Ultrasound Flashcards
Harmonic can improve what type of resolution?
Both lateral and axial resolution as it causes the beam to become narrower, and you are using higher frequency.
Compression of a sound wave is what type of pressure?
High pressure
What is the equation for speed of a wave?
Speed = wavelength x Freq.
Therefore wavelength = 1/freq
When the speed of sound changes in the body what happens to the frequency and wavelength?
Frequency stays the same
Wavelength changes!
What is the attenuation coeffient of sound in soft tissue?
0.5dB/Mhz/cm
What is the attenuation equation of sound in soft tissue?
Attenuation = 0.5 x Distance x freq
FOr solving for distance = dB (attenuation)/freq(Mhz) x 0.5 (dB/cm/Mhz)
What is the half value thickness (HVT) intensity loss?
The tissue thickness that reduces the US intensity by 3 dB
What is a high Q factor?
Light damping and therefore long SPL
- low bandwidth
- Poor sp resolution
- Doppler used this
Low Q factor is what?
Heavy damping and therefore short SPL
- increase bandwidth
- harmonics
- Sp resolution increase
What is the duty factor?
% time trasmitting sound
What is the fresnel zone and the fraunhofer zone?
Fresnel = near field
Fraunhofer = far field
How is RI (resistive index) calculated?
Systolic - diastolic/systolic
How is PI (pulatile index) calculated?
Systolic - diastolic/mean velocity
What the portal velocities for the dog? cat?
10-25cm/s = dog
10-18 cm/s = cat
What are the four interaction with matter sound can have?
Reflection
Refraction
Scatter
Absorption
What is the equation for impedence?
Z = density x speed of sound (1540m/s)
What is the equation for refraction?
sin angle 1/sin angle 2 = Speed 2/Speed 1
What is the classic example of refraction?
Edge shadow artifact
What causes the bending of the sound wave during refraction?
Change in speed
Angle of incidence
Hence the equation of Snell.
What are the two types of scattering?
Specular (smooth)
Non-specular
What type of scatter relies on angle of incidence?
Specular or smooth
Non-specular or rough, the angle has no influence
What type of scatter relies on frequency?
Non-specular or rough. The higher the frequency or smaller the wave length will reflex more off of surface - this give organs their echo texture.
What is the unit for impedence?
Rayl
What determines the amplitude of the echo?
Angle and Impedence(speed)
What is impedance?
The stiffness of tissue
The frequency is determinded by what factor of the probe?
The thickness of the PZT material = 1/2 wavelength
What is the matching layer of the probe?
The layer between the transducer and the tissue
Minimizes acoustic impedance differences between probe and patient (1/4th the wavelength)
What are the two different types of probe and how do they differ?
- Sequenced: Linear or curvalinear
- Big probe
- Each element fires individually with no steering noted
- Phased
- Uses electronic steering
- Small probe
- Elements work together
Near field (Fresnel Zone) size is dependent on what?
Higher frequency = Longer near field
Larger diameter element = longer near field
What effects the axial resolution?
Spatial pulse length (Low Q = shorter pulses and High Freq = shorter pulses)
Axial res = SPL/2
If an object is closer than SPL/2 it will not be resolved
So… you have an object that is 2mm in diameter and you have a SPL of 5mm… will the object be resolved? Well 5/2 = 2.5mm and since that is larger than 2mm then the object will NOT be resolved.
What is lateral resolution based off of?
- Beam width.
- Focal zone will help this
- High Frequencies shrink beam width (harmonics does this too)
- More scan lines increase it
- Narrow width
Why do higher frequencies not penetrate as much?
They are attenuated more.
Distance = dB/(Freq x 0.5) — the higher the freq the smaller the distance.
What is the elevation resolution based off of?
Heigh of transducer
The side-lobe artifact occurs in what type of probe?
Linear… no steering of the beam
The classic look of the side-lobe artifact
Pseudosludge in the gallbladder
How is the beam width artifact made?
As the beam diverages after the focal point it passes the width of the probe… anything it encounters it will send it back towards the probe and the probe will place it in the main beam.
Adjust your focal zone
What is the difference between a comet-tail, ring down and reverb artifact?
Comet tail = metal encounter or the highly reflective objects are super close and therefore cannot distinguish between them
Ring-down = tetrahedron air bubble
Reverb = between two highly reflective objects - separate white lines
Speed displacement artifact is the same as?
Progation speed error - Speed of sound slows down in fat— so if for example there is a fat lobe in the liver… the farther border of the liver will look farther away.
Where does refraction artifact happen alot?
Between the rectus muscles and the midline fat… the sound coming back gets bent by the lobes of the muscles and thus causing the deep structures to move.
What is the best angle for doppler?
30-60 degrees
90 = 0 speed and less than 20 will cause refractopm and signal loss.
What is spectral doppler?
Pulse wave and Continuous doppler
What is the BART scheme?
Blue away
Red towards
In doppler
Power doppler - each pixel represents what and how does this affect the image?
Frequency shifts - so the intensity of the change. Not direction or anything.
- No aliasing
- Independent of the angle
- Supersensitive to flow.
What is the nyquist limit (kHz) in Doppler?
1/2 pulse repetition frequency.
How can you fix aliasing on Doppler?
Decreasing doppler shift - with increasing angle or lowering the frequency.
Increase scale
Increase PRF
The twinkle artifact has a greater sensisitivity for small stones than what other artifact?
Acoustic shadowing
What is color bleed?
Color is extending beyond vessel wall.
Decrease the color gain improves this.
Why can harmonics not be used in the near field?
It takes time for the tissue to create the harmonics.
What artifacts does harmonics improve?
- Reverb
- Less side lobe
- Less scatter
- Less slice thickness
Removes clutter and noise
Decreases depth.
Shadowing increases in width
Better Spatial res and contrast
What does a harmonic make a solid structure look like?
Cyst (more anechoic)
What artifacts does spatial compounding do?
- Shadowing is gone
- Reverb is better but not gone
Sharpens edges
Spatial compounding makes cysts look what?
Solid - by removing posterior features.
The higher the frequency… the longer the what?
Near field
What can you decrease to look for slow flow on doppler?
PRF
What is the difference between shear wave and strain elastography?
Shear wave: uses perpendicular shear waves and their velocities to check thickness
Strain: Uses pre and post compression frequencies
Both are put on a color map with red/yellow = hard
Blue/green = soft
What is the distance to reflector or range equation?
Distance = 1/2 x speed x time of round trip
What is the pulse repetition period?
1/PRF
What is the difference been reverb and ring-down?
Reverb has distinct repetitive linear interfaces
Ring-down does not.
Discuss US contrast agents…ie what are they made of and how are they eliminated?
Bubble of inert fluorocarbon gas surrounded by lipoprotein shell. Easily distorted or destroyed.
Inert gas is breathed out
Lipoprotein is excerted in the bile.
What does yellow flow in doppler likely represent?
Aliasing.
What is Anisotropy when refering to US?
It means the echo amplitude is highly dependent on the angle of insonation
Ex: 90º to a tendon makes it look hyperechoic and normal…however, as you go off angle then you start to lose that echogenicity.
What is the difference between side-lobe artifact and grating lobe artifact?
Side lobe is seen in all probes and is due to lateral expansion
Grating is only seen in linear probs and is due to constructive interference of the ultrasound beam from adjacent elements
What does spatial compounding do to nephroliths?
More hyperechoic
Less distinct margins
Weaker acoustic shadow
What is the difference between pulse repetition frequency and pulse repetition period?
PRF = Number of pulses per second
PRP = length of time between pulses
PRP = 1/PRF
Duty factor: Pulse duration/PRP
