D

damping (transducer)

Any mechanism that removes mechanical or electrical energy from the transducer. This can include internal absorption in the transducer as well as electrical or mechanical loading of a transducer to produce a more rapid decrease in the trailing edge of the transmitted acoustic pulse and the received echoes. Damping is used to improve axial resolution, usually at the expense of sensitivity (see Figure 20).

Related term: backing layer

damping ratio

Ratio of the actual damping to the critical damping.

dashpot

A damping element in a viscoelastic model in which the resulting force is proportional to the rate of change of strain with respect to time. In simpler terms, a dashpot is a component in ultrasound transducer and wave propagation models that represents energy dissipation, helping to explain transducer behavior and performance. The graphical symbol for a dashpot (η) is:

dB

Abbreviation for decibel.

dead time

The time interval between the start of the transmission pulse and the reception of the first identifiable echo. The dead time is determined by such factors as the ring-down time of the transducer and the recovery time of the receiving amplifier.

dead zone

Region close to the transducer in which imaging cannot be performed due to echo arrival during dead time.

Related term: dead time

decay rate

Time rate at which the sound pressure level decreases at a given frequency.

decibel

A unit representing a ratio, used to express how much larger or smaller a quantity is with respect to a reference quantity. The precise definition is 10 · log10(I/I0), where I is the intensity of interest, and I₀ is the reference intensity. The abbreviation is dB. This unit can be used for other than intensity ratios and is frequently used for defining the magnitude of echoes. Because sound amplitude is proportional to the square root of intensity, the formula for decibel becomes 20 · log10(A/A₀) when comparing echo amplitudes, where A is the amplitude of interest, and A₀ is the reference amplitude. Note the formulas for amplitude and corresponding intensity give the same value in decibels. For example, an amplitude ratio of ½ and an intensity ratio value of ¼ are both represented by –6 dB. Although the bel is the primary unit, a decibel is 1/10 of a bel and is used more often.

Related term: bel

delay line

System that enables the time delay of a signal (in analog or digital form) used for delayed transmission and reception of signals of individual elements of array transducers.

delay and sum beamformer

A delay and sum receive beamformer adjusts the excitation delay and propagation path delay from each transducer array element to concentrate the energy at a selected focal point and produce an amplitude gain there.

Related terms: beamformer, receive beamformer, transmit beamformer

delivered acoustic energy

The product of acoustic power and exposure time at the prescribed site.

delivered acoustic pulse energy

See energy per pulse.

demodulation

Removal of the carrier signal from a modulated wave to produce a signal representing the desired information (eg, the Doppler shift or the amplitude of the echo). The process of demodulation is a type of detection.

demodulator, Doppler

The electrical device in the Doppler receiver responsible for shifting the center frequency of the received signal from that of the transmitted signal to DC (direct current), so that echoes from stationary tissue may be eliminated by high-pass filtering.

density

Mass per unit volume.

density, photographic

See optical density.

depth

The distance along the sound path from the point of entry into the patient, or other object being examined, to the point of interest. Distance along the time axis of the display is assumed to correspond proportionally to depth in the tissue.

depth-gain compensation

See time-gain compensation.

depth of field

The range around the focal length in which targets remain in focus. For circularly symmetric lenses in optics, it represents a distance between the points at which the axial intensity drops by –3 dB (a ratio of 1/2). This relation does not hold for ultrasound imaging, where rectangular arrays are used as well as different types of focusing in the azimuth and elevation planes. A criterion based on narrowness of the beam in a specified plane is more appropriate: the distance between two points along the beam axis, which are defined by the locations where the –6-dB beamwidths increase over the minimum beamwidth by a factor of 2 (see Figure 7).

Related terms: array, azimuth plane, beamwidth, elevation plane, depth of field, focus, focal zone, lens

depth of focus

See focal lenght

depth of penetration

The maximum distance from the transducer into the medium along the beam axis from which echoes can be detected. This distance will increase with ultrasound beam intensity and decrease with greater attenuation of the medium. The depth of penetration is usually given in centimeters (cm) and less frequently in wavelengths.

depth resolution

See axial resolution.

destruction-replenishment (or destruction-reperfusion) technique

A method of perfusion quantification based on microbubble disruption within a selected area of interest and subsequent assessment of its refilling by new, incoming microbubbles.

detectability, Doppler

The ability of a Doppler-shifted echo from moving targets (such as blood in a small vessel) to be detected by the system. Related to Doppler sensitivity and dynamic range.

Related terms: Doppler, dynamic range

detector

A device that gives a measurable response on receipt of energy and is used to detect the presence of a signal.

derated quantities

Physical quantities, such as pressure and intensity, that are measured in water and reduced in value by the product of a predetermined acoustic attenuation coefficient times a predetermined path length. Most commonly, for regulatory purposes, the predetermined acoustic attenuation is 0.3 dB/cm/MHz, and the predetermined path length is the distance from the transducer to the measurement point of the ultrasound beam. The purpose of derating is to estimate in situ values from quantities measured in water. Note, this is an approximate method because absorption and nonlinear effects can distort the shape of waveforms as well as reduce their amplitude.

Related terms: absorption, absorption coefficient, acoustic output, attenuation, in situ

diagnostic ultrasound

Use of ultrasound waves to produce signals or images that help diagnose (detect and/or identify) a medical condition.

diametrical beam scan

A set of measurements of the hydrophone output voltage made in a straight line passing through the beam center and normal to the beam alignment axis. The diametrical beam scan may extend to different distances on either side of the beam center.

diastole

The relaxation period in the cardiac cycle in which the ventricles fill and the aortic and pulmonary valves are closed.

Related term: Contrast with systole

DICOM

A file formatting and communications standard for medical images. Acronym for the Digital Imaging and Communications in Medicine standard.

dicrotic notch

A brief, abrupt upswing in pressure during the deceleration phase of systole, forming a “notch” in the pressure and velocity waveforms. The dicrotic notch and the small undulations following the notch, known as the dicrotic wave, can be traced to reflections from the distal arterial tree.

differential scattering cross section

Power scattered per unit of solid angle per unit of incident intensity. The unit is square centimeter per steradian (cm2/steradian). In simpler terms, it is a measure of how strongly an object scatters ultrasound, which is important for understanding image formation and quality.

differentiation

A mathematical operation for calculating the rate of change of a variable. In ultrasound imaging, differentiation is used to process signals to enhance edges of the structures. This is accomplished by using circuits whose output depends on the rate of change of the input or by software.

Related terms: integration (antonym)

diffraction

The general term for various phenomena in which waves from different parts of a source add or subtract. It is the shaping of an ultrasound field caused by the contributions of waves radiated by a source. The source can be primary such as a transducer or secondary such as a scatterer. The diffraction phenomenon occurs for sources or objects whose size or surface irregularities are on the order of the wavelength.

Related terms: beam, scatterer

diffraction constant

Ratio of sound pressure acting on a transducer to the free-field sound pressure at that place in the absence of the transducer.

diffraction loss

That part of the detected loss of acoustic power that is due to diffraction (spreading) of the propagating wave from a transducer. Also called divergence loss or spreading loss.

Related terms: divergence loss, spreading loss

diffractive scattering

Waves that are reradiated from objects whose size or surface irregularities are comparable to the insonifying acoustic wavelength. The direction of the scattered wave deviates from that of the incident wave in proportion to the ratio of the acoustic speeds in the materials on each side of the reflecting surface. This type of scattering contributes to the texture observed in ultrasound images, especially when imaging tissues with small variations in density.

Related terms: acoustic wavelength, intermediate scattering

diffuse enhancement

Echo enhancement distributed throughout a lesion. It can be homogeneous or heterogeneous. In liver contrast studies, diffuse enhancement may occur in the arterial, portal, and sinusoidal phases.

diffuse reflector

Reflecting interface having surface irregularities of dimensions on the order of or greater than the ultrasonic wavelength. In contrast to the case of a specular reflector, echoes from a diffuse reflector travel in different directions. This allows echoes from a diffuse reflector to be detected even when it is not perpendicular to the ultrasound beam.

diffuse sound field

Sound field that in a given region has statistically uniform energy density, for which the directions of propagation at any point are randomly distributed.

diffusive scattering

Waves that are reradiated from objects whose size or curvature is much smaller than the insonifying acoustic wavelength. The reflected waves spread out in a wide angular range.

Related terms: acoustic wavelength, Rayleigh scattering; contrast with specular scattering, diffractive scattering

digital (signal)

A signal whose values consist of discrete numbers. For example, a binary signal is a digital signal that can assume at any moment only one of two possible values: 0 or 1. Contrast with an analog signal, whose values can consist of any value between its minimum and maximum values.

digital beamforming

The formation of a desired transmit or receive beam pattern by digital means. Digital transmit beamforming typically uses synchronized and precisely timed digitally delayed excitations across transducer array elements to achieve transmit focusing and/or steering of the ultrasound beam. Receive beamforming is commonly done by the delay-and-sum of digitized echo signals from multiple elements of an array ultrasound transducer. Digital receive beamforming is flexible and can include other methods such as plane wave compounding.

Related term: beamforming

digital signal processing

The mathematical operation or numerical manipulation of a discrete-valued time or space varying process (signal) to modify it or produce new information. An algorithm that manipulates a series of discrete-valued numbers. The discrete Fourier transform (DFT) is an example of digital signal processing.

Related terms: discrete Fourier transform, Fourier transform.

digitization (quantization) noise

The noise introduced when a continuously varying (analog) signal is converted (sampled) into a signal possessing only discrete values (a digital signal). The amplitude of this noise is equal to the difference between the amplitudes of the analog and digital signals.

See quantization

dimensionless

The property of a physical quantity or parameter whose magnitude does not have any units (such as meters, seconds, kilograms, or volts) in its complete description.

dipole (acoustical)

A pair of acoustical sources, usually separated by one half-wavelength, that generate sound pressure waves of approximately equal amplitudes and 180º out of phase. Acoustical dipoles are a fundamental concept in how ultrasound transducers create focused beams. By carefully controlling the timing and strength of these sources, the transducer can shape the ultrasound beam, reducing unwanted signals and improving image clarity. This beam shaping is essential for optimizing image resolution and minimizing artifacts during sonographic examinations.

dipole (electrical)

Positive and negative electrical charges at a short distance from each other.

directional Doppler system

Doppler equipment that measures not only the speed of the bloodstream in a vessel but also the direction (unidirectional) or directions (bidirectional) of the stream as well.

directional filter

A Fourier transform-based signal processing method that separates shear waves based on their propagation directions.

Related term: Fourier transform

directional gain

See directivity factor.

directional hydrophone

Hydrophone the response of which is dependent on the direction of sound incidence.

Related term: Compare with omnidirectional hydrophone

directional pattern

See directivity pattern.

directional response

See directivity function.

directivity factor

A ratio of the intensity in a given direction over that for an omnidirectional radiator of the same power.

directivity function

The quantitative description of the dependence of irradiated acoustic intensity or sensitivity of a transducer on direction (see Figure 10).

directivity pattern

The directivity function plotted in polar coordinates.

discrete Fourier transform (DFT)

Digital version of Fourier transform. A computational technique used in ultrasound signal processing to analyze frequency components of echoes, aiding in image formation and Doppler analysis.

Related term: Fourier transform

discrete operating mode

Mode of operation of medical diagnostic ultrasonic equipment in which the purpose of the excitation of the ultrasonic transducer or ultrasonic transducer element group is to use only one diagnostic method. Note: Examples of discrete operating modes are M-mode (M), B-mode (B), continuous wave Doppler (cwD), pulsed Doppler (D).

Related term: combined operating mode

dispersion

The variation in phase velocity with frequency in a medium.

dispersive medium

A medium in which the phase velocity of an acoustic wave is a function of frequency.

displacement

The amount by which an object, such as a tissue element, moves.

display

Visible presentation of ultrasound parameters. For example, B-mode or Doppler. Also an electronic device for the visual presentation of data.

Related terms: display format, cathode ray tube, liquid crystal display, light-emitting diode display

display dynamic range

See dynamic range.

display format

The manner in which information is presented to the diagnostician, eg, A-mode, B-mode, C-mode, etc.

Related terms: A-mode, B-mode, C-mode

dissipation

Any loss of energy, generally by conversion into heat.

dissipation factor

Ratio of sound energy dissipated as heat to the energy of the incident sound wave.

distortion

Undesired change of a waveform. Note: Distortion may result from (1) a nonlinear relation between the input and output, (2) nonuniform transmission at different frequencies, (3) a phase shift not proportional to the frequency or (4) absorption and dispersion.

disturbed flow

Deviations from laminar flow consisting of oscillatory variations in direction or the formation of vortices. Disturbance of blood flow may be caused by high velocities, by curving, branching, and divergence of vessels, or by projections in the vessel lumen.

divergence (beam)

The continual widening of a sound beam as it propagates.

divergence loss

Related term: diffraction loss

Doppler angle

The angle between the direction of propagation of the ultrasound beam and the direction of flow. As an approximation, the angle between the axis of the ultrasound beam and the axis of the vessel lumen is generally used.

Doppler

Contraction (colloquial) for the Doppler effect or Doppler ultrasound. Also used colloquially as a catchall term for describing any method of measuring flow or the movement of a cluster of objects or tissue by any ultrasound means. The concept of Doppler effect was first observed by Christian Andreas Doppler in 1842.

Related terms: Doppler effect, Doppler ultrasound

Doppler, continuous wave

Doppler processing using continuous wave ultrasound.

Related terms: continuous wave (CW) ultrasound, continuous wave (CW) Doppler system

Doppler effect

An apparent change in the observed sound frequency caused by relative motion between the sound source or reflector and the observer.

Doppler frequency shift

The difference between the frequencies of transmitted and received waves. The shift in frequency is proportional to the velocity of relative motion between the transducer and the reflector.

Doppler frequency shift, maximum

The highest Doppler shift frequency at a moment in time or in an individual Doppler spectrum. This corresponds to the fastest moving target in the Doppler sample volume. Because of the effect of noise, some form of signal conditioning is usually performed before the maximum frequency is measured.

Doppler frequency shift, mean

The average Doppler shift frequency in a given power spectrum.

Doppler frequency shift, median

The Doppler shift frequency above and below which one-half of the total power in the spectrum resides.

Doppler frequency shift, mode

The Doppler shift frequency with the greatest power in a given spectrum.

Doppler, pulsed

See pulsed Doppler.

Doppler shift frequency

See Doppler frequency shift.

Doppler spectrum

Spectrum of a Doppler signal.

Doppler ultrasound

Application of the Doppler effect in ultrasound to detect movement of scatterers by the analysis of the change in the frequency of the returning echoes.

Doppler velocity signal

A signal whose instantaneous voltage is proportional to the instantaneous Doppler frequency shift, derived by a frequency-to-voltage conversion of the Doppler signal.

dosimetry

The quantitative determination of the amount of acoustic radiation absorbed.

duplex mode

An imaging mode in which two modes such as real-time B-mode and Doppler are displayed simultaneously.

Related term: duplex scanner

duplex scanner

An ultrasound instrument that has duplex mode processing and display capabilities.

Related term: duplex mode

duty factor

The product of the pulse duration and the pulse repetition rate. The fraction of time the transducer is active when excited by a sequence of pulses.

dwell time

The total time that a specified anatomic structure is continuously exposed to the ultrasound beam during an ultrasound examination.

dynamic contrast-enhanced ultrasound (DCE-US)

The application of microbubble contrast media to traditional ultrasound imaging. Acquisition of a contrast- enhanced ultrasound image sequence describes the time history of intravascular contrast agent flow as an indicator of blood flow.

Related terms: contrast agent, intravascular ultrasound

dynamic focusing

A method of extending the resolution and depth of focus during real-time imaging. For receive dynamic focusing, the receive beam is refocused at nearly each depth within the limits of the receive aperture available. For transmit, the depth of focus can be extended by splicing together an image from short depth ranges, each one with an appropriate transmit focal length within the range.

Related terms: depth of field, receive aperture, receive beamforming

dynamic imaging

Imaging of an object in motion, a technique that is frequently referred to as real-time imaging.

dynamic range

The ratio of the minimum to maximum signal amplitude that can be applied to a device without producing distortion in the output signal (expressed in decibels [dB]). The largest signal is usually limited by saturation and the smallest by the background noise level. This concept applies to amplifiers, displays, and other devices.

dynamic range compression

Related terms: compression, dynamic range

dynamic range expansion (elastography)

A technique by which the dynamic range of an elastogram may be increased. This is done by successively applying increasing amounts of strain to the tissue and selecting only those strain estimates from each resulting elastogram that are of acceptable quality. These estimates are then combined to generate a composite elastogram that depicts a large dynamic range of strain.

Related terms: dynamic range, elastography, strain

Recommended Ultrasound Terminology

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