Fetal Echocardiography & Ultrasound Physics Review

Flashcards covering ultrasound physics basics, wave behavior, and pulsed-wave parameters from the lecture notes.

What is sound?

A mechanical longitudinal wave that carries energy, requires a medium, and cannot travel through a vacuum; produced by the vibration of a moving object.

What are the three acoustic variables and their common units?

Pressure (Pascals, Pa), Density (kg/cm³), Distance/particle motion (cm or m).

What are the seven acoustic parameters?

Period, Frequency, Amplitude, Power, Intensity, Speed, and Wavelength.

What is a transverse wave?

A wave in which particles move perpendicular to the direction of wave propagation.

What is a longitudinal wave?

A wave in which particles move parallel to the direction of wave propagation.

What does it mean for waves to be in phase?

Peaks (and troughs) occur at the same time; leads to constructive interference.

What does it mean for waves to be out of phase?

Peaks do not coincide; can produce destructive interference.

What is the relationship between period and frequency?

Period = 1 / frequency; frequency = 1 / period; they are reciprocals.

What is wavelength?

The distance of one complete cycle; λ = speed / frequency. In soft tissue, λ ≈ 1.54 mm / frequency (MHz).

What is propagation speed?

The speed at which sound travels through a medium; determined by the medium's density and stiffness (elastic properties); frequency does not determine speed.

How do density and stiffness affect propagation speed?

Speed increases with higher stiffness and decreases with higher density.

What is the general rule for speed among gas, liquid, and solid?

Speed of sound is slowest in gas, faster in liquid, fastest in solid; density and stiffness determine speed.

What is amplitude?

The strength of a sound beam; the difference between the maximum and minimum values of an acoustic variable; decreases with propagation; can be expressed in dB.

What is power in ultrasound?

The rate at which energy is transferred; units are watts; related to amplitude (power ∝ amplitude²).

What is intensity?

The concentration of energy in a sound beam; calculated as power divided by beam cross-sectional area; units: W/cm²; decreases with propagation and is key for bioeffects.

What is SPL (Spatial Pulse Length)?

The distance that a pulse occupies in space; SPL = number of cycles in the pulse × wavelength; shorter SPL improves axial resolution.

What is Pulse Duration (PD)?

The time from the start to the end of a single pulse; units in seconds; determined by the source; not changed by imaging depth.

What is Pulse Repetition Period (PRP)?

The time from the start of one pulse to the start of the next; PRP = 1 / PRF; determined by the source and imaging depth.

What is Pulse Repetition Frequency (PRF)?

The number of pulses transmitted per second; PRF = 1 / PRP; adjustable by depth; PRF × PRP = 1.

What is Duty Factor (DF)?

The fraction of time the system transmits a pulse; DF(%) = (PD / PRP) × 100; DF increases with shorter PRP (shallower depth) and decreases with deeper imaging.

What is frequency in ultrasound?

The number of acoustic cycles per second; units Hz; typical diagnostic frequencies are in the MHz range; frequency is determined by the source and is inversely related to period.

How is wavelength related to speed and frequency?

Wavelength λ = speed / frequency; in soft tissue speed ≈ 1540 m/s; λ (mm) ≈ 1.54 / f(MHz).

What is the typical frequency range for diagnostic ultrasound and its trade-off?

Typical range ~2–15 MHz (commonly 2–10 MHz); higher frequency improves axial resolution but reduces penetration.

What are common speeds of sound in different media?

Air ~330 m/s; Lung ~300–1200 m/s; Fat ~1450 m/s; Soft tissue ~1540 m/s; Bone ~2000–4000 m/s.

What are the main pulsed-wave parameters?

Pulse Duration (PD), Spatial Pulse Length (SPL), Pulse Repetition Period (PRP), Pulse Repetition Frequency (PRF), Duty Factor (DF).

What is PD relationship to cycles and period?

PD = (# cycles in the pulse) × (period of each cycle); provides the actual on-time; not adjustable by the sonographer; determined by source.

How does imaging depth affect PRP/PRF/DF?

Increasing depth increases PRP and decreases PRF; DF decreases with deeper imaging and increases with shallow imaging.

What is the difference between acoustic variables and acoustic parameters?

Acoustic variables describe instantaneous properties (pressure, density, distance). Acoustic parameters describe features of the wave (period, frequency, amplitude, power, intensity, speed, wavelength).