Week 2 Assessment (6,7,8) Flashcards
logarithms
Count the zeros.Ex.log of 100 is 2.
Positive decibels and negative decibels
Positive:Intensity doubles for every 3bD
Ex. 9dB (two 3s to get 9 so 2 x 2= 4 times larger)
Negative: -3dB:1/2 ..example—6dB=1/2x1/2=1/4
-10dB: 1/10…ex.9dB=1/2x1/2x1/2=1/8
Attenuation is directly related to
Distance and frequency
longer distances, higher frequencies= more attenuation
3 components of attenuation
Absorption, scattering and reflection
Media with least attenuation
water
Media with most attenuation
Air
what’s the conversion of acoustic energy into heat?
Absorption
Primary cause of attenuation in soft tissue (80%)
Absorption
Absorption is_________related to distance and frequency
directly
Sound that returns back to the transducer after striking a boundary
Reflection
The 2 forms of reflection are
Specular:
(smooth reflector, one direction, smaller wavelength than boundary irregularities)
Diffuse/backscatter:
(rough reflector, random directions, wavelength same size as boundary irregularities)
Boundary’s irregularities are same size to bit smaller than wavelength, wave redirected in all directions.High frequencies have more …ex.Red blood cells are this
Rayleigh scattering
Thin half value layer
Bone
high freq,high attenuation rate
thick half value layer
Fluids
low freq,low attenuation rate
what other names half value layer known by
penetration, depth of penetration, half boundary layer
sound beam strikes boundary exactly at 90 degrees
Normal incidence
Any angle other than 90 degrees is
Oblique incidence
acute<90
Obtuse >90
Formula for incident intensity
Reflected intensity + Transmitted intensity
% reflected for soft tissue,bone and air
soft tissue: less than 1%
Bone : 50%
Air : 99%
Formula for IRC(%)–reflection with normal incidence
(Z2-Z1/Z2+Z1)^2
Z1= reflection #
Z2= Transmission #
Reflection with normal incidence (90 degrees) occurs when (IRC)
there are different impedances on either side of the boundary
Transmission with normal incidence occurs (ITC)
When the Two media have same impedance
% from reflected sound - 100%= ITC
Transmission with oblique transmission
100% IRC +ITC
transmission difficult to predict
Refraction cannot occur with
Normal incidence or with identical propagation speeds.
Refraction is and requires
when the direction of wave propagation is changed from 1 medium to another
requires :
Oblique incidence and diff propagation speeds of 2 media
Snell’s slaw is the reason
-Refraction occurs.
-Greater the change in the speeds of the media the greater the change in the angle of refraction
sin(transmission angle)/sin (incident angle)=propagation speed 2/propagation speed 1
medium 1 and medium 2 refraction (oblique angle)
If md 1 is > md 2 travel in same direction
md 1 < md 2 ,md 2 is faster,shorter
md 1 > md 2 ,md 2 is slower, longer
Thickness of matching layer
1/4
thickness of active element (pzt)
1/2
3 consequences of using backing material
decreased sensitivity
wide bandwidth
low quality factor
Resonant frequency
ringing at single frequency
Main frequency of transducer known as
center, resonant,primary or natural frequency
Quality factor
Main frequency/ bandwidth
ex.7hz/8hz-6
transducer that has Low Q due to backing material
Imaging transducers
therapeutic imagining has
HIGH Q
Speed of sound in PZT and frequency are
directly related
Thickness in PZT and frequency are
inversely related
