Midterm Review Notes Flashcards
What is attenuation
The weakening of sound as it propagates through the tissues
How does attenuation limit the image
Limits the penetration therefore the depth
What are the three contributing factors for attenuation
Absorption, reflection, scattering
What is the dominant contributing factor for attenuation
Absorption
What is absorption
The conversion of sound into heat
What is a decibel
Is a logarithmic unit —> the log of the ratio of the ratio between two quantities such as the ratio between of 2 powers/amplitudes/intensities
What is the attenuation coefficient
The attenuation that occurs with each centimetre of sound travel
What is the attenuation in soft tissue
0.5 DB/cm x MHz
What does hvlt stand for
Half value layer thickness
What is HVLT
The thickness of a medium that decreases the intensity of the beam by half
In soft tissue there is approx ____dB of attenuation per cm for each MHz of frequency
0.5 dB
Attenuation in bone is higher or lower than in soft tissue
Higher
A reflected beam is known as
An echo
When a perpendicular beam is reflected, what happens?
When the beam hits an interface and is reflected, it goes back to the reciever and is picked up to form our images on the screen
When the perpendicular beam is reflected, what happens
The beam moves through the 2nd medium in the same direction as the incident sound beam
What is acoustic impedance
Is a measure of the resistance of a medium to the transmission of sound
What is the equation to impedance, with units
Z= c (m/s) x p(kg/m3)
What does it mean if there is no reflection at a boundary
All is transmission and no echo occurs
This happens when the medium impedances are equal
What must the reflective/transmission coefficients add up to?
1 or 100%
How is oblique incidence different from perpendicular incidence
Oblique is any other angle other than perpendicular
What is refraction
A change in teh direction of sound as it crosses a boundary
What determines refraction
It is determined by the different acoustic velocity/impedance in the different mediums.
Why is refraction important
It contributes to the formation of our image
What does snells law tell us about
The law tells us the relationship between angle of incidence/ transmission and the velocity of sound of the 2 media
What is a specular reflector
It is a flat smooth boundary that reflects well (mirror like).
What anatomy would produce a specular reflector
Artery walls or diaphragm
What is scattering
Is the formation of echo in many directions by rough surfaces or by heterogenous media or small objects
What does scattering depend on
The surface of the interface and the size of the object compared to the size of the wavelength
Describe why tgc is needed
Because echoes do not come back in equal amplitude as they have different travel times
What is the TGC slope
TGC displayed graphically with increasing deflection to the righ t, the slope compensates less at the shallowest portion and more at the deepest portion of the organ
What does the operator need to keep in mind when setting the TGC
The slope should correspond to the attenuation coefficient in the tissue. Should be about 1db/cm-MHz, because the attenuation coefficient in soft tissue is approximately 0.5db/cm, and the sound must travel “there and back”.
What is the difference between TGC and overall gain
TGC is the depth specific; field of view broken into zones and is controlled with a slide bar (pods). The overall gain is the echoes from all depths, its “One” big zone and is controlled with a knob
What are the factors affecting TGC setting
Same things that affect the TGC setting
Tissue comp
Attenuation coefficient
Transducer frequency
If a medium has an attenuation coefficient of 2db/cm, what would be the best choice for the slope of the TGC?
4db/cm. TGC slope compensates for attenuation but must do so at twice the rate since it must compensate both for pulse attenuation as the pulse travels to the reflector and for echo attenuation as the echo returns to the transducer. So the slope for the TGC should be approximately double the attenuation coefficient
What is speckle
Speckle is a form of acoustic noise and shoes as the graininess of tissue within the image
How is speckle produced
It is produced by constructive/destructive interference patterns within the returning echoes producing darker and brighter spots on the screen
What is CEUS
Contrast enhanced ultrasound
What are contrast agents
Microbubble polymers introduced through IV into the boodstream to highlight particular isoechoic abnormal tissue
What are 5 characteristics of contrast agents
Stable, small enough to perfuse, different z to surrounding medium, non toxic, easy admin
How does the contrast agent function?
Depending on the type, it will perfuse through the blood into the necessary tissue and due to the difference in impedance (impedance mismatch), will brighten those tissues. This will either brighten the lesion or the surrounding tissue differently and show more contrast and allow better delineation of the lesion. Can also pick out the harmonic frequencies since they produce stronger harmonics.
How are contrast agents used to show perfusion
Increased US amplitude can pop the bubbles and we can see the tissues and the rate at which they fill up again with the agent
What is the units for amplitude
Pascal, N/cm2, kg/cm3, volts
