Recommended Ultrasound Terminology

AcronymsFigures

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oblique image plane

An image plane that is not exactly axial, sagittal, or coronal. Often used when a structure lies diagonally.

offset distance

The distance between the near point of a device and its active transducer aperture, measured along the beam axis (see Figures 2, 7, and 9).

off time

The duration of time that an acoustically coupled transducer is not emitting sound. For pulsed systems, it is the sum of the time intervals between pulses during the total time that the transducer is acoustically coupled to the subject or object of interest.

Related terms: Contrast with duty factor, echo listening time, on time

Ohm’s law

The relationship between voltage, V, and current, I, usually given for a resistance load, R: V = IR.

omnidirectional hydrophone

Hydrophone the response of which is substantially independent of the direction of sound incidence.

Related term: Contrast with directional hydrophone

on time

The time in which the transducer is actively emitting sound. For pulsed systems, it is the sum of pulse durations during the total time that the transducer is acoustically coupled to the subject or other object of interest.

Related terms: Compare with duty factor, echo listening time, off time

opacification

An increased level of ultrasonic backscatter (image brightness), typically associated with the presence of an ultrasonic contrast agent. The term originates from x-ray imaging but should be used cautiously in ultrasound, where 'enhancement' is often a more appropriate term.

Related term ultrasound contrast agent

open-circuit mechanical impedance

Input mechanical impedance when the electric output is connected to an infinite load: an open circuit.

operating condition

Any unique combination of control settings for a selected operating mode.

operating mode

Any one or combination of the possible modes of an ultrasound instrument. Examples are A-mode, M-mode, B-scan, pulsed Doppler, and duplex mode.

optical hydrophone

A hydrophone whereby the sensitive element for measuring ultrasound pressure is an optical glass fiber. The ultrasound signal is detected by pressure-induced changes in the optical properties of the fiber. It is used for measuring high pressures such as those used in therapeutic ultrasound.

output beam area

Area of the ultrasonic beam derived from the output beam dimensions. The unit is square centimeter (cm2).

output beam dimension

Dimension of the ultrasonic beam (eg, –6-dB pulse beamwidth) in a specific direction normal to the beam alignment axis at the transducer output face. The unit is millimeter (mm).

output display standard (ODS)

Also known as the standard for real-time display of thermal and mechanical indices on diagnostic ultrasound equipment. It provides a method for visual feedback on diagnostic ultrasound imaging systems of the potential for bioeffects through thermal and mechanical indices displayed on the imaging screen for the particular control settings selected to implement the ALARA principle. The ODS provides visual feedback on diagnostic ultrasound imaging systems of the potential for the induction of adverse biological effects. The likelihood of an adverse bioeffect occurring because of a thermal mechanism is indicated by a thermal index (TI), and the likelihood of an adverse bioeffect due to a nonthermal mechanism, such as cavitation, is indicated by a mechanical index (MI) (AIUM/NEMA, 1998).

Related terms: thermal index (TI), mechanical index (MI), ALARA principle

output power

Time-average ultrasonic power radiated by an ultrasonic transducer. The unit is watt (W).

overall gain (general engineering)

The gain of an entire system. The overall gain (in decibels [dB]) equals the sum of the individual gains (in dB) of each of the system’s individual components.

overall gain (ultrasound)

Amount of amplification applied to echo signals over the entire field of view. In most systems, overall gain can be varied by the operator using a single control; it differs from time-gain compensation, where gain varies with depth in the image, and lateral gain compensation, where gain varies with lateral position in the image.

Related terms: automatic gain control, lateral gain control, time-gain compensation

overshoot

Those portions of a transient signal whose magnitudes are greater than the steady-state value (see Figure 13).