Definitions

Acoustic Impedance

The resistance sound encounters as it travels through a medium; mismatch causes reflection at boundaries.

Aperture

The active surface area of the transducer that determines beam width and focus.

Array Transducer

A transducer made up of multiple small crystal elements that can be activated individually or in groups to create, steer, or focus sound beams.

Axial Resolution

The ability to distinguish two structures that lie along the path of the sound beam; equals ½ the spatial pulse length (SPL).

Backing Material (Damping Material)

The material behind the crystal that absorbs stray sound waves and shortens pulse duration to improve axial resolution.

Bandwidth

The range of frequencies emitted by the transducer; wider bandwidth means shorter pulses and better resolution.

Beam Width

The width of the ultrasound beam; directly affects lateral resolution.

Brightness Mode (B-Mode)

Imaging mode that converts returning echo amplitudes into brightness dots on the screen.

Composite Transducer

A piezoelectric element made of PZT and a polymer, resulting in lower acoustic impedance, lighter weight, wider bandwidth, and improved sensitivity.

Continuous Wave (CW)

A mode where the crystal is constantly transmitting and receiving sound; used in Doppler, not imaging.

Converse Piezoelectric Effect

When electrical voltage causes the crystal to change shape, producing sound waves.

Curie Temperature

The temperature at which the crystal is polarized; if heated above this later, the crystal loses its piezoelectric properties.

Curvilinear (Convex) Array

A curved linear array that produces a fan-shaped image with scan lines diverging as depth increases.

Delay Lines (Time Delays)

Electronic timing differences used to steer or focus the ultrasound beam in phased array systems.

Depth of Field

The range of depths over which the beam remains in good focus.

Destructive Interference

When sound waves out of phase cancel each other, reducing amplitude.

Dynamic Receive Focusing

Automatic focusing during echo reception using variable time delays; adjusts focus for echoes returning from different depths in real time.

Electronic Focusing

Using electronic time delays to focus the beam without mechanical movement.

Electronic Steering (Phasing)

Steering of the beam direction by adjusting timing of excitation pulses across array elements.

Elevational Resolution (Slice Thickness)

The ability to distinguish structures in the plane perpendicular to the image plane; determined by beam thickness in the elevational direction.

Element

A single crystal within a transducer array that acts as an independent sound source.

Far Field (Fraunhofer Zone)

The region beyond the focus where the beam diverges and intensity decreases.

Focal Point

The location along the beam where intensity is maximum and beam width is minimum.

Focal Length (Focal Depth)

The distance from the transducer face to the focal point.

Focal Zone

The region around the focal point where beam intensity is within 3 dB of its maximum.

Focusing Techniques

Methods to narrow the beam and improve lateral resolution: lens, curved crystal, mirror, or electronic focusing.

Half-Wave Crystal Thickness

Crystal thickness set at half the wavelength of sound to reduce internal destructive interference.

Hanafy Lens

A crystal designed with variable thickness to improve elevational focusing.

Huygens’ Principle

Each point on a wavefront acts as a source of secondary wavelets; their interference forms the overall beam pattern.

Imaging Transducer

A transducer that produces a 2D image using pulsed waves.

Intensity

The power of the ultrasound beam per unit area; highest at the focal point.

Lateral Resolution

The ability to distinguish two structures side-by-side; equals beam width.

Lens (Acoustic Lens)

A physical focusing device placed in front of the crystal to converge sound waves and improve focus.

Linear Array

An array of crystals arranged in a straight line; scan lines are produced sequentially and parallel to each other.

Linear Phased Array

Combines sequencing and phasing to produce high-resolution images, commonly used in vascular imaging.

Line Density

The number of scan lines per image frame; affects lateral resolution and frame rate.

Matching Layer

Material between the crystal and patient’s skin that reduces acoustic impedance mismatch; ideally ¼ wavelength thick.

Mechanical Steering

Beam movement achieved by physically oscillating or rotating a crystal.

Multidimensional Arrays (1.5D & 2D)

Arrays that allow electronic focusing and steering in both lateral and elevational planes.

Near Field (Fresnel Zone)

The region between the transducer and focus where the beam converges and intensity fluctuates.

Near Zone Length (NZL)

The distance from the transducer face to the natural focus.

Non-Imaging Transducer

A probe used only for Doppler that transmits and receives sound but does not produce an image.

PEDOF Transducer (Pulsed Echo Doppler of Flow)

A blind (non-imaging) transducer used for continuous-wave Doppler; extremely sensitive to blood flow.

Phased Array

A transducer that electronically steers and focuses the beam.

Piezoelectric Effect

The property of certain crystals to generate an electric voltage when pressure is applied (and vice versa).

PZT (Lead Zirconate Titanate)

The most common piezoelectric ceramic used in medical transducers; high efficiency and strong electromechanical properties.

Pulse Wave (PW)

Ultrasound mode where sound is transmitted in short bursts, allowing for depth selection and image formation.

Pulse Repetition Frequency (PRF)

The number of sound pulses emitted per second.

Pulse Length (Spatial Pulse Length, SPL)

The physical length of a pulse; shorter SPL means better axial resolution.

Quarter Wavelength Matching Layer

Matching layer thickness of ¼ wavelength to minimize interference and maximize transmission into tissue.

Receive Focusing

The process of focusing echoes as they return to the transducer.

Resolution

The ability to clearly distinguish two separate structures.

Sector Scan

A fan-shaped image format created by steering beams from a single point of origin.

Segmental Linear Array

A linear array that fires groups of elements at a time to produce a longer near field.

Sequential Linear Array

A linear array that activates elements one by one; creates parallel scan lines.

Simultaneous Steering and Focusing

Combining delay profiles to both steer and focus the beam at the same time.

Single Element Transducer

A transducer with one crystal that transmits and receives; used in older or specialized applications.

Spatial Pulse Length (SPL)

The physical distance a pulse occupies; equals the number of cycles multiplied by the wavelength.

Step-Down Segmental Array

A linear array where each new firing sequence overlaps slightly with the previous group.

Steering by Phasing

Beam steering by applying precise time delays across the transducer elements.

Temporal Resolution

The ability to accurately display moving structures in time; affected by frame rate.

Transducer

A device that converts electrical energy to mechanical (sound) energy and vice versa.

Transmit Focusing

Beam focusing during sound transmission; operator-controlled and can slow frame rate if multiple focal zones are used.

Trapezoidal Scanning

Image format created by combining phasing and sequencing to widen the field of view.

Wave Interference

The combination of two or more waves; can be constructive or destructive.

Zone (Focal Zone)

Region where the beam is narrowest and resolution is best, typically at or near the focal point.