Acoustic Impedance
The resistance a medium presents to sound transmission. Acoustic impedance is calculated as density multiplied by propagation speed: Z = ρ × c.
Impedance differences at tissue boundaries determine the amount of reflected sound.
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Review the ultrasound physics terms that appear repeatedly across the ARDMS Sonography Principles and Instrumentation examination.
Ultrasound Physics Glossary
Definitions, equations, and board-focused reminders for the core ultrasound physics concepts tested on the ARDMS SPI exam.
Axial Resolution
The ability to distinguish two structures positioned along the path of the sound beam. Axial resolution equals one-half of the spatial pulse length: SPL / 2.
Shorter spatial pulse length produces better axial resolution.
Lateral Resolution
The ability to distinguish two structures positioned side by side, perpendicular to the sound beam.
Lateral resolution is best at the beam focus, where beam width is smallest.
Duty Factor
The fraction of time an ultrasound system is actively transmitting sound. Duty factor equals pulse duration divided by pulse repetition period.
Increasing imaging depth increases listening time and generally decreases duty factor.
Nyquist Limit
The highest Doppler frequency shift that can be measured without aliasing. The Nyquist limit equals one-half of the pulse repetition frequency: PRF / 2.
Increase PRF or shift the baseline to help reduce aliasing.
Attenuation
The weakening of an ultrasound beam as it travels through tissue because of absorption, reflection, scattering, and refraction.
Higher frequency sound attenuates more rapidly and has less penetration.
Cavitation
The interaction of ultrasound with microscopic gas bodies that may expand, contract, or collapse in response to acoustic pressure.
Cavitation risk is associated with the Mechanical Index.
Snell's Law
The relationship that describes how the direction of a sound wave changes when it crosses an interface between media with different propagation speeds.
Refraction requires oblique incidence and different propagation speeds.
Turn definitions into exam-ready recall.
Use the free practice test to apply these concepts under question pressure, then continue with the full SonoPrep preparation system.