Ultrasound Flashcards
(39 cards)
What is Half Value Tissue Thickness in US?
Tissue thickness that decreases signal by 3 dB
What is Impedance?
- Stiffness
- Density X Speed of sound
- Bigger impedance= bigger reflection
- Units= Rayl
What is Refraction?
Bending of sound waves due to change in speed
-Depends on Speed and change in angle
Do smooth (specular) or Non specular surfaces depends on angle of incidence?
-Specular reflectors depend on angle of incidence
How does frequency affect scatter?
higher frequency–> Smaller wavelength–> Surfaces appear non specular–> More Scatter
How does frequency affect HVT?
Increased frequency–> Decreased HVT
Equation for attenuation?
0.5 (dB/cmMHz) X Frequency of Tranducer X Distance (round trip)
How does the crystal thickness relate to probe frequency?
- Inversely proportional
- Increased crystal Thickness= Lower Frequency (Increased Wavelength)
- Decreased Crystal Thickness= Higher Frequency (Smaller Wavelength)
What is the Dampening Block?
Absorbs energy to change the SPL
- Thin Block: Long Spatial Pulse Length, narrow bandwidth (Doppler)
- Thick Block: Short SPL, Broad Bandwidth (better axial resolution)
What is the Matching layer?
Minimizes Acoustic Impedance between Transducer and Tissue
Optimal thickness if 1/4 Wavelength.
How does the transducer Frequency and Diameter affect the near field?
Increased Frequency= Longer Nearfield
Increased width= Longer Nearfield
Higher Frequency= Less DIvergence
What is the Spatial pulse length?
Number of Cycles emitted per pulse X Wavelength?
What is axial resolution?
- Ability to discriminate two objects in the direction of the beam.
- Two objects must be at least 1/2 SPL apart
- Does not depend on depth
What is lateral resolution?
- Ability to discriminate things perpendicular to the beam
- Higher Frequency (longer near field and narrower focal spot)= better lateral resolution
- Higher Scan Line Density= Better lateral resolution
- Wider Transducer Width= Better lateral resolution
What is Elevational resolution?
- Lateral resolution in the orthogonal plane
- Smaller Transducer element height= Better Elevation resolution
- Higher Frequency= Better elevational resolution
- Better with 1.5 dimension array
How do you improve axial resolution?
- Shorter SPL
- Thick Dampening Crystal
- Higher Frequency Probe (lower wavelength, lower SPL)
What is Side Lobe Artifact?
- Strong Reflector outside the main beam generates echos detectable by the transducer
- Ex: GB Sludge
- Assumption: All detected echos originate from the main beam
What is Beam Width Artifact?
- Object in the divergent field sends echos back to the main beam
- Ex: Bladder Debris
- Assumption: All detected echos originate from the main beam
- Fix: Correct the focal spot or place the tranducer at the center of the image
What is reverberation Artifact?
- Two parallel reflectors bounce echos back and forth before returning to detector
- Assumption: All echos return to detector after one reflection
What is comet tail artifact?
- Type of reverberation artifact
- Assumption: All echos return to detector after one reflection
- Triangle because later echos are more attenuated
What is Ring Down Artifact?
- Tetrahedron of air bubbles surround fluid and continuously send signal back
- Assumption: All echos return to detector after one reflection
- Ex: Air/Fluid in the duodenum
What is mirror artifact?
-Hit a strong reflector and echos bounce back and forth
-Assumption: All echos return to detector after one reflection
Ex: Lung/Liver interface
What is speed displacement Artifact?
- Slower velocity in tissues will result in longer return echo time (speed is slower in fat)
- Assumption: Velocity is same in all tissues
- Ex: Fat in liver will make the liver border look discontinuous bc it took longer in that area for it to return
What is Refracton Artifact?
- Due to the angle change and speed change in different tissues
- Assumption: All echos return to detector after one reflection
- Ex: Duplicated SMA
