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nanobubbles

Nanoscale gas-filled bubbles used as contrast agents and therapeutic enhancers in ultrasound applications.

nanodroplets

lNanoscale phase-changing contrast agents used in ultrasound imaging and therapy that can be acoustically activated to transition from liquid-core, nanoscale particles into gas microbubbles.

nanoparticle

A solid or liquid particle with a diameter in the nanometer range, typically 1 to 500 nm. May be used as an ultrasound contrast agent or for drug delivery.

Related and current term: nanobubbles, nanodroplets

narrow-bandwidth

An adjective referring to the ability of a device to pass only a very limited range of frequencies in a signal. The resulting frequency spectrum usually consists of a single narrow peak.

narrow-bandwidth transducer

A transducer with a bandwidth that is less than 15% of its center frequency.

natural focus (archaic)

Related and current term: beam minimum

natural frequency

Frequency of a freely oscillating (undriven) system. For a multiple-degree-of-freedom system, the natural frequencies are the frequencies of the normal modes of oscillation.

Navier-Stokes equations

A set of equations that describe the motion of fluid substances such as liquids and gases. These equations establish that changes in momentum (acceleration) of fluid particles are simply the product of changes in pressure and dissipative viscous forces (similar to friction) acting inside the fluid. These viscous forces originate molecular interactions and dictate how sticky (viscous) a fluid is. Thus, the Navier-Stokes equations are dynamic statements of the balance of forces acting at any given region of the fluid.

near field (Fresnel zone)

For a nonfocusing transducer, the region within the transition distance from the transducer in a specified longitudinal plane. This region of the ultrasound field undergoes rapid fluctuations in the shape of the beam near the transducer. The region of the acoustic field close to the transducer (colloquial). For a given frequency, the near field is the region closest to the transducer, characterized by its inhomogeneity and where the particle velocity and instantaneous acoustic pressure are not in phase. For a circular transducer, the transition from the near field to the far field occurs approximately at a range of r_²/λ, where r is the radius of the transducer, and λ is the wavelength. For planar transducers, this is at a distance less than (A₁/πλ), where Aλ is the effective area of the transducer (see Figure 3).

Related terms: transition distance; contrast with far field

near Fresnel zone (archaic)

See preferred term: prefocal zone (see Figure 3).

near gain

Amplification of echo signals arising from structures close to the transducer. These signals tend to be larger than those received from more distant structures because of attenuation in the propagating medium.

near point

The near point of a transducer or transducer assembly is the point in the source aperture plane for which the integral over the source plane of the product of the acoustic pressure and the vectorial displacement from that point is zero. For a transducer or a transducer assembly with an active transducer aperture with symmetry in the source aperture plane, the near point is in the center of the source aperture.

Related terms: source aperture plane, source plane, transducer assembly, active transducer aperture

near sound field

See near field.

near transition distance

The distance past which the focal zone begins for a focusing transducer (see Figure 6).

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

neper

A logarithmic unit used for expressing the amplitude ratio of two quantities having the same units. If the two amplitudes are A1 and A2, then the corresponding number of nepers is ln(A1/A2). The symbol ln is the logarithm to the base e (1 neper [Np] = 8.686 decibels [dB]).

neurosonography

Ultrasound imaging of the nervous system.

newton

A unit of force. One newton is the force required to cause an object with a mass of 1 kg to accelerate at a rate of 1 m/s2. The abbreviation is N.

newtonian fluid

An incompressible fluid in which the viscosity is constant. Consequently, the shear stress at any point is proportional to the rate of strain.

node

Point, line, or surface in a standing wave where some specified characteristic of the wave field has essentially zero amplitude. In practice, this amplitude is generally not zero but simply a minimum. The node is then said to be partial. The appropriate modifier should be used before the “node” to signify the type that is intended: eg, displacement node, particle velocity node, or sound pressure node.

nodule-in-nodule appearance

Postcontrast recognition of a nodule of different (usually increased) echogenicity within a larger one.

noise

Unwanted and usually random signals.

noise (digitization)

See digitization noise.

noise (electrical)

Noise signals that are present in electrical circuits even in the absence of intended signals.

noise (quantization)

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

noise (thermal)

Electric noise produced by thermal agitation of electrons in conductors and semiconductors. Also known as Johnson noise or resistance noise.

nominal frequency

A convenient value used to represent the actual frequency or frequencies emitted by a transducer. The nominal value is usually specified by a manufacturer and is approximately equal to the actual value.

nonfocusing transducer

A transducer that does not produce a focused field (see Figures 2 and 3).

noninertial cavitation

Any form of cavitation that is not inertial. It includes various types of sonically induced activities involving bubbles or gas bodies such as translational motions, surface distortions, jet formations, rectified diffusion, and acoustic microstreaming. In contrast to inertial cavitation, free radicals are not produced, and temperature and pressure elevations are much lower.

Related term: inertial cavitation

nonlinear

A condition in which the amplitude of the output or response of a system or medium is not proportional to the input signal.

nonlinear amplification

Amplification in which the output signal is not directly proportional to the input signal. An example is logarithmic amplification.

nonlinear mode

Sonographic techniques detecting and forming images using nonlinear components of echoes. Opposite to the fundamental mode, in which a linear portion of the response is used to form the image.

nonlinear propagation

Passage of a sound wave in which the waveform becomes progressively distorted due to the medium. This usually occurs when the amplitude of the sound wave is large.

nonlinear propagation parameter

An index that indicates the degree of distortion of an ultrasound wave due to nonlinear propagation. This parameter will vary with the propagation medium.

nonlinearity

A relationship between two variables in which each one is not proportional to the other.

nonscanning mode

A mode of operation of a system that is restricted to one acoustic path. M-mode and pulsed Doppler are examples of nonscanning modes.

normal

(1) At right angles, eg to a line or surface. Synonym: perpendicular. (2) Usual, typical, expected, or standard.

normalized cross-correlation function

The cross-correlation function whose amplitude is normalized by the square root of the product of the autocorrelation functions of the respective signals.

Related term: cross-correlation function

normal mode of oscillation

Assuming sufficiently small displacements, this is a free (undriven) characteristic oscillation of a mechanical system in which all parts oscillate at the same frequency, called the natural frequency of the normal mode of oscillation. For a given system, there can be more than one normal mode of oscillation, each mode having a natural frequency generally different from that of any other normal mode of vibration of the system. For sufficiently small time-dependent displacements, any motion of a system corresponds to a linear superposition of its normal mode oscillations.

Nyquist criterion

The requirement that a band-limited signal can only be unambiguously represented by instantaneous samples if the sampling frequency is more than twice the maximum frequency present in the spectrum of the signal.

Related terms: sampling theorem, Nyquist limit

Nyquist limit

The highest frequency in a waveform that can be represented unambiguously by a sampled signal, equal to one half of the sampling frequency. In Doppler systems, this is one half of the pulse repetition rate when detecting flow in both directions.

Related terms: sampling theorem, Nyquist criterion

Recommended Ultrasound Terminology

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