value of less than 1 would indicate a linear condition, and a value greater than 3 would indicate a marked nonlinear condition.
shock wave
The shape of a large-amplitude acoustic waveform that is maximally distorted by nonlinear propagation effects and looks like the letter N.
short-axis scan (echocardiography)
A cardiac scan in which the scanning plane is at right angles to the long axis of the heart (base to apex).
short-circuit acoustic impedance
Input acoustic impedance of an electroacoustic transducer, when the electric output is connected to zero impedance: a short circuit.
SI
The internationally accepted version of the metric system of units. The abbreviation comes from the French Système International d’Unités. This modernized metric system is based on 7 base units, including the meter, kilogram, and second. All other physical units are derived units, formed by multiplying and dividing units within the system.
side band
The components of a signal whose frequencies are either above (upper side band) or below (lower side band) the frequency of the transmitted (carrier) signal.
side lobe
Secondary beam regions generated by a transducer and deviating from the direction of the main beam or grating lobes. Usually the maximum intensity of the side lobe is significantly less than that of the main lobe. The presence of side lobes may be responsible for introducing artifactual echoes into an ultrasound image. A side lobe is narrower and weaker than the main lobe and is a result of the finite size and weighting of the transducer aperture. The lower the level of the side lobes, the better the contrast resolution of the image (see Figure 10).
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Related term: Contrast with grating lobe
signal (electrical)
The component of a varying electrical voltage or current that contains information, as opposed to containing noise.
signal enhancer
Deprecated term for contrast agent.
signal-to-noise ratio
The ratio of the signal power to the noise power over a given bandwidth. The larger the signal-to-noise ratio, the easier it is to detect and measure a signal. The sensitivity of any device is ultimately limited by the signal-to-noise ratio. This ratio is usually expressed in decibels (dB).
signal type
Classification of the acoustic pressure waveform into continuous wave, tone burst, or broadband.
Related terms: broadband, continuous-wave mode, narrow bandwidth, tone burst
simple sound source
See point source of sound.
single-level (or single-slice) technique
Contrast-enhanced sonographic acquisition modality done without any movement of the transducer over the patient’s body surface. Used for lesion characterization and functional studies.
sinusoidal phase of enhancement
The phase of contrast medium circulation when contrast-enhanced blood leaves the liver through its venous drainage (from 90 to 120 seconds after contrast medium injection to about 300 seconds after contrast medium injection, depending on the contrast medium used). Liver enhancement progressively declines to the baseline appearance. The sinusoidal phase is sometimes referred to as the late vascular or extended portal venous phase.
slice thickness
The width of the point spread function (PSF) in the elevation direction perpendicular to the scan plane, usually given in decibels (dB), most commonly as the –6-dB width. This dimension gives the “thickness” of an ultrasound beam “slice” through the imaged tissue and varies with imaging or axial depth (see Figure 33).
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Related term: point spread function
slice thickness artifact
A sonographic artifact caused when the beam slice thickness is larger than the thickness of the anatomic structure, introducing echoes originating outside the scan plane into the scan plane.
small parts
See superficial.
small-parts scanner
Scanning device adapted to diagnostic examination of small and superficial organs (eg, eye and thyroid).
Related term: superficial
Snell’s law
When sound travels from one medium into another, the incident and refractive rays (1) lie in one and the same plane with the normal to the surface, (2) are on opposite sides of the normal, and (3) make angles with the normal whose sines have a constant ratio to one another. The angle of the beam transmitted at the interface into a medium is related to the angle of incidence at the interface and to the speed of sound of the two media by (sin θ1)/v1= (sin θ2)/v2, where θ1 is the incidence angle in medium 1, θ2 is the refraction (transmission) angle in medium 2, v1 is the speed of sound in medium 1, and v2 is the speed of sound in medium 2 (see Figure 6).
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soft copy
A document or record that exists in electronic form, on a video screen, on magnetic tape, or stored in computer memory.
Related term: Contrast with hard copy
soft limiting
A technique by which the gain of a system is a function of the output amplitude for the purpose of preventing the output signal from exceeding a predetermined value. The output waveform is therefore distorted but not clipped.
Related term: Contrast with clipping
soft tissue thermal index (TIS)
The thermal index developed for ultrasound exposures limited to soft tissue.
Related terms: thermal index, bone thermal index (TIB), transcranial thermal index (TIC)
solid angle
A 3-dimensional angle (colloquial). A surface formed by rays with a common origin, the vertex of the solid angle, and passing through a closed curve. The unit is steradian. The total solid angle about a point is equal to 4π steradians.
Related term: steradian
sonar
The term “sonar” is the generic term for echo ranging and imaging underwater, usually at frequencies below 1 MHz. Acronym for sound navigation and ranging.
sonic
An adjective relating to sound.
sonoelasticity imaging
A method for imaging the mechanical response of tissue to the passage of an applied shear wave therein. The method involves the detection of continuous wave vibration amplitudes using Doppler imaging.
Related term: sonoelastography
sonoencephalography
See echoencephalography.
sonogenetics
A method using ultrasound to activate ultrasonically sensitized neurons
sonogram
The pictorial record of an ultrasound examination.
sonographer
A highly skilled professional qualified by technological education to provide patient services using diagnostic ultrasound under the supervision of a doctor of medicine or osteopathy. Sonographers assist physicians in performing ultrasound examinations and gathering sonographic data necessary to reach diagnostic decisions.
sonography
Any imaging method using sound and yielding an image or a graphic representation of the subject. This is a more inclusive term than echography inasmuch as transmission sonography is also included.
sonohysterography (SHG)
An ultrasound examination, usually performed endovaginally, to evaluate the female pelvis during and after the instillation of fluid into the endometrial cavity.
Related term: Hysterosonography (HSG)
sonologist
The physician who interprets a sonogram.
sonolucent
The property of a medium allowing easy passage of sound; ie, it has very low attenuation. Sonolucent is a misnomer for anechoic.
See anechoic.
sonoluminescence
Production of light by a medium when subjected to intense levels of sound. Byproduct of an inertial cavitation event.
Related term: inertial cavitation
sonomyography
A dynamic and functional assessment of muscle using ultrasound imaging.
sonopaque
The property of a medium that obstructs the passage of sound; ie, it has very high attenuation (very high absorption and/or very high scattering). Sonopaque is a misnomer for a region on a B-scan completely filled in with echoes.
Related term: Compare with sonolucent
sonoporation
The use of ultrasound energy to increase cell membrane permeability to enhance extracellular molecular internalization. Typically, stable cavitation is used to modify permeability. A former definition is the use of ultrasound to propel a chemical agent or drug subcutaneously into tissue.
sound
Vibrational energy that propagates through a medium. Liquids and gases support longitudinal (compressional) waves. Solids support other vibrational modes in addition to longitudinal waves.
sound-absorbing material
A material characterized by a relatively large capability of absorbing sound.
sound absorption
See absorption.
sound absorption coefficient
See absorption coefficient.
Related terms: absorption, statistical sound
sound analyzer
Apparatus for the determination of the spectrum of a sound source.
sound energy
Acoustic or elastic energy of propagating sound waves in units of joules (J).
sound field
Region of an elastic medium containing sound waves.
sound intensity
Rate of energy flow (power) of a propagating acoustic or elastic wave through a unit area perpendicular to the direction of propagation. The unit of intensity is watts per square meter (W/m2) or, more commonly, watts per square centimeter (W/cm2).
sound power
See acoustic power.
sound power density
See sound intensity.
sound power of a source
Total sound energy radiated by the source in a specified frequency band over a certain time interval divided by the interval.
sound pressure
See acoustic pressure.
sound pressure amplitude
The deviation from the ambient value of the pressure in a medium due to the presence of an acoustic or elastic wave.
sound pressure reflection coefficient
See reflection coefficient (amplitude).
sound propagation speed
See speed of sound.
sound reflection coefficient
See reflection coefficient.
sound spectrum
Representation of the magnitudes (and sometimes phases) of the components of a complex sound as a function of frequency.
sound wave velocity
See velocity of sound.
source
The origin of the wave in question. For example, a primary source would be the transmitting transducer, and a secondary source would be the surface or object reflecting or scattering the wave.
source aperture
An equivalent aperture for measurement purposes formed by the intersection of the radiated acoustic energy from the active transducer aperture and the source aperture plane (see Figures 2, 7, and 9).
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source aperture average pressure
This parameter is one-half of the peak-to-peak acoustic pressure, averaged over the –20-dB beam area in the source aperture plane. The region for this averaging includes all points where the pressure is at least 1/10 of the maximum pressure in the source aperture plane and is bounded by the continuous curve of minimum length that encloses these points.
source aperture plane
The closest possible measurement plane to the active transducer aperture that is perpendicular to the direction of propagation of the emitted ultrasound (see Figures 2, 7, and 9)).
source aperture width
In a specified longitudinal plane, the –20-dB beamwidth along the line of intersection between the designated longitudinal plane and the source aperture plane (see Figure 2).
sparse array
An array in which few of the elements are active. For example, in a sparse 2D array in a 2D ultrasound transducer, most of its elements are inactive, or signals from these elements are not recorded.
spatial-average
An variable averaged over the beam cross-sectional area.
spatial-average intensity
See this entry under intensity.
spatial-average pulse acoustic pressure
For non–auto-scanning systems, equal to the beam-average pulse acoustic pressure. Note: Spatial-average pulse acoustic pressure does not apply to automatic scanning systems; beam-average pulse acoustic pressure applies only. The unit is pascal (Pa).
spatial-average pulse-average intensity (ISAPA)
See this entry under intensity.
spatial-average root mean square acoustic pressure
For non–automatic-scanning systems, equal to the beam-average root mean square (RMS) acoustic pressure. For automatic scanning systems, equal to the RMS acoustic pressure averaged over the scan area, where the RMS acoustic pressure is taken over the scan repetition period. The unit is pascal (Pa).
spatial-average temporal-average intensity (ISATA)
See this entry under intensity.
spatial coherence
The correlation between signals at different points in space.
Related term: correlation
spatial compounding
Acquiring and combining ultrasound echo data from the same location but with different acoustic or synthetic scan line orientations. For linear and curvilinear arrays, this may be done by combining electronic beam steering and beam translation, resulting in overlapping scan lines. Thus, internal anatomic structures are scanned from many directions.
See compound imaging and frequency compounding (see Figure 34).
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spatial-peak
The maximum value of a variable in an acoustic field or in a specified plane.
spatial-peak pulse acoustic pressure
The maximum value of the pulse acoustic pressure in an acoustic field or in a specified plane. The unit is pascal (Pa).
spatial-peak pulse-average intensity (ISPPA)
See this entry under intensity.
spatial-peak pulse intensity integral
The maximum value of the pulse intensity integral in an acoustic field or in a specified plane. The unit is joules per square meter (J/m2).
spatial-peak root mean square acoustic pressure
The maximum value of the root mean square acoustic pressure in an acoustic field or in a specified plane. The unit is pascal (Pa).
spatial-peak temporal-average intensity (ISPTA)
See this entry under intensity.
spatial-peak temporal-peak acoustic pressure (or peak sound pressure)
The larger of the peak-positive acoustic pressure or the peak-negative acoustic pressure. The unit is pascal (Pa).
spatial-peak temporal-peak intensity (ISPTP)
See this entry under intensity.
spatiotemporal image correlation (STIC)
A 4-dimensional (4D) scanning technique to visualize the beating fetal heart. During the volume acquisition phase, which may take 10 seconds or so, the fetal heart may beat 20 to 25 times. To provide the best imaging of the heart, the B-mode frames are reordered according to their spatial and temporal relation to the fetal heartbeat. The 4D visualization can be performed with the heart frozen at a particular phase of the heartbeat or viewed with the heart in normal motion.
specific absorption rate (SAR)
The rate at which energy is absorbed per unit mass of tissue. SAR is expressed in units of watts per kilogram (W/kg).
specific acoustic admittance
Reciprocal of the specific acoustic impedance. Note: The real part is the specific acoustic conductance, and the imaginary part is the specific acoustic susceptance.
specific acoustic impedance
The ratio of instantaneous acoustic pressure to instantaneous particle velocity. For a plane progressive (forward propagating) wave in a lossless medium, the specific acoustic impedance is numerically equal to the characteristic acoustic impedance. For a negative propagating wave, this impedance is negative.
specific acoustic reactance
Imaginary part of specific acoustic impedance.
specific acoustic resistance
Real part of specific acoustic impedance.
speckle
The granular appearance of an ultrasound image due to the coherent addition (ie, constructive and destructive interference) of detected echo signals arising from unresolved distributed scatterers within a sample volume.
speckle reduction
The smoothing of tissue texture in an ultrasound image by various methods such as spatial or frequency compounding.
speckle tracking
Determination of local ultrasound displacement and/or movement by using the spatially invariant or deterministic properties of speckle over short periods of time. Typically, A-lines or images at different sequential times are cross- correlated to obtain local changes in displacement. Major applications include time domain Doppler, elastography, and assessing thermally induced changes in tissue.
spectral analysis
Estimation of amplitude at all frequencies contained in the spectrum corresponding to a time signal (obtained by Fourier analysis or by a Doppler measurement).
spectral broadening
The increase of the range of Doppler shift frequencies either by occurrence of turbulence in the bloodstream (such as after passing a stenosis) or due to the finite size of the sampling volume (transit time spectral broadening).
spectral bubble noise
An artifact associated with ultrasound contrast agent studies, where the Doppler spectrum contains very large excursions caused by bubble destruction within the sample volume. It is always associated with a loud “crackling” noise in the audio signal. Spectral bubble noise is of minor clinical importance.
Compare to acoustic emission
spectral density/spectrum density
Limit, as the bandwidth approaches zero, of the mean square value of a field quantity divided by bandwidth. The kind of field quantity must be specified, such as sound pressure, particle velocity, or particle acceleration. Also called spectrum density.
spectral Doppler
A name commonly used to refer to the combination of either continuous wave or pulsed Doppler with a spectral display, usually of measured velocities.
spectral width
See bandwidth.
spectrum
The equivalent representation of a temporal ultrasonic or electrical signals by a distribution of their frequency components. Each frequency is represented as a complex number with amplitude and phase. Often a spectrum is displayed as magnitude components on a linear or dB scale.
Related terms: Fourier analysis, Fourier transform
spectrum density
See spectral density.
specular highlight (ultrasound)
Bright, high-amplitude echo arising from a localized region on a tissue interface when the region is perpendicular or nearly perpendicular to the incident beam. In pulse-echo imaging, specular highlights are often seen for small regions on curved interfaces and for isolated areas on the surface of tightly curved structures when the region or area is oriented perpendicularly to the beam.
specular reflection
Reflection of sound energy from a smooth surface that is large compared to the wavelength. Specular reflection implies that the angle of reflection equals the angle of incidence.
Related terms: scatterer, diffractive scattering, diffusive scattering, intermediate scattering, Rayleigh scattering, specular scattering
specular scattering
Waves that are reradiated from objects whose size or surface irregularities are much greater than the insonifying acoustic wavelength.
Related terms: acoustic wavelength, specular reflection
speed of sound
The rate at which an acoustic wave propagates through a material. The speed of sound depends on the density and the elastic properties of the medium. The unit is meters per second (m/s).
spherical wave
A wave that emanates from a point source and propagates evenly in all directions, producing a spherical wavefront. The wavefronts are concentric spheres.
spider sign
See centrifugal arterial blood supply.
spleen-specific phase of enhancement
See parenchymal phase of enhancement.
spoked wheel–shaped pattern
See centrifugal arterial blood supply.
spot-poled region
A small region in a ferroelectric material that has been poled.
Related term: poling
spreading loss
See diffraction loss.
stable cavitation
A form of noninertial cavitation characterized by a steady oscillation of a bubble due to the passage of a sound wave. Mechanical movements, such as acoustic streaming, occur in the vicinity of the bubble.
Related terms: cavitation, inertial cavitation, noninertial cavitation, transient cavitation
stand-alone
Being able to operate without support or control from another system.
standard linear viscoelastic model
Preferred name: Kelvin model
Related terms: Burgers model, dashpot, elastic spring, Maxwell model, Kelvin model, Kelvin-Voigt model, Voigt model
standing wave
Periodic wave having a fixed distribution in space that is the result of interference of progressive waves of the same frequency and kind. Note: Such waves are characterized by the existence of nodes or partial nodes and antinodes that are fixed in space.
Related term: standing wave field
standing wave field
Pattern of a standing wave in a medium. When two sound waves of equal magnitude and frequency travel in opposite directions, a pattern of stationary pressure peaks with nodes occurring at half-wavelength intervals results. In this situation, there is no net flow of acoustic energy.
Related term: standing wave
standoff
A device placed between a transducer and the skin so that ultrasound imaging can be performed so that focusing can occur near the region of interest, usually near the skin surface. A secondary benefit of this device is to reduce artifacts that would arise in contact scanning. Standoff devices are particularly helpful in scanning superficial structures, such as the thyroid gland, and in maintaining good acoustic coupling on sharply curved surfaces, such as the neck. The material within the standoff, whether liquid or solid, should be very low in attenuation and should be well matched acoustically to the body.
starlike pattern
See centrifugal arterial blood supply.
statistical sound absorption coefficient
Sound power absorption coefficient measured or calculated with plane waves at randomly distributed angles of incidence.
See absorption, sound absorption coefficient.
steady state
The existence of a situation stable for sufficiently long periods to allow variations with time to be neglected.
steady-state oscillation
Oscillation that continues without change.
steering
The changing of the direction of an ultrasound beam. Steering is produced in a transducer array by exciting the transducer elements not synchronously, but with increasing delays in the firing of successive elements such that the emerging beam leaves the transducer in a chosen direction. Continuously changing this type of excitation delay pattern results in a sector scan. Also known as electronic beam steering (see Figure 35).
Related term: phased array and scan
stellate pattern
See centrifugal arterial blood supply.
stenosis
The local narrowing of a vessel lumen or other structure, usually as the result of a disease process.
steradian
The unit of measure of a solid (3-dimensional) angle. One steradian equals the solid angle, which, having its vertex in the center of a sphere, subtends an area on the surface equal to the radius squared. There are 4π steradians in the solid angle comprising a complete sphere.
stereoscopic
Pertaining to 3-dimensional viewing with depth perception.
stiffness
An elastic coefficient matrix determined by the vector ratio of stress to strain. In general, there are components of stress generated in other directions in addition to those of the applied strain. The unit is pascal (Pa).
stiffness (tissue)
The firmness of a tissue element that is due to its elastic modulus as well as its geometric position, shape, and boundaries.
stimulus excitation
Archaic term for external force, or other input, applied to a system.
stippled pattern of enhancement
In contrast-enhanced sonography, diffuse chaotically disposed discrete vessels within a lesion. This is a typical color Doppler and contrast-enhanced sonographic pattern seen in hepatocellular carcinoma.
strain
The fractional change in the shape or size of a structure due to an imposed stress. For example, in one dimension, strain is the ratio of the change in length divided by the original length. The strain is termed elastic if the material returns to its original shape and size after removal of the stress. The strain is called plastic if the material does not return to its original shape and size.
strain filter
A statistical function depicting the upper bound of the elastographic signal-to-noise ratio that can be theoretically attained in an elastogram of a uniformly elastic medium as a function of the applied strain. The shape of the strain filter resembles a bandpass characteristic in the strain domain and is influenced by several mechanical, acoustical, and signal-processing parameters.
strain imaging (echocardiography)
An imaging method in which the degree of tissue deformation (ie, strain) is displayed.
strain-induced spectral broadening
The increase in the spectral bandwidth of a received echo signal that is a result of incurring a small compressive strain. Conversely, the bandwidth is narrowed when the strain is tensile.
strain-induced spectral shift
The upward shift in the center frequency of a wideband pulse spectrum as a result of incurring a small compressive strain. Conversely, the spectral shift is downward when the strain is tensile.
strain rate imaging (echocardiography)
An imaging method in which the rate of tissue deformation (ie, strain) is displayed.
strain shadowing
A misnomer for strain concentration.
Related term: strain concentration
strain tensor
A tensor in which each of its 9 elements represents a strain. The elements of the strain tensor are usually represented by the symbol ij, where i is the row, and j is the column position. i = 1, 2, or 3, indicating the direction of the strain, and j = 1, 2, or 3, indicating the direction of the surface undergoing the strain.
streamline
A line that is everywhere parallel to the direction of fluid flow at a given instant.
strength of a simple sound source
Maximum instantaneous volume velocity produced by a simple source, small compared with the wavelength, emitting a wave with sinusoidal variation with time.
stress
The force per unit area applied to a structure. The unit is pascal (Pa).
stress concentration
An increase in the stress in a selected area in an elastogram due to changes in internal or external boundary conditions.
stress decay
The diminution of the stress as a function of distance. The exact behavior of the stress decay depends on the geometry and the boundary conditions and on the details of the applied stress.
stress dilution
A decrease in the stress in a selected area in an elastogram due to changes in internal or external boundary conditions.
stress tensor
A tensor in which each of its 9 elements represents a stress. The elements of the stress tensor are usually represented by the symbol Sij, where i is the row, and j is the column position. i = 1, 2, or 3, indicating the direction of the stress, and j = 1, 2, or 3, indicating the direction of the surface acted on by the stress.
subelement
An element forming part of a group or subgroup of elements which together function as an array element.
subharmonic
The signal with a harmonic frequency below the fundamental frequency. The nth subharmonic of a fundamental frequency F is a signal with frequency F/n, where n is a positive integer.
Related term: integer
subharmonic imaging
Method of imaging in which ultrasound is transmitted at a fundamental frequency and is detected at subharmonic frequencies. Subharmonics are generated by nonlinear reflectors such as contrast microbubbles. The resulting subharmonic is displayed as an image to improve the signal-to-noise ratio.
subharmonic response
Periodic response of a system at a frequency that is a submultiple fraction of the fundamental frequency.
superficial (sonographic)
Applies to sonographic examinations of tissues near or just under the skin. Includes breast, muscles, testicular, thyroid, and other tissues. Formerly called “small parts.” Superficial sonographic examinations are performed using higher-frequency transducers.
Related term: small-parts scanner
superharmonic
Synonymous with ultraharmonic.
superposition principle
A principle stating that if a linear physical system is acted on by a number of independent influences, the resultant response is the sum of the responses from each of the influences acting individually.
super-resolution ultrasound imaging
See ultrasound localization microscopy (ULM)
supersonic shear wave imaging
A shear wave elasticity imaging technology that uses shear waves produced by rapidly moving the focus of a push beam at a speed higher than the shear wave propagation speed within the medium being imaged. The technology can generate shear wave fronts with different axial extents and inclination angles to suit different shear wave imaging applications.
supplying artery
An artery entering into an anatomic structure. It may be seen as an enhanced vessel outside the nodule (malignant lesions) or even central to the nodule (focal nodular hyperplasia).
suppression
The reduction of undesired noise in a noisy signal. Rejection eliminates weak signals below a threshold level. Logarithmic amplification suppresses large signals relative to weak ones. Squaring the signals suppresses weak signals relative to strong ones.
surface-rendered
See surface rendering.
surface rendering
An imaging display technique in 3-dimensional (3D) ultrasound in which the echo information obtained by volume acquisition is modified to produce a smooth continuous superficial surface of an object. Surface rendering is especially helpful in producing realistic 3D views of the fetal face.
survey scan
An initial sonographic scan that provides an overview and orientation before the diagnostic images are recorded. Surveying is recommended at the start of every type of ultrasound examination.
swept gain
The process by which the gain of a pulse-echo system is varied with time to compensate for the effects of attenuation.
Related terms: time-gain compensation, attenuation compensation
symmetry axis of the scan plane
An imaginary line in the scan plane on each side of which an equal number of ultrasonic scan lines occur. Synthetic aperture.
See synthetic aperture ultrasound imaging.
synthetic aperture imaging
See synthetic aperture ultrasound imaging.
synthetic aperture ultrasound imaging
A method in which high-resolution images are obtained by transmitting on a single or a subset of all available transducer elements and reception is on all elements. For synthetic aperture using single elements, beams are half the width, (twice the resolution) of a focused beam at the same location produced by an unapodized phased array that has the same number of elements as the total number of elements in the synthetic aperture. An added advantage is that the synthetic aperture method achieves dynamic focusing on both transmit and receive throughout the image, whereas conventional delay and sum beamforming must create an image line by line and achieves transmit and receive focusing at only one depth. A disadvantage is that synthetic aperture has poorer signal to noise ratio; however, this ration can be improved by using groups of elements instead of single elements.
Related terms: delay and sum beamformer, dynamic focusing, signal to noise ratio
synthetic scan line
A reconstruction of echo information along an arbitrarily specified direction within a collection of acoustic scan lines. The direction of the scan line may not correspond to the direction of original acoustic scan line.
Related terms: acoustic scan line, scan line
systole
The portion of the cardiac cycle during which myocardial contraction occurs.
Related term: Contrast with diastole
systolic
See systole; window, systolic.
systolic window
See window, systolic.
