Flashcards in SPI-Lecture 7 Deck (13)
The angle of incidence = the angle of reflection.
-Angle of incidence is NOT 90 degrees
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.
REQUIREMENTS FOR REFRACTION
1. Oblique incidence
2. Different propagation speeds between the media.
- Allows us to calculate the angle of refraction.
SNELL'S LAW IN ULTRASOUOND
Sine refracted angle propagation speed medium 2 ------------------------------ = -------------------------------------------
Sine incident angle propagation speed medium 1
WHAT IS SINE?
Refers to ratio of 2 of the sides of a right triangle
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.
- multiples of the fundamental (incident) frequency.
- caused by the distortion of the fundamental wave AS
- the fundamental wave is removed, only the harmonic
- the fundamental wave is the first harmonic.
- we are concerned with the second harmonic wave,
which is used for harmonic imaging.
- 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.
- generate less side and grating lobes.
- only generated by strong signals, not weak ones.
- side and grating lobes are weaker than the main
- therefore, the harmonic waves are generated in the
- very few side or grating lobes are generated because
they are weak.
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)