Variables of Propagation 2

Variables of Propagation

  • Presenter: Pascual Nava BS, RDMS, RVT

Elasticity

  • Refers to the ability of a solid object to return to its original shape after distortion by force.

  • Important concept in understanding how materials behave under pressure or force applications.

Compressibility

  • Definition: The compressibility of a material is a measure of how much the volume of the material changes for a given distorting force, such as pressure.

  • High Compressibility: Implies that a material can be easily compressed.

  • Relation to Elasticity: Compressibility is relative to elasticity; materials with high elasticity typically have low compressibility.

Stiffness

  • In the context of ultrasound, stiffness is used to imply the inverse of elasticity or compressibility.

  • Characteristic: Stiff materials cannot be easily compressed.

  • Relevant in medical imaging where the stiffness of the medium affects sound wave propagation.

Bulk Modulus

  • Defined as the decrease in the ratio of the stress to the strain in a material.

  • Stress: Change in pressure applied.

  • Strain: Percent of change in volume as a result of the applied stress.

  • High Bulk Modulus indicates the material is incompressible and stiff, while low Bulk Modulus suggests the material is compressible and elastic.

Bulk Modulus Characteristics

  • High Bulk Modulus:

    • Incompressible

    • Inelastic

    • Stiff

  • Low Bulk Modulus:

    • Compressible

    • Elastic

    • Not stiff

Propagation and Bulk Modulus

  • Formula relating to propagation speed of ultrasound waves:

    • Propagation (c) is inversely proportional to density (ρ) and compressibility.

    • Propagation speed is influenced by bulk modulus (βmodulus).

  • Equation: C = √(Bulk Modulus / Density)

    • Indicates that propagation speed is highest in denser and more rigid materials, such as bone.

Density and its Effect on Propagation Velocity

  • General trends in how density, stiffness, and bulk modulus affect propagation velocity:

    • Increased Density: Can lead to decreased propagation speed if bulk modulus remains unchanged.

    • Increased Stiffness/Bulk Modulus: Leads to increased propagation velocity.

Control of the Wavelength

  • Wavelength Effects: Impacts critical parameters in diagnostic ultrasound, including:

    • Reflection: How ultrasound waves bounce back from different mediums.

    • Resolution: Ability to distinguish between closely spaced objects.

  • Increasing propagation speed stretches the wavelength proportionally to the change in medium.

  • Increasing frequency results in shorter wavelengths due to more cycles per pulse.

  • Wavelength characteristics are important for:

    • Types of reflections and signal strength

    • Types of artifacts

    • Resolution and harmonics