The measurable properties of a sound wave in tissue: frequency, wavelength, period, and amplitude.
wavelength = speed / frequency
Acoustic Parameters (Reference)SPI Domain 1
Two states compared
3 MHz in soft tissue wavelength = 0.51 mm period = 0.33 µs
10 MHz in soft tissue wavelength = 0.15 mm period = 0.10 µs
wavelength (mm) = speed (m/s) / frequency (Hz) × 1000
Key Concept: Higher frequency = shorter wavelength = better resolution but less penetration depth. In soft tissue, the speed of sound is always 1540 m/s. The transducer controls frequency; wavelength follows.
Acoustic Impedance and ReflectionSPI Domain 1 | Physical Principles
Acoustic Impedance
A tissue's resistance to sound propagation. At boundaries, impedance mismatch determines how much sound reflects.
R = ((Z2 - Z1) / (Z2 + Z1))²
Acoustic Impedance (Reference)SPI Domain 1
Two boundary examples
Soft Tissue to Bone
Strong reflection. 43% reflected.
Liver to Kidney
Minimal reflection. 0.01% reflected.
R = ((Z2 - Z1) / (Z2 + Z1))² | Z = density × speed
Key Concept: The greater the impedance mismatch between two tissues, the stronger the reflection at their boundary. This is why coupling gel eliminates the air interface, and why bone and gas create acoustic shadows.
Doppler Effect and Angle DependenceSPI Domain 4 | Doppler Principles
Doppler Effect
Frequency shifts in reflected ultrasound measure blood flow. The beam-to-flow angle is the critical variable.
fd = (2 × f0 × v × cosθ) / c
Doppler Effect (Reference)SPI Domain 4
Three angle examples
30°
cos(30) = 0.87 Strong signal
60° (max recommended)
cos(60) = 0.50 Half signal
90°
cos(90) = 0 No signal
fd = (2 × f0 × v × cosθ) / c
Key Concept: Doppler shift depends on the cosine of the angle between the beam and flow direction. At 90 degrees, cosine equals zero, and there is no Doppler signal at all. Always keep the Doppler angle at or below 60 degrees.