Ultrasound Physics Vocabulary

A comprehensive vocabulary flashcard set covering foundational ultrasound physics, wave parameters, transducers, resolution, image processing, artifacts, hemodynamics, Doppler, and safety.

Ultrasound

Sound waves with frequencies above the upper limit of human hearing, typically above $20{,}000\,Hz$ ($20\,kHz$).

Frequency

The number of cycles of a wave that occur in one second, measured in hertz ($Hz$); in diagnostic imaging, it typically ranges from $2 - 15\,MHz$.

Period

The time required to complete one full wave cycle; calculated as $\text{period} = \frac{1}{\text{frequency}}$.

Wavelength

The distance of one complete wave cycle; in soft tissue, it is calculated as $\text{wavelength} = \frac{1.54\,mm}{\text{frequency}}$.

Amplitude

The maximum displacement or variation of an acoustic wave from its equilibrium position.

Power

The rate at which energy is transferred, measured in watts ($W$); directly proportional to intensity and to amplitude squared ($\text{power} \propto \text{amplitude}^2$).

Intensity

The concentration of energy in a sound beam, calculated as $\text{intensity} = \frac{\text{power}}{\text{area}}$ and measured in units of $W/cm^2$.

Propagation Speed

The speed at which a sound wave travels through a medium; in soft tissues, ultrasound waves travel at approximately $1{,}540\,m/s$.

Pulse Duration (PD)

The time it takes for one pulse to occur from beginning to end; calculated as $\text{PD} = \#\,\text{cycles} \times \text{period} = \frac{\#\,\text{cycles}}{\text{frequency}}$.

Spatial Pulse Length (SPL)

The physical length of space over which one pulse occurs; calculated as $\text{SPL} = \#\,\text{cycles} \times \text{wavelength}$.

Pulse Repetition Frequency (PRF)

The number of ultrasound pulses emitted by the transducer in one second; inversely related to PRP ($\text{PRF} = \frac{1}{\text{PRP}}$).

Pulse Repetition Period (PRP)

The time from the start of one pulse to the start of the next pulse; inversely related to PRF ($\text{PRP} = \frac{1}{\text{PRF}}$).

Duty Factor

The percentage or fraction of time that the ultrasound system actively transmits sound waves, calculated as $\text{Duty Factor (\%)} = \left(\frac{\text{Pulse Duration}}{\text{Pulse Repetition Period}}\right) \times 100$.

Near Field (Fresnel Zone)

The portion of the ultrasound beam extending from the transducer face to the focal point.

Far Field (Fraunhofer Zone)

The portion of the ultrasound beam starting at the focus and extending deeper into tissue.

Focal Length

The distance from the transducer face to the focal point/focus.

Focus

The point or location along the sound beam where the beam width is most narrow and concentrated.

A-Mode (Amplitude Mode)

A one-dimensional display mode where echo amplitude is represented on the y-axis and reflector depth on the x-axis.

B-Mode (Brightness Mode)

A two-dimensional display mode where variations in echo brightness represent reflector depth on the x-axis and signal amplitude on the z-axis.

M-Mode (Motion Mode)

A display mode presenting a one-dimensional slice of tissue over time, with time on the x-axis and reflector depth on the y-axis.

SPTA Intensity

Spatial Peak Temporal Average intensity; the crucial parameter used in ultrasound bioeffects analysis to ensure tissue heating remains safe.

Echogenicity

The inherent ability of biological tissue to generate echoes or reflections in response to an ultrasound beam.

Hyperechoic

Describing a structure that appears brighter on an ultrasound image due to increased reflection of sound waves relative to surrounding tissues.

Hypoechoic

Describing a structure that appears darker than surrounding tissues due to fewer reflected echoes.

Isoechoic

Describing structures that exhibit the same level of brightness as adjacent or surrounding tissue.

Anechoic

Describing a completely echo-free structure that appears black on an ultrasound image.

Homogeneous Echogenicity

An imaging texture characterized by a uniform echo pattern throughout a structure.

Heterogeneous Echogenicity

An imaging texture characterized by an irregular or non-uniform echo pattern indicating varied tissue composition.

Attenuation

The progressive weakening of sound waves as they propagate through tissues, measured in decibels ($dB$).

Specular Reflection

A reflection occurring when sound strikes a smooth, large surface at a consistent angle, reflecting the beam in a single direction.

Diffuse Reflection

A reflection occurring when sound strikes a rough or irregular surface, reflecting sound waves in multiple directions.

Rayleigh Scattering

Scattering that occurs when tissue structure size is much smaller than the wavelength, directing sound equally in all directions; proportional to frequency to the fourth power ($\text{frequency}^4$).

Absorption

The process by which acoustic energy is converted into thermal energy (heat) as ultrasound passes through tissue.

Refraction

The bending of a sound beam as it crosses a boundary between two media with different propagation speeds at oblique incidence.

Acoustic Impedance (Z)

The acoustic resistance offered by a medium to sound wave propagation, defined as $\text{Impedance (Z)} = \text{density} \times \text{propagation speed}$ and expressed in rayls.

Amplification (Receiver Gain)

The first receiver function, which uniformly amplifies weak electrical signals to adjust overall image brightness.

Time Gain Compensation (TGC)

The second receiver function, which increases amplification progressively at deeper tissue levels to ensure uniform image brightness despite attenuation.

Compression

The third receiver function, which reduces the dynamic range of received signals to adjust grayscale contrast.

Demodulation

The fourth receiver function, which extracts information from returning echoes to format them for monitor display; non-adjustable by the sonographer.

Rejection (Reject)

The fifth receiver function, which discards low-level signals below a chosen threshold to eliminate image noise.

Output Power

The strength or intensity of the sound beam emitted into patient tissues by the transducer.

ALARA Principle

'As Low As Reasonably Achievable'; the governing safety mandate to minimize ultrasound exposure to patients while obtaining diagnostic images.

Dynamic Range

The ratio of the largest to smallest signal intensity that an ultrasound system can accurately display, expressed in decibels ($dB$).

PACS

Picture Archiving and Communication System; a computer network designed for the storage, retrieval, distribution, and display of medical images.

DICOM

Digital Imaging and Communications in Medicine; a standardized networking protocol for transmitting, storing, and sharing medical imaging data across systems.

Pre-Processing

The manipulation or processing of ultrasound image data before it is stored in system memory.

Post-Processing

The manipulation or refining of ultrasound image data performed after frames are frozen or stored in memory.

Write Magnification

A pre-processing zoom technique performed in real-time that rescans the selected area to improve line density and spatial resolution.

Read Magnification

A post-processing zoom technique applied to a frozen image that enlarges pixel sizes without altering spatial resolution.

Coded Excitation

An advanced signal technique using long, complex coded pulses to improve penetration, contrast resolution, and signal-to-noise ratio.

Edge Enhancement

An image processing algorithm that sharpens and accentuates boundaries between adjacent tissue structures.

Spatial Compounding

An ultrasound technique combining frames acquired from multiple steering angles into a single image to reduce speckle and shadowing.

Frequency Compounding

An ultrasound technique combining images created from different frequency sub-bands to reduce noise and speckle artifacts.

Temporal Compounding (Persistence)

An image processing technique that averages multiple sequential frames over time to produce a smoother image and reduce noise.

Fill-In Interpolation

A computational technique estimating signal values between known scan lines to construct missing data and enhance spatial resolution.

Piezoelectric Effect

The property of active materials (PZT) to create an electrical charge when mechanically stressed and to deform when voltage is applied.

Matching Layer

A component attached to the PZT crystal face ($\frac{1}{4}$ wavelength thick) that reduces impedance mismatch between crystal and body tissue.

Backing Material (Damping Element)

A component placed behind the PZT crystal that absorbs backward sound waves, shortens pulse duration, and improves axial resolution.

Continuous Wave (CW) Transducer

A transducer with two separate elements continuously transmitting and receiving sound, detecting high velocities without range specificity.

Pulsed Wave (PW) Transducer

A transducer with a single element alternating between transmitting and receiving pulses, providing range specificity.

Mechanical Transducer

A transducer using a motor-driven single crystal to sweep the sound beam, generating a sector image with fixed focus.

Linear Phased Array Transducer

A small-footprint probe using electronic time delays (phasing) across a linear group of elements to steer and focus a sector-shaped beam.

Linear Sequential Array Transducer

A large-footprint probe with elements arranged in a line that fire in sequential groups, producing a rectangular image.

Curvilinear Array Transducer

A probe with elements arranged along a curved arc that fire in groups, generating a blunted sector image with a wide field of view.

Vector Array Transducer

A transducer combining linear sequential and linear phased array technologies to produce a trapezoidal image shape.

Axial Resolution

The ability to distinguish two distinct reflectors lying parallel to the ultrasound beam along the same line of sight (Synonyms: LARRD).

Lateral Resolution

The ability to distinguish two distinct reflectors lying side-by-side perpendicular to the ultrasound beam (Synonyms: LATA); best at the focal point.

Temporal Resolution

The system's ability to precisely display moving structures over time; determined primarily by frame rate.

Frame Rate

The number of ultrasound image frames created and displayed per second, inversely related to imaging depth and line density.

Elevational Resolution

Resolution in the slice-thickness dimension perpendicular to the 2D scanning plane.

Harmonic Imaging

An imaging technique created by non-linear propagation that constructs images using reflections at twice the fundamental transmitted frequency ($f_{\text{harmonic}} = 2 \times f_{\text{fundamental}}$).

Mechanical Index (MI)

A parameter quantifying the potential for bioeffects and cavitation, calculated as $\text{MI} = \frac{\text{peak rarefaction pressure}}{\sqrt{\text{frequency}}}$.

Acoustic Shadowing

An artifact presenting as an anechoic or hypoechoic region extending distally beyond a strongly attenuating structure (e.g., gallstone, bone).

Acoustic Enhancement

An artifact presenting as a bright hyperechoic region distal to a weakly attenuating fluid structure (e.g., cyst, urinary bladder).

Reverberation Artifact

An artifact presenting as multiple, equally spaced, parallel hyperechoic lines caused by sound bouncing repeatedly between two strong reflectors.

Comet Tail Artifact

A form of reverberation presenting as a solid, bright, vertical hyperechoic line extending distally from a highly reflective object.

Apodization

The process of varying electrical voltage excitation across array elements to reduce side lobes and grating lobes.

Hemodynamics

The study of physical forces, pressures, and principles involved in blood circulation within the cardiovascular system.

Phasic Flow

Blood flow that accelerates and decelerates in response to respiratory changes, commonly observed in venous circulation.

Pulsatile Flow

Blood flow that accelerates and decelerates with cardiac contractions during systole and diastole, typical of arterial circulation.

Laminar Flow

A smooth, streamlined blood flow pattern where fluid layers slide parallel to one another with maximum speed at the center; associated with a Reynolds number $< 1500$.

Turbulent Flow

A chaotic, disorganized blood flow pattern with swirling eddies and cross-currents; associated with a Reynolds number $> 2000$.

Reynolds Number

A dimensionless index predicting whether fluid flow will be laminar ($< 1500$) or turbulent ($> 2000$).

Stenosis

An abnormal narrowing within a blood vessel that causes localized velocity elevation, pressure drop, and post-stenotic turbulence.

Bernoulli's Principle

A fluid dynamics principle stating that an increase in fluid velocity occurs simultaneously with a decrease in static pressure.

Doppler Shift

The difference between the transmitted and reflected ultrasound frequency caused by moving red blood cells, expressed as $\text{Doppler Shift} = \frac{2 \times v \times f_0 \times \cos(\theta)}{c}$.

BART Rule

A color Doppler mnemonic where color maps indicate flow direction relative to the transducer: 'Blue Away, Red Towards'.

Power Doppler

A color Doppler mode mapping signal energy amplitude rather than velocity or direction, offering high sensitivity to low flow without aliasing.

Spectral Doppler

A technique presenting blood flow velocities over time as a graphical spectral waveform showing peak systolic velocity (PSV) and end-diastolic velocity (EDV).

Resistive Index (RI)

A quantitative Doppler ratio measuring vascular bed resistance, calculated as $\text{RI} = \frac{\text{PSV} - \text{EDV}}{\text{PSV}}$.

Pulsatility Index (PI)

A quantitative Doppler ratio measuring vascular bed resistance over the cardiac cycle, calculated as $\text{PI} = \frac{\text{PSV} - \text{EDV}}{\text{Mean Velocity}}$.

Wall Filter

An adjustable filter that eliminates low-frequency, high-amplitude Doppler signals caused by moving tissue or vessel wall clutter.

Sample Gate

A virtual area positioned on a 2D image in pulsed wave Doppler specifying the exact location where flow velocities are measured.

Nyquist Limit

The maximum measurable Doppler frequency shift without aliasing in pulsed wave Doppler, calculated as $\text{Nyquist Limit} = \frac{\text{PRF}}{2}$.

Aliasing

A pulsed Doppler artifact occurring when flow velocities exceed the Nyquist limit, causing the spectrum or color to wrap around the display in the wrong direction.

Ghosting Artifact

A color Doppler artifact where low-frequency tissue motion causes color signals to bleed outside true vessel boundaries.

Crosstalk Artifact

A spectral Doppler mirror-image artifact presenting identical flow waveforms symmetrically above and below the baseline.

Tissue-Mimicking Phantom

A quality assurance phantom with soft tissue acoustic properties (attenuation, speed $1{,}540\,m/s$, scattering) used to test grayscale performance.

Dead Zone

The region closest to the transducer face where no useful imaging reflections can be produced due to transducer ringing time.

Cavitation

A non-thermal bioeffect mechanism involving acoustic wave interaction with microscopic dissolved gas bubbles in biological tissue.