Ultrasound Physics

1
Ultrasound Physics

• 03AReflections

97
2
Perpendicular Incidence

• Sound beam travels perpendicular to boundary
  between two media

90o Incident Angle
1
2
Boundary between media
3
Oblique Incidence

• Sound beam travel not perpendicular to boundary

Oblique Incident Angle (not equal to 90o)
1
2
Boundary between media
4
Perpendicular Incidence

• What happens to sound at boundary?
• reflected
• sound returns toward source
• transmitted
• sound continues in same direction

1
2
5
Perpendicular Incidence

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

Acoustic Impedance Density X Speed of Sound
6
An Aside about Reflections

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

Medium 1
Medium 2
7
Intensity Reflection Coefficient
(IRC)Intensity Transmission Coefficient (ITC)

• IRC
• Fraction of sound intensity reflected at
  interface
• lt1
• ITC
• Fraction of sound intensity transmitted through
  interface
• lt1

Medium 1
IRC ITC 1
Medium 2
8
IRC Equation
For perpendicular incidence
reflected intensity z2 - z1 IRC
------------------------ ----------
incident intensity z2 z1

• Z1 is acoustic impedance of medium 1
• Z2 is acoustic impedance of medium 2

Medium 1
Medium 2
9
Reflections

• Impedances equal
• no reflection
• Impedances similar
• little reflected
• Impedances very different
• virtually all reflected

10
Why Use Gel?
reflected
intensity z2 - z1
IRC ------------------------ ----------
incident intensity
z2 z1
Fraction Reflected 0.9995

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

11
Rayleigh Scattering

• redirection of sound in many directions
• caused by rough surface with respect to
  wavelength of sound

12
Diffuse Scattering Rough Surfaces

• heterogeneous media
• cellular tissue
• particle suspension
• blood, for example

13
Scattering

• Occurs if
• boundary not smooth
• Roughness related to frequency
• frequency changes wavelength
• higher frequency shortens wavelength
• shorter wavelength roughens surface

14
Specular Reflections

• Un-scattered sound
• occurs with smooth boundaries
• similar to light reflection from mirror
• opposite of scatter from rough surface
• wall is example of rough surface

15
Backscatter

• sound scattered back in the direction of source

16
Backscatter Comments

• Caused by
• rough surfaces
• heterogeneous media
• Depends on scatterers
• size
• roughness
• shape
• orientation
• Depends on sound frequency
• affects wavelength

17
Backscatter Intensity

• normally ltlt 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