D2- SPI, Attenuation, Reflection, Refraction, Transmission

reflection

• occurs due to impedance - the resistance to the propagation of sound through a medium

  • when there is a difference in impedance, reflection occurs

  • the ↑ IMPEDANCE = ↑ REFLECTION

• the amount of reflection from a boundary is determined by the acoustic impedance mismatch (diff in size and tissue)

  • ↓ diff in acoustic impedance = ↓ reflection & significant transmission

  • ↑ diff in acoustic impedance = ↑ reflection & minimal transmission

Types of Reflection

1. Specular Reflection

2. Diffuse Reflection (Scattering)

3. Rayleigh Scattering (Back Scattering)

1. Specular Reflection Criteria

Criteria:

1. Angle dependent (90 degrees)

   1. normal incidence, orthogonal, right angle, perpendicular

2. Large smooth surface with a ↑ propagation speed (ex: bone, thyroid, clavicle)

   1. the size of the reflector (large smooth surface) is larger than the wavelength of the incident beam

3. Obeys Luez’s Law - angle of incidence = angle of reflection

   1. only applies with specular reflectors

1. Specular Reflection (cont. 1)

• mirror like reflections created when a sound wave strikes a large, smooth surface at a 90 degree angle of incidence

• occurs along organ boundaries - sound bounces back only in one direction

• oblique angle = echoes may not return to TDR Specular Reflection

• 2 criteria:

  • (1) difference in acoustic impedance between 2 tissue media = z (rayls) = pV

  • (2) sound beam must strike boundary between 2 media at 90 degrees (normal, perpendicular, orthogonal incidence)

• the size of the reflector (large smooth surface) is larger than the wavelength of the incident beam

As a sound wave meets a boundary between 2 media, there is:

(Specular Reflection, (cont. 2))

• Angle of Incidence (incident beam)

  • angle at which which a sound beam hits a tissue interface (Θi)

• Angle of Reflection (reflected beam)

  • angle at which a beam bounces off a tissue surface (Θr)

  • returns to the transducer if at 90 degrees or away from transducer if oblique

• Transmission (transmitted beam)

  • portion of a sound beam that travels through a tissue interface

*what is reflected back depends on what is transmitted*

2. Diffuse Reflection/Scattering

• Scattering- permits the ability to see through tissues that are not smooth (ex: thyroid, liver, pancreas)

  • lower than specular intensity

  • not angle dependent compared to specular

  • increases with higher frequency transducer

• small irregularities in tissue accentuate the texture of the image

• some terminology includes: back scatter, diffuse scattering

3. Rayleigh Scattering

• occurs when the reflector is smaller than the sound wavelength (ex: RBC)

  • useful for blood flow detection applications of Doppler Ultrasound

  • useful in detecting contrast bubbles (injected in heart chamber)

• Acoustic energy reflected to transducer is reflected in multiple equal directions

• Rayleigh Scattering ∝ Frequency^4

  • ↑ freq. = ↑ scattering by the 4th power

• NOT angle dependent

Specular vs Non Specular Table

Acoustic Impedance (Z)

• medium characteristics: SIZE DIFF BETWEEN 2 MEDIUMS

• resistance to ultrasound through a medium

• the amount of impedance (Z) is related to:

  • medium density

  • propagation speed through a medium

  • ↓ diff in acoustic impedance = ↓ reflection & significant transmission

  • ↑ diff in acoustic impedance = ↑ reflection & minimal transmission

Impedance (cont.)

• Impedance = Density x Propagation Speed

  • Z = p x c

• the amount of reflection from a boundary is determined by the acoustic impedance mismatch (how different the tissues are)

• Z (impedance) ↑ when p (density) ↑

  • reflection occurs when there is a difference in density and speed

• reflections can still occur even if 2 media have the same densities but different propagation speeds

Impedance EXs

• No Impedance (Z) = NO reflections

• Large Impedance (Z) = STRONG reflections

• Small Impedance (Z) = WEAK reflections

Impedance mismatches

small impedance mismatches can result in the inability to visualize a structure, even if the structure is relatively large (isoechoic)

ex: mass can be missed bc it can blend into the tissue if impedance differences are small

Oblique Incidence

• incident beam strikes at a large non-perpendicular angle

• angle of incidence ≠ 90 degrees

• reflected sound does not return to transducer, travels in another direction, does not produce an image on display

Reflection and Transmission Intensities

• Incident Intensity- initial intensity (always = 100%)

• Reflected Intensity- intensity that returns to the transducer

• Transmitted Intensity- intensity that is continues through the medium

• Incident Intensity = Reflected Intensity + Transmitted Intensity

Reflection and Transmission Coefficients

• Intensity Reflection Coefficient (IRC)

  • percentage of incident sound intensity reflected back

  • ↑ impedance mismatch = stronger reflection

  • If tissue impedances are the same —> there is 100% transmission and reflection = 0%

  • If tissue impedances are different —> there is 1% transmission and reflection = 99%