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 (or size of the crystal/array) 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 (front-to-back); 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 = 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 (around 300°C or 572°F). 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 (thin center, thick edges) 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 (perpendicular to the beam); 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, forming a rectangular image.

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; used in early mechanical transducers.

Multidimensional Arrays (1.5D & 2D) – Arrays that allow electronic focusing and steering in both lateral and elevational planes, enabling 3D and 4D imaging.

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, calculated as:

NZL ≈ D² × f / 6

Non-Imaging Transducer – A probe used only for Doppler (e.g., PEDOF) 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 by applying microsecond or nanosecond time delays to each element.

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 = better axial resolution.

Quarter Wavelength Matching Layer – Matching layer thickness of ¼ wavelength to cause 180° out-of-phase reflections, minimizing interference and maximizing transmission into tissue.

Receive Focusing – The process of focusing echoes as they return to the transducer, often dynamically controlled.

Resolution – The ability to clearly distinguish two separate structures; includes axial, lateral, and elevationalcomponents.

Sector Scan – A fan-shaped image format created by steering beams from a single point of origin (typical of phased arrays).

Segmental Linear Array – A linear array that fires groups of elements at a time instead of one, producing a longer near field and better lateral resolution.

Sequential Linear Array – A linear array that activates elements one by one; creates parallel scan lines but limited depth and poor far-field resolution.

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 (M-mode, ophthalmic, intravascular).

Spatial Pulse Length (SPL) – The physical distance a pulse occupies; equals the number of cycles × wavelength.

Step-Down Segmental Array – A linear array where each new firing sequence overlaps slightly with the previous group, improving temporal and lateral resolution.

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 (more focal zones = lower 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 (for the outer “wings”) and sequencing (for the central area) to widen the field of view.

Wave Interference – The combination of two or more waves; can be constructive (add together) or destructive (cancel out).

Zone (Focal Zone) – Region where the beam is narrowest and resolution is best, typically at or near the focal point.