Recommended Ultrasound Terminology

AcronymsFigures

F

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false echoes

Related term: artifact

far field (Fraunhofer zone)

The region of the ultrasound beam sufficiently far from the transducer so that wavefronts arriving from different regions of the transducer surface reinforce each other. An ultrasound beam is conveniently divided into two regions. The region farthest from the transducer is called the far field and sometimes called the Fraunhofer zone. For an unfocused circular transducer, the transition from the near field to the far field occurs at approximately r²/λ, where r is the transducer radius, and λ is the wavelength. 

Related term: Contrast with near field

far Fresnel zone (archaic term)

Preferred term: postfocal zone

far transition distance

The distance past which the postfocal zone begins for a focusing transducer (see Figure 5).

Related terms: near transition zone, focal zone, prefocal zone, postfocal zone

fast Fourier transform (FFT)

An efficient method of computing the Fourier transform of a function. The Fourier transform is a mathematical operation that decomposes a function into an equivalent group of sinusoidal functions of various frequencies and amplitudes. The amplitudes of the frequency components are often displayed as a plot of relative amplitude versus frequency, known as the amplitude spectrum. The FFT is frequently used to compute the frequency components of the Doppler signal.

Related terms: Doppler, Fourier transform

ferroelectricity

A dielectric phenomenon, analogous to ferromagnetism, characterized by spontaneous polarization of crystalline domains. The remanent polarization of a material may be reversed or reoriented by the application of an external electric field.

ferroelectric material

A crystalline or ceramic material that exhibits a remanent polarization, which can be reversed or reoriented by the application of an external electric field. Some ferroelectric materials can be rendered piezoelectric and used as ultrasound transducers.

fiber-optic hydrophone

See optical hydrophone.

field, far

See far field.

field, near

See near field.

filling in

See centripetal enhancement.

filling out

See centrifugal enhancement.

filter

A device or material used for suppressing acoustic or electromagnetic waves of certain frequencies while allowing others to pass. In digital computing, a filter is often an algorithm or a program that is used to eliminate of modify specific types of information in a file while leaving the remaining data more or less intact.

filter, bandpass

A device or material that allows only a limited range of frequencies to pass.

filter, high-pass

A device or material that allows high-frequency but not low-frequency signals to pass. An example is the electrical filter used in Doppler devices to eliminate low-frequency Doppler shifts caused by arterial wall motion.

filter, low-pass

A device or material that allows low-frequency but not high-frequency signals to pass. By preferentially attenuating sound at higher frequencies, most tissues act as low-pass filters.

finite amplitude

A term associated with an ultrasonic field whose pressure amplitude is relatively large. Typically used to contrast with an infinitesimal amplitude. Usually meant to describe amplitudes capable of generating nonlinear fields and nonlinear propagation.

first-order quantity

In acoustics, this refers to the fundamental parameters pertaining to wave propagation. Particle velocity, acoustic pressure, and material density are first-order quantities. In contrast, intensity and power are second-order quantities because the first-order quantities appear as squared terms in the expressions defining them.

fixed focus

A unchangeable mechanism capable of focusing in only one location. Usually applied to focusing by use of a physical lens or a curved transducer surface.

Related term: Contrast with dynamic focusing

flash (beamforming)

The use of a transmitted plane wave to illuminate a region of interest as opposed to line-by-line acoustic transmission.

flash (energy)

A sudden rise in acoustic power that is triggered by the operator to better assess a given area or to destroy the microbubbles filling the insonated parenchymal region and obtain functional data from the refilling kinetics of microbubbles.

See destruction-replenishment technique.

flaw detector

Echographic equipment for examining (solid) materials for structural defects.

flicker

A visibly apparent brightness fluctuation due to a display frame rate that is too low for visual perception of continuous brightness. Typically this will occur when the frame rate is below 15 frames per second. Other factors that affect flicker include phosphor persistence and brightness.

flow rate

The rate of transfer of volume of fluid with time. Measured in liters/second or milliliters/minute.

flow separation

A phenomenon occurring when flowing fluid travels at a high velocity adjacent to fluid traveling at a lower velocity. At the flow separation boundary, both a high-velocity gradient and high shear stress are present. Flow separation often occurs just distal to a vessel stenosis.

fluid (flow) resistance

Proportionality constant (R) between volume flow (Fv) and pressure difference at two positions along a blood vessel: ∆p = (p₂ – p₁) = RFv.

flux density

See intensity.

FM

Frequency modulation or frequency-modulated.

f-number

The focal length of a focused transducer divided by the diameter or length of its radiating surface. This ratio is a measure of the focusing power of the lens.

focal area

The area of the focal surface.

Related term: focal surface

focal diameter

For a sound beam with a circular cross section, the diameter of the beam at –6 dB (or –20 dB) of the peak amplitude level in the focus.

focal dimension

See focal diameter, and focal n-dB beam area.

focal Fraunhofer zone (archaic)

Preferred term: postfocal zone

focal length

The axial distance from the geometric center of the radiating surface of a focusing transducer assembly to the focal surface. In other words, the distance from the surface of the transducer assembly to the narrowest point (focus) of the beam.

Preferred term: geometric focal length

focal n-dB beam area

The smallest of areas containing all the points of a plane perpendicular to the sound beam axis where the received signal exceeds a value of n-dB below the maximum signal value.

focal pressure

One half of the maximum peak-to-peak acoustic pressure at the pressure focus.

focal ratio

See f-number.

focal surface

The surface perpendicular to the beam axis containing the smallest of all beam cross-sectional areas of a focusing transducer assembly at –6 dB relative to the maximum signal value or at another specified level.

focal zone

Region along the beam axis that is bounded by the near transition distance and the far transition distance. Equivalently, the region of an ultrasound field about a geometric focus in a specified longitudinal plane (see Figures 3, 4, and 5).

Related terms: near transition distance, far transition distance

focus (focal point)

The point on the axis of an ultrasonic beam where the width of the beam has a minimum value. Generally, all the waves passing through the focus are in phase in relation to the surface of the transducer.

Related terms: geometric cylindrical focus, geometric focal point, geometric focal length

focus, depth of

See depth of focus.

focal zone

The region of a focused beam where the beam is the narrowest and is bounded axially by the near- and far-field transition distances (usually defined in a longitudinal plane; see Figures 3 and 5). For circular transducers, the elevational focus and azimuthal focus will be the same; for noncircular transducers, the elevation plane focus is different than the azimuth plane focus.

Related terms: azimuth plane, elevation plane

focused field

A deliberate narrowing of the ultrasound beam to create an increased acoustic pressure at a specific point on the beam axis. This narrowing is significantly more than would occur with a flat transducer with the same aperture (see Figures 3, 4, 5, 7, 8, and 9).

Related term: focusing transducer

focused reflector

A device that produces by reflection an acoustic pressure at some point along the beam axis that is greater than a specified factor (eg, 2.5) above the root mean square acoustic pressure averaged over the geometric area of the transducer.

focusing transducer

Electro-acoustic device that produces a narrower –6-dB beamwidth in a longitudinal plane that is less than that of a flat transducer with the same source aperture size (and apodization, if any) and same excitation at a distance less than the transition distance from the source aperture plane (see Figures 2, 3, 4, and 5).

Related terms: apodization, beamwidth, longitudinal plane, source aperture, transition distance

focused ultrasound

The concentration of ultrasound energy in a selected region. It can be used for diagnostic, therapeutic, or neuromodulation applications. Although it is commonly used interchangeably as FUS with HIFU at high energy levels or LIFU at low energy levels for therapy and neuromodulation application.

Related term: lens, HIFU, LIFU, FUS

focusing

See focused field.

focusing, dynamic

See dynamic focusing.

focusing gain (geometric)

Ratio of pressure at geometric focal point on beam axis to average pressure on the transducer face.

footprint

The area covered on a surface when an object is applied to that surface. For example, the footprint of a transducer is the skin surface area in contact with the transducer when scanning.

force constant

The constant of proportionality between the applied force and the change in length when an object is stretched.

F = k · x, where F is the force, k is the force constant, and x is the displacement.

forced oscillation

Oscillation driven by an external force.

forward problem (elastography)

The mathematical problem of estimating the distribution of the strains in a target from knowledge of the applied strain or stress and the known distribution of the elastic moduli therein.

Fourier analysis

A mathematical technique for the representation of a function (such as a time-varying waveform) as a sum of sinusoidal functions of different frequencies. For a periodic function, each of these constituent functions has a frequency that is an exact multiple of the fundamental frequency. Fourier analysis can be used to describe aDoppler signal in terms of the relative amplitude of the various Doppler shift frequencies of which it is composed. This type of analysis and the Fourier transform were developed by Joseph Fourier.

Related terms: discrete Fourier transform, Doppler effect, Fourier transform

Fourier transform

A mathematical integral or algorithm that converts a waveform into a spectrum.

fractional bandwidth

A relative measure of bandwidth equal to the ratio of bandwidth to center frequency.

frame

A single image in a sequence of many individual images.

Related term: frame rate

frame rate

The rate at which pictures are refreshed on the display of a real-time system, otherwise known as the scan repetition rate. In some systems, such as television, a single frame is composed of two interlaced fields. The frame rate of standard television in the United States is 30 frames per second.

Related term: scan repetition rate

Fourier transform

A mathematical algorithm that converts a waveform into its spectrum. The waveform could be in space or time or be multidimensional.

Fraunhofer focal zone

Preferred term: postfocal zone

Fraunhofer zone

Preferred term: far field

free field

Sound field in a medium with negligible interference from extraneous sources of sound or reflections.

free-field current sensitivity

Of a hydrophone, for a specified frequency and a specified direction of sound incidence, the ratio of the short- circuit current at the output terminals of the hydrophone to the sound pressure in the undisturbed free field.

free-field pressure sensitivity

As applied to a hydrophone, the voltage induced per unit of applied acoustic pressure. The preferred unit is volts per pascal (V/Pa).

free-field voltage sensitivity

Of a microphone, for a specified frequency and a specified direction of sound incidence, the quotient of the output open-circuit voltage by sound pressure in the undisturbed free field. Note: The load impedance must be specified if other than open-circuit.

freehand scanning

Acquiring a volume of echo information through manual movement of the transducer without restriction on the possible motion.

freehand scanning (3-dimensional)

Acquiring a volume of echo information through manual movement of the transducer without restriction on the possible motion without sensing the position of the image plane. Quantitative measurements in such data sets may be inaccurate.

free oscillation

Oscillation that continues after removal of any external excitation.

free progressive wave

Wave propagating in a medium, free from boundary effects.

free sound field

See free field.

freeze

The status of sonographic equipment that causes cessation of the acoustic output, and the most recent image is stored in a scan converter memory and displayed continuously on a monitor.

Related term: cine loop

freeze frame

Control setting of sonographic equipment that causes the most recent image to be stored in a scan converter memory and continuously displayed on a monitor.

frequency

Number of cycles of a periodic process per unit of time, expressed in hertz (Hz) or multiples such as megahertz (l MHz = 10⁶ Hz). 1 Hz = 1 cycle per second.

See angular frequency.

frequency ambiguity

See aliasing.

frequency compounding

Acquiring and combining echo data from the same location but at different ultrasonic frequencies. The signals may be acquired using transmit pulses at different frequencies or by filtering echo signals following a broad-bandwidth transmit pulse. Envelope data at the various frequencies are combined at each location.

Related terms: compound image, spatial compounding

frequency domain

A mathematical framework in which a signal’s magnitude and phase are characterized in terms of frequency. In this domain, variables are described in terms of frequency rather than in terms of time or space.

frequency domain processing

A signal-processing scheme that operates on the spectral representation of signals rather than the temporal representation of the signals.

frequency, Doppler shift

See Doppler frequency shift.

frequency interval

The difference between two frequencies.

frequency, received

The frequency of the signal detected by the receiving transducer.

frequency response

A measure of the effectiveness with which a circuit, device, or system transmits the different frequencies applied to it. It is the ratio of the output signal to the input signal, considered as a function of frequency. Often frequency response is considered only between bounding frequencies of interest.

See bandwidth.

frequency modulation (FM)

Encoding of information into a carrier wave by variation of its instantaneous frequency in accordance with the amplitude of an input signal.

frequency spectrum, instantaneous

The frequency spectrum of a signal at a given point in time.

frequency spectrum, mean

The average of any set of frequency spectra.

Fresnel zone

See near field.

front end

The section of a medical ultrasound system (usually an imaging system) that interfaces directly with each transducer element. It sends the desired waveforms to each transducer element and captures the incoming echoes from each transducer element. Transmit and receive beamforming and amplification such as time-gain compensation are usually included in this section.

full-width half-maximum

The minimum separation between two half-maximum amplitude points, abbreviated as FWHM. Example of beam FWHM in Figure 25. FWHM can also be used for envelopes (see Figure 26).

functional ultrasound (fUS)

The medical imaging of physiologic activities, such as changes in metabolism or blood flow, by ultrasound.

functional ultrasound localization microscopy (fULM)

A method that uses the principles of ultrasound localization microscopy to measure hemodynamic responses induced by neural activities at super-resolution.

Related terms: ULM, super-resolution, fUS

fundamental component

See fundamental frequency.

fundamental frequency

The center frequency of the transmitted acoustic wave.

fundamental mode (imaging)

“Conventional” (grayscale) sonographic and Doppler techniques based on the transmission frequency of a sonography system. The echoes chosen to create the image are at the same frequency as that of the transmitted frequency.

Related term: Contrast with harmonic imaging

fundamental mode of oscillation

Mode of oscillation of a system having the lowest natural frequency.