Chapter 6 – Interaction of Sound and Media (Vocabulary)

A comprehensive set of vocabulary flashcards summarizing key terms and relationships from Chapter 6, covering attenuation, reflection, scattering, refraction, impedance, and related ultrasound physics concepts.

Attenuation

Decrease in sound wave power, amplitude, and intensity as it propagates through tissue; measured in decibels (dB).

Path Length

Distance the sound has traveled; directly related to attenuation.

Frequency-Attenuation Relationship

Higher frequency = more attenuation; lower frequency = less attenuation.

Absorption

Conversion of acoustic energy into heat; largest contributor to attenuation in tissue.

Scattering

Redirection of sound in many directions when the beam strikes small or rough structures.

Reflection

Redirection of part of the sound beam back toward the source when it hits an interface.

Specular Reflection

Mirror-like reflection from a large, smooth boundary at 90° incidence; angle-dependent and produces strong echoes.

Diffuse (Backscatter) Reflection

Disorganized reflection from a rough interface similar in size to wavelength; not angle-dependent and weaker than specular.

Nonspecular Reflection

Another term for diffuse or backscatter reflection.

Rayleigh Scattering

Omnidirectional scattering produced by structures much smaller than the wavelength (e.g., red blood cells); intensity ∝ frequency⁴.

Acoustic Speckle

Granular image texture caused by constructive and destructive interference of scattered echoes.

Decibel (dB)

Logarithmic unit expressing relative change in power or intensity; negative dB indicate attenuation.

Attenuation Coefficient

Rate of sound attenuation per centimeter; ≈ 0.5 dB/cm/MHz in soft tissue.

Total Attenuation

Path length (cm) × attenuation coefficient (dB/cm).

Half-Value Layer (Half Intensity Depth)

Tissue thickness that reduces beam intensity by 50% (-3 dB); inversely proportional to frequency.

Impedance (Z)

Resistance to sound in a medium; Z = density × propagation speed; units = Rayls.

Impedance Mismatch

Difference in acoustic impedances of two media; greater mismatch yields stronger reflection.

Incident Beam

Portion of the ultrasound beam that leaves the transducer and enters the body.

Reflected Beam

Portion of the incident beam returned to the transducer due to reflection.

Transmitted Beam

Portion of the incident beam that continues beyond an interface.

Normal (Perpendicular) Incidence

Sound strikes the boundary at 90° (orthogonal) angle; required for predictable reflection formulas.

Oblique Incidence

Sound strikes a boundary at any angle other than 90°; angle of incidence ≠ 90°.

Critical Angle (2D Imaging)

Perpendicular insonation angle that yields total reflection from a smooth surface.

Intensity Reflection Coefficient (IRC)

Percentage of incident intensity reflected at a boundary; IRC = [(Z₂−Z₁)/(Z₂+Z₁)]².

Intensity Transmission Coefficient (ITC)

Percentage of incident intensity transmitted across a boundary; ITC = 1 − IRC.

Conservation of Energy (Interface)

Incident intensity = Reflected intensity + Transmitted intensity (IRC + ITC = 100%).

Refraction

Change in direction (bending) of the transmitted beam at an interface with oblique incidence and different propagation speeds.

Snell’s Law

Relates incident and transmitted angles to propagation speeds: sin θ₁ / c₁ = sin θ₂ / c₂.

Propagation Speed–Angle Rule

If c₂ > c₁, transmitted angle > incident angle; if c₂ < c₁, transmitted angle < incident angle.

Transmission With No Refraction

Occurs when propagation speeds are equal or incidence is 90° (regardless of impedance).

Bone Attenuation

High attenuation mainly by absorption; causes acoustic shadowing.

Air Attenuation

Extremely high attenuation due to absorption; nearly total reflection at tissue-air interface.

Lung Attenuation

Greater than soft tissue; predominantly due to scattering from air bubbles.

Attenuation Order (Lowest→Highest)

Water < Blood < Soft tissue < Bone < Air.

Diffraction

Spreading of the beam as it passes small openings or edges; increases grating lobes with beam steering.

Huygen’s Principle

Each point on a transducer face acts as a small wavelet source; interference of wavelets forms the beam’s hourglass shape.

Range Equation

Distance to reflector = (propagation speed × time of flight) / 2; 13 µs rule for soft tissue gives 1 cm depth per 13 µs round-trip.

13 Microsecond Rule

In soft tissue, each 13 µs of round-trip travel time corresponds to 1 cm depth.

dB/cm/MHz Factor

Soft-tissue attenuation coefficient (≈ 0.5 dB/cm/MHz).

Speckle Reduction

Post-processing algorithm to smooth granular image appearance caused by interference patterns.

Zone of Sensitivity

Area where transmitted and reflected pulses overlap; crucial for echo detection.

Mirror (Specular) Dependence

Specular echoes vanish when beam strikes interface at non-perpendicular angle.

Acoustic Shadowing

Signal dropout behind highly attenuating or reflecting structures (e.g., stones, bone).

Posterior Enhancement

Increased echo brightness behind weakly attenuating structures (e.g., cysts) due to relative lack of attenuation.

Acute Angle

Angle < 90°.

Obtuse Angle

Angle > 90° but < 180°.

Orthogonal

At right angles (90°) to a surface; synonymous with normal or perpendicular incidence.

Raleigh Scatterer

Reflector whose dimensions are much smaller than wavelength (e.g., RBC).

Acoustic Gel Function

Reduces impedance mismatch between transducer face and skin, minimizing reflection at the skin-probe interface.

Scattering Intensity-Frequency Rule

Scatter increases as operating frequency increases.

Attenuation in Water

Very low; ultrasound propagates with minimal energy loss.

Percent Reflection at Tissue-Air

Approximately 99% of beam intensity is reflected.

Percent Reflection at Tissue-Bone

Roughly 50% of intensity reflected at soft-tissue–bone interface.

Attenuation Coefficient Formula (Soft Tissue)

α (dB/cm) = frequency (MHz) / 2.

Half-Value Layer Dependence

Determined by medium and frequency; independent of path length.

Scatter Definition

Redirection of the beam in several directions away from the transducer by small, rough, or heterogeneous structures.

Beam Intensity Reduction

Attenuation lowers beam intensity as it travels; −3 dB equals 50% intensity loss.

Red Blood Cells

Classical Rayleigh scatterers producing Doppler signals.

Duty Factor (Unrelated to Attenuation)

Fraction of time the transducer emits sound; does not affect attenuation.

Output Power (Imaging)

Controls emitted beam intensity; reflection amount at interfaces depends on impedance, not power.

Density

Mass per unit volume of a medium; helps determine impedance and propagation speed.

Stiffness

Resistance to compression; with density, determines propagation speed and impedance.

Acoustic Window

Body location or structure allowing improved sound penetration due to lower attenuation (e.g., full bladder for pelvic scan).

Beam Uniformity Coefficient

Quantifies intensity distribution in the beam; unrelated to attenuation per centimeter.

Snell’s Law Conditions

Needs oblique incidence and velocity mismatch; no refraction with equal speeds or 90° incidence.

Refraction-Free Interfaces

Boundaries where propagation speeds are equal or beam is perpendicular; transmitted beam continues straight.

Grating Lobes

Secondary off-axis beams promoted by diffraction; degrade image quality.