SPI-Lecture 7

Question 1

LUEZ’S LAW

Answer 1

The angle of incidence = the angle of reflection.

Question 2

OBLIQUE INCIDENCE

Answer 2

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

Question 3

REFRACTION

Answer 3

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.

Question 4

REQUIREMENTS FOR REFRACTION

Answer 4

1. Oblique incidence

2. Different propagation speeds between the media.

Question 5

SNELL’S LAW

Answer 5

• Allows us to calculate the angle of refraction.

Question 6

SNELL’S LAW IN ULTRASOUOND

Answer 6

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

Question 7

WHAT IS SINE?

Answer 7

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

Question 8

THINGS TO REMEMBER ABOUT SNELL’S LAW

Answer 8

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.

Question 9

HARMONICS

Answer 9

• 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.

Question 10

HARMONICS

Answer 10

• 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.

Question 11

HARMONICS

Answer 11

• 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.

Question 12

ADVANTAGES OF HARMONICS

Answer 12

• 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)

Question 13

DISADVANTAGES OF HARMONICS

Answer 13

• lower power output (harmonic waves are weaker

than the fundamental waves)