Absorption – the conversion of sound to heat because of its interaction with tissue.

Acoustic enhancement – an increase in reflection amplitude that occurs when a wave strikes a medium that is located behind a tissue of weak acoustic impedance.

Acoustic impedance – the resistant properties of a medium to sound transmission.

Amplification – Increasing small echoes into larger ones.

Amplitude – the maximal variation within a sound wave.

Anechoic – any medium that does not produce an echo.

Artifact – any phenomenon that affects the acquisition or interpretation of an ultrasound image.

Attenuation – the decreasing amplitude of a sound wave as it propagates through tissue.

Attenuation coefficient – the degree of attenuation measured per unit length of wave travel.

Axial resolution – the ability of an ultrasound system to distinguish separate structures that are in close proximity to each other along the same axis.

B mode – creation of an ultrasound image in which the display records brightness for each echo based on the strength and time the echoes are received.

Beam – the sum of all the sound waves generated by the transducer.

Color flow Doppler – a two dimensional, real-time Doppler shift that is superimposed on a real-time Bmode image.  Flow towards and away from the transducer are represented as different colors on the display.

Cosine – in a right triangle, it is the length of the adjacent divided by the length of the hypotenuse.

Coupling medium – gel used to provide transmission of sound between the transducer and skin.

Cycle – one complete compression and rarefaction in a sound wave.

Damping – material placed behind the transducer elements to reduce pulse variation and duration.

Diffuse reflector – (also called scattering) returning echoes that are forced to deviate from a straight-line trajectory due to small, localized non-uniformities in the tissue.

Doppler angle – the angle between flow and the ultrasound wave.

Doppler shift – the change in frequency caused by motion.  It is the reflected frequency minus the incident frequency.

Echo – a reflection.

Echogenicity – the degree of reflection caused by varying degrees of acoustic impedance within a tissue.

Focal region – the area where an ultrasound beam is at minimum diameter and area.

Focus – the ability to concentrate a sound beam in an area where it normally would not occur.

Frame – the single image that results from one complete scan of the sound beam.

Frame rate – the number of frames that can be displayed per unit of time.

Frequency– the number of cycles that occur per second, measured in Herz (Hz).

Gray scale – the complete range of brightness between white and black that is displayed in a Bmode image.

Heterogeneous – tissues consisting of various impedances, such as muscle.

Hertz (Hz) – unit of frequency, one cycle per second; unit of pulse repetition frequency, one pulse per second.

Homogeneous – tissues that consist of the same impedance; such as the thyroid gland and liver, which contain the same specialized cells.

Hyperechoic – “hyper” echoes; those tissues that cause increased reflection.

Hypoechoic – “hypo” echoes; those tissues that are not significant reflectors.

Incident angle – the angle created between the incident sound beam and a line drawn perpendicular to the medium.

Incident beam – the ultrasound beam that originates from the transducer.

Interface – the boundary between two tissues with different acoustic impedances.

Kilohertz (kHz) – one thousand Hertz.

Lateral resolution – the ability of an ultrasound system to differentiate between two objects that are perpendicular to a sound beam.

Linear array transducer – a transducer made up of rectangular elements lined up side-by-side.

Longitudinal Wave – movement of particles in the same direction as the direction of wave propagation.

Matching layer – material placed in front of the transducer elements to reduce the acoustic impedance between the transducer and skin.

Megahertz (MHz) – one million hertz.

Mirror-image artifact – the duplication of an object on the opposite side of a strong reflector.  In Doppler, it is the duplication of the spectrum on the other side of a strong reflector.

Near zone – the region of the sound beam where the diameter decreases as the distance from the transducer increases.

Period – the amount of time required to complete one cycle.

Phantom – materials that have characteristics of tissue.

Piezoelectric effect – the conversion of electrical energy to mechanical energy and vice versa.

Probe – a transducer.

Propagation velocity – the speed through which a wave will travel in a particular medium.

Pulse – a few cycles of a sound wave.

Pulse-echo ultrasound – imaging and flow measurement that utilize the transmission of pulses to generate a display.

Pulse repetition frequency – the number of pulses per unit of time.

Rayleigh scattering – the uniform distribution of a sound wave in all directions when it comes in contact with tissue that has smaller dimensions than the wave (e.g. red blood cells)

Reflection – the mirror-like redirection and return of a propagating sound wave towards the transducer that follows a standard law of reflection; for example, specular reflection results in the reflected angle being equal to the incident angle of the energy propagation.

Refraction – a change in the direction of wave propagation when traveling from one medium to another with different propagation speeds according to Snell’s Law.

Resolution – the ability to distinguish between two structures that lie close to one another.

Reverberation – multiple reflections of the same object that creates the illusion of many objects.

Scattering – see diffuse reflection.

Signal – with regards to sound, it is the acoustic conveyance of information.

Sound – a longitudinal, mechanical wave of acoustic variables.

Sound wave – traveling variation of acoustic variables.

Spectral analysis – the process of separating Doppler signals into a spectrum based on their velocities and direction of flow.

Specular reflection – return of echoes in a singular direction after contacting a medium that has a large smooth surface (e.g. bone).

Time Gain Compensation (TGC) – the ability of an ultrasound system to equalize differences in reflection amplitude caused by attenuation and reflector depth.

Transducer – a device that converts energy from one form to another.

Transducer array – a transducer that contains more than one transducer element.

Ultrasound – sound frequencies greater than 20 kHz.

Velocity – the sound speed and direction of motion specified.

Wavelength – the distance over which the acoustic disturbance repeats itself at any instant in time during a cycle.


Kremkau F W. Doppler Ultrasound: Principles and Instruments. Philadelphia, PA: W.B. Saunders Company; 1990; 221-233.

Chan VW, Abbas S, Brull R, et al. Ultrasound imaging for regional anesthesia; a practical guide. 3rd ed. Toronto. Toronto Printing Company; 2010:4-5.

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