Ultrasound (Erin) Flashcards
Velocity of USS Waves in a medium equation: C = what?
_________________________
_/(Bulk Modulus/density)
C (velocity) = What? X What?
C (velocity) = frequency x wavelength
We set frequency of USS Waves but the velocity depends on the …?
Medium. The wavelength changes to compensate.
Diagnostic USS Waves travel at what frequencies in general?
2-15MHz
Humans can only hear what frequency range?
20-Hz - 20kHz
In soft tissue, Uss waves travel at what speed?
1540m/s
Pulse Echo: Time taken for USS wave to go there an back = what?
2xDepth / velocity
What relationship of frequency shift to Pulse Repetition Frequency causes Aliasing?
Aliasing occurs when PRF is less than half of the frequency shift.
ie Frequency shift > 1/2 PRF
State the Doppler shift equation
2 x frequency x velocity of blood x cos(theta)
—————————————————
velocity of sound
Thermal index refers to the power needed to increase temperature by how many degrees?
1 degree
Mechanical Index must be less than what number to avoid risk of cavitation?
0.7
True/false: mechanical bio effects are more likely to occur in the lung and intestines than soft tissues
True - risk of bruising and cavitations
There is (increased/decreased) likelihood of acoustic cavitation at lower frequencies
Increased
Thermal index should be kept below what number (generally)
1
Thermal index should be kept below what number for obstetrics?
0.7
Thermal index: temperature rise depends on…(4)
Frequency
Power
Duration of scan
Tissue type
Mechanical index should be kept below what level for neonates?
0.3
How safe are microbubble USS contrast agents?
Very safe. Can use in renal/liver failure, even in pts with allergies to iodine/gadolinium.
What is the effect of USS Harmonics on:
- lateral resolution
- axial resolution
Improves lateral resolution
Reduces axial resolution
USS Harmonics makes what structures appear clearer?
Cystic and fluid-filled structures
Acoustic impedance equation?
Z = p x c
Acoustic impedance = density of tissue x speed of sound
What is Snell’s law?
Angle of refraction at tissue boundaries.
sin(theta-1)/sin(theta-2) = c1/c2
The near field/Fresnel zone of the USS beam can be calculated as…?
Diameter^2 / 4x wavelength
What are the 3 methods of USS Beam focusing?
- Time-delays of firing waves from different transducer elements
- Curved crystals in transducer
- Lens to mould the beam
What is the speed of ultrasound in air?
300m/s
around x% of the signal is reflected at the kidney-perinephric fat boundary
1%
USS waves travel through Bone, Fat Muscle, liver and Air and in the following order from fastest to slowest:
Bone
Muscle
Liver/kidney
Fat
Air
True/false: The velocity of sound is independent of body temperature
False. Temp affects density.
T/F: Aliasing occurs if the Nyquist criteria are satisfied
False.
T/F: Aliasing occurs less at higher angles
True. higher angle = cos theta closer to 0 (between 0-90 degrees)
The wavelength of ultrasound in soft tissue is approximately…
0.1- 1.5 mm
Artefacts: Acoustic enhancement occurs when…
the sound travels through a structure which is heavily fluid-based.
Artefact: Ring down artefact is caused by…?
resonance within gas bubbles
Artefact: Reverberation happens close to…
strongly reflective structures
Artefact: Comet tail artefact is…
a kind of reverberation artefact seen underneath small foci of highly reflective material such as the crystals found in the gallbladder wall
Full destructive USS wave interference only happens if…
two waves are of the same wavelength and opposite phase when they meet
Mechanical Index is calculated using which 2 figures?
peak rarefaction pressure
Square root of Frequency
What is an acceptable frame per second rate?
30 fps
How many lines per frame makes a good ultrasound image
100 lines per frame
What percentage of the gas/tissue interface is reflected?
99.9%
What percentage of the bone-soft tissue interface is reflected?
Around 40%
What percentage of the fat to muscle interface is reflected
1%
What percentage of the liver to muscle interface is reflected
0.01%
What is the acoustic impedence equation
Acoustic Impedence (Z) = density (ρ) x speed of sound in the tissue (v)
What unit is acoustic impedence measured in
Rayls
In doppler what happens to frequency if the blood is travelling towards the transducer
Increase in frequency
In doppler what happens to frequency if the blood is travelling away from the transducer
Decrease in frequency
How is velocity measured in doppler ultrasound
Measure of the change of frequency
What transducer angle does doppler work best/worst at?
best at 0 degree (theoretically can’t in actual practice)
Worst at 90 degrees
In Duplex Ultrasound what colour is given to higher frequencies (representing flow towards the transducer)
Red
(BART - blue away, red towards)
In Duplex Ultrasound what colour is given to lower frequencies (representing flow away from the transducer?
Blue
(BART - blue away, red towards)
Does duplex scanning produce quantitative date?
No
Define Thermal Index
The Thermal Index (TI) is a figure which compares the power being emitted to that which would cause a temperature rise of 1oC
At what level is the TI considered definitely safey
TI<0.5
At what TI should care be made to keep scanning time to a minimum
TI >0.7
What is the effect of a higher frequency on temperature rise in tissue
Higher effect
What is the effect of a lower frequency on temperature rise in tissue
Lower effect
How is MI calculated?
Calculated as the peak rarefaction (NEGATIVE) pressure divided by the square root of the frequency of ultrasound being used.
At what MI level does the risk of cavitation start?
MI >0.7
At what MI does the risk of Neonatal lung and intestinal damage occur?
MI >0.3
What is the risk with increasing mechanical index
Cavitation
At what TI is fetal scanning not recommended, no matter how brief?
TI >3
How is mechanical damage caused by ultrasound
At subcellular level, the acceleration of particles within cells can damage the cell wall
Alignment of subcellular structures is a mechanism of damage caused by USS T o r F?
True
Acoustic streaming – repeated passage of waves in one direction cause alignment of subcellular structures in the direction of the sound travel. This can have a negative effect on the cell membranes.
What does 1Hz stand for
1 Hz is 1 wavelength per second
How is acoustic impedence (Z) calculated?
Acoustic impedence (Z) = density (p) x speed of sound in that tissue (V)
How is the reflected portion calculated?
(Z1-Z2)2 / (Z1 + Z2)2
What is the unit of acoustic impedence Z?
Rayls
What is the velocity of USS waves in air?
330 ms-1 (approx)
What is the velocity of USS waves in soft tissue?
Approve 1540 ms -1
What is the velocity of USS waves in bone?
4000ms-1/5080 ms-1
Are USS waves faster in air or bone?
Bone
Are USS waves faster in soft tissue or bone?
Bone
Are USS waves faster in soft tissue or in air?
Soft tissue
How is USS velocity calculated?
_________________________
C = _/(stiffness/density)
Speed = square root of (stiffness/density)
If the stiffness increases what happens to the speed of USS waves
IncreasesI
If the density increases what happens to the speed of the USS waves
Decreases
What is the effect of increased temperature on the speed of ultrasound waves
Increases
Describe constructive interference
Interaction that occurs when the two waves are exactly in step and their amplitudes add up
Describe destructive interference
Interaction that occurs when the two waves are out of phase
Results in reduction in intensity
If equal and exactly out of phase they completely cancel each other out
What is frequency of a sound wave?
How many times per second the compression phases passes ant single point in the medium
Define acoustic impedence
Resistance experienced by an ultrasound beam in the medium
What is the main crystal type used in ultrasound?
Lead Zirconate Titanate (PZT)
What is the curie temperature for PZT?
350 degrees C
What is the effect of tissue harmonic imaging on lateral resolution
improves lateral resolution
What is the effect of tissue harmonics on axial resolution
Reduces axial resolution
What is the effect of using tissue harmonic imaging on resolution when scanning patients with a large BMI?
Improved resolution
What is the effect of tissue harmonics on reverberation and side lobe artefacts
Reduces side lobe and reverberation artefacts
What is the effect of tissue harmonics on imaging tissue structures
Makes tissue structures seem clearer
What is the wavelength of ultrasound in soft tissue
Around 0.1-1.0mm
How are contrast agents given in ultrasound?
IV injectiona
What is the contrast used in ultrasound?
Microbubbles
Is iodine used in ultrasound for contrast
No
What is the wavelength of 1.5MHz ultrasound wave in soft tissue?
1.0mm
What is the wavelength of 15MHz ultrasound wave in soft tissue?
0.1mm
Does bone have a high Z or low Z (acoustic impedence)
High Z
Does PZT crystal have a low or high Z (acoustic impedence)
High Z
Does air have a high or low Z (acoustic impedence)?
Low
Does lung have a high or low Z (acoustic impedence?)
Low