Glossary of Ultrasound Terminology* A Mode (Amplitude-mode): A method of echo signal display in which time is represented along the horizontal axis and echo amplitude is displayed along the vertical axis. AB Mode (Amplitude brightness-mode): A method of data presenta­ tion on the oscilloscope screen in which one coordinate represents time (depth) and the amplitude of the echo signal is displayed as both a deflection along the other coordinate and as a brightening of the display spot. Absolute Maximum (from AIUM/NEMA Standard): This means the largest possible value of a specified quantity either for an individual instrument or for all instruments of a given generic type. This value shall include effects of inaccuracies and imprecision of the measure­ ment process(es) used to determine it. Absolute Minimum (from AIUM/NEMA Standard): This means the smallest possible value of a specified quantity either for an individual instrument or for all instruments of a given generic type. This value shall include effects of inaccuracies and imprecision of the measure­ ment process(es) used to determine it. Absorbed Dose: The thermal energy imparted to matter by absorption of acoustic radiation per unit mass (or per unit volume) of irradiated material at the site of interest. Absorption: The process by which acoustic radiation imparts energy locally to the medium through which it propagates, by conversion of acoustic energy to heat. Absorption contributes to attenuation. Acoustic, Acoustical: The qualifying adjectives "acoustic" and "acous­ tical" mean containing, producing, arising from, actuated by, related to, or associated with sound. Acoustic (note 1) is used when the term being qualified designates something that has the properties, dimen­ sions, or physical characteristics associated with sound waves; acous­ tical (note 2) is used when the term being qualified does not desig- *Selected terms taken from AlUM Recommended Nomenclature [Reflections 6 0),3219801 with permission. Definitions modified from this reference are markedt. 305 306 GLOSSARY nate explicitly something that has such properties, dimensions, or physical characteristics. Note 1: The following examples qualify as having the properties or physical characteristics associated with sound waves and hence would take acoustic: impedance, output (sound power), energy wave, medium, signal, transducer. Note 2: The following examples do not have the requisite physical characteristics and therefore take acoustical: method, engineer, symbol, problem, measurement. Note 3: As illustrated in the preceding notes, usually the generic term is modified by acoustical, whereas the specific technical impli­ cation calls for acoustic. Acoustic Attenuation*: Amplitude reduction of the acoustic signal as a function of time or of propagation distance. Attenuation includes the effects of absorption, scattering, reflection, refraction, and diffraction. Acoustic Energy: Mechanical energy transported by an acoustic wave. The units are those of acoustic power times time, the joule (J) (Watt­ second) in standard international units. Acoustic Field: A distribution (in space and time) of acoustic energy. Acoustic Holography: See Holography. Acoustic Impedance: A vector quantity formed by taking the ratio of the instantaneous acoustic pressure at a surface to the instantaneous volume velocity at the surface in an acoustic field. (See also specific acoustic impedance, characteristic acoustic impedance). Acoustic Impedance Match (Colloquial): The condition of equality of Characteristic Acoustic Impedances of contiguous media, avoiding reflection of acoustic energy at the interface (see Impedance Ratio). *In this (and other similar definitions in this section), the adjective "acoustic" may be deleted when the acoustic context of the term is understood; or it may be replaced by the equivalent term "sonic" or, when appropriate, the more specific adjective "ultrasonic." These terms, which refer to the sound pressure amplitude, can also be defined in terms of intensity. The choice would be based upon the measurement technique. For example, a hydrophone probe will yield data in terms of sound pressure amplitude, and a thermocouple probe will give data in terms of intensity. The magnitude of this attenuation (or attenuation coefficient) depends upon many characteristics of the measurement system, such as the size and orientation of the transducer or transmitting and receiving transducer element, the orientation of the tissues, and electrical signal processing. The acoustic attenuation of tissue or other material in an ultrasound beam is the ratio of the signal received by the transducer relative to the signal that is received when the tissue or other material in the propagation pathway is replaced by a specified lossless medium (water, at a given temperature). GLOSSARY 307 Acoustic Impedance Mismatch: A condition of unequal Characteris­ tic Acoustic Impedances of contiguous media, causing reflection of acoustic energy at the interface (see Impedance Ratio). Acoustic Intensity: See Intensity. Acoustic Lens: A refractive element employed to redirect acoustic waves or rays in order to increase or decrease acoustic energy den­ sity in a prescribed volume or to modify phase coherence at a receiving transducer. Acoustic Power: Acoustic energy transported per unit time (usually a temporal average is quoted; e.g. I J/s) (See Average Acoustic Power). Acoustic Pressure: The instantaneous value of the total pressure mi­ nus the ambient pressure. Acoustics: Acoustics is the science of sound, including its production, transmission, and effects. Acoustic Shadow: A manifestation of reduced acoustic signal ampli­ tude in or returning from regions lying beyond an attenuating ob­ ject. It is important to distinguish between acoustic shadows and re­ gions of low reflectivity. Acoustic Streamingt: An acoustically generated time-independent transport of fluid within the body of the sonicated fluid or tissue. Acoustic Wave: A mechanical disturbance that progagates through a continuous medium. Acoustic Waveform: See Waveform. Acoustic Wavefront: The surface of equal phase in a propagating wave. Acoustic Wavelength: The acoustic wavelength is the distance between any two adjacent points at which the phase, at the same instant, dif­ fers by 21T; it corresponds to the distance traveled by the wave during one cycle; A = clf, where A is the wavelength, c is the speed of sound, and f is the frequency. For water or tissue at I MHz, the wavelength is approximately 1.5 mm. Amplitude: The magnitude of the envelope of a first-order electrical or acoustic waveform (e.g., voltage or acoustic pressure in linear acoustics. ) Amplitude Modulation Factor: The value of the expression 100 X (IAI - IBI) I IAI, expressed as a percentage, where IAI and IBI are the absolute maximum and minimum amplitudes of the envelope of a modulated acoustical or electrical carrier (first-order quantity), respectively. Amplitude Modulated Waveform: A waveform in which the carrier wave is modified in amplitude by a signal wave as a means of trans­ mitting information. For measurement purposes the AIUM-NEMA 308 GLOSSARY standard defines it as a waveform in which the amplitude modula­ tion factor is greater than 5%. Angle of Incidence: The angle between the axis of an ultrasound beam encountering an interface and the vector normal (perpendicular) to the interface. Attenuation: See Acoustic Attenuation. Attenuation Coefficient: The relative change in the acoustic wave am­ plitude (acoustic attenuation) per unit path length in a medium. Commonly employed units are dB/cm, and Np/cm where Np = Neper. Attenuator: A device that reduces the signal by a specified amount, e.g., 10 dB steps. Average Acoustic Power: The power output from an acoustic trans­ ducer averaged over a period of time that is either long com pared to, or exactly to, the period of pulses or variations in power. Average Intensity: See Intensity. Axial Resolution: The minimum separation of reflectors, required along the direction of sound travel, such that each can be separately distinguished on the display (same as Depth Resolution, Longitudi­ nal Resolution, and Range Resolution). Azimuthal Resolution: The minimum angular separation between ad­ jacent reflectors at the same range such that each can be separately distinguished in the display. This resolution is limited by the beamwidth of the transducer at that range of the targets (see Lateral Resolution). B-Mode (Brightness-Mode): A method of display on an oscilloscope screen in which the intensity of the echo is represented by modula­ tion of the brightness of the spot and in which the position of the echo, displayed in the x-y plane, is determined from the position of the transducer and the transit time of the acoustic pulse. B-Scan: A misnomer for a B-mode scan or image; scanning with B-mode display. Backscattered Energy: The portion of the incident acoustic energy re­ flected from a small (compared to the wavelength) target back to­ ward the reflector source; to be distinguished from specular reflec­ tion, where the reflector dimension may be large compared to the wavelength. Bandwidth: The transmitted bandwidth is the difference in the fre­ quencies, Fr and F2, at which the magnitude in the acoustic pressure spectrum is 71% (-3 dB) of its maximum value. The pulse-echo bandwidth is the difference in frequencies, Fr and F2, at which the magnitude of the pulse-echo response from a planar reflector at a specified range is 50% (-6 dB) of its maximum value.