NAMU CMU key terms

The resistance a tissue offers to passing sound, determined by its density and the speed of sound within it. Large mismatches between two tissues cause strong reflections at their boundary.

The progressive loss of ultrasound beam strength as it travels through tissue, caused by absorption, reflection, and scattering. Higher-frequency beams attenuate faster, limiting how deep useful imaging can reach.

The smallest distance between two reflectors along the beam path that can be displayed as separate echoes. It improves as pulse length shortens, so higher-frequency, heavily damped transducers give the best axial resolution.

The ability to distinguish two reflectors lying side by side at right angles to the beam. It depends mainly on beam width and is best at the focal zone, where the beam is narrowest.

The number of ultrasound pulses emitted per second. It sets the maximum Doppler shift that can be measured without aliasing and, together with pulse duration, influences the displayed depth range.

The highest Doppler frequency shift that can be detected unambiguously, equal to half the pulse repetition frequency. Shifts above this value wrap around and appear as flow in the opposite direction.

A Doppler artifact in which blood velocities exceeding the Nyquist limit are displayed as flow in the wrong direction or as a confusing color wrap. Raising the velocity scale, lowering frequency, or shifting the baseline can correct it.

A display mode that superimposes mean blood velocity and direction on the grayscale image using a color map. It rapidly surveys vessels and flow patterns before spectral Doppler sampling.

A Doppler mode that displays the amplitude of the Doppler signal rather than velocity or direction. It is more sensitive to slow flow in small vessels but does not show flow direction or speed.

A technique that builds the image from higher-frequency echoes generated within tissue as the beam propagates. It reduces side-lobe and reverberation artifacts and improves contrast, especially in obese patients.

Repeated back-and-forth echoes between two strong reflectors that create evenly spaced false lines or copies of a structure. It commonly appears behind gas or near the abdominal wall.

A dark band behind a structure that reflects or absorbs sound strongly, such as bone or a gallstone. Clean shadowing suggests a dense object; dirty shadowing suggests gas.

Increased brightness deep to a fluid-filled structure because sound passes through it with little attenuation. It helps confirm that a lesion is cystic rather than solid.

An on-screen estimate of the potential tissue heating produced by absorbed ultrasound energy, expressed as a ratio to the level believed to raise temperature by one degree Celsius. It guides output reduction during prolonged scanning.

An estimate of the likelihood of cavitation, calculated from peak negative pressure divided by the square root of frequency. It rises with higher output and lower frequency and matters most with contrast agents.

The practice of keeping acoustic output and exposure time as low as reasonably achievable while still obtaining diagnostic information. It underlies output display standards and scanning habits in every examination.

The standard two-dimensional grayscale display in which echo brightness represents reflector strength along each scan line. It forms the anatomical basis for nearly all other ultrasound modes.

A user control that selectively amplifies echoes returning from greater depths to offset attenuation. Correct settings produce uniform brightness across tissues at different depths without hiding real differences.

The depth range where the transmitted beam is narrowest and lateral resolution is best. The sonographer moves it to the level of the structure of interest before measuring.

A display of Doppler shift frequencies over time as a waveform, allowing measurement of peak velocities and resistance patterns. It is used to grade stenosis and assess flow direction and pulsatility.

A calculated value describing downstream resistance, obtained by subtracting end-diastolic velocity from peak systolic velocity and dividing by peak systolic velocity. Values near one indicate high resistance.

A Doppler index expressing the spread of velocities within a cardiac cycle relative to the mean velocity. It is used to describe waveform shape in organs such as the kidney and liver.

An imaging technique that maps tissue stiffness by measuring how much it deforms under compression or shear wave excitation. Stiffer areas appear differently and may indicate fibrosis or malignancy.

A quantitative stiffness method that generates shear waves in tissue and measures their speed, reporting results in kilopascals or meters per second. It is operator-independent compared with manual compression techniques.

A stiffness technique that compares tissue deformation before and after gentle compression, displaying relative strain as a color map. Results depend heavily on the amount and uniformity of applied pressure.

A technique using microbubble agents and low mechanical index settings to show blood flow within lesions in real time. The bubbles remain intravascular, making it useful for characterizing liver nodules and other masses.

The assembly of piezoelectric elements that generates and receives ultrasound. Curved arrays widen the field of view for abdominal work, while linear arrays give high resolution for superficial structures.

A water-based medium applied between the transducer and skin to eliminate air and allow sound to enter tissue. Failure to apply it adequately causes poor image quality and signal dropout.