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Flashcards in SPI-Lecture 7 Deck (13)
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1

LUEZ'S LAW

The angle of incidence = the angle of reflection.

2

OBLIQUE INCIDENCE

-Angle of incidence is NOT 90 degrees
-Reflection uncertain
-Transmission uncertain

3

REFRACTION

Bending of a beam of sound or change in direction of a beam of sound as it passes from one medium to another.

*Refraction is a possible source of image artifacts.

4

REQUIREMENTS FOR REFRACTION

1. Oblique incidence
2. Different propagation speeds between the media.

5

SNELL'S LAW

- Allows us to calculate the angle of refraction.

6

SNELL'S LAW IN ULTRASOUOND

Sine refracted angle propagation speed medium 2 ------------------------------ = -------------------------------------------
Sine incident angle propagation speed medium 1

7

WHAT IS SINE?

Refers to ratio of 2 of the sides of a right triangle

8

THINGS TO REMEMBER ABOUT SNELL'S LAW

1. When the velocity in medium 2 is greater than velocity in medium 1, the transmission (refracted) angle is greater than the incidence angle.

2. When the velocity in medium 2 is less than the velocity in medium 1, the transmission (refracted) angle is less than the incidence angle.

9

HARMONICS

- multiples of the fundamental (incident) frequency.
- caused by the distortion of the fundamental wave AS
IT TRAVELS.
- the fundamental wave is removed, only the harmonic
is processed.
- the fundamental wave is the first harmonic.
- we are concerned with the second harmonic wave,
which is used for harmonic imaging.

10

HARMONICS

- much of the noise of the reflection is encountered
within the first few centimeters of the skin due to high
energy and drifting fascia.
- the harmonic wave begins with low intensity, then
increases in a NON-LINEAR fashion.
- because of this, the harmonic wave picks up less
noise than a fundamental wave.

11

HARMONICS

- generate less side and grating lobes.
- only generated by strong signals, not weak ones.
- side and grating lobes are weaker than the main
beam.
- therefore, the harmonic waves are generated in the
main beam.
- very few side or grating lobes are generated because
they are weak.

12

ADVANTAGES OF HARMONICS

- better resolution (higher frequencies used)
- narrower beams ( higher frequencies = longer NZL's,
gradual tapering of the beam)
- less divergence in the far field.
- a better signal to noise ratio (clearer signal)

13

DISADVANTAGES OF HARMONICS

- lower power output (harmonic waves are weaker
than the fundamental waves)