Recommended Ultrasound Terminology
P
Pa
The abbreviation for pascal.
Related term: pascal
packet length
See ensemble length.
PACS
A medical imaging system designed to acquire, store, retrieve, display, and distribute medical images digitally. Picture archiving and communication system (PACS) facilitates workflow efficiency in radiology and sonography by replacing traditional film-based image management with universally agreed format, digital storage and retrieval systems.
parallel receive beamforming
The parallel processing of incoming signals from different receive elements of an array to form multiple beams from the same data.
Related terms: beamforming, pulse echo
parametric imaging
A technique that visually represents variations in a quantitative parameter, such as blood flow or perfusion, using a color scale. In contrast-enhanced ultrasound, parametric imaging is commonly used to highlight differences in perfusion dynamics within tissues or lesions. Also referred to as color coding.
parasagittal plane
A plane parallel to the sagittal plane.
Related term: sagittal plane
parenchymal phase of enhancement
Some sonographic contrast media show first an intravascular phase and then a more or less pronounced phase of extravascular retention within liver and spleen parenchyma. To image the liver during this phase, scans are taken from 2 to 5 minutes after contrast medium injection up to 6 to 10 minutes, when the enhancing effect vanishes (depending on the type of contrast medium used). The parenchymal phase is also described as “late,” “delayed,” “liver-specific,” or “postvascular,” but “parenchymal phase of contrast enhancement” is the most appropriate description.
particle
Any small subdivision of matter. In ultrasonics, the particle is usually taken to have a homogeneous composition and a diameter that is much less than the wavelengths involved.
particle displacement
Instantaneous displacement of a particle from its mean position.
particle velocity
The speed of a particle or mathematically, the derivative with respect to the time of particle displacement.
pascal
The unit for pressure. One pascal equals 1 N/m2. The abbreviation is Pa.
Pascal’s law
The statement that pressure applied to the surface of a fluid is transmitted unchanged to all parts of the fluid.
peak compressional pressure (pc)
The spatial-peak temporal-peak positive pressure amplitude (see Figure 32).
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Related term: peak positive pressure
peak instantaneous intensity
See this entry under intensity.
peak intensity
See this entry under intensity.
peak negative (or peak rarefactional) acoustic pressure
The maximum of the modulus of the negative instantaneous acoustic pressure in an acoustic field or in a specified plane. Peak negative acoustic pressure is expressed as a positive number. The unit is pascal (Pa). The symbol is p– (or pr) (see Figure 32).
Related term: peak rarefactional pressure
peak particle displacement
Greatest instantaneous particle displacement during a given time interval.
peak positive (or peak compressional) acoustic pressure
The maximum positive instantaneous acoustic pressure in an acoustic field or in a specified plane. The unit is pascal (Pa). The symbol is p+ (or pc).
Related term: peak compressional pressure
peak rarefactional pressure (pr)
The spatial-peak temporal-peak negative pressure amplitude.
Related term: peak negative pressure
peak sound pressure
Greatest absolute value of instantaneous sound pressure during a given time interval.
penetration
See depth of penetration.
perfect reflector
A planar interface for which the reflection coefficient equals 1.
Related terms: scan plane, reflection, reflection coefficient (amplitude), reflection coefficient (intensity)
perfusion
Total blood flow, including capillary flow, to an organ. Perfusion of a tumor includes its microcirculation, but it is not a synonymous term.
perfusion defect
An area of poor or absent enhancement within parenchyma due to infarction or ischemia, appearing within normally enhanced surrounding parenchyma.
perfusion imaging
Imaging of flow at the capillary level. Usually used as synonymous with continuous mode, allowing real-time demonstration of parenchymal and lesional perfusion.
peripheral enhancement
An enhancement pattern found in ultrasound images such as near or at an outer tissue or tumor boundary.
persistence
The degree to which images or other data acquired sequentially in time are combined or averaged. Persistence can reduce the appearance of speckle and other variations, such as noise, in the displayed image or data. Also referred to as frame-to-frame averaging.
phantom
A device that simulates some parameters of the human body, allowing measurements of ultrasound system parameters or visualization of simulated anatomic features. Phantoms are useful for assessing the clinical performance of a diagnostic or therapeutic ultrasound system and for teaching.
phantom, digital
A digital phantom which displays an image determined by the tracked position and attitude of a dummy transducer on a curved surface and retrieved from a stored volume image data base. There is nothing below the surface (air or homogeneous solid) so it is a phantom.
phase
The fraction of the period of a periodic signal that has elapsed relative to a specified origin. The origin is usually taken to be the last previous passing through zero amplitude from the negative to the positive direction.
Related term: Compare with phase angle
phase angle
The measure of progression of a periodic signal in time or space from a specified origin. The phase angle is obtained by multiplying the phase by 360° or by 2π radians. The unit is degree or radian.
Related term: Compare with phase
phased array
A transducer configuration that consists of multiple independent elements that can be controlled individually. Using proper time delays of the excitations, a wavefront of a desired configuration can be synthesized. Earlier systems used phase differences instead of time delays; hence the name. Phased arrays are used for electronic beam steering and dynamic focusing (see Figure 35).
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Related terms: transducer element, phase difference
phase difference
The difference between the phase angle of a sinusoidally varying quantity and the phase angle of a second quantity that varies sinusoidally at the same frequency. The unit is degree or radian.
Related terms: Compare with phase, phase angle
phase domain processing
Any signal-processing scheme that operates on signals according to their phases.
phase inversion imaging
See pulse inversion imaging.
Related term: pulse inversion imaging
phase quadrature processing
A signal-processing technique using a reference sinusoidal signal and another reference signal equal to the first but shifted through 90° of phase angle. The input signal is then multiplied by each reference signal.
phases of contrast enhancement
In dynamic computed tomography and magnetic resonance imaging, the concept of phase is limited to the peak of enhancement in a given phase of contrast-enhanced blood distribution. In contrast-enhanced sonography, instead, the term “phase” refers to the entire duration of a given phase. Particularly for the liver, which has a double blood supply system, there is partial overlap between the final part of the arterial phase and the initial part of the portal phase, the latter starting when the enhancement of portal branches starts. For the liver, phases for contrast-enhanced sonography should be termed as “arterial” (vascular, ranging approximately from 15 to 40 seconds after peripheral contrast medium injection), “portal sinusoidal” (vascular, ranging approximately from 40 to 300 seconds after contrast medium injection), and eventually “parenchymal” (postvascular, for those contrast media showing late intraparenchymal uptake, lasting up to 10 minutes after contrast medium injection).
phase-change contrast agents (PCCAs)
A class of ultrasound contrast agents that exist as encapsulated liquid droplets under ambient conditions but are triggered (activated) using ultrasound energy to undergo phase transition into gaseous microbubbles that are suitable for both intravascular and extravascular ultrasound imaging studies.
phase shift
A change in the phase of a signal or in the phase difference between two signals.
phase tracking
A technique for following changes in the phase of a signal.
phase velocity
The velocity of a point that moves with a wavefront in the direction of propagation in such a way that the phase at the point is constant.
photoacoustic effect
The generation of sound waves in a material by the absorption of light.
photoacoustics
A method for imaging opaque (and biological) systems using the photoacoustic effect. Photoacoustic contrast is mainly determined by contrast in the optical absorption coefficient. It is often applied to the visualization of vessels because of the high optical absorption of blood.
Related term: thermoelastic expansion
piezoelectric effect
The property of some materials by which a mechanical deformation produces an electrical voltage across the material and vice versa. The production of voltage by the application of mechanical stress is called the direct piezoelectric effect. The production of a mechanical deformation by an applied voltage is called the reverse piezoelectric effect.
The direct piezoelectric effect is used in the detection of ultrasound echoes, and the indirect piezoelectric effect is used in the generation of ultrasound pulses.
pink noise
Noise whose power spectral density is inversely proportional to frequency.
pitch
(1) Distance between adjacent array elements. (2) The perceived apparent frequency of a sound.
pixel
A contraction of “picture element.” It is the smallest spatial unit or cell size of a digitized 2-dimensional array representation of an image. Each pixel has an address, (a set of x- and y-coordinates corresponding to its position in the array) and a specific brightness level.
plane
A flat surface.
plane strain
Strain that is confined within a plane.
plane stress
Stress that is confined within a plane.
plane wave
Wave in which the wavefronts are parallel planes normal to the direction of propagation.
Related terms: plane wave angular compounding, plane wave imaging
plane wave angular compounding
An ultrasound imaging technique that uses a collection of unfocused plane wave transmissions over a range of angles to form a composite ultrasound image frame after summation.
Related terms: plane wave, plane wave imaging
plane wave imaging
A fast ultrasound imaging technique that uses plane wave transmission to form an ultrasound image.
Related terms: plane wave, plane wave angular compounding
plasma monitor
Monitor containing many tiny cells in the screen, each having an inert mixture of a noble gas such as neon or xenon. When an electrical signal representing the brightness or color of a spot on the image is applied, the gas in the cells is turned into a plasma, that is, an ionized gas, which excites phosphors that emit light.
Related terms: Contrast with cathode ray tube, light-emitting diode display, liquid crystal display monitor
plug flow
A region of flow with a flat velocity profile, ie, in which nearly all the flow is at a single velocity, except for the boundary layer.
PMUT
Piezoelectric micromachined ultrasound transducers (PMUTs) that contain miniature capacitors, backed by gas, whose membranes, augmented with a thin piezoelectric film, 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 MEMS or silicon technology.
Related terms: CMUT, MEMS
pocket point of care ultrasound
Point of care ultrasound employing a pocket ultrasound device.
Related terms: POCUS, pocket ultrasound, handheld ultrasound
pocket ultrasound device
An ultrasound imaging system, small enough to fit easily in the palm of a hand. It connects to a display, often a smart phone or tablet display, by a small cable or wirelessly.
Related terms: handheld ultrasound, POCUS
point source of sound
Source that is small compared to the wavelength that radiates sound equally in all directions in a free field.
Related term: Huygen’s principle
point spread function
The output response of a system to a point source is a point spread function. This is a convenient characterization of an imaging system. The combined spatial and temporal response of a system or the beamforming part of an ultrasound system in three dimensions to a point scatterer. Pulse envelopes along the time axis of propagation are measured in two spatial dimensions (usually azimuth and elevation) perpendicular to the pulse.
Related terms: azimuth axis, diffractive scattering, diffusive scattering, elevation axis, specular scattering, specular reflection
point-of-care ultrasound (POCUS)
The use of a mobile ultrasound device where it is needed such as at a patient’s bedside or a physician’s office for imaging or therapy, or even outside the clinical environment. The use of portable/mobile ultrasound devices at the patient’s bedside or point of care to provide immediate imaging for diagnostic or procedural guidance, commonly used in emergency medicine, critical care, and outpatient settings.
point scatterer
A scattering object that is much smaller than the insonifying acoustic wavelength. A point scatterer scatters sound equally in all directions.
Related terms: diffractive scattering, diffusive scattering, specular scattering, specular reflection
poise
The cgs unit of viscosity. One poise is equal to 10 pascal seconds (Pa · s).
Poiseuille flow
The steady flow of an incompressible fluid parallel to the axis of a long circular vessel. It is produced by a pressure gradient along the vessel. The flow velocity is greatest in the center and is zero at the vessel wall, resulting in a parabolic velocity profile.
Related terms: velocity profile, parabolic
Poiseuille’s law
The law stating that the volume flow of an incompressible fluid, such as blood, through a circular tube is proportional to the pressure difference between the ends of the tube times the fourth power of the tube’s radius divided by the product of the tube’s length and the viscosity of the fluid. The equation is F = [π · (P1 – P2) · r4]/[8 · L · η], where F is the volume flow, P1 is the pressure at the beginning of the tube, P2 is the pressure at the end of the tube, r is the tube radius, L is the length of the tube, and η is the viscosity of the fluid.
Poisson elastogram
An elastogram depicting the distribution of Poisson’s ratio in a medium.
Poisson’s ratio
The fractional reduction in width divided by the fractional increase in length when a material is subjected to a tensile stress. The ratio between the lateral and axial strain in a medium.
polar coordinates
A point in a plane can be represented by coordinates (r, Θ), where Θ is the angle between the positive x-axis of a cartesian (x, y) system and the line from the origin to the point, and r is the length of that line; r and Θ are the polar coordinates of the point.
poling
A means of conferring piezoelectric properties by alignment of the dipoles in a suitable material through the application of an electric field while the temperature of that material is raised above the Curie point and then lowered below it.
poro-elasticity
The property of a material that is both porous and elastic.
portable imaging systems
Imaging systems that can be easily transported.
portal venous phase of enhancement
The phase of contrast medium circulation when contrast-enhanced blood reaches the liver through the venous portal system (from 40 to 50 seconds after contrast medium injection to 100 to 120 seconds after contrast medium injection). Since the liver has a double feeding system, arterial and portal, the portal phase of enhancement is not exactly the same concept as the venous phase in other organs. This is the phase when liver parenchyma (properly, small intrahepatic portal vessels) reaches its peak of enhancement.
position-sensed freehand scanning (3-dimensional)
Acquiring a volume of echo information through manual movement of the transducer without restriction on the possible motion while sensing the position of the image plane to provide geometrically accurate reconstruction.
postfocal zone
The region beyond the focal zone for an ultrasonic focused transducer in a specified longitudinal plane (see Figures 3, 4, and 5).
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postnatal
See postpartum.
postpartum
The period beginning immediately after birth and lasting 6 weeks. Also called the postnatal period.
postprocessing
Processing of image data after it has been assembled and stored in a 2- or 3-dimensional format and before display on the monitor.
Related term: preprocessing
postvascular phase of enhancement
Some sonographic contrast media show first an intravascular phase and then a more or less pronounced phase of intraparenchymal accumulation.
See parenchymal phase of enhancement.
potential energy
The capacity to do work that a body or system has by virtue of its state, position, or configuration.
Related term: Compare with kinetic energy
potentiometer
A device for changing and controlling electrical resistance. It consists of a resistor having a continuously adjustable sliding contact. Also known as a pot (archaic slang as this slang term now is used to refer to marijuana).
Pourcelot’s index
See resistive index.
power
The rate of doing work per unit time. The unit of power is the watt (W), and 1 W = 1 joule per second (J/s).
power cross-spectrum
The spectral density of two real or complex input signals usually computed in the frequency domain by various methods such as taking the Fourier transform of the cross-correlation functions of two signals. It is used for estimating attenuation from backscattered ultrasound signals.
power Doppler
A Doppler imaging mode in which the signal displayed is based on an estimate of the integrated Doppler signal power over a period of time at a given position. The signal represents the energy in the Doppler signal and is usually displayed on a color scale. This color value is related to the volume of blood present in the resolution volume, rather than its velocity. Typically, black represents no detectable blood flow and orange represents maximum detected blood volume.
power Doppler imaging
A form of pulsed Doppler in which a large number of estimates of Doppler signal power or amplitude are color coded and overlaid in the location of their detection on the grayscale image. Power Doppler imaging systems operate within the range of “real-time” frame rates.
Related terms: color Doppler imaging, color flow imaging, power modulation imaging, color amplitude imaging
power modulation imaging
Nonlinear techniques in which two separate pulses are transmitted down each scanning line, the second being at one-half of the acoustic pressure of the first. The second returned signal is doubled and subtracted from the first. The symmetric echoes from linear structures such as tissues cancel each other, whereas those from contrast- induced, harmonic components (nonlinear scattering) produce the image line. Other ratios can also be used; the first pulse is usually in the linear range.
power spectral density
Power per unit frequency interval in a power spectrum. The unit is watt per megahertz (W/MHz).
power spectrum
A frequency distribution of the power contained in a signal. It is often displayed as a graph showing the relative power of each frequency component contained in a signal. One way of calculating power is taking the square of the magnitude of the Fourier transform of the signal. Alternatively, the power spectrum can be calculated as the Fourier transform of the autocorrelation function of the signal.
power-up mode
A defined state of a system corresponding to the mode of operation and system settings automatically established when power to the system is first turned on.
preamp
Abbreviation for preamplifier.
preamplifier
A low-noise amplifier that is connected to a low-level signal source and precedes the main amplifier. It provides amplification and electrical impedance matching with minimal noise.
prefocal zone
The region before the focal zone for an ultrasonic focused transducer in a specified longitudinal plane (see Figures 3, 4, and 5).
preprocessing
Processing of ultrasound signals before they have been stored in a 2- or 3-dimensional image format as pixels or voxels. Preprocessing improves the quality and representation of the original pulse echo or other signals such as power Doppler before conversion to image format. In preprocessing, the original sequence in which the signals were received need not be lost.
Related term: postprocessing
presets
Preprogrammed operating conditions for specific diagnostic applications.
Related term: operating conditions
pressure
Force per unit area. The unit is pascal (Pa) (see Figures 31 and 32).
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Related term: Compare with acoustic pressure
pressure amplitude
The value of the envelope of the acoustic pressure oscillation (equals one-half of the peak-to-peak value in case of continuous wave ultrasound. In pulsed ultrasound, pressure amplitude usually refers to the peak value of the envelope (see Figures 26, 31, and 32).
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pressure focal area
The area in the pressure focal plane bounded by the locus of all points where the pulse pressure-squared integral is at least one-quarter of that at the pressure focus; otherwise known as the - 6 dB level or contour. If there is more than one closed region defined by these points, only the area of the region containing the pressure focus should be taken.
Related terms: decibel, pressure focus, pulse-pressure-squared integral
pressure focal gain
The square root of the pulse pressure-squared integral at the pressure focus divided by the source aperture average pressure.
Related terms: pressure focus, pulse pressure-squared integral, source aperture average pressure
pressure focal plane
The plane perpendicular to the beam axis that contains the pressure focus (see Figures 2 and 9).
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Related terms: beam axis, pressure focus
pressure focus
The position of the maximum pulse pressure-squared integral throughout the field (see Figures 2 and 9).
Related terms: pulse pressure-squared integral
pressure gradient hydrophone
A hydrophone whose output voltage is proportional to the pressure gradient.
pressure gradient microphone
Microphone that responds substantially to the gradient of the sound pressure.
pressure sensitivity
Of a hydrophone, for a given frequency, quotient of the output open-circuit voltage by the actual sound pressure existing over the region of the hydrophone designed to receive sound. Note: The load impedance must be specified if other than open circuit.
pressure waves
See acoustic waves.
principal acoustic axis
See beam axis.
principal axis
Geometric reference axis passing through the reference point that serves as the origin of angular coordinates for description of directional characteristics of an electroacoustic transducer. The direction of the maximum response at a stated frequency is usually chosen as the principal axis, which is often an axis of geometric symmetry.
probe assembly
An assembly consisting of one or more ultrasound transducers together with the driving mechanism and/or with associated electronics in a single housing.
probe hydrophone
Hydrophone adapted to explore a sound field without significantly disturbing it.
puddle enhancement
See globular enhancement.
pulsatility index (PI)
A parameter used to convey the pulsatility of a time-varying waveform such as the Doppler shift frequency of the signal from an artery. Indices of pulsatility are usually ratios of Doppler shift frequencies and are hence, independent of Doppler angle. The most common definition of the PI of a waveform is the difference between the maximum and minimum values divided by the mean value of the waveform over the cardiac cycle. Other types of PIs are also used.
Related term: Doppler frequency shift
pulse
A signal or disturbance of finite time duration.
Related term: pulse duration, pulse pressure-squared integral
pulse acoustic pressure
The square root of the ratio of the pulse pressure-squared integral to the pulse duration at a particular point in an acoustic field. The unit is pascal (Pa).
pulse-average intensity
See this entry under intensity.
pulse beamwidth
See beamwidth (reception), beamwidth (transmission).
pulsed Doppler (range-gated Doppler)
Doppler interrogation using a pulsed mode of transmission. Temporal gating of the returning echoes allows determination of the Doppler shift from within a specific region at a known distance from the transducer. This region is known as the sample volume.
Related term: multigate pulsed Doppler system
pulse duration
Time required for the instantaneous value of an excitation to rise from some stated fraction of its maximum value and to decay to the same fractional value.
pulse echo
A received reflected or backscattered sound signal usually caused by a repetitively transmitted pulse of sound.
pulse-echo imaging
Process of forming a sonogram. Short-duration ultrasound pulses are transmitted into tissue, and echoes from scatterers and reflectors are detected and displayed. At each location on the image, display brightness indicates the echo amplitude from the corresponding location in tissue.
Related term: sonogram
pulse-echo response profile
Graph of an echo signal parameter, such as echo amplitude or frequency content, as a function of the position, relative to the transducer, of a well-defined reflector.
pulse-echo technique
See echo ranging.
pulse intensity integral
The time integral of intensity, for any specific point and pulse, integrated over the time in which the envelope of acoustic pressure or hydrophone signal for the specific pulse is nonzero. For a transducer assembly operating in a non–auto-scanning mode, this is equivalent to the product of the temporal-average intensity and pulse repetition period.
pulse length
See pulse duration.
pulse (or phase) inversion mode imaging
Nonlinear techniques in which two separate pulses are transmitted down each scanning line, the second being an inverted copy of the first (ie, two pulses phase shifted by 180°). On summation of the two returned signals, the symmetric echoes from linear structures such as tissues cancel each other, whereas those from contrast-induced, harmonic components (nonlinear scattering) are summed to produce the image line. Tissue pulse inversion imaging and contrast-enhanced coded pulse inversion harmonic imaging are both possible. Contrast-enhanced pulse inversion may be used either as a continuous mode or an intermittent mode.
Related terms: contrast-enhanced harmonic imaging, harmonic imaging, nonlinear, nonlinear mode
pulse pair covariance
Particular type of autocorrelation processing in which signals obtained from successive pulses are correlated.
pulse-peak cycle
A single cycle of an acoustic pulse waveform between two points of zero instantaneous acoustic pressure. This single cycle is composed of the half-cycle containing the temporal peak acoustic pressure and whichever of the two adjacent half cycles contains the larger peak instantaneous acoustic pressure.
pulse pressure
The difference between the peak systolic and minimum diastolic pressures in the cardiac cycle.
pulse pressure (physiologic)
The difference between the maximum and minimum pressures in the cardiac cycle. It equals the systolic pressure minus the diastolic pressure.
pulse pressure-squared integral
Time integral of the square of the instantaneous acoustic pressure at a particular point in an acoustic field integrated over the acoustic pulse waveform. The unit is square pascal second (Pa2s).
pulse repetition frequency (PRF)
In a pulsed system, the number of ultrasonic pulses emitted by the transducer per second. Typically, the PRF of a diagnostic ultrasound system may be in the range of 0.5 to 15 kHz.
pulse repetition period
The time interval between two successive pulses or tone bursts. The unit is second.
pulse repetition rate
See pulse repetition frequency.
pulse rise time
Interval of time required for the leading edge of a pulse to rise from some specified small fraction to some specified larger fraction of the maximum value.
pulse stretching
Increase in pulse duration due, for example, to the imposition of a bandwidth limitation.
pulse waveform
The amplitude of a sound pulse as a function of time.
pure (sound/tone)
Sinusoidal acoustic oscillation.
push beam
A high-intensity pulsed ultrasound beam that is used to move objects in the direction of beam propagation. The forward motion of the object is caused by the radiation force of the ultrasound beam.
Related term: acoustic radiation force imaging (ARFI)
