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Flashcards in SPI-Lecture 6 Deck (18)
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1

When sound hits a boundary, it may be:

Transmitted
Reflected
Scattered
Refracted

2

Ultrasound can be used for imaging because:

-Reflection and scattering at organ boundaries

-Scattering in heterogeneous tissues.

3

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.

4

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.

5

2 types of Scattering:

-Backscatter (Non-specular - large, rough surface) - sound returns toward transducer.

-Rayleigh Scatter (small,

6

Incident Sound

-Sound that is coming from the sound source and going into the medium.

-Incident sound = Transmitted Sound + Reflected Sound.

7

Transmitted Sound

-Sound that continues to propagate in the direction it was going, after hitting a boundary or interface.

-Majority of sound is transmitted.

8

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.

9

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.

10

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.

11

Transmitted Intensity

The intensity the sound has after it encounters a boundary or an interface and continues to travel.

12

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.

13

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.

14

IRC and ITC Relationship

ALL sound must be accounted for so:
IRC and ITC MUST equal 100% of original intensity.

15

Two factors required for reflection to occur:

1. Angle at which sound strikes the boundary must be at 90 degrees.

2. The two tissues or media must have different acoustic impedances.

16

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!

17

ITC Equation

ITC = 1 - IRC

18

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.