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  • Absorption: Conversion of ultrasound energy into heat.
  • Active element: Integral part of all ultrasound transducers. Also called a crystal, it is made of piezoelectric material (lead zirconate titanate or PZT) that converts electrical energy into ultrasound and vice versa.
  • Acoustic variables: Parameters that define a sound wave, such as pressure and density that change rhythmically.
  • AIUM: American Institute of Ultrasound in Medicine.
  • AIUM 100 mm test object: Standard phantom used for quality assurance.
  • Akinetic: Organ or its part that should be moving, but is not.
  • ALARA: As Low As Reasonably Achievable: AIUM principle, limiting possible bioeffects of acoustic radiation.
  • Aliasing: Sampling error characteristic of the inability of pulsed-wave Doppler to accurately measure high-flow velocities.
  • Ambiguity (range): Characteristic of continuous-wave Doppler describing its inability to define the position of the sample. Caused by an overlap between transmitting and receiving beams.
  • A-mode ultrasound: Antiquated mode of ultrasound used to depict the position of a reflector as well as the strength of the returning echo by its amplitude. Seldom used in modern practice.
  • Amplification (receiver gain): Increases signal strength in the receiver of the ultrasound system and therefore overall brightness of the image.
  • Amplitude: The difference between the average value of the acoustic variable and its maximum value through the duration of the sound wave; the “loudness” of the ultrasound.
  • Analog image: Image on the screen of the cathode-ray tube (TV screen) prior to any computer processing.
  • Anechoic: Area producing no-echo reflections and appearing black on the ultrasound image.
  • Archiving: Storage of images.
  • Array transducer: Transducer with multiple active elements, arranged in a certain order.
  • Artifact: Image errors or any image that differs from true anatomy of the reflector. Can be caused by malfunction of the ultrasound system, physical limitations of ultrasound, or operator error.
  • Attenuation: Reduction of amplitude of an ultrasound wave, as it propagates through the medium.
  • Attenuation coefficient: Attenuation in negative decibels per one centimeter (cm) travel. In soft tissues, 0.5 dB/cm/MHz.
  • Augmentation: Increase in venous flow with distal compression; a sign of venous patency.
  • Axial resolution: The minimal distance between two objects positioned along a line parallel to the ultrasound beam where both can be distinguished as separate objects. Defines longitudinal or depth resolution or the distance between two reflectors, measured in millimeters (mm), at which the reflectors are still imaged as separate. It is measured as a half of the ultrasound pulse length, with typical values in diagnostic ultrasound of 0.05–0.5 mm.
  • Backing material: Backing also known as damping material consists of the layer of epoxy resin impregnated with tungsten and placed behind the active element of the ultrasound transducer. It improves axial resolution by decreasing pulse duration (after-ringing), much like a hand placed on a guitar string.
  • Banding: Hyperechoic artifact within the focal zone. Appears as a bright, horizontal stripe.
  • Beam (Ultrasound beam): Bundle of acoustic radiation transmitted by the transducer, caused by wavelet interactions, and shaped like an hourglass.
  • Bernoulli equation (simplified): Converts maximal flow velocity ...

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