Week 5 - Reflection, Transmission, Refraction Flashcards
What happens to sound beam’s energy when it enters tissue?
Some energy is reflected, some energy is transmitted, and some energy is changed.
Most of the ultrasound energy is what?
changed to heat
Describe a specular reflector?
a large, flat, smooth surface; with dimensions larger than the wavelength
What is an example of a specular reflector?
the wall of an artery or vein
What is incidence?
the angle the sound beam hits a specular reflector
an angle of 90 degrees is know as __________
normal incidence
When scanning a specular reflector, what angle gives you the clearest image?
90 degrees
What technique can be used with the probe to obtain a normal incident angle?
heel and toe
What influences how much energy is reflected at a perpendicular incident?
the difference of the tissues at the interface
The ________ the difference in ___________ ___________ , the greater percentage of E that is reflected.
bigger, acoustic impedance
What is an interface?
a boundary or border
What is acoustic impedance?
a characteristic of tissue that affects the strength of an echo
Proportion. Acoustic Impedance and density of tissue.
Directly
Proportion. Acoustic impedance and speed of particle vibration.
Directly
What is the unit of acoustic impedance?
Rayl
What is the reflection coefficient?
% of wave strength that is reflected at an interface
What are the 2 types of reflection coefficient?
Amplitude reflection coefficient and Intensity reflection coefficient
What does ARC stand for?
Amplitude Reflection Coefficient
What is ARC?
the percentage of the strength of the wave that is reflected at an interface
What is the equation for ARC?
ARC% = (Z2 - Z1) / (Z2 + Z1)
The _________ the difference in Z of two adjacent media, the greater the reflected percentage
larger
What is the equation for IRC?
IRC% = [ (Z2 - Z1) / (Z2 + Z1) ] squared
If 60% of the energy is reflected a normal incident, how much is transmitted?
40%
When does diffuse reflection happen?
when the interface is not smooth
the most important source of echoes in the body is what?
acoustic scattering
What direction do scatterers reflect the beam?
all directions
What is an example of a diffuse reflector?
a plaque surface
What are two organs that can be described as scatterers?
the liver and the kidney
What is a Rayleigh Scatterer?
a reflector that is MUCH smaller in size than the wavelength of the beam
What is an example of a Rayleigh scatterer?
RBCs
How do Rayleigh scatterers appear on screen? Why?
Black, because the echoes are scattered in all directions, therefore, very week echoes
Rayleigh scattering is proportional to ________
frequency to the 4th power
Areas of increased level of scattering compared to surrounding tissue will result in _________ echoes.
brighter
When echoes appear brighter, they are know as __________.
hyperechoic
Hypoechoic echoes appear _________.
darker
term meaning “echoes present”
echogenic
term meaning “no echoes”
echolucent
homogeneous echoes will appear ___________.
similar in strength
heterogeneous echoes will appear ___________.
bright and dark
Refraction occurs when what happens?
an ultrasound beam meets an interface of tissues with different propagation speeds at a non-perpendicular angle
Slow to fast C yields a __________ angle of transmission
higher
Fast to slow C yields a __________ angle of transmission
lower
State Snell’s law
sin (transmitted angle) / sin (incident angle) = C2 / C1
a large, flat, smooth surface; with dimensions larger than the wavelength is a what?
specular reflector
the wall of an artery or vein is what
an example of a specular reflector
normal incidence = what?
90 degrees
a characteristic of tissue that affects the strength of an echo is what?
acoustic impedance
% of wave strength that is reflected at an interface
reflection coefficient
what happens when the interface is not smooth
diffuse reflection
a plaque surface is an example of what?
a diffuse reflector
a reflector that is MUCH smaller in size than the wavelength of the beam
Rayleigh Scatterer
darker echoes are _________
hypoechoic
echogenic means what?
echoes are present
echolucent means what?
no echoes present
echoes that are similar in strength
homogeneous
echoes that have bright and dark areas
heterogeneous
a higher angle of transmission results from what?
Slow to fast C
a lower angle of transmissions results from what?
Fast to slow C
