SPI-Lecture 6 Flashcards
When sound hits a boundary, it may be:
Transmitted
Reflected
Scattered
Refracted
Ultrasound can be used for imaging because:
- Reflection and scattering at organ boundaries
- Scattering in heterogeneous tissues.
Specular Reflection
- Arise from large smooth boundaries.
- Similar to your reflection in mirror.
- Sound must encounter boundary at a 90 degree angle (perpendicular)
- Produces stronger echos than non-specular reflectors.
Scattering
- Occurs when sound encounters irregular, rough, heterogeneous, or small (equal to or smaller than wavelength) boundary or tissue.
- Sound is redirected into many directions.
2 types of Scattering:
- Backscatter (Non-specular - large, rough surface) - sound returns toward transducer.
- Rayleigh Scatter (small,
Incident Sound
- Sound that is coming from the sound source and going into the medium.
- Incident sound = Transmitted Sound + Reflected Sound.
Transmitted Sound
- Sound that continues to propagate in the direction it was going, after hitting a boundary or interface.
- Majority of sound is transmitted.
Reflected Sound
- Sound that is reflected at the boundary or interface and returns to the transducer.
- Only 1% or less of sound is reflected at a soft tissue interface.
Ways that Incident Sound can encounter the interface:
Perpendicular - Normal, orthogonal, 90 degree incidence, right angle incidence. The direction of sound travel is perpendicular (at 90 degrees) in relation to boundary.
Oblique- Sound does not encounter the boundary at 90 degrees.
Incident Intensity
The intensity of the sound just after it encounters a boundary or and interface and is reflected.
Incident Intensity = Reflected Intensity + Transmitted Intensity.
Transmitted Intensity
The intensity the sound has after it encounters a boundary or an interface and continues to travel.
Intensity Reflection Coefficient (IRC)
The percentage of the sound’s intensity that is reflected when sound hits a boundary or tissue interface between two media.
Intensity Transmission Coefficient (ITC)
The percentage of the sound’s intensity that is transmitted when sound hits a boundary or tissue interface between two media.
IRC and ITC Relationship
ALL sound must be accounted for so:
IRC and ITC MUST equal 100% of original intensity.
Two factors required for reflection to occur:
- Angle at which sound strikes the boundary must be at 90 degrees.
- The two tissues or media must have different acoustic impedances.
IRC Equation
R = (Z2 - Z1)² Watts/cm²
—————
(Z2 + Z1)² Watts/cm²
IRC = Reflected Intensity
(medium 2 impedance - medium 1 impedance)²
DIVIDED BY
Incident Intensity
(medium 2 impedance + medium 1 impedance)²
Reflection depends on impedance difference!
ITC Equation
ITC = 1 - IRC
Examples of impedance mismatch:
- Highest IRC occurs between an air/soft tissue interface. (99%)
- Lower IRC occurs between a soft tissue/bone interface. (50%)
- Lowest IRC occurs between 2 similar soft tissue interfaces, such as the liver and kidney.