Acoustics and Medical Sonography Notes

Acoustics

• Acoustics is a branch of physics that studies sound and sound waves.
• It involves the generation, propagation, and reception of sound waves.
• Ultrasound refers to sound frequencies beyond the range of normal human hearing, which is 20 Hz to 20 kHz.
• Therefore, ultrasound frequencies are >20 kHz.
• Sound results from mechanical energy, creating alternating compression and rarefaction in a conducting medium as it travels as a wave.
• A wave is the propagation of energy that vibrates back and forth at a steady rate.
• Diagnostic ultrasound uses short sound pulses at frequencies of 1 to 20 MHz to examine soft-tissue anatomic structures.
• Ultrasound waves are created by a vibrating crystal within a ceramic probe.
• Waves travel through the tissue and are partly reflected at each tissue interface.
• As the transducer element vibrates, waves undergo compression and rarefaction, pulling molecules apart.

Applications of Sound Frequency

• Infrasound: 0-25 Hz, produced by electromagnetic vibrators, used for vibration analysis of structures.
• Audible: 20 Hz-20 kHz, produced by electromagnetic vibrators and musical instruments, used for communications and signaling.
• Ultrasound:
  • 20-100 kHz: Produced by air whistles and electric devices, used in biology and sonar.
  • 100 kHz-1 MHz: Produced by electric devices, used for flaw detection and biology.
  • 1-20 MHz: Produced by electric devices, used for diagnostic ultrasound.

Acoustics in Medical Sonography

• A piezoelectric vibrating source (ceramic element) within the transducer vibrates in response to an electrical signal.
• The vibrating motion of the ceramic element in the transducer causes particles in the surrounding tissue to vibrate.
• The transducer converts electrical energy into mechanical energy.
• As a sound beam is directed into the body, reflection, absorption, and scatter cause the returning signal to be weaker than the initial impulse.
• The velocity of propagation is constant for a given tissue and is not affected by the frequency or wavelength of the pulse.
• In soft tissues, the assumed average propagation velocity is 1540 m/sec.
• Acoustic impedance is the measure of a material's resistance to the propagation of sound.
• Stiffness and density of the medium determine how fast sound waves will travel through it.
• The more closely packed the molecules, the faster the speed of sound.
• The velocity of sound differs greatly among air, bone, and soft tissue but varies only a little from one soft tissue to another.
• Sound waves travel slowly through gas (air), at intermediate speed through liquids, and quickly through solids (metal).

Characteristic Acoustic Impedance and Velocity of Ultrasound

• Acoustic impedance is measured in (g/cm^3
  ormal{x} cm/sec
  ormal{x} 10^5).
• Material and their respective Acoustic Impedance and Velocity:
  • Air: 0.0001, 331 (Air-filled structures impede sound transmission.)
  • Fat: 1.38, 1450
  • Water: 1.50, 1430
  • Blood: 1.61, 1570
  • Kidney: 1.62, 1560
  • Liver: 1.65, 1550
  • Muscle: 1.70, 1580
  • Skull: 7.80, 4080 (Sound is attenuated through most bony structures.)
• Small differences among fat, blood, and organ tissues observed on an ultrasound image may be better delineated with high-frequency transducers that improve resolution.