Ultrasound Sound Beam Anatomy

30 vocabulary flashcards covering key terminology and relationships related to ultrasound sound-beam shape, zones, focal depth, divergence, and factors that influence beam behavior.

Sound Beam

The column of ultrasound energy emitted by a transducer, whose width and shape vary with distance.

Beam Width Rule

Narrower beams produce higher-quality ultrasound images.

Focus

Location where the sound beam reaches its minimum diameter (narrowest point).

Near Zone (Fresnel Zone)

Region between the transducer face and the focus where the beam converges; length equals focal depth.

Focal Depth (Focal Length, Near Zone Length)

Distance from the transducer face to the focus.

Far Zone (Fraunhofer Zone)

Region beyond the focus where the beam diverges.

Focal Zone

Area surrounding the focus where the beam remains relatively narrow; half lies in the near zone and half in the far zone.

Beam Diameter at Transducer

Equal to the physical diameter (aperture) of the transducer crystal.

Beam Diameter at Focus

One-half the diameter of the transducer aperture.

Beam Diameter at Two Near-Zone Lengths

Approximately equal to the transducer aperture diameter.

Natural Focus

Intrinsic narrowing of an unfocused transducer’s beam produced by its geometry and frequency.

Beam Divergence

Spread of the sound beam in the far field; wider divergence lowers lateral resolution.

Aperture (Element Diameter)

Physical width of the active transducer element; larger apertures yield deeper focus and less divergence.

Frequency Effect on Focus

Higher frequency ultrasound produces a deeper natural focus.

Frequency Effect on Divergence

Higher frequency beams diverge less in the far zone.

Near Zone Length Relationship

NZL is proportional to aperture squared and directly proportional to frequency.

Lateral Resolution

Ability to distinguish two structures that lie side-by-side; best where the beam is narrowest.

Dynamic Focusing

Electronic technique that adjusts focus during reception or transmission to reduce divergence and improve resolution.

Acoustic Lens

External or built-in lens placed on the transducer face to converge the beam and lengthen the near zone.

Reducing Beam Divergence

Achieved by using a higher frequency, larger diameter transducer, dynamic focusing, or an acoustic lens.

Lower Frequency Transducer

Provides greater penetration and a shorter focal depth but produces more beam divergence.

High Frequency, Large Diameter Probe

Combination that offers the deepest natural focus and least beam divergence.

Huygens’ Principle

Concept that individual point sources on the crystal create V-shaped waves that sum to form the overall beam.

Unfocused Beam Divergence

Natural widening of the beam occurring in the Fraunhofer (far) zone when no external focusing is applied.

Intensity Uniformity Myth

Beam intensity is not uniform across its cross-section; greatest variations occur near the focus and diminish in the far field.

Near Zone Length Design Goal

Should be long enough to encompass all regions of interest but not excessively long to avoid unnecessary focal depth.

Large Diameter – Less Divergence

Wider elements create narrower far-field beams, improving lateral resolution.

Small Diameter – Shallow Focus

Reducing aperture shortens the near zone length and increases beam divergence.

Fraunhofer Zone Synonym

Alternate term for the far zone of the ultrasound beam.

Fresnel Zone Synonym

Alternate term for the near zone of the ultrasound beam.