George DavidAssociate Professor

Ultrasound PhysicsUltrasound Physics

03A:Reflections

‘97

George DavidAssociate Professor

Perpendicular IncidencePerpendicular Incidence

• Sound beam travels perpendicular to boundary between two media

90o

IncidentAngle

1

2Boundarybetweenmedia

George DavidAssociate Professor

Oblique IncidenceOblique Incidence

• Sound beam travel not perpendicular to boundary Oblique

IncidentAngle

(not equal to 90o)

1

2

Boundarybetweenmedia

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Perpendicular IncidencePerpendicular Incidence

• What happens to sound at boundary?reflected

» sound returns toward source

transmitted» sound continues in

same direction

1

2

George DavidAssociate Professor

Perpendicular IncidencePerpendicular Incidence

• Fraction of intensity reflected depends on acoustic impedances of two media

1

2

Acoustic Impedance =Density X Speed of Sound

George DavidAssociate Professor

An Aside about ReflectionsAn Aside about Reflections

• Echoes occur at interfaces between 2 media of different acoustic impedancesspeed of sound X density

Medium 1

Medium 2

Intensity Reflection Coefficient (IRC)&

Intensity Transmission Coefficient (ITC)

Intensity Reflection Coefficient (IRC)&

Intensity Transmission Coefficient (ITC)

• IRCFraction of sound intensity reflected at

interface<1

• ITCFraction of sound intensity transmitted through

interface<1

Medium 1

Medium 2IRC + ITC = 1

IRC EquationIRC Equation

• Z1 is acoustic impedance of medium #1

• Z2 is acoustic impedance of medium #2

2 reflected intensity z2 - z1

IRC = ------------------------ = ----------

incident intensity z2 + z1

For perpendicular incidence

Medium 1

Medium 2

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ReflectionsReflections

• Impedances equal no reflection

• Impedances similar little reflected

• Impedances very different virtually all reflected

2 reflected intensity z2 - z1

Fraction Reflected = ------------------------ = ----------

incident intensity z2 + z1

Why Use Gel?Why Use Gel?

• Acoustic Impedance of air & soft tissue very different• Without gel virtually no sound penetrates skin

2 reflected intensity z2 - z1

IRC = ------------------------ = ----------

incident intensity z2 + z1

Acoustic Impedance

(rayls)

Air 400Soft Tissue 1,630,000

Fraction Reflected: 0.9995

George DavidAssociate Professor

Rayleigh ScatteringRayleigh Scattering

• redirection of sound in many directions

• caused by rough surface with respect to wavelength of sound

Diffuse Scattering & Rough Surfaces

Diffuse Scattering & Rough Surfaces

• heterogeneous media

• cellular tissue

• particle suspension blood, for example

ScatteringScattering• Occurs if

boundary not smooth

• Roughness related to frequencyfrequency changes wavelength

» higher frequency shortens wavelength

» shorter wavelength “roughens” surface

George DavidAssociate Professor

Specular ReflectionsSpecular Reflections

• Un-scattered soundoccurs with smooth

boundaries

• similar to light reflection from mirroropposite of scatter from

rough surfacewall is example of rough

surface

George DavidAssociate Professor

BackscatterBackscatter

• sound scattered back in the direction of source

George DavidAssociate Professor

Backscatter CommentsBackscatter Comments

• Caused byrough surfacesheterogeneous media

• Depends on scatterer’ssizeroughnessshapeorientation

• Depends on sound frequencyaffects wavelength

George DavidAssociate Professor

Backscatter IntensityBackscatter Intensity

• normally << than specular reflections

• angle dependance specular reflection very angle dependent backscatter not angle dependent

» echo reception not dependent on incident angle

• increasing frequency effectively roughens surface

higher frequency results in more backscatter