Physics Board Exams
Disadvantages of using power doppler?
- lowers frame rate due to more signal averaging - no directional information no hard to identif regions of flow disturbance - more susceptible to artifacts because of the averaging techniques used. (color bleeding and color flash)
What are the advantages of using power doppler?
- more signal averaging so we can visualize trickle flow/areas of low velocity flow - no aliasing because there is no directional information - because there is no directional information we can visualize tortorous vessels
What does the amount of heating depend on?
- scan duration - beam path , (tissue? bone? lung?) - transmit frequency - amount of tissue perfusion -ultrasound beam power and intensity
What are the Acoustic parameters as an U/S pulse moves through tissue?
- tissue particles move back and forth -temperature rises and falls -pressure rises and falls - density goes up and down
Describe diffuse reflection
- weak , not angle dependent and comes from a rough surface
When is power doppler especially useful?
-trying to identify trickle flow - is the vessel fully occluded? - visualizing torturous vessels - seeing if a mass has blood supply (torturous vessel with low flow?)
What are the accepted measurements of exposure by AIUM and Health Canada?
1) Acoustic power 2) Acoustic intensity 3) Peak compression pressure and peak rarefraction pressure
Name 4 ways to reduce aliasing when using PW doppler
1) Increase the PRF (scale or decrease depth) 2) Change doppler angle closer to 90 degrees to reduce the signal 3) shift the baseline 4) reduce the frequency of the doppler pulses to make fd > PRF/2
What methods are used to determine direction of flow in CW imaging?
1) Phase quadrature 2) Single Sideband 3) Heterodyne detection
By using phase shift autocorrelation what 3 pieces of information are we given?
1) The average velocity of the pulses within each color sample 2) The variance or variation in velocities in each sample volume 3) The strength (amplitude) of the doppler echoes in each sample volume.
What information does phase shift autocorrelation give us?
1) average velocity in each sample 2) strength of doppler echoes from each sample 3) variance or variation of the average velocities from each pulse.
What are 3 characteristics of harmonics?
1) they are weak 2) The higher the pressure amplitude of the original pulse the stronger the harmonic 3) They occur after the pulse travels through some tissue - no harmonics are formed in shallow tissues.
How many pulses are launched along each line in color doppler?
4- 16 : similiar to b-mode in that it sends pulses along one line at a time.
What is the AIUM recommendation for MI?
< 1.9
Reynolds number is ___
> 2000
Heterodyne detection
A fixed frequency (fshift) is added to the ftrans and fecho. If the fbeat is > fshift the doppler shift is positive (flow coming towards the transducer) if fbeat< fshift the doppler shift is negative (flow moving away from transducer)
What is ultrasound?
A longitudinal mechanical wave with a frequency above 20 KHZ
Where are harmonics generated?
Along the center of the beam where pressure amplitude is the highest.
What technique is used in color doppler to provide us with an overall map of blood flow?
Autocorrelation
What has the best spatial resolution? 2nd best?
Axial resolution Lateral resolution
What mode is cavitation a greater risk?
B-mode
Where will the greatest temperature rise occur?
Before the focus but closer to the transducer because of attenuation.
Capacitane vs compliance?
Capacitance - the change in volume over the change in time Compliance - the change in pressure over the change in time
How is frequency related to speed of sound?
Directly proportional
What are harmonics?
Extra higher frequency echoes that are produced by non linear interactions between the tissues and the ultrasound pulse.
What technique is used in pulsed doppler to determine the doppler frequency in each echo?
FFT - Fast Fourier Analysis/
What technique is used to separate the individual doppler frequencies that make up the signal in pulsed doppler?
Fast Fourier Analysis/ FFT
What is stable cavitation?
Formation of bubbles that change size as ultrasound passes through them.
What is Transient/Inertial cavitation?
Formation, transformation and collapse of gas bubbles as ultrasound passes through them.
Near field AKA Far field AKA
Fresnel zone Fraunhofer zone
What are the 2 conditions of refraction?
Has to be an oblique angle of incidence speed of sound in both tissues needs to be different
A ___ pass filter cuts out the low frequencies that are associated with tissue motion/noise
High pass
Who's principle describes Diffraction?
Hyugen
How can we improve axial resolution?
Increase our frequency
How is acoustic impedance related to the intensity of an U/S beam?
Indirectly proportional
What are the positives and negatives of high PRF mode?
It allows you to measure velocities in deep tissue but introduces range ambiguity.
What transducer will provide a uniform display when using color doppler and why?
Linear vectory array - the doppler angle is the same through out the color box.
Do you use low frequencies or high frequencies in CW doppler and why?
Low frequenices - RBC's are rayleigh scaterres and produce weak echoes, attentuation increases with frequency. Low frequncies are better for doppler
What happens when the sonographer increases the axial length of the sample box in pulsed doppler?
More velocities will fill in the spectral trace
What is electronic dampling?
One pulse is sent out and the inverse of that pulse is sent immediately after to stop the PZT from vibrating - creates short duration pulses
What is the formula for RI?
PSV - EDV / PSV
What is the formula for PI?
PSV - EDV / mean velocity
What causes spectral broadening and mirroring?
PW gain too high
Whose law describes laminar flow?
Pouisellie's Law
What mode can increase the effect of twinkle artifact?
Power doppler
Backing material
Prevents the PZT elements from vibrating , creating short pulse durations.
What measurements attempt to quantify the resistance to flow?
Pulsatility index and resistivity index
What mode has the highest potential for thermal effects?
Pulsed modes - CW
What is the formula for the reynolds number?
Re = (density)(speed)(diameter)/viscosity
What is the "sensitive region" where harmonics are the strongest?
Some distance into the tissue but before they are attenuated too much in the tissue.
What is the color packet/pulse packet?
The # of ultrasound pulses per beam line
What is refraction?
The changing of direction of an ultrasound beam as it passes through a boundary between tissues
What does the PRF depend on?
The depth we are imaging to The speed of sound
What is scattering?
The distribution of ultrasound energy into the tissue parenchyma.
What is acoustic intensity?
The power we are depositing over the area we are depositing it into.
What is acoustic power?
The rate at which we deposit energy into the tissue. Energy/time
What is diffraction?
The spreading of ultrasound waves as they pass through an opening.
Single Side band?
There are two channels that receive the pulses coming back to the transducer. If the frequency of the echo is higher than ftrans it goes to one channel; if lower than ftrans it goes to the other. Velocities are calculated electronically
What are possible bioeffects?
Tissue heating Cavitation Micro streaming of fluids, intracellular + extracellular pressure waves Tissue shearing
What causes color noise?
Too high of color gain - noise associated with tissue motion is being perceived as blood flow.
When will we see twinkle?
When the US encounters a strong granular reflector; kidney stones, gallstones, calcification.
What is the formula for acoustic impedance?
Z = (Density)(speed of sound)
How is the beat frequency produced?
by adding or multiplying the ftrans and fecho
3 ways to increase frame rate when using color?
decrease color box width decrease overall 2D depth decrease the scan line density ( # of beams sent out in the color box )
Rarefraction are areas of _____
decreased pressure, density, temperature - sound wave speed is decreased
What determines the speed of sound in different tissues?
density and stiffness
PZT effect ____ Reverse PZR effect
detects ultrasound pulses. transmits/produces ultrasound pulses
Thermal index is ______ related to frequency
directly
How is frequency related to the near field length?
directly proportional
What are side lobes?
extra weak echoes that appear on the side of the main ultrasound beam due to diffraction.
Matching layer
has same acoustic impedance as PZT elements to prevent a large reflection.
Components of a single element transducer
housing material backing layer matching layer PZT element Impedance matching layer
How can we increase scattering?
increase our frequency - this decreases our wavelength giving us a stronger signal.
How to eliminate aliasing with color doppler?
increase scale change angle closer to 90 degrees decrease transmit frequency adjust baseline to force velocities into a certain range.
Compression are areas of ____
increased pressure, density and temperature - sound wave speed increases
Mechanical index is _____ related to frequency
indirectly
A shallow curve of PZT elements will cause a ____ near field length. How will it effect the lateral resolution?
longer NFL - decreases the lateral resolution
What type of filter is used in CW?
low pass filter.
Longer pulse durations provide better sensitivity to ______
low velocities
What is the variance?
range of average velocities within each color packet
What are the interactions between ultrasound and tissue?
rarefraction diffraction absorption scattering reflection
What do red pixels, blue pixels and green pixels display on color doppler?
red - flow coming towards the transducer blue - flow going away from transducer green - variance
4 ways to reduce color bleed and color noise
reduce gain increase scale decrease transmit power adjust color write priority
Coupling gel
reduces friction has an acoustic impedance the same as soft tissue (eliminates reflection) speed of sound is the same as soft tissue (eliminates rarefraction)
The bigger the mismatch in acoustic impedance the stronger the ____
reflection.
How can we focus our lateral resolution?
set by the width of the U/S beam
A sharper curve of the PZT elements will cause a _______ near field length. How will it effect the lateral resolution?
shorter increase the lateral resolution
What is a phased array used for?
small aperature, wide field of view.
3 traits of scattering
smaller than the wavelength, produces very weak echoes, isotropic (equal in all directions)
Power doppler displays the _____
strength of the echoes
Directional power doppler displays the ______
strength of the echoes, color encoded to show the direction of flow.
Describe specular reflection
strong, very angle dependent (90 degrees) comes from a smooth surface
3 factors affecting the speed of sound
temperature pressure rising and falling frequency
CDI suffers from a poor type of ______ resolution?
temporal
What do we lose even further when using power doppler? Why?
temporal resolution - frame rate Power doppler uses more frame averaging.
What is twinkle?
the appearance of quickly fluctuating colors distal to a strong granulor reflector
What does the color-write priority set?
the threshold value/strength of the echo. If the echo strength is below the threshold it will be interpreted with color and displayed as color. If its below it will be interpreted like b-mode and will be displayed with grey scale
A high variance can indicate _______
turbulent or disturbed flow
If our spectral window is filled in what can this indicate?
we are catching a lot of velocities - spectral window placement? turbulent flow?
When does color mirror image occur?
when the ultrasound encounters a strong reflector (bone, lung) echoes can bounce several times and a mirror image can occur due to the reflection of echoes.
