Physics 1 Module 5 & 6 Flashcards
The ultrasound beam is created by many waves each emitted by their own source. What is this source called?
Huygens’ sources
What are the waves called that are emitted from each Huygens’ source?
Huygens’ wavelets
When two waves that are in phase interact and results in increased amplitude and intensity
Constructive interference
When two out of phase waves interfere and results in decreased amplitude and intensity or they may cancel each other out.
Destructive interference
When two waves are neither completely in phase or out of phase
Beat frequency
Two waves propagating in the same medium but in opposite directions, resulting in ares of nodes (low pressure) and anti-nodes (high pressure)
Standing waves
1 bel = ____ decibels?
10
A 3 dB drop results in ____ the original intensity?
1/2
A 10 dB drop results in ____ the original intensity?
0.1
The amount of attenuation that occurs with each one centimetre travelled
Attenuation coefficient
What is the attenuation coefficient in soft tissue?
0.5 dB of attenuation occurs for every one centimetre travelled per 1 MHz therefore,
Attenuation coefficient = 1/2frequency
What is the total attenuation equation?
TAtt = Att. Coefficient x path length (cm)
In soft tissue:
TAtt = 1/2f x path length (cm)
The distance sound must travel to reduce the intensity to half its original value
Half-value layer
What 3 factors affect attenuation?
- Nature of the tissue (necrotic, fluid, etc)
- Frequency of the ultrasound
- Depth
The five kinds of attenuation
- Absorption
- Reflection
- Refraction
- Scatter
- Wave-front divergence
Conversion of sound energy to heat & accounts for around 80% of attenuation
Absorption
What factors affect absorption?
- Viscosity (more viscosity = more friction = more absorption)
- Relaxation time (higher time = more energy required to reverse direction = more heat)
- Frequency (higher frequency = less time for molecules to relax = more absorption)
A large surface greater than one wavelength in diameter
Specular reflector
Why would a reflection of sound not make it back to the probe?
Non-perpendicular angle of incidence
What is acoustic impedance?
Z = density(p) x velocity(c)
The larger the difference in Z values, the ______ the reflection
Bigger
Why do Z values differ within the tissues in the body?
Because there are differences in density and stiffness
What is an example of a tissue with a very high Z value and one with a very small Z value?
Bone is very high
Air is very small
Is it possible for 2 tissues to have different sound velocities but have the same Z value?
Yes
What happens to sound when the velocities differ across the interface and the angle of incidence is non-perpendicular?
Refraction
What happens if the velocities across the interface are equal?
No refraction will occur
What happens when the velocity of the first medium is greater than the second?
Sound will bend towards the normal (towards y axis)
What happens if the velocity of the first medium is less than the second?
The sound will refract away from the normal (towards x axis)
How does refraction relate to Z values?
It doesn’t
Sound interacting with interfaces smaller than a wavelength and that are rough & is responsible for the internal texture of organs
Scatter
Type of scatter when sound is directed back to its origin
Backscatter
Type of scatter that happens when sound interacts with RBCs
Rayleigh scatter
Why does the intensity of the beam weaken as it diverges?
Due to the relationship between intensity and area (as area increases, intensity decreases), so as the beam spreads apart, it gets weaker
The fixed points of amplitude as the result of a standing wave
Nodes
Point of maximum fluctuation or amplitude as the result of a standing wave
Anti-nodes
Waves spread out as a result of passing through a narrow aperture
Diffraction
