chapter 6 - 7

Vocabulary flashcards covering decibels, attenuation mechanisms, reflection/refraction principles, and the range equation from Chapters 6–7.

Attenuation

Decrease in intensity, power, and amplitude of a sound wave as it propagates; depends on distance and frequency, measured in negative dB.

Decibel (dB)

Logarithmic, relative unit that compares two intensities, powers, or amplitudes; expresses how much a signal has increased or decreased.

Logarithm

Mathematical value indicating how many 10s are multiplied to obtain a number; e.g., log 1000 = 3 because 10×10×10 = 1000.

Positive Decibels

dB values that indicate signal growth; +3 dB doubles intensity, +10 dB increases it ten-fold, +20 dB one-hundred-fold, etc.

Negative Decibels

dB values that indicate signal loss; −3 dB halves intensity, −10 dB reduces it to one-tenth, −20 dB to one-hundredth, etc.

Absorption

Conversion of ultrasound energy into heat within tissue; major contributor to attenuation and increases with frequency.

Scattering

Redirection of sound in many directions from a rough or small interface; produces image texture and rises with frequency.

Specular Reflection

Mirror-like reflection from a smooth, large boundary; returns in one direction and is angle-dependent (best at 90°).

Diffuse Reflection (Backscatter)

Reflection from an irregular surface that radiates sound in many directions; weaker than specular but less angle-dependent.

Rayleigh Scattering

Uniform scattering by structures much smaller than wavelength (e.g., RBCs); intensity ∝ frequency⁴.

Attenuation Coefficient

Amount of attenuation per centimeter; in soft tissue ≈ 0.5 dB/cm/MHz, so higher frequency ⇒ larger coefficient.

Total Attenuation

Product of path length and attenuation coefficient; expressed in decibels (dB).

Acoustic Impedance (Z)

Property of a medium equal to density × propagation speed; measured in Rayls and determines reflection strength.

Normal Incidence

Wave strikes boundary at 90° (perpendicular); reflection depends solely on impedance mismatch.

Oblique Incidence

Wave strikes boundary at any angle other than 90°; reflection and transmission become unpredictable.

Incident Intensity

Sound intensity just before hitting a boundary.

Reflected Intensity

Portion of incident intensity that returns toward the source after striking a boundary.

Transmitted Intensity

Portion of incident intensity that continues forward past the boundary.

Intensity Reflection Coefficient (IRC)

Percentage (or fraction) of incident intensity that is reflected; increases with larger impedance differences.

Intensity Transmission Coefficient (ITC)

Percentage (or fraction) of incident intensity that is transmitted; equals 1 − IRC for normal incidence.

Refraction

Change in beam direction at a boundary due to oblique incidence and differing propagation speeds between media.

Snell’s Law

Relationship governing refraction: (sin θₜ / sin θᵢ) = (c₂ / c₁), where θ = angle and c = speed in each medium.

Range Equation

Distance = (go-return time × speed) / 2; determines reflector depth from pulse travel time.

Time-of-Flight (Go-Return Time)

Elapsed time for a pulse to travel to a reflector and back; directly proportional to depth.

13-Microsecond Rule

In soft tissue, every 13 µs of go-return time corresponds to 1 cm of one-way depth (2 cm total path).

Pulse Repetition Period (PRP)

Time from the start of one pulse to the start of the next; PRP (µs) = imaging depth (cm) × 13 µs/cm.

Pulse Repetition Frequency (PRF)

Number of pulses emitted per second; PRF (Hz) = 77,000 cm/s ÷ imaging depth (cm); inversely related to depth.