Recommended Ultrasound Terminology
L
laminar flow
Flow in which there is smooth and gradual variation of velocity with position. Flow may be thought of as comprising a series of individual laminae, each moving at one velocity, with viscous cohesion maintaining the flow of adjacent laminae at nearly the same velocity.
late phase of enhancement
See parenchymal phase of enhancement and sinusoidal phase of enhancement.
Related terms: parenchymal phase of enhancement, sinusoidal phase of enhancement
lateral gain control (LGC)
A means of altering receiver gain with the lateral position in the image. This is often used to compensate for a loss in echo amplitude arising from laterally varying attenuation properties such as bright or dark objects at the same depth.
Related terms: automatic gain control, lateral gain control, time-gain compensation
lateral resolution
The minimum separation between two equally reflecting point reflectors in a direction perpendicular to the beam axis at which the individual reflectors can be distinguished in the image display. Alternatively, the distance, across the principal acoustic axis, between the two points farthest on each side from the point of maximum response at which the response of a pulse echo system is at a specified level, such as –6 dB relative to the maximum.
Related term: Compare with azimuthal resolution
lateral strain elastogram
An elastogram that depicts the distribution of the lateral strain.
LCD monitor
See liquid crystal display monitor.
lead metaniobate
A ferroelectric ceramic used for piezoelectric elements in transducer assemblies. The abbreviation is PbNiO.
lead zirconate titanate
A ferroelectric ceramic used for piezoelectric elements in transducer assemblies. The abbreviation is PbZrTiO.
lens
An object that produces the focus or divergence of an ultrasound beam by refracting the acoustic rays. Most lenses in diagnostic imaging are used to bring an ultrasound beam to a focus at a desired point in the body. Lenses in therapeutic ultrasound are used to focus the ultrasound beam on the desired target to concentrate energy on the therapeutic target and minimize the energy applied to surrounding normal tissue. An acoustic lens can be simulated by electronically delaying echo signals arriving at or emitted by various transducer elements (see Figure 36).
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Related term: focused ultrasound
LIFU (Low Intensity Focused Ultrasound)
LIFU, or low-intensity focused ultrasound, refers to a class of ultrasound techniques that deliver focused acoustic energy to a target region at intensities low enough to avoid significant tissue heating or destruction, distinguishing it from high-intensity focused ultrasound (HIFU), which ablates tissue. LIFU is typically used to induce nonthermal bioeffects such as neuromodulation and blood-brain barrier opening.
Related term: focused ultrasound
light-emitting diode display
An electronic display that uses light-emitting diodes (LEDs). Light-emitting diodes are semiconductor light sources that emit light when activated.
Related terms: Contrast with cathode ray tube, liquid crystal display, plasma monitor
lighthouse artifact
See comet tail artifact (see Figure 23).
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limiting
A technique that does not permit a voltage level to exceed a specified value. This may be an inherent limitation of a component or a system rather than a technique per se.
linear amplification
Amplification where the output signal is proportional to the input signal.
linear amplifier
An amplifier for which the output amplitude varies linearly with the input amplitude over a considerable range of the input signal.
linear array
A transducer assembly consisting of transducer elements arranged in a row, which can be electrically configured to control the characteristics, focusing and scanning of acoustic beams along the array length. With linear arrays, the beam axis is translated sequentially along the array (see Figure 27).
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Related term: Compare with phased array
linear scan
The motion of an ultrasound beam axis along a straight line, usually at right angles to the beam.
line density
Number of scan lines per unit length or angle used in formation of an image.
line spectrum
Spectrum containing only discrete frequency components.
liquid coupling
A technique using a liquid, such as water, to couple the transducer to the object.
Related term: coupling method
liquid crystal display monitor
Monitor containing a liquid crystal display (LCD) to view data or images. In an LCD, a liquid crystal is positioned between sheets of polarizing material. Passage of light through spots on the material is controlled by an electric charge that can be stored in tiny components positioned adjacent to the material. Some LCD displays are used for soft menu control of the device.
Related terms: Contrast with cathode ray tube, light-emitting diode display, plasma monitor
lithotripsy
The use of high-pressure ultrasound waves to break up kidney stones and gallstones.
liver-specific (late) phase of enhancement
See parenchymal phase of enhancement.
loaded acoustic impedance
Input acoustic impedance of an electroacoustic transducer, when the electric output is connected to a normal load.
logarithmic amplifier
An amplifier for which the output is proportional to the logarithm of the ratio of an input voltage and a reference voltage. Logarithmic amplifiers selectively enhance the distinguishability of small echoes in grayscale displays.
logarithmic decrement
Natural logarithm of the ratio of any two successive maxima of like sign in the decay of a single-frequency oscillation.
logarithmic frequency interval/interval
Logarithm of the ratio of two frequencies.
longitudinal
Parallel to the beam axis or to the long axis of the body or other object of interest: eg, a longitudinal scan or longitudinal sound wave.
longitudinal image plane
Any image plane that passes through or is parallel to the long axis of the body. Sagittal and coronal planes are both specific types of longitudinal image planes (see Figure 1).
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longitudinal plane
A plane containing the beam axis. It is defined by the beam axis and a specified orthogonal axis.
longitudinal resolution
See axial resolution.
longitudinal wave
Wave motion for which the particle displacement in the medium is parallel to the direction of wave propagation.
lossless medium
A medium that is nonattenuating.
lossy
An adjective describing a material that attenuates an ultrasound wave.
low-flow lesion
A lesion showing very slow enhancement over time.
lymphosonography
A contrast-enhanced imaging of lymph nodes and lymphatic vessels with ultrasound.
