[Ultrasound Physics] TYPES OF INCIDENCE

[Contents]

 

1. Normal Incidence (Perpendicular Incidence)

2. Oblique Incidence

3. Refraction

4. Acoustic Impedance (z)

5. Important Terms

1. Normal Incidence (Perpendicular Incidence)

1) Incident sound beam encounters a boundary between two media at a 0° incident angle

- The sound beam is perpendicular to the boundary.

 

2) Reflected sound returns in the same direction as the incident sound

 

3) Transmitted sound continues on in the same direction as the incident sound

 

4) There is no Refraction (bending of sound)

2. Oblique Incidence [★]

 

 

1) When the incident sound beam encounters the boundary between two media at an angle

- The incident angle is something other than 0°

 

2) The reflected angle is equal to incident angle

 

3) Transmitted sound will also continue on an angle (Refraction)

 

4) The angle of reflection will be oriented away from the transducer

- resulting in decreased visualization of the structure

3. Refraction

1) A change in the direction of sound after encountering a boundary (bending of sound)

 

2) Requirements for refraction

- Oblique Incidence

: Perpendicular/Normal Incidence = No refraction

- Mismatch in Propagating speeds (c) of two media

- M2>M1 than T>I: the transmission angle is greater than the incidence angle

- M2<M1than T< I: the transmission angle is less than the incident angle

 

3) Matching PS and mismatched Impedance causes reflection but not refraction

4. Acoustic Impedance (z)

 

1) physical property of tissue

- Resistance to travel that a sound beam encounters as it passes through a medium [rayls]

Z [rayls] = d [kg/m³] x c [m/s] [★★★★]

- affected by tissue stiffness, density and soundwave speed      

: not) frequency 

               - density = most responsible factor

 

2) An Impedance mismatch determines reflection

- Tissue to Tissue: Reflection ≤2% (mostly transmitted = weakest reflected signal)

- Tissue to Air: mostly reflected without coupling medium

- Diagnostic application to adult brain is limited

: great acoustic impedance mismatch between cranium and soft tissue

: causing most sound to be reflected at interface

- With oblique incidence reflection can occur without refraction

: when there are mismatched impedances, but matching propagating speeds

5. Important Terms

1) Intensity Reflection Coefficient (IRC)

- The fraction of incident intensity that is reflected

: IRC = Ir/Ii (reflected intensity/incident intensity)

- IRC and impedance mismatch: proportional

: ↑z mismatch → ↑IRC

: ↓z mismatch → ↓IRC

- If impedance of the two media are the same: no reflection

 

2) Intensity Transmission Coefficient (ITC)

- The fraction of incident intensity that is transmitted into the second medium

               : ITC = It/Ii (transmitted intensity/incident intensity)

: ITC = 1 – IRC

- IRC and ITC should always equal one

- IRC ↑ → ITC ↓, vice versa.

 

6) Scattering

- The diffusion or redirection of sound in several directions upon encountering a rough surface

- Backscatter: Sound scattered back in the direction from which it originally came

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