Recommended Ultrasound Terminology
C
calibration
Determination of the accuracy of an instrument, usually by measurement of its variation from a standard, to ascertain necessary correction factors.
calipers (electronic)
Two or more electronic cursors generated on a display, which can be manipulated to coincide with echoes of interest on the screen. On an image, the distance between the cursors is calibrated to provide the correct dimensions of the structure of interest. On an M-mode or Doppler display, calipers may be used to measure such quantities as time, frequency, and velocity.
Related terms: Doppler, M-mode
carbon dioxide (enhanced) sonography
Transabdominal imaging of the liver during transcatheter injection of carbon dioxide in the hepatic arterial system. Vascularized lesions become hyperechoic for up to 10 to 15 minutes.
cathode ray tube (CRT)
An electron beam tube formerly used for 2-dimensional display of signals as a function of their coordinates in space, time, or both. It consists of an electron source (gun), a means for deflecting the electron beam in the horizontal and vertical directions, and a phosphor-coated screen on which the position of the electron beam is visible. The brightness of the display can be modulated by varying the current to the electron source or the voltage on the grid of the CRT.
Related terms: light-emitting diode display, liquid crystal display
cavitation
The creation of a mechanical response of one or more cavities in a liquid or other medium of interest subjected to stress produced by acoustic, hydrodynamic, or other means. Cavitation may be inertial or noninertial.
cavitation, inertial
See inertial cavitation.
cavitation, noninertial
See noninertial cavitation.
cavitation nucleus
A cavity that can serve as a site for cavitation.
cavitation, stable
See stable cavitation.
cavitation, transient
Deprecated term.
See inertial cavitation.
cavity
A volume filled with gas, vapor, or both.
cavity, Rayleigh
See Rayleigh cavity.
center frequency
The average frequency over the bandwidth of a signal. The sound leaving a transducer in a pulse-echo system contains not one but a spectrum of frequencies. The center frequency is usually determined by [(ƒ1+ ƒ2)/2], where ƒ1 and ƒ2 are the frequencies used in defining the bandwidth. In some pulse-echo systems, the median frequency is used for the center frequency.
Related term: bandwidth
center of gravity
A unique point in a structure where the total weight of the structure may be thought to be concentrated. Any structure, if supported at its center of gravity, can be balanced.
central axis
See beam axis.
central scan line
For automatic scanning systems, the scan line closest to the symmetry axis of the scan plane.
centrifugal arterial blood supply
Color Doppler and contrast-enhanced ultrasound image pattern of intralesional macrovascularization consisting of a vessel that branches from the center toward the periphery of a lesion. Discrete arteries showing progressive enhancement from the center to periphery, with an overall stellate distribution. It is a common pattern of focal nodular hyperplasia. The overall appearance is sometimes referred to as spoked wheel-like, stellate, starlike, or spiderlike.
Related term: Contrast with centrifugal enhancement
centrifugal enhancement
Progressive enhancement of a lesion from the center to periphery, eg, during a contrast-enhanced ultrasound study. This is an uncommon feature of liver hemangioma and peliosis hepatis.
centripetal enhancement
Progressive enhancement of a lesion from the periphery to the center, eg, during a contrast-enhanced ultrasound study A common pattern of liver hemangioma, where globular enhancement is followed by progressive filling in and predominantly during the arterial or portal phase.
centroid
A weighted average of a function or a quantity. For example, the centroid of a frequency spectrum is the sum of all of the frequencies in the spectrum that are weighted by their individual relative amplitudes divided by the range of frequencies. It can also be used to determine a weighted average across a beam (see Figures 7 and 9).
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Related term: center of gravity
cgs
The system of units based on the centimeter, gram, and second. Although the cgs system is still commonly used, the SI system, based on the meter, kilogram, and second, is preferred.
Related term: SI
characteristic acoustic impedance
The product of the speed of sound and density of a medium in which sound is traveling. Changes in acoustic impedance are responsible for the echoes on which ultrasound imaging and color flow Doppler are based.
Related terms: reflection, reflection coefficient
chirp pulse (ultrasound)
Transmit pulse in which the ultrasound frequency varies over the duration of the pulse. Chirp pulses are used in some systems to increase the energy of the acoustic pulse while not increasing the amplitude, eg, to improve the echo signal-to-noise ratio. Short-duration echo signals can be recovered in such systems with the use of pulse compression techniques, such as matched filtering.
cine loop
The continuously repeated display of a record of images or frames from memory.
Related term: cine memory
cine memory
The set of images temporarily retained for display as individual images or cine loops until replaced as new images are required.
Related term: cine loop
clipping
A technique by which the amplitude of a signal is prevented from exceeding a predetermined value. In clipping, the amplitude is set to a predetermined maximum value whenever the signal would otherwise exceed that value. Clipping results in distortion of the signal. Also known as “hard limiting.”
Related term: Contrast with soft limiting
clutter
Unwanted components in images or Doppler signals generally caused by undesired backscattered signals from interfering structures that obscure the desired signal. In Doppler measurements, clutter is often caused by high- amplitude, stationary, or slowly moving structures, such as vessel walls.
Related term: Doppler
C-mode (constant depth mode)
An ultrasound imaging technique that generates a cross-sectional view of 3-dimensional echo data from a constant depth within the body. The imaged plane is parallel to the transducer surface and perpendicular to the ultrasound beam, capturing echoes exclusively from a fixed distance. C-mode provides a "slice" of anatomy at a specific depth, offering detailed spatial relationships of structures in that plane and complementing other imaging modes like B-mode for comprehensive evaluation
Related term: pulse echo, A mode, B mode, 3-dimensional imaging
CMUT
Capacitive micromachined ultrasound transducers (CMUTs) that contain miniature capacitors, backed by gas, whose membranes generate outgoing ultrasound waves with applied voltages and also generate voltages in response to incoming ultrasound waves. They are often made in 2-dimensional arrays with silicon technology reproducibly in large numbers and can be integrated with microelectronics on the same substrates.
Related term: PMUT, MEMS
coded excitation (ultrasound)
A method of altering the shape of the transmit waveform such that returning echoes can be uniquely detected, resulting in increased penetration depth and signal-to-noise ratio. Coded excitation is used in some systems to increase the energy of the acoustic pulse, thereby improving the signal-to-noise ratio; to enhance signals from contrast agents; or to detect blood flow. In this approach, low-amplitude, short-duration backscattered signals can be recovered with the use of special signal-processing techniques, such as matched filtering, that use features encoded in the transmitted pulse sequences. Examples of codes used are chirp, Golay, and Barker.
Related terms: Barker code, chirp pulse, Golay code
coefficient of nonlinearity (β)
A measure of the nonlinearity of a material (gas, solid, or fluid) and defined as β = 1 + B/2A. For water it equals 3.5; blood, 4.0; fat, 6.2; and liver, 4.3.
Related term: B/A
coherence
The degree of phase agreement among the signals making up a composite wave; if all of the signals are in phase, the wave is said to be coherent. There are various degrees of coherence that describe waves with less than full coherence. If no phase agreement exists, the wave is called incoherent.
Related term: incoherence
collimator
An acoustic lens used to focus an acoustic beam at infinity, ie, to produce plane parallel waves. In practice, collimators may provide weak focusing.
color amplitude imaging
An archaic term for power Doppler imaging
Related term: power Doppler imaging, color Doppler imaging, color flow imaging
color blooming
Related term: color blooming artifact
color blooming artifact
A color Doppler imaging artifact in which grayscale pixels are seen changing to a color display in regions where flow is not present. It results when the color signal exceeds the display capability of a pixel and “spills over” into surrounding pixels. In contrast-enhanced sonography, a color blooming artifact sometimes occurs at the moment when contrast-enhanced blood arrives in the scanned area. To avoid this phenomenon, it is necessary to decrease color gain, decrease output power, or inject the contrast medium more slowly.
Related term: color Doppler imaging
color coding
See parametric imaging.
color Doppler imaging
A form of pulsed Doppler imaging in which a large number of estimates of the mean Doppler frequency shift and direction are color coded and overlaid in the location of their detection on the grayscale image. The overlaid color is often depicted as a velocity with units of centimeters per second (cm/s) assuming a 0º Doppler angle throughout the imaging plane. Color Doppler imaging systems operate within the range of “real-time” frame rates (see Figure 19).
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Related terms: color flow imaging, power Doppler imaging
color flow Doppler
The 2-dimensional presentation of Doppler shift information superimposed on a real-time grayscale anatomic cross-sectional image. Flow directions toward and away from the transducer, ie, positive and negative Doppler shifts, are presented as different colors on the display.
color flow imaging
See color flow Doppler.
Related terms: color Doppler imaging, power Doppler imaging
combined operating mode
Mode of operation of a system that combines more than one discrete operating mode. Note: Examples of combined operating modes are real-time B-mode combined with M-mode (B + M), real-time B-mode combined with pulsed Doppler (B + D), color M-mode (cM), and real-time B-mode combined with real-time flow-mapping Doppler (B + rD), ie, flow-mapping in which different types of acoustic pulses are used to generate the Doppler information and the imaging information.
Related terms: B-mode, color Doppler imaging, M-mode, power Doppler imaging, pulsed Doppler
comet tail artifact
A sonographic artifact that appears as a dense tapering trail of echoes just distal to a strongly reflecting structure. The tail is caused by reverberations within an object that appear brighter near the actual object and decrease in amplitude along its length due to decaying reverberations. Typical targets that cause these tails are air bubbles, air pockets or interfaces, and small metal objects. The key thing that distinguishes it from other reverberation-type artifacts is that its geometric shape is similar to a comet tail (see Figure 23).
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complex impedance
Related terms: acoustic impedance, acoustic reactance, acoustic resistance
complex number
A combination of two numbers, usually written in the form a + ib, where i is the square root of –1. The first number, a, is called the real part, and ib, the second, is called the imaginary part. In mathematical equations, a complex number is handled as a single entity and is therefore a convenient way to manipulate two quantities at the same time.
Related terms: complex parameter, imaginary, real
complex parameter
Any parameter requiring the specification of two quantities for its complete characterization. For example, acoustic impedance is a complex parameter that is characterized by a complex number consisting of a real part (resistance) and an imaginary part (reactance).
Related terms: acoustic impedance, acoustic reactance, acoustic resistance, complex impedance, imaginary, real
compliance
The rate of change of the volume of a distensible vessel with pressure (volume compliance), the rate of change of the cross-sectional area with pressure (area compliance), or the rate of change of the diameter with pressure (diameter compliance).
compliance (of a vessel)
Relative change of the cross-sectional area of a blood vessel per unit of pressure change: C = (ΔV/V)/Δp, where ΔV = ΔA x L, and where A is the cross-sectional area, and L is the length of the segment and Δp is the change in pressure. The unit is reciprocal pascal (Pa).
compound image
An ultrasound image combining echo information using different parameters (eg, overlapping scan lines and/ or multiple frequencies) to enhance echoes from real structures and reduce speckle to provide an overall improvement in image contrast.
Related terms: contrast (image), pulse echo, scan line, speckle
compressibility
The fractional change in the volume of a medium or an object with change in pressure is sometimes called volume compliance; the fractional change in cross-sectional area with change in pressure is sometimes called area compliance; and the fractional change in diameter with change in pressure is sometimes called diameter compliance. Volume compliance (C) = (ΔV/V)/Δp, where ΔV/V is the fractional change in volume, and Δp is the change in pressure.
compression (elastography)
The reduction in size when an object is subjected to a compressive stress. It is the opposite of tension.
Related term: tension
compression (image processing)
The reduction in bits used to represent an image. Compression is used primarily to reduce the image size needed for transmission and storage while preserving the information as much as possible.
compression (of dynamic range)
Decrease of the total amplitude range of echo signals by means of a nonlinear amplifier (eg, a logarithmic amplifier) or by using a nonlinear lookup table in a case of a digitized signal.
compression (signal processing)
The reduction in bits used to represent a signal. Compression is used to reduce the number of bits required to transmit and store the signal while preserving the information as much as possible.
compressional wave
Wave in an elastic medium that causes an element of the medium to change its volume and density without undergoing rotation. Note: Mathematically, a compressional wave is one whose velocity field has zero curl.
See longitudinal wave.
compressive modulus
The ratio of compressive stress to compressive strain.
compressive strain
The fractional reduction in the length of an object when subjected to a compressive stress (dimensionless).
compressive stress
The force per unit area that when applied to an object causes a dimension of the object to shorten. The unit is pascal (Pa).
Related term: tensile stress
compressor
A device capable of applying a compressive stress to an object or material.
condensations
When a sound beam passes through a material, some regions of the material are compressed, while other regions expand. The compressed regions are called condensations.
Related term: rarefaction
confocal
Lenses or transducers positioned so that their focal points coincide.
conjugate impedances
Impedances whose real components (resistances) are equal and whose imaginary components (reactances) are equal but opposite in sign. Note: Conjugate impedances are expressed by conjugate complex quantities.
Related terms: acoustic impedance, acoustic reactance, acoustic resistance, complex impedance
constitutive equation (elasticity)
An equation that relates the response of a material to an applied force in which the response depends on the properties of the material. For example, the equation that relates all the tensor components of stress to all the tensor components of strain through a matrix of elastic constants is a constitutive equation pertinent to elastography.
Related terms: elastography, strain, stress
constitutive equation (general)
A relationship between two physical quantities (eg, stress and strain) that incorporates a specific property of a material or substance. It is typically used in combination with other equations representing physical laws to express or solve a specific physical problem.
Related term: constitutive equation (elasticity)
contact coupling
Acoustic coupling of a transducer by direct contact with the skin, using liquid or gel to exclude air from the space between the transducer and skin.
continuous mode
Uninterrupted real-time acquisition of sonographic images.
continuous spectrum
A spectrum whose components are continuously distributed over a given frequency range.
continuous wave (CW) Doppler system
An ultrasonic system that detects Doppler-shifted signals by continuous and simultaneous transmission and reception of sound waves. The CW Doppler system provides no range resolution.
continuous wave (CW) ultrasound
A wave of constant or nearly constant amplitude that persists for a large number of cycles. Also known as an eternal sine wave.
Related term: tone burst signal
contour enhancement
Improvement of contour outlining of anatomic structures by electronic means (preprocessing) or by image processing after storage in a scan converter (postprocessing).
contour plot
A topographic representation of a quantity over a 2-dimensional plane in which amplitudes or intensities of the same value are represented by one or more closed curves that are either identified by a value (such as –6 dB) or color coded. The plot is usually plotted in decibel values with reference to the maximum value in the plane. Most often used to represent an ultrasound beam in a selected plane. Analogous to topographic maps (see Figure 4).
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contrast (image)
In the image display, the ratio of the mean perceived luminance (brightness) within a region of interest (ROI) to the mean luminance of an equally sized area adjacent to the ROI. It is usually expressed in decibels (dB). This parameter can also be expressed in percent as the difference in luminance divided by the luminance of the adjacent ROI.
contrast (object)
The ratio of the mean backscattered intensity from within a target to the mean backscattered intensity from within an equally sized volume of surrounding background material. It is usually expressed in decibels (dB) as 10log10(Iobj/Isur) where I denotes the intensity of the object (subscript obj) and surroundings (subscript sur), respectively. Object contrast is a property of the target and the surrounding medium. Also called backscatter contrast.
contrast (target)
The ratio of the backscattered intensity from within a target volume to the backscattered intensity from the background.
contrast agent
A material that when introduced into a region of interest results in an increased or a decreased signal strength compared to its surroundings. A contrast agent is used to enhance the contrast between an anatomic structure and that of the surrounding tissue, thereby making the structure easier to detect (ie, increases the contrast resolution level).
contrast agent, vascular
A collection of gas-filled microbubbles (1–10 µm in diameter). Following intravenous injection, the microbubbles circulate in the bloodstream and provide a marked increase in the blood-to-clutter signal-to-noise ratio.
contrast arrival (imaging at)
Assessment of contrast-enhanced ultrasound image intensity during the first passage of a contrast agent flowing through an organ or tissue.
contrast arrival time
Time intervening between the beginning of contrast agent injection and arrival within a given organ vessel as measured using contrast-enhanced ultrasound imaging.
contrast detail analysis
A test method to measure an imaging system’s ability to resolve structures based on their size and object contrast. At a specified transducer frequency, set of display settings, and depth range, this test measures the smallest diameter target that can be differentiated from the background at a specified level of confidence (detection threshold). The contrast detail analysis yields a plot of the object contrast versus target diameter, the object contrast being the minimum such that the corresponding diameter can be detected.
contrast detail detectability
The minimum diameter target that is just detectable, at a specified level of confidence, as a function of the contrast of the target with the background.
contrast-enhanced ultrasound
Any ultrasound technique performed after injection of a contrast agent.
contrast-enhanced harmonic imaging
Contrast-specific imaging mode where the scanner is configured to transmit at one frequency and receive at harmonic or subharmonic frequencies. This scheme accentuates the nonlinear behavior of tissue and contrast microbubbles to generate images that contain fewer artifacts.
See contrast agent, contrast subharmonic, contrast-enhanced sonography, harmonic imaging, and subharmonic imaging, ultraharmonic imaging
contrast enhancement
The increase in echo amplitude from tissue lying distal to a weak reflector such as a cyst or the increase in echo intensity in an organ or lesion due to the presence of a contrast medium.
contrast extravasation
Detection of contrast agent signals from extravasated blood after intravenous injection.
contrast gradient
The ratio of the change in contrast to the change in distance.
contrast harmonic imaging
See contrast-enhanced harmonic imaging
contrast medium
Related term: contrast agent
contrast pulse sequencing (CPS)
An ultrasound imaging method for detecting and processing the nonlinear signal generated by ultrasound contrast agents. In this approach, for example, the ultrasound scanner transmits a given pulse waveform 3 times scaled in amplitude by the coefficients 0.5, –1, and 0.5, respectively, before summing the corresponding echoes that suppress the linear component and enhance the nonlinear terms.
contrast ratio
Related terms: contrast (image), contrast (object)
contrast ratio (image)
Related term: contrast (image)
contrast resolution
The ability of an imaging system to distinguish a target with low contrast in an image from its surrounding background.
contrast subharmonic contrast-enhanced sonography
Experimental technique using that portion of the returned signal having a frequency one-half of that of the transmission frequency.
Related term: harmonic imaging
contrast transfer efficiency (CTE)
The ratio of the contrast ratio of the strain between an inclusion and the surrounding medium to the contrast ratio of their elastic moduli. The CTE is a dimensionless number that is generally less than or equal to 1. For stiff circular inclusions in a softer medium, the CTE is approximately equal to 0.5.
contrast transit time
Time interval between the arrival of a contrast agent in the artery feeding an organ and its arrival within the related venous drainage as measured using contrast-enhanced ultrasound imaging.
Related term: contrast agent
control settings
The settings of the controls on an ultrasound instrument. Such controls would include the power output control, the focal zone control, the imaging range control, etc.
conventional sonography
See unenhanced sonography.
convex array
A curvilinear array transducer with cylindrical outward curvature, producing a sectorlike image with the top part of the image defined by the arc of the curvilinear transducer (see Figure 30).
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Related term: curvilinear array
coronal image plane
Any plane perpendicular to a sagittal plane and parallel to the long axis of the body. Each coronal image plane extends from left to right through the body (see Figure 1).
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coronal plane
See coronal image plane.
correlation
For ultrasound, a mathematical procedure that is used to quantify the similarity between two signals.
Related terms: autocorrelation, cross-correlation
coupled modes
Modes of oscillation that are not independent but influenced by the transfer of energy from one mode to the other.
coupling
The formation of a transmission path for a sound wave between the transducer and the object being imaged. This is usually accomplished by replacing the air between the transducer and the object with a fluid or gel.
Related term: contact coupling
coupling agent
A medium used to enhance transmission of sound from the transducer into the body.
Related term: contact coupling
coupling method
The method by which ultrasound is transmitted from a transducer to the subject and vice versa. These methods include contact coupling, immersion coupling, and liquid coupling.
Cramér-Rao lower bound (CRLB)
The CRLB sets a lower bound on the variance of any unbiased estimator, such as a strain estimator. It is often used in calculating the best possible performance of such estimators.
cranial thermal index (TIC)
The thermal index developed for ultrasound exposures for which the acoustic beam passes through bone near the beam entrance into the body. Such exposures include the neonatal and adult skull but not the in-utero fetal skull.
Related terms: bone thermal index (TIB), soft tissue thermal index (TIS), thermal index
crawling wave
A slowly moving shear wave interference pattern induced by a pair of mechanical sources vibrating at slightly offset (“beat”) frequencies.
Related term: shear wave interference pattern
crawling wave elastography
An elastographic method in which slowly moving interference patterns produced by two oppositely directed shear waves with a slight difference in frequency are used. The apparent spatial wavelength from the interference pattern is proportional to the underlying shear wave speed, which can be used to estimate the tissue shear modulus.
Related term: crawling wave
critical angle
The angle of incidence of a sound beam on an interface at which the angle of transmission just reaches 90° with respect to the normal to the interface. A critical angle can only be reached if the medium on the proximal side of the interface has a sound speed that is less than that of the medium on the distal side of the interface. At and above the critical angle, no transmission of the sound beam into the distal material occurs beyond the interface (see Figure 6).
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Related terms: angle of incidence, angle of reflection, angle of refraction, Snell’s law
critical damping
Damping that will allow an oscillating electrical or mechanical system to return to rest in the minimum amount of time.
critical Reynolds number
The Reynolds number above which the transition from laminar to turbulent flow takes place.
Related term: turbulence
critical stenosis
Stenosis of sufficient diameter reduction that the flow rate and pressure are dangerously diminished. The term implies that the flow rate is so reduced that a stroke or occlusion may be imminent. In most modern stenosis grading schemes, critical stenosis corresponds to a diameter reduction of 80% or greater.
cross-correlation
A mathematical procedure whereby one waveform is multiplied by successively time-shifted versions of another waveform and the results are summed. Cross-correlation of successive ultrasound scan lines can be used to quantify movement of tissue, including blood.
cross-correlation function
A mathematical function that involves the summation (or integration) of products of successively shifted samples of a first signal with respect to a second signal and plotting the amplitude of the result as a function of the shift (lag) between the signals. The cross-correlation function may be used to study the shift between signals and the degree of similarity. Two-dimensional cross-correlation can be applied in image processing.
CRT
Abbreviation for cathode ray tube.
crystal
A colloquial term for the piezoelectric element of the transducer. Most piezoelectric elements are made of crystalline materials, such as quartz and certain ceramics.
C-scan
A class of data acquisition geometry in which echo information is acquired from points lying in a scanned plane, or other section, intersecting an interrogating principal acoustic axis.
Related term: C-mode
Curie point
The temperature above which a material loses its piezoelectric properties.
curtain artifact
A noisy appearance of the strain in an elastogram that is a result of excessive lateral displacements of tissue elements at the lateral edges of the elastogram. These artifacts may be reduced by compensation for the undesired lateral displacements.
Related term: elastography
curvilinear array
A curvilinear array consists of in-line transducer elements on a curved surface, which can be electrically configured to control the characteristics and/or direction of an acoustic beam. A convex array is a type of curvilinear array.
Related terms: convex array
cutoff frequency
Frequency at which the transmission through an electronic filter is decreased by –3 dB or some other specified amount with respect to the maximum amplitude at any frequency.
cylindrical wave
Wave of which the wavefronts are coaxial cylinders.
