Understanding Ultrasound Physics - Vocabulary Flashcards

A comprehensive vocabulary review set covering foundational concepts, parameters, transducers, resolution, display modes, hemodynamics, Doppler, artifacts, and safety principles from Sidney K. Edelman's Understanding Ultrasound Physics (4th Edition).

Acoustic Propagation Properties

The effects of the medium upon the sound wave traveling through it.

Biologic Effects

The effects of the sound wave upon the biologic tissue through which it passes.

Pressure

An acoustic variable defined as the concentration of force in an area, measured in pascals ($\text{Pa}$).

Density

An acoustic variable defined as the concentration of mass in a volume, measured in $\text{kg/cm}^3$.

Distance

An acoustic variable measuring particle motion, expressed in units of length such as $\text{cm}$, feet, or miles.

Transverse Wave

A wave in which particles move in a direction perpendicular (at right angles) to the direction that the wave propagates.

Longitudinal Wave

A wave in which particles move back and forth in the same direction that the wave propagates.

In-Phase Waves

A pair of waves whose peaks (maximum values) and troughs (minimum values) occur at the same time and at the same location.

Out-of-Phase Waves

A pair of waves whose peaks and troughs occur at different times and locations.

Interference

The combination of multiple sound beams arriving at an identical location at the exact same time to form a single wave.

Constructive Interference

The combination of a pair of in-phase waves that results in the formation of a single wave of greater amplitude than either of its components.

Destructive Interference

The combination of a pair of out-of-phase waves that results in the formation of a single wave of lesser amplitude than at least one of its components.

Period

The time it takes a wave to vibrate a single cycle, or the time from the start of one cycle to the start of the next cycle.

Frequency

The number of particular events or cycles that occur in a specific duration of time (in diagnostic ultrasound, cycles per second).

Infrasound

Sound waves with frequencies less than $20\,\text{Hz}$, which are below the threshold of human hearing.

Audible Sound

Sound waves with frequencies between $20\,\text{Hz}$ and $20,000\,\text{Hz}$, which can be heard by humans.

Ultrasound

Sound waves with frequencies higher than $20,000\,\text{Hz}$ ($20\,\text{kHz}$), which humans cannot hear.

Amplitude

The difference between the maximum value and the average or undisturbed value of an acoustic variable, describing the bigness of a wave.

Peak-to-Peak Amplitude

The difference between the maximum and minimum values of an acoustic variable, equal to twice the amplitude.

Power

The rate of energy transfer or the rate at which work is performed, expressed in units of watts.

Intensity

The concentration of energy in a sound beam, calculated by dividing the beam's power by its cross-sectional area.

Wavelength

The distance or length of one complete cycle of a sound wave.

Propagation Speed

The rate at which a sound wave travels through a medium.

Stiffness

The ability of an object to resist compression (also referred to as bulk modulus).

Pulse Duration

The actual time from the start of an ultrasound pulse to the end of that pulse, representing a single transmit or 'on' time.

Spatial Pulse Length

The distance that an ultrasound pulse occupies in space from the start to the end of a pulse.

Pulse Repetition Period (PRP)

The time from the start of one pulse to the start of the next pulse, including one pulse duration plus one listening time.

Pulse Repetition Frequency (PRF)

The number of pulses that an ultrasound system transmits into the body each second.

Duty Factor

The percentage or fraction of time that an ultrasound system transmits a pulse.

Attenuation

The decrease in intensity, power, and amplitude as sound travels through a medium.

Specular Reflection

Reflection created when a sound wave strikes a smooth boundary and is reflected in only one direction in an organized manner.

Diffuse Reflection

Reflection created when a sound wave reflects off an irregular surface and radiates in more than one direction (also called backscatter).

Scattering

The random redirection of sound in many directions when the tissue interface is small (equal to or less than the wavelength).

Rayleigh Scattering

A special form of scattering that occurs when structure dimensions are much smaller than the wavelength, redirecting sound equally in all directions.

Attenuation Coefficient

The number of decibels of attenuation that occurs when sound travels one centimeter ($\text{dB/cm}$).

Half-Value Layer Thickness

The distance sound travels in a tissue that reduces the intensity of sound to one-half its original value.

Impedance

The acoustic resistance to sound traveling in a medium, calculated as density multiplied by propagation speed ($Z = \text{density} \times \text{speed}$).

Normal Incidence

Occurs when the incident sound beam strikes a boundary at exactly $90^\circ$ (synonymous with perpendicular, orthogonal, and right angle).

Oblique Incidence

Occurs when the incident sound beam strikes a boundary at any angle other than $90^\circ$.

Refraction

A change in direction of wave propagation when traveling from one medium to another ('transmission with a bend').

Time-of-Flight

The elapsed time from pulse creation to pulse reception (also called go-return time).

13-Microsecond Rule

A rule stating that for every $13\,\mu\text{s}$ of go-return time in soft tissue, the object creating the reflection is $1\,\text{cm}$ deeper in the body.

Piezoelectric Effect

The property of certain materials to create a voltage when mechanically deformed or when pressure is applied to them.

Matching Layer

A transducer component in front of the PZT that increases the efficiency of sound transfer between the active element and skin by having an intermediate impedance.

Backing Material

A damping element bonded to the back of the active element that restricts PZT deformation and reduces ringing, creating short duration pulses.

Bandwidth

The range of frequencies in an ultrasound pulse, calculated as the difference between the highest and lowest frequencies.

Quality Factor

A unitless number inversely related to bandwidth, calculated as the main frequency divided by the bandwidth.

Focus

The location where an ultrasound sound beam is at its narrowest width (also called focal point).

Huygens' Principle

The principle stating that a large active element may be thought of as millions of tiny, distinct sound sources creating V-shaped wavelets that interfere to form an hourglass-shaped beam.

Axial Resolution

The ability of a system to display two structures that are very close together when parallel to the sound beam's main axis.

Lateral Resolution

The ability to distinctly identify two structures that are very close together when side by side or perpendicular to the sound beam's main axis.

A-mode

An amplitude display mode appearing as a series of upward spikes, where the x-axis represents reflector depth and the y-axis represents reflection amplitude.

B-mode

A brightness display mode appearing as a line of dots of varying brightness, where the x-axis represents depth and dot brightness represents reflection amplitude.

M-mode

A motion display mode appearing as horizontal wavy lines representing changing reflector depth (y-axis) over time (x-axis).

Linear Phased Array

A compact array transducer that steers and focuses sound beams electronically using phasing (phase delays).

Dynamic Receive Focusing

A technique where the receiver introduces variable time delays to returning signals to focus the reflected sound at many depths automatically.

Annular Phased Array

An array transducer with concentric ring-shaped elements that utilizes mechanical steering and multi-focus capability.

Linear Sequential Array

An array transducer with rectangular elements arranged in a line that creates rectangular images by firing small groups of elements simultaneously.

Elevational Resolution

Resolution measured in a direction perpendicular to the imaging plane or above-to-below the imaging plane (also called slice thickness resolution).

Apodization

A process that reduces side and grating lobes by exciting inner array elements with stronger electrical spikes and outer elements with weaker spikes.

Frame Rate

The system's ability to create numerous frames each second, determined by sound speed and imaging depth.

Temporal Resolution

The ability to precisely position moving structures from instant to instant, determined by frame rate.

Pulser

An ultrasound system component that creates electrical signals to excite PZT crystals and generate sound beams during transmission.

Beam Former

A device in array systems that coordinates firing delay patterns, apodization, dynamic receive focusing, and dynamic aperture.

Amplification

The first receiver function (receiver gain) where each electronic signal returning from the transducer is made larger by an equal amount.

Compensation

The second receiver function (TGC) that corrects for attenuation to create an image of uniform brightness from top to bottom.

Compression

The third receiver function that reduces the dynamic range of signal levels to keep them within the accuracy range of electronics and human vision.

Demodulation

The fourth receiver function that changes electrical signals into a displayable form using rectification and smoothing.

Reject

The fifth receiver function (threshold or suppression) that allows control over whether low-level gray scale noise appears on the display.

ALARA Principle

The principle ('As Low As Reasonably Achievable') stating that output power or receiver gain adjustments should minimize patient ultrasound exposure.

Dynamic Range

A method of reporting the extent to which a signal can vary and still be accurately measured, expressed in decibels ($\text{dB}$).

Fundamental Frequency

The frequency of sound created by the transducer and transmitted into the body.

Harmonic Frequency

Twice the fundamental frequency (second harmonic), created from nonlinear sound propagation or contrast bubble interaction.

Tissue Harmonics

Harmonic signals created during transmission in tissue as sound travels faster in compressions and slower in rarefactions.

Contrast Agents

Gas bubbles encapsulated in a shell injected or ingested to create strong reflections that delineate blood vessels and heart chambers.

Laminar Flow

A flow pattern characterized by parallel streamlines traveling at individual speeds without mixing.

Turbulent Flow

Chaotic blood flow characterized by streamlines moving in many directions and speeds, featuring eddy currents or vortices.

Reynolds Number

A calculated number that predicts whether blood flow is laminar ($< 1500$) or turbulent ($> 2000$).

Bernoulli's Principle

The principle stating that with steady flow, the sum of all forms of energy is constant, causing pressure energy to drop where kinetic energy and velocity increase.

Hydrostatic Pressure

Pressure related to the weight of blood pressing on a vessel measured relative to the level of the heart.

Doppler Shift

The change or shift in frequency resulting from relative motion between a sound source and receiver.

Aliasing

An artifact in pulsed Doppler where very high velocities above the Nyquist limit are displayed as flowing in the opposite direction.

Nyquist Limit

The highest Doppler frequency or velocity that can be measured without aliasing, equal to one-half of the pulse repetition frequency ($\text{PRF}/2$).

Fast Fourier Transform

A digital technique used to process pulsed and continuous wave Doppler signals, displaying all individual component velocities.

Autocorrelation

A digital technique used to analyze color flow Doppler signals rapidly.

Quality Assurance

The routine, periodic evaluation of an ultrasound system to guarantee optimal image quality.

Hydrophone

A small transducer or microprobe used to measure acoustic pressure and sound beam characteristics at specific locations.

Informed Consent

The process by which patients are educated about the essentials of a medical procedure to make a voluntary, knowledgeable health decision.

Standard Precautions

Guidelines based on treating all patients as potentially infectious to minimize health care worker exposure to bloodborne pathogens.