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KEY PHYSICS CONCEPTS—KNOW THE LANGUAGE

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Concept Details Why It Matters
Frequency

# of cycles/second—measured in Hertz. A megahertz (MHz) = 106 Hz. Human hearing is in the range 20-20,000 Hz.

Probe Selection: low frequency/long wavelength, provide better penetration with less resolution, good for deep structures. High frequency/short wavelength, provide better resolution, less penetration.

Resolution

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Schematic showing ultrasound beam edge-on

Lateral resolution is the smallest distance that can be distinguished with a given probe and machine at any given depth. This explains why focal depth and the number of crystals in the transducer are important. Axial resolution allows differentiation parallel to the beam, and is dependent on frequency: higher frequencies give higher axial resolution.

Pulse Echo Principle

Ultrasound waves leave the probe, strike an object, and are reflected back. Depth on the screen is measured by the time for this echo to return to the transducer.

Basics of machine function. Also important to understand several artifacts, including reverberation and shadowing.

Impedance or Density/Stiffness

When ultrasound encounters material of different impedance (from liquid to gas, or a change in tissue stiffness), it reflects.

Explains reflection of ultrasound by gas and solids, and the artifact of posterior acoustic enhancement.

Echogenicity

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Hyperechoic: brighter echoes than surrounding structures

Isoechoic: equal in echoes to surrounding structures

Hypoechoic: darker echoes than surrounding structures

Anechoic: completely black

Heterogeneous: nonuniform texture

Homogeneous: uniform texture

Important Artifacts

Acoustic Shadow

When sound cannot pass through a dense/reflective object and all signal is either returned to the probe or absorbed, the absence of ultrasound posteriorly creates the appearance of a black shadow. This image shows gallstones (arrows) with shadowing (between arrowheads).

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Video 02-01: Acoustic Shadowing

Fanning through the gallbladder in the transverse view, several hyperechoic stones can be seen casting dense acoustic shadows.

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Posterior Acoustic Enhancement

Occurs when sound travels through a material with low attenuation (like urine, bile, effusions), resulting in objects behind the material appearing brighter than other tissues of the same echodensity at the same depth. The image shows that tissues between arrowheads are brighter than similar retro-orbital tissue.

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Video 02-02: Posterior Acoustic Enhancement

This video quickly fans through the ocular anatomy and demonstrates the soft tissue deep to the globe is notably brighter than that on either side of the eyeball due to posterior acoustic enhancement.

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Reverberation

Sound bouncing back and forth between two highly reflective parallel surfaces that are perpendicular to the direction of the ultrasound beam results in the machine displaying multiple versions of the two surfaces at increasing depth, as in the case of the needle and the reverberations (arrowheads in this image).

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