Ultrasound Physics Practice Flashcards: Sound Beams, Resolution, Display Modes, 2D Imaging, & Real-Time

Comprehensive 300 vocabulary flashcards covering sound beams, axial & lateral resolution, A/B/M display modes, array transducers, and real-time imaging parameters.

PZT (Lead Zirconate Titanate)

Synthetic ceramic material used as the active element or crystal in ultrasound transducers that converts sound into electricity and vice versa.

Transducer Aperture

The diameter or physical width of the active PZT element as the sound beam exits the transducer face.

Unfocused Continuous Wave Sound Beam

An ultrasound beam produced by a single disc-shaped crystal operating without focusing, which changes width as it travels through tissue.

Beam Width at Transducer

The diameter of the sound beam as it leaves the transducer, which is exactly equal to the active element diameter or transducer aperture.

Focus (Focal Point)

The location where the ultrasound beam reaches its minimum diameter and where lateral resolution is best.

Beam Width at Focus

For a continuous wave disc-shaped crystal, the width of the beam at the focus is exactly one-half the width of the active element.

Near Zone (Fresnel Zone)

The region or zone of the sound beam extending from the transducer face to the focus, where the beam gradually converges.

Near Zone Beam Behavior

The characteristic narrowing or convergence of the sound beam diameter as sound travels from the transducer toward the focus.

Focal Length (Focal Depth / Near Zone Length)

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

Far Zone (Fraunhofer Zone)

The region or zone of the sound beam located deeper than the focus, extending beyond the near zone.

Far Zone Beam Behavior

The spreading or divergence of the sound beam as sound travels deeper than the focus into the far field.

Beam Width at Two Near Zone Lengths

The point in the far zone at a depth of two near zone lengths where the beam diameter expands back to the exact size of the active element aperture.

Beam Width Deeper Than Two Near Zone Lengths

The region of the sound beam beyond two near zone lengths where the beam diameter becomes wider than the transducer aperture.

Focal Zone

The region surrounding the focus where the sound beam is relatively narrow, producing the most accurate images.

Focal Zone Composition

The focal zone includes a portion of the near zone immediately before the focus and a portion of the far zone immediately after the focus.

Beam Dimension Example at Transducer Face

In Fig. 9.4, a disc-shaped crystal with a $12\,mm$ aperture creates a beam with an initial diameter of $12\,mm$ at the transducer exit.

Beam Dimension Example at Focal Depth

In Fig. 9.4, at a focal depth of $8\,cm$, a $12\,mm$ diameter crystal produces a focal beam width of $6\,mm$.

Beam Dimension Example at Two Near Zone Lengths

In Fig. 9.4, at a depth of $16\,cm$ ($2$ near zone lengths), the sound beam expands back to a diameter of $12\,mm$.

Focal Zone Depth Boundaries in Fig. 9.4

In Fig. 9.4, with an $8\,cm$ focal depth and a $4\,cm$ focal zone length, the focal zone begins at $6\,cm$ and ends at $10\,cm$.

Focal Zone Length

The total axial depth span around the focus over which the sound beam remains relatively narrow.

Phased Array Focal Depth

An adjustable focus ultrasound system that allows the sonographer to electronically change the location of the focus.

Fixed Focal Depth Determinants

In non-adjustable single element transducers, focal depth is determined strictly by transducer crystal diameter and ultrasound frequency.

Relationship: Transducer Diameter and Focal Depth

Transducer crystal diameter and focal depth are directly related; a larger crystal diameter creates a deeper focus.

Relationship: Frequency and Focal Depth

Ultrasound frequency and focal depth are directly related; higher frequency sound produces a deeper focus.

Shallow Focus Combination

A shallow focal depth is created by a transducer with a small crystal diameter and a low ultrasound frequency.

Deep Focus Combination

A deep focal depth is created by a transducer with a large crystal diameter and a high ultrasound frequency.

Intensity at Deep Focus vs Shallow Focus

Beams with a deep focus have lower intensity at the focus compared to beams designed with a shallow focus.

Sound Beam Divergence

The spreading out or expansion of the ultrasound beam in the deep far zone beyond the focus.

Far Zone Beam Divergence Determinants

The degree of divergence in the far zone is determined combinedly by active element diameter and sound frequency.

Relationship: Transducer Diameter and Beam Divergence

Transducer diameter and far-field beam divergence are inversely related; smaller diameter crystals diverge more in the far field.

Relationship: Frequency and Beam Divergence

Sound frequency and far-field beam divergence are inversely related; lower frequency sound diverges more in the deep far zone.

Less Beam Divergence Conditions

Less divergence in the far field occurs with a larger aperture or larger diameter crystal and a higher ultrasound frequency.

More Beam Divergence Conditions

More divergence in the far field occurs with a smaller aperture or smaller diameter crystal and a lower ultrasound frequency.

Far-Field Lateral Resolution and Divergence

Less divergence produces a narrower beam in the far field, improving far-field lateral resolution.

Degraded Far-Field Lateral Resolution

Substantial beam divergence creates a wider far-field sound beam, degrading lateral resolution in deep tissues.

Spherical Waves

V-shaped sound waves created when a tiny sound source, approximately the size of a sound wavelength, emits ultrasound.

Synonyms for Spherical Waves

Spherical waves are also called diffraction patterns or Huygens' wavelets.

Huygens' Source

Each tiny, individual particle on the surface of a large active element that acts as an independent sound emitter.

Huygens' Wavelet

A small V-shaped sound wave generated by an individual Huygens' source on a transducer crystal.

Huygens' Principle

The principle stating that the overall hourglass shape of an ultrasound beam results from constructive and destructive interference of numerous tiny V-shaped Huygens' wavelets.

Constructive Interference in Huygens' Principle

The overlap of wavelets where wave peaks align to reinforce sound energy, defining the main hourglass sound beam.

Destructive Interference in Huygens' Principle

The overlap of wavelets where wave peaks cancel out troughs, eliminating sound energy outside the main hourglass beam.

Hourglass Sound Beam Formation

The continuous interference pattern produced by multiple small particles across a disc-shaped crystal face.

Focus Summary Feature

The location of minimum beam width where beam diameter equals half the aperture and lateral resolution is best.

Near Zone Summary Feature

The region between the transducer face and focus where the sound beam gradually converges toward the focus.

Far Zone Summary Feature

The region deeper than the focus where the sound beam diverges and spreads out wider.

Focal Zone Summary Feature

The zone surrounding the focus that contains portions of both near and far fields where the beam is narrowest.

Large Crystal Diameter Impact on Divergence

Larger diameter crystals result in less beam divergence in the deep zone, preserving image resolution.

High Frequency Impact on Divergence

Higher frequency sound beams diverge less in the far field than lower frequency beams.

Direct Relationships in Beam Geometry

Both transducer diameter and frequency are directly related to focal depth.

Inverse Relationships in Beam Geometry

Both transducer diameter and frequency are inversely related to far-field beam divergence.

End of the Near Zone

An alternate anatomical name for the focus or focal point of a sound beam.

Beginning of the Far Zone

An alternate anatomical name for the location where the focus ends and divergence begins.

Middle of the Focal Zone

The exact central point within the focal zone corresponding to the beam's minimum diameter.

Continuous Wave Beam Source

A sound beam formed continuously by a single disc-shaped active element.

Beam Convergence Zone

The Fresnel zone, where beam width narrows from transducer diameter to half transducer diameter.

Beam Divergence Zone

The Fraunhofer zone, where beam width expands from half transducer diameter back to aperture size and beyond.

Aperture Equivalent Distance

At a depth of two near zone lengths, beam diameter equals the initial transducer aperture.

Expanded Beam Depth

At depths greater than two near zone lengths, the beam diameter exceeds the initial transducer aperture.

Huygens' Wavelet Wave Shape

A characteristic V-shaped wave produced by a tiny point source.

Axial Resolution

The ability of an ultrasound system to display two structures that are close together along or parallel to the sound beam's main axis.

Axial Resolution Orientation

Front-to-back or parallel to the ultrasound sound beam path.

LARRD

Mnemonic for synonyms of axial resolution: Longitudinal, Axial, Range, Radial, Depth.

Longitudinal Resolution

Synonym for axial resolution representing front-to-back reflector discrimination.

Range Resolution

Synonym for axial resolution describing accuracy along the reflector path depth.

Radial Resolution

Synonym for axial resolution referring to resolution along the ray axis.

Depth Resolution

Synonym for axial resolution emphasizing depth-wise structure separation.

Axial Resolution Unit

Distance, expressed in units such as millimeters ($mm$) or centimeters ($cm$).

Axial Resolution Value Quality

Lower numerical values represent shorter pulse lengths and superior axial resolution.

Typical Axial Resolution Values

In clinical ultrasound, axial resolution typical values range from $0.1\,mm$ to $1.0\,mm$.

Spatial Pulse Length (SPL) and Axial Resolution

Axial resolution is directly determined by spatial pulse length; shorter pulse lengths produce smaller numerical values and better resolution.

Pulse Duration and Axial Resolution

Short pulse durations correspond to short spatial pulse lengths, yielding superior axial resolution.

Sonographer Adjustability of Axial Resolution

Axial resolution is not directly adjustable by the sonographer because spatial pulse length is fixed per transducer.

Formula: Axial Resolution from SPL

$$\text{Axial Resolution (mm)} = \frac{\text{SPL (mm)}}{2}$$

Formula: Axial Resolution from Wavelength and Cycles

$$\text{Axial Resolution (mm)} = \frac{\lambda \times \text{\# of cycles}}{2}$$

Formula: Axial Resolution in Soft Tissue

$$\text{Axial Resolution (mm)} = \frac{0.77 \times \text{\# cycles in pulse}}{\text{Frequency (MHz)}}$$

Less Ringing

The creation of short pulses with few cycles per pulse, achieved through damping material.

Damping (Backing) Material Role

Material bonded to the transducer active element that restricts crystal ringing, shortening pulse duration and SPL to improve axial resolution.

Cycles per Pulse in Imaging Transducers

Ultrasound imaging transducers typically create pulses containing $2\text{ to }3$ cycles.

Frequency Relation to Axial Resolution

Higher frequency ultrasound produces shorter wavelengths, resulting in shorter pulse lengths and superior axial resolution.

Axial Resolution Depth Independence

Axial resolution remains constant and does not change with imaging depth.

Optimal Axial Resolution Conditions

Axial resolution is optimal using transducers with the highest frequency and fewest cycles per pulse.

Worst Axial Resolution Conditions

Axial resolution is worst when using low frequency transducers with a large number of cycles per pulse.

Reflector Separation for Axial Resolution

In Fig. 10.2, two closely spaced front-to-back reflectors are resolved as distinct echoes by a short red pulse but merged by a longer green pulse.

Pulse Cycle Comparison in Fig. 10.3

A 1-cycle pulse is shorter than a 2-cycle pulse of the same frequency due to reduced ringing.

Lateral Resolution

The minimum distance that two structures positioned side-by-side or perpendicular to the sound beam can be apart and still produce two distinct echoes.

Lateral Resolution Orientation

Side-by-side or perpendicular to the ultrasound sound beam path.

LATA

Mnemonic for synonyms of lateral resolution: Lateral, Angular, Transverse, Azimuthal.

Angular Resolution

Synonym for lateral resolution referring to beam angle spreading across reflectors.

Transverse Resolution

Synonym for lateral resolution emphasizing cross-beam structure separation.

Azimuthal Resolution

Synonym for lateral resolution describing side-to-side reflector discrimination.

Lateral Resolution Unit

Distance, expressed in millimeters ($mm$) or centimeters ($cm$).

Formula: Lateral Resolution

$$\text{Lateral Resolution (mm)} = \text{Beam Diameter (mm)}$$

Determinant of Lateral Resolution

Lateral resolution is determined directly by the width of the sound beam; narrower beams produce better lateral resolution.

Location of Best Lateral Resolution

Lateral resolution is best at the focus, where beam diameter is narrowest.

Lateral Resolution Behavior with Depth

Lateral resolution changes continuously with depth because beam diameter varies with depth.

Overlapping Reflectors in Lateral Resolution

When two side-by-side structures are closer together than the beam diameter, they appear as a single wide reflection on the image.

High Frequency Effect on Lateral Resolution

Higher frequencies create narrower beams in the far field due to decreased divergence, improving far-field lateral resolution.

Clinical Comparison: Axial vs Lateral Resolution

Axial resolution is superior to lateral resolution in clinical imaging because ultrasound pulses are shorter than they are wide.

High Frequency Resolution Benefits

High frequency sound improves axial resolution everywhere in the image and improves lateral resolution only in the far field.