echoes Flashcards
___ is an Echo, a Portion of sound returned from a media boundary or interface that create our image
reflection
why do reflections occur?
Impedance changes cause reflections to occur.
.
define impedance
Impedance – an acoustic property of a medium that is related to its density and propagation speed
___ is Directly proportional to density and propagation speed. what is it measured in?
what is the avg in soft tissue?
what is its formula?
impedance rayles
Average for soft tissue is 1,630,000 R (1.63 MR)
Z (r) = Density (kg/m3) x C (m/s)
What is the impedance of a medium if the density is 1000 kg/m3 and the propagation speed is 1540 m/s?
what are these?
impedances of selected tissue
Greater the change of impedance, greater the _____
_____ is point where impedance changes and a reflection occurs. AKA Boundary
____ is the media boundary that produces a reflection
reflection
Interface
Reflector
what is the impedance change in each image?
1st one is no impedance change
____ is reflection from a large (relative to the wavelength), flat, smooth boundary
specular
____ is – diffusion or redirection of sound in several directions upon encountering a particle suspension (small boundaries) or a rough surface
Scatter (Diffuse, non- specular)
e.g. light off a mirror = specular; light off a white wall = diffuse
____ have Reflectors larger than wavelength
Wavelength small compared to reflector
Examples: diaphragm, wall of vessel, femur, heart valve
Cause more artifacts
Angle dependant
Specular
____ are Reflectors are smaller than wavelength
Wavelengths large compared to reflector
Examples: liver, kidney, placenta, muscle
Cause less artifacts
Angle independent
diffuse
what is raleigh scattering
Sound scattering in all directions
“Back scatter” the sound that returns to the transducer to create the image
Often referred to with Doppler and blood
what are Specular Reflectors -
what types are there?
Incidence
Perpendicular incidence (normal, orthagonal) Oblique incidence (non- perpendicular)
___ is the – angle that strike interface
Calculate strength of reflection with _____
Calculate angle of reflection with _______
Types of incidence
perpendicular
oblique
The greater the change of impedance the ____ the reflection
stronger
what type of incidence
Beam hits interface at 90 degree
what is Reflected intensity - ?
Transmitted intensity – ?
Will calculate these by determining the “fraction” or “percent” of sound reflected
perpendicular
reflection comes straight back to transducer
sound that continues
what is IRC?
what is its formula?
what is it measured in?
Intensity Reflection Coefficient – fraction or percent of sound reflected
IRC = [Z2 –Z1 / Z2 + Z1]2
unitless
What is the intensity reflection coefficient if medium one has an impedance of 1 MR and medium two has an impedance of 2 MR?
What is the intensity reflection coefficient at this bile (water) – gall stone (bone) interface
what is reflected intensity?
what is the formula?
Intensity of reflection (reflected intensity)
IR mW/cm2 = II mW/cm2 x IRC
What is the reflected intensity if medium one has an impedance of 1 MR and medium two has an impedance of 2 MR and the incident intensity is 100 mW/cm2?
Already calculated IRC to be .09 (or 9%)
What is the reflected intensity if medium one has an impedance of 1 MR and medium two has an impedance of 4 MR and the incident intensity is 100 mW/cm2?
Already calculated IRC to be .36 (or 36%)
What is the reflected intensity if medium one has an impedance of 1 MR and medium two has an impedance of 4 MR and the incident intensity is 200 mW/cm2?
Already calculated IRC to be .36 (or 36%)
what is intensity transmitted coefficient
it’s formula? it’s unit?
ITC – fraction or percent of sound transmitted
Will be everything that was NOT reflected
1 = IRC + ITC
ITC = 1 – IRC
Unitless
IT mW/cm2 = II mW/cm2 x ITC
IT mW/cm2 = II mW/cm2 –IR mW/cm2
What is the intensity reflection coefficient if medium one has an impedance of 1 MR and medium two has an impedance of 2 MR?
IRC was .09 (or 9%)
What is the intensity transmitted coefficient?
What is the reflected intensity if medium one has an impedance of 1 MR and medium two has an impedance of 4 MR and the incident intensity is 200 mW/cm2?
IRC is .36 (36%)
IR is 72 mW/cm2
What is the transmitted intensity?
able to calculate based on specular and perpendicular only
What is the intensity transmitted coefficient at this bile (water) – gall stone (bone) interface?
Already calculated IRC of .46
If Z is = then no reflection
Large difference in Z, large IRC and strong reflected intensity (bone; air)
perpendicular incidence
what happens w/ oblique incidence?
Beam hits interface at < 90 degree
Can not calculate intensities – too complex
Will calculate angle of reflection and transmission
Causes artifacts – missing reflector, mirror image, anisotropy, etc.
Beam hits interface at < 90 degree describe the difference.
Angle of Incidence (
Angle of reflection (
Angle of transmission (
what does the angle of r always equal?
1
and angle of r = angle of i
When angle of incidence is ___degrees the reflection may not return to the transducer
Results in missing reflectors
Results in diminished or missing reflection (hypoechoic) esp. when involving tendons, ligaments, muscles referred to as anisotropy.
<90
what is rarefraction?
what law provides the formula? what is it?
Refraction - Change in direction of sound on passing from one medium to another
Snell’s law
Sine (remove sine for our puposes)
If the angle of incidence is 45o at an interface between medium one with a propagation speed of 1.3 mm/us and medium two with a propagation speed of 1.7 mm/us. What is the angle of reflection and angle of transmission?
Refraction causes ____because transmitted sound changes direction
edge shadows
hits the edge, changes direction, wave doesn’t bounce back. refraction of the beam when it hits a specular reflector w/ 2 mediums w/ different propagation speeds at an obliqued angle
what is a rarefraction mirror image
goes through rectus abdominous muscle which is a specular reflector . which bends the beam. in this case as it bounces back it hits the sac causing a refraction. another beam hits the sac straight on allowing the beam to travel back hence you have 2 sacs
what is this? how can u tell?
rarefraction mirror image
what does this describe?
Missing reflections – large shadow
Grazing Incidence (critical angle)
If the angle of incidence is 60o at an interface between medium one with a propagation speed of 1.0 mm/us and medium two with a propagation speed of 1.5 mm/us. What is the angle of reflection and angle of transmission?
90 degrees
If oblique incidence and change in propagation speed = refraction Need to image specular reflectors at __ degrees to see them
90
describe diffuse reflectors. what is another name for these?
scatter or rayleigh scattering
Rough surfaces and small reflectors
Scatterers
Rayleigh scattering - redirection of sound in all directions by the diffuse reflectors
Often used to describe scattering from RBC
Wavelength of 5MHz .3 mm, RBC .008 mm
___ is – sound that returns to transducer to create image. Able to image diffuse reflectors at any angle and it looks the same
Back scatter
what are diffuse reflectors dependent upon? how?
frequency
Increased frequency = increased scatter b/c
Increased frequency = smaller wavelength; therefore
Wavelength is < reflector
An 8 MHz transducer will create more scatter than a 4 MHz
Scattering from___ scatterers increased to the 4th power of frequency
Doubling the frequency = increases scattering by 16 times (24)
How much will the scattering increase when changing from a 2 MHz to a 8 MHz?
Rayleigh
