sound chapter 2 flashcards

Sound Waves and Their Properties

• Sound DMS (Diagnostic Medical Sonography)

  • Generated by transducers.

  • Sound pulses travel through biological tissue media.

  • Reflections of sound waves return to the transducer to form images.

• Energy Carrier:

  • Sound waves carry energy that includes:

  • Heat

  • Sound

  • Magnetic energy

  • Light

• Nature of Sound:

  • Sound is defined as a mechanical wave.

  • Particles in the medium move and vibrate back and forth from a fixed position.

  • Sound movement characteristics:

  • Compressed: Particles squeezed together.

  • Rarefied: Particles stretched apart.

  • Sound travels in a straight line.

• Acoustic Propagation Properties:

  • Speed of Sound ($c$): The rate at which electrical pulses are converted into acoustic energy and travel through a medium.

  • Determined by the medium's stiffness and density.

  • Average speed in soft tissue is approximately $1540$ m/s.

  • Stiffer media (e.g., bone) typically have faster speeds; denser media (e.g., fat) typically have slower speeds.

  • Frequency ($f$): The number of cycles per second ($Hz$).

  • Inversely related to wavelength.

  • Determines image axial resolution and penetration depth.

  • Wavelength ($\lambda$): The spatial length of one complete cycle of a wave ($mm$).

  • Formula: $\lambda = c/f$

  • Period ($T$): The time it takes for one complete cycle to occur ($\mu s$).

  • Formula: $T = 1/f$

  • Amplitude: The maximum variation of an acoustic variable (e.g., pressure) from its average value.

  • Related to the strength or intensity of the sound wave.

  • Power ($P$): The rate at which work is done or energy is transferred by the sound wave ($Watts$).

  • Proportional to the amplitude squared ($P \propto A^2$).

  • Intensity ($I$): The concentration of power in a sound beam ($W/cm^2$).

  • Formula: $I = P/Area$

  • Directly related to the heating of tissue.

  • Attenuation: The progressive weakening of the sound beam as it travels through a medium.

  • Caused by absorption (conversion of sound energy to heat), reflection, and scattering.

  • Higher frequency waves attenuate more quickly, leading to decreased penetration.

• Biologic Effects (Bioeffects) of Ultrasound:

  • The potential effects of ultrasound on living tissue, primarily due to absorption and mechanical interaction.

  • Thermal Mechanism:

  • Occurs due to the absorption of sound energy, which is converted into heat.

  • Can lead to localized tissue heating, especially at high intensities or long exposure times.

  • The Thermal Index (TI) is a commonly used indicator of potential thermal bioeffects.

  • Mechanical Mechanism (Cavitation):

  • Involves the interaction of the sound wave with microscopic gas bubbles within the tissue.

  • Stable Cavitation: Bubbles oscillate in size but do not burst, potentially causing microstreaming (localized fluid movement) and fluid stresses.

  • Transient (Inertial) Cavitation: Bubbles expand rapidly and then collapse violently, generating shock waves, very high localized temperatures, and free radicals. This mechanism is generally considered to pose a higher risk of tissue damage.

  • The Mechanical Index (MI) is a common indicator of potential cavitation-related bioeffects.

• Types of waves:

  • Longitudinal Waves:

  • Particles move in the same direction that the wave propagates.

  • Example: Sound waves.

  • Transverse Waves:

  • Particles move perpendicular to the direction of wave propagation.

  • Example: Shaking a string up and down creates transverse waves.

Wave Interference

• In Phase Waves:

  • Two waves are considered in phase when their peaks (maximum values) occur at the same time at the same location.

  • This also applies to the minimum values of the waves.

• Out of Phase Waves:

  • Two waves are out of phase if their peaks do not align and occur at different points in time.

• Interference of Waves:

  • One sound beam can project multiple beams, causing the waves to lose their individual characteristics and combine into a single wave, a phenomenon known as interference.

• Constructive Interference:

  • Occurs when a pair of in-phase waves combine to create a single wave of greater amplitude than either of the original waves.

• Destructive Interference:

  • Occurs when a pair of out-of-phase waves combine to create a single wave with a lower amplitude than that of one of the waves in the pair.

  • A pair of two out-of-phase waves of equal amplitude can cancel each other out.

• Interference of Waves with Different Frequencies:

  • When waves of different frequencies interfere with each other, both constructive and destructive interference can occur at different times, leading to a complex wave pattern.