2. George David Associate Professor Ultrasound Physics 03A: Reflections ‘97

3. George David Associate Professor Perpendicular Incidence Sound beam travels perpendicular to boundary between two media 90 o Incident Angle 1 2 Boundary between media

4. George David Associate Professor Oblique Incidence Sound beam travel not perpendicular to boundary Oblique Incident Angle (not equal to 90 o ) 1 2 Boundary between media

5. George David Associate Professor Perpendicular Incidence What happens to sound at boundary?  reflected »sound returns toward source  transmitted »sound continues in same direction 1 2

6. George David Associate Professor Perpendicular Incidence Fraction of intensity reflected depends on acoustic impedances of two media 1 2 Acoustic Impedance = Density X Speed of Sound

7. George David Associate Professor An Aside about Reflections Echoes occur at interfaces between 2 media of different acoustic impedances  speed of sound X density Medium 1 Medium 2

8. Intensity Reflection Coefficient (IRC) & Intensity Transmission Coefficient (ITC) IRC  Fraction of sound intensity reflected at interface  <1 ITC  Fraction of sound intensity transmitted through interface  <1 Medium 1 Medium 2 IRC + ITC = 1

9. IRC Equation Z 1 is acoustic impedance of medium #1 Z 2 is acoustic impedance of medium #2 2 reflected intensity z 2 - z 1 IRC = ------------------------ = ---------- incident intensity z 2 + z 1 For perpendicular incidence Medium 1 Medium 2

10. George David Associate Professor Reflections Impedances equal  no reflection Impedances similar  little reflected Impedances very different  virtually all reflected 2 reflected intensity z 2 - z 1 Fraction Reflected = ------------------------ = ---------- incident intensity z 2 + z 1

11. Why Use Gel? Acoustic Impedance of air & soft tissue very different Without gel virtually no sound penetrates skin 2 reflected intensity z 2 - z 1 IRC = ------------------------ = ---------- incident intensity z 2 + z 1 Acoustic Impedance (rayls) Air400 Soft Tissue1,630,000 Fraction Reflected: 0.9995

12. George David Associate Professor Rayleigh Scattering redirection of sound in many directions caused by rough surface with respect to wavelength of sound

13. Diffuse Scattering & Rough Surfaces heterogeneous media cellular tissue particle suspension  blood, for example

14. Scattering Occurs if  boundary not smooth Roughness related to frequency  frequency changes wavelength »higher frequency shortens wavelength »shorter wavelength “roughens” surface

15. George David Associate Professor 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

16. George David Associate Professor Backscatter sound scattered back in the direction of source

17. George David Associate Professor Backscatter Comments Caused by  rough surfaces  heterogeneous media Depends on scatterer’s  size  roughness  shape  orientation Depends on sound frequency  affects wavelength

18. George David Associate Professor Backscatter 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