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What are the 2 forms of harmonic imaging?
Tissue harmonics & Contrast harmmonics
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
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.
When are contrast harmonics created?
During REFLECTION of sound.
proportional or symmetrical
irregular, disproportional, asymmetrical--behaves unevenly
Contrast agents (microbubbles)
- -administered by injecting
- -enclosed in a shell
- -have different acoustic fingerprints which result in stronger reflections that light up areas
Harmonic frequency sound arises from ____________ behavior.
The creation of an image from sound reflections at TWICE the fundamental frequency (or 2nd harmonic frequency) of the transmitted sound.
The sound created by the transducer and transmitted into the body.
Advantages of harmonics:
- -Reduction of acoustic noise
- -Enhances contrast resolution
- -improves borders
- -demonstrates shadowing & enhancement
What increases mechanical index (MI)?
- -Lower frequency
- -Stronger sound waves
- -High pressure variation
Mechanical Index (MI)
Estimates the amount of contrast harmonics that will be produced.
What level of MI creates linear behavior backscatter?
What level of MI creates non-linear behavior (resonance)?
Higher MI (medium)
What level of MI creates disruption and extreme non-linear behavior? (bubble disruption)
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.
When are tissue harmonics created?
During TRANSMISSION of sound.
-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.
With regards to tissue harmonics, non-linear behavior is _________ through compression and ________ through rarefaction.
Where are the strongest tissue harmonics created?
In the MAIN beam.
Which is stronger: Contrast harmonics or Tissue harmonics?
With regards to tissue harmonics, the farther the beam travels, the _________________ the harmonics.
Where can harmonic frequencies NOT exist?
At superficial depth
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
Is Color Flow information qualitative or quantitative?
Qualitative (an exact #)
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.
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
What does color flow represent?
the MEAN velocity from the sample volumes and the angle between the ultrasound beam and the blood flow
What type of Doppler is color flow?
A mathematical technique that detects the Doppler shift
What does autocorrelation determine?
The mean and variance of the Doppler shift info. at each location along the scan line.
What is faster: FFT used in Spectral display or autocorrelation used in color flow?
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.
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.
What does an increase in luminance of color on color display mean?
stronger signal (amplitude)
What does the saturation of the color indicate on color display?
Less saturated = faster velocity
more intense saturation = slower velocity
Describe a velocity map:
-Colors are arranged vertically
-Slowest flow towards baseline
-Faster flow at ends of scale
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
The maximum frequency that can be detected with color flow is limited by ____________________.
The sampling frequency
The highest frequency that can be displayed with color flow without aliasing = ?
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
What is the relationship between PRF and High Pass Clutter Filter?
Higher PRF = Higher filter
# of images produced per second.
Color flow has a _____________frame rate.
What does frame rate affect?
What factors is frame rate dependent on?
- -Size of color box
- -Position of color box
- -Line density
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
Average of consecutive images to improve signal to noise, creating a smoother color
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
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
What is the main difference between Power Doppler and Color Flow Doppler?
Power Doppler does not show the direction of blood flow.
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.
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
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
Occurs when Doppler sampling rate is too low and/or when Doppler Shift frequency exceeds half the PRF (Nyquist limit)
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
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.
How can you fix Mirror image or ghosting?
Moving usually fixes this artifact
Mirror Image: Spectral Doppler (Crosstalk)
ELECTRONIC duplication of spectral info. Spectral info. appears on both sides of the baseline.
How can you eliminate Mirror Image: Spectral Doppler (crosstalk?)
Tun the gain down or increase power
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
How do you remedy Ghosting (Flashing?)
-Increase wall filter
-Reduce width of the color box.=
Color Blossoming (Bleed)
The extension of color beyond the region of flow to the adjacent tissue
How do you eliminate color blossoming (bleed?)
Decrease the color gain or transmit power.
Color Noise or Clutter
Random variations in signal detection causes areas without flow to be embedded with color.
How do you eliminate Color Noise/Clutter?
Increase the filter or threshold to eliminate the low level echoes.
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
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.
Where can visible color bruits be seen?
AV fistulae, arterial stenosis, and psuedoaneurysms
Spectral Doppler Bruit
Small, strong signal prominent above and below the baseline. (Occurs in systole)