[Ultrasound Physics] REFLECTORS

[Contents]

1. Reflection

2. Specular Reflection

3. Non-Specular Reflection (=Diffuse reflection, Scattering)

1. Reflection [★]

1) Must have an Impedance (z) mismatch to have any reflection

- All of the sound will be transmitted if two media have the same impedance.

 

2) the sound traveling back towards the source (transducer) after encountering a boundary

- Speed it travels back is based on medium it is traveling through

- Sound-tissue interaction necessary to form an ultrasound image

- Reflections from different frequencies have identical transit times

 

3) ex: shadow of a renal stone is a result of reflection

2. Specular Reflection [★★]

1) Must have mismatch of impedance (Z)

 

2) Mirror-like (reflection comes right back)

- No refraction

- Returned from large flat surfaces

- Boundary is larger than beam (Beam is smaller than boundary)

- Boundary is larger than wavelength (wavelength smaller than boundary)

- Most angle-dependent:  Must be 90⁰ angle (Perpendicular/Normal Incidence)

- Strength of received signal depends on

               : Difference in acoustic impedance

               : Angle of incidence

 

3) ex: diaphragm, pericardium are specular reflectors

 

4) Good reflectors: Bright smooth reflections                                                                                    

 

5) Limitation: if not normal incidence not as nice of reflection

- Operator dependent

 

6) When sound strikes specular reflector at an oblique angle

- Angle of reflection equals to the angle of incidence

 

3. Non-Specular Reflection (=Diffuse reflection, Scattering) [★]

-

 

1) Must have mismatch of impedance (Z)

 

2) Not straight reflections (often): Defuses all over (scatter)

- Does not come right back

- refraction (+), scatter (+), absorption (+)

- Rough irregular surfaces

- Beam larger than Boundary (Boundary smaller than beam)

- Boundary smaller than wavelength (Wavelength larger than boundary)

- Not angle-dependent   

 

3) ex: Liver tissue

- Scattering: primarily responsible for imaging internal structure of organs

 

4) Directly related to frequency

- Higher frequency (f↑) = more scatter (scatter ↑)

 

5) Backscatter

- Scatter that returns back to source

- directly related to frequency

- These will be low amplitude reflections

 

6) Rayleigh scatter [★]

- reflector is smaller than the wavelength of sound beam

 - Scattering intensity is proportional to frequency raised to the fourth power

-ex) RBC

Reference

 

* Davies Ultrasound Physics review

* https://sites.google.com/site/lindadmsportfolio/ultrasound-physics/

* https://sites.google.com/site/nataljasultrasoundphysics/

* https://sites.google.com/site/ektasphysicseportfolio/doppler