1. What Is Sound?
Ultrasound is a mechanical wave, so it needs a medium and cannot travel through a vacuum. In conventional pulse-echo imaging, tissue particles oscillate parallel to propagation: a longitudinal pressure wave. The particles move locally; they do not travel from the probe to the organ. Solids can also support other wave modes. Sound propagation.
Air limits direct visualization of aerated lung, but lung ultrasound uses pleural findings and artifacts. Consolidation reaching the pleural surface and pleural fluid offer additional imaging targets. International lung-ultrasound consensus.
📡 2. How Ultrasound Uses Sound
The transducer sends a pulse and receives echoes. The scanner estimates depth from the round-trip travel time: depth = assumed speed × time / 2. It conventionally assumes 1540 m/s in soft tissue. Echo amplitude contributes to B-mode brightness; receiver gain and processing also affect the display. ACEP physics overview.
Worked example: an echo returning after 100 microseconds is displayed about 7.7 cm deep: 1540 × 0.0001 / 2 = 0.077 m. This calculation assumes a direct out-and-back path and the stated sound speed.
📈 3. Acoustic Variables: What Changes in the Medium
As a pressure wave passes, local pressure and density rise and fall, while particles move around their resting positions. Pressure is force per area, measured in pascals (Pa); density is mass per volume, measured in kg/m³. Particle displacement is a distance. NIST units.
🔢 4. Acoustic Parameters: How We Describe a Wave
| Parameter | Meaning | Unit or relationship |
|---|---|---|
| Frequency, f | Cycles per second | Hz; 1 MHz = 1 million Hz |
| Period, T | Time per cycle | Seconds; T = 1/f |
| Wavelength, λ | Distance per cycle | Metres; λ = c/f |
| Propagation speed, c | Wave travel distance per time | m/s |
| Pressure amplitude | Magnitude of pressure variation | Pa |
| Power | Acoustic energy per time | W |
| Intensity | Power per area perpendicular to propagation | W/m² or W/cm² |
Decibels express a level or ratio relative to a reference, rather than a pressure unit. Definitions: AIUM acoustic terms; intensity.
🧬 5. Frequency & Wavelength: The Resolution–Depth Tradeoff
Higher frequencies generally provide finer detail but attenuate more with depth. A high-frequency linear probe suits superficial vessels and soft tissues; a lower-frequency curvilinear or phased-array probe often suits deeper abdominal targets. Select the highest frequency that adequately shows the target. Probe selection and image optimization.
Frequency alone does not determine resolution. Axial detail depends on pulse length; lateral detail depends on beam width. At an assumed 1540 m/s, wavelength is about 0.31 mm at 5 MHz and 0.15 mm at 10 MHz. These are wavelengths, not guaranteed image-resolution values. AIUM axial-resolution terminology.
🧪 6. Sound Speed in Different Media
Speed depends on the medium’s elastic properties and density. For a fluid model, c = √(bulk modulus / density): greater stiffness increases speed only when density is held fixed, and greater density decreases speed only when stiffness is held fixed. Density alone cannot rank tissues.
At a stationary boundary in the basic linear model, frequency stays unchanged while speed and wavelength can change. Increasing the selected frequency in the same medium mainly shortens wavelength; it does not make the wave travel faster. Medium, speed and wavelength.
Do not confuse speed with penetration. Bone and gas can obstruct deeper views through attenuation or strong reflection even though their sound speeds differ. Changing the acoustic window may help; increasing receiver gain cannot recover echoes that never reach the probe.
🌊 7. Interference: When Waves Meet
Overlapping waves combine according to phase and amplitude. In-phase waves reinforce; opposite phases reduce the result, with complete cancellation requiring equal amplitudes. Speckle is the granular pattern produced by interference among echoes from unresolved scatterers. It is not a direct map of individual tissue structures. Interference; speckle.
✅ Key Takeaways (Clinical & Exam-Ready):
Match the probe and frequency to target depth, then optimize focus and receiver gain. Gain changes received signals; acoustic output changes transmitted energy. Use the lowest output and shortest examination time that yield the information needed, consistent with ALARA.
