SPI-Lecture 6

Question 1

When sound hits a boundary, it may be:

Answer 1

Transmitted
Reflected
Scattered
Refracted

Question 2

Ultrasound can be used for imaging because:

Answer 2

• Reflection and scattering at organ boundaries

- Scattering in heterogeneous tissues.

Question 3

Specular Reflection

Answer 3

• 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.

Question 4

Scattering

Answer 4

• Occurs when sound encounters irregular, rough, heterogeneous, or small (equal to or smaller than wavelength) boundary or tissue.
• Sound is redirected into many directions.

Question 5

2 types of Scattering:

Answer 5

• Backscatter (Non-specular - large, rough surface) - sound returns toward transducer.
• Rayleigh Scatter (small,

Question 6

Incident Sound

Answer 6

• Sound that is coming from the sound source and going into the medium.
• Incident sound = Transmitted Sound + Reflected Sound.

Question 7

Transmitted Sound

Answer 7

• Sound that continues to propagate in the direction it was going, after hitting a boundary or interface.
• Majority of sound is transmitted.

Question 8

Reflected Sound

Answer 8

• 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.

Question 9

Ways that Incident Sound can encounter the interface:

Answer 9

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.

Question 10

Incident Intensity

Answer 10

The intensity of the sound just after it encounters a boundary or and interface and is reflected.

Incident Intensity = Reflected Intensity + Transmitted Intensity.

Question 11

Transmitted Intensity

Answer 11

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

Question 12

Intensity Reflection Coefficient (IRC)

Answer 12

The percentage of the sound’s intensity that is reflected when sound hits a boundary or tissue interface between two media.

Question 13

Intensity Transmission Coefficient (ITC)

Answer 13

The percentage of the sound’s intensity that is transmitted when sound hits a boundary or tissue interface between two media.

Question 14

IRC and ITC Relationship

Answer 14

ALL sound must be accounted for so:

IRC and ITC MUST equal 100% of original intensity.

Question 15

Two factors required for reflection to occur:

Answer 15

1. Angle at which sound strikes the boundary must be at 90 degrees.
2. The two tissues or media must have different acoustic impedances.

Question 16

IRC Equation

Answer 16

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!

Question 17

ITC Equation

Answer 17

ITC = 1 - IRC

Question 18

Examples of impedance mismatch:

Answer 18

• 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.