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What are the 2 forms of harmonic imaging?
Tissue harmonics & Contrast harmmonics
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What are the 5 requirements for contrast agents?
- -They are safe.
- -Metabolically inert (doesn't interact)
- - Long-lasting
- -Strong reflector of ultrasound
- -small enough to pass through capillaries
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How does contrast harmonics work?
With interaction of microbubbles, a small amount of energy is converted from fundamental to harmonic frequency. This increases the frequency so you get a better image.
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When are contrast harmonics created?
During REFLECTION of sound.
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Linear
proportional or symmetrical
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Non-linear behavior
irregular, disproportional, asymmetrical--behaves unevenly
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Contrast agents (microbubbles)
- -administered by injecting
- -enclosed in a shell
- -have different acoustic fingerprints which result in stronger reflections that light up areas
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Harmonic frequency sound arises from ____________ behavior.
non-linear
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Harmonic imaging
The creation of an image from sound reflections at TWICE the fundamental frequency (or 2nd harmonic frequency) of the transmitted sound.
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Fundamental frequency
The sound created by the transducer and transmitted into the body.
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Advantages of harmonics:
- -Reduction of acoustic noise
- -Enhances contrast resolution
- -improves borders
- -demonstrates shadowing & enhancement
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What increases mechanical index (MI)?
- -Lower frequency
- -Stronger sound waves
- -High pressure variation
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Mechanical Index (MI)
Estimates the amount of contrast harmonics that will be produced.
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What level of MI creates linear behavior backscatter?
Low MI
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What level of MI creates non-linear behavior (resonance)?
Higher MI (medium)
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What level of MI creates disruption and extreme non-linear behavior? (bubble disruption)
Highest MI
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What does the relationship between the nature of the outer shell and the gas within the bubble (contrast) determine?
The stability and longevity in circulation.
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When are tissue harmonics created?
During TRANSMISSION of sound.
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Tissue harmonics
-Conversion of miniscule amount of energy from fundamental frequency to harmonic frequency.
-Non-linear behavior of speed caused by sound travelling.
-Harmonic frequency caused by variation in speeds.
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With regards to tissue harmonics, non-linear behavior is _________ through compression and ________ through rarefaction.
faster, slower
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Where are the strongest tissue harmonics created?
In the MAIN beam.
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Which is stronger: Contrast harmonics or Tissue harmonics?
Contrast harmonics
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With regards to tissue harmonics, the farther the beam travels, the _________________ the harmonics.
stronger
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Where can harmonic frequencies NOT exist?
At superficial depth
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What is the relationship between sound beam strength and the amount of tissue harmonics?
Weak beam = no harmonics
Medium beam = tiny amount of harmonics
Strong beam = Significant amount of harmonics
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Is Color Flow information qualitative or quantitative?
Qualitative (an exact #)
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What is Color Doppler?
Imaging that presents 2-D, cross-sectional, real-time blood flow or tissue motion information along with 2-D, cross-sectional, real-time gray scale anatomic imaging.
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For each color image, what determinations does the equipment make?
-How long it takes for the sound beam to travel to and from the site of the echo
-How strong the echo is
-If there is a Doppler shift present
-The magnitude of the Doppler frequency shift
-The direction of the Doppler shift
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What does color flow represent?
the MEAN velocity from the sample volumes and the angle between the ultrasound beam and the blood flow
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What type of Doppler is color flow?
Pulsed Doppler
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Autocorrelation
A mathematical technique that detects the Doppler shift
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What does autocorrelation determine?
The mean and variance of the Doppler shift info. at each location along the scan line.
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What is faster: FFT used in Spectral display or autocorrelation used in color flow?
Autocorrelation
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Describe the "phase shift" approach of autocorrelation:
Compares 2 consecutive pulses returning from a given sample volume. Uses time delay to send out the pulses. The phase shift between 4 or more pulses are used to estimate mean velocity which is used to display the corresponding color.
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Describe the colors in the display of Doppler information:
Red indicates flow towards the transducer. Blue indicates flow away from the transducer. Darker colors indicate slower speeds, whereas lightest/brightest colors indicate faster speeds.
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What does an increase in luminance of color on color display mean?
stronger signal (amplitude)
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What does the saturation of the color indicate on color display?
Less saturated = faster velocity
more intense saturation = slower velocity
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Describe a velocity map:
-Colors are arranged vertically
-Slowest flow towards baseline
-Faster flow at ends of scale
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Describe a variance map:
-Additional colors added on side
-Side-to-side changes relate to laminar vs. turbulent flow
-Green or yellow used to tag turbulent flow
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The maximum frequency that can be detected with color flow is limited by ____________________.
The sampling frequency
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The highest frequency that can be displayed with color flow without aliasing = ?
1/2 PRF
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Describe High Pass Clutter Filter:
-affects lowest frequency that can be displayed
-Only allows frequencies greater than cutoff frequency to be displayed
-Eliminates low flow during diastole
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What is the relationship between PRF and High Pass Clutter Filter?
Higher PRF = Higher filter
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Frame Rate
# of images produced per second.
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Color flow has a _____________frame rate.
slower
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What does frame rate affect?
Temporal resolution
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What factors is frame rate dependent on?
- -Size of color box
- -Position of color box
- -Line density
- -PRF
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What can you do to get a better frame rate?
Decrease the # of lines, decrease color box width, put the color box at a shallower depth
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Smoothing (Persistence)
Average of consecutive images to improve signal to noise, creating a smoother color
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What are the advantages of Color Imaging?
-Can visualize vessels too small to be seen in gray scale
-Aids in determining where the peak systolic velocities are in order to place sample volume
-Documentation of pathologies such as occlusions, severe stenosis, and turbulent flow
-Can see flow in the entire vessel lumen
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What are the disadvantages of Color Imaging?
-Qualitative (average, not exact) information
-Lowers PRF and frame rate (degrades images, diminishes spatial resolution, more aliasing, and diminishes temporal resolution)
-Blood flow is angle dependent
-Flow direction is arbitrary
-Color may obscure pathology within a vessel
- Color flash
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What is the main difference between Power Doppler and Color Flow Doppler?
Power Doppler does not show the direction of blood flow.
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Power Doppler (color power/energy mode)
The strength (amplitude) of the reflected signal is processed. The brightness is directly related to the number of moving blood cells. Displayed in amplitude.
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What are the advantages of Power Doppler?
-Improved sensitivity to flow (or slow velocity)
-Unaffected by Doppler angles
-No aliasing since velocity information is ignored
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What are the disadvantages of Power Doppler?
-VERY slow frame rate (lower temporal resolution)
-Susceptible to motion (flash artifact)
-No flow direction
-No velocity measurement
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Aliasing
Occurs when Doppler sampling rate is too low and/or when Doppler Shift frequency exceeds half the PRF (Nyquist limit)
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How can you correct aliasing?
-Adjust the scale to its maximum (increases PRF)
- Select lower frequency transducer
-Reposition transducer so area of interest is shallower (increases PRF)
- Shift the base line
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Mirror Image or Ghosting
Duplicates structure on the other side of a strong reflector. (common around diaphragm and lungs) The image closest to the transducer is usually real.
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How can you fix Mirror image or ghosting?
Moving usually fixes this artifact
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Mirror Image: Spectral Doppler (Crosstalk)
ELECTRONIC duplication of spectral info. Spectral info. appears on both sides of the baseline.
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How can you eliminate Mirror Image: Spectral Doppler (crosstalk?)
Tun the gain down or increase power
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Ghosting or Flash
Occurs in Color Doppler. Anything in the field of view that moves is shown in color--this generates blotches of color that can obscure the field of view
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How do you remedy Ghosting (Flashing?)
-Increase wall filter
-Decrease persistence
-Reduce width of the color box.=
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Color Blossoming (Bleed)
The extension of color beyond the region of flow to the adjacent tissue
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How do you eliminate color blossoming (bleed?)
Decrease the color gain or transmit power.
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Color Noise or Clutter
Random variations in signal detection causes areas without flow to be embedded with color.
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How do you eliminate Color Noise/Clutter?
Increase the filter or threshold to eliminate the low level echoes.
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Color Bruit (Visible)
Useful flow phenomenon: mixture of color is seen within the soft tissue adjacent to the blood vessel as a result of the vessel wall vibration
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Why are visible color bruits useful?
They can act as a marker of vascular pathology. (Wall vibration is an indication of a severe flow disturbance.
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Where can visible color bruits be seen?
AV fistulae, arterial stenosis, and psuedoaneurysms
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Spectral Doppler Bruit
Small, strong signal prominent above and below the baseline. (Occurs in systole)
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