Physics, Artifact, M-mode Flashcards
What are the types of cavitation?
- Squeeze flow
- acceleration of blood as occluder approaches stop causing drop in pressure
- Vortex flow
- turbulence at edge of rapidly moving occluder
What is velocity (propagation speed) error artifact?
- sound propogates through some structures at velocity other than 1540 m/sec
- takes longer to travel through a structure - which is assumed by the ultrasound machine
What is specular reflection?
- mirror-like reflection of waves from a surface, in which waves from a single incoming direction (a ray) is reflected into a single outgoing direction.
- angle of reflection equals angle of incidence for objects > 1 wavelength diameter (~0.5mm)
What is cavitation?
- rapid formation of vaporous microbubbles in a fluid due to a local reduction in pressure
What are High Intensity Transient Signals (HITS)?
- under some conditions (vortices), vaporous microbubbles can coalesce into larger bubbles
- degassing: removal of gasses (principally CO2 in blood) from liquid medium
- larger bubbles persist and ar visible on echo
- no demonstrated neurologic sequelae from HITS microbubbles
Why are bubbles such good reflectors? Why do we see them with prosthetics?
- they have a very low acoustic impedance
- acoustic impedance (velocity x density)
- interface between materials with dissimilar acoustic impedances (velocity x density) reflects sound
- 23 (air) vs. 1465 (water)
To reduce aliasing on color-flow Doppler, a sonographer should perform this action?
- Changing the baseline shift
- allows velocities to be displayed up to twice th original Nyquist limit
What is pulse repetition frequency (PRF)?
- number of pulses of a repeating signal per unit time
- inversely related to PRP (PRF = 1 / PRP)
- inversely related to imaging depth (PRF x 1/d)
What environment can sound waves not travel through?
- vacuum
- pressure waves can only be transmitted through physical media consisting of molecules that interact with each other
What is the upper limit of human hearing?
20,000 Hz or 20 kHz
The frequency of a sound wave is measured in Hz as the:
- number of times particles vibrate each second in the direction of wave propogation
- 1/s
- the number of times a particle in a conducting medium vibrates per unit time
Ultrasound imaging is usually performed in what frequency range?
- 1-30 MHz
- lower frequencies (greater penetration) –> larger organs or deeper structures
- higher frequencies (better spatial resolution) –> smaller, more superficial structures
Define wavelength
the distance a wave travels during a single cycle
What type of tissue results in the fastest loss of ultrasound wave strenght?
- Lung
- because of the high content of air and the abundance of highly reflective tissue/air interfaces, the sound waves dissipate in the lung so fast that the lungs are virtually opaque to ultrasound
What is the main goal of the gel used during ultrasound imaging?
- Improve contact between the transducer surface and the skin
- eliminates any tissue/air interfaces which are highly reflective and thus prevent ultrasound transmission
What are Piezoelectric crystals?
materials that respond to electric signals by vibrating and generating acoustic waves and vice versa
To better assess rapidly moving structures, a sonographer should perform this action?
- Narrow scan sector width and decrease imaging depth = increased frame rate
- Higher frame rate is required to increase temporal resolution
- some machines enable direct manipulation of frame rate, others can be adjusted by sector width and imaging depth
- Higher frame rate is required to increase temporal resolution
How do Piezoelectric crystals generate ultrasound images?
- transmit waves by “exciting” the crystals in the transducer by an electrical stimulus,
- then receiving the ultrasound waves reflected by structures inside the body,
- translating them back into electrical signals that are used to form an image of the reflecting structures
Define Doppler shift?
- change in the frequency of a sound wave reflected by a moving target
- negative / red shift
- object moving away
- wavelength made longer (increased)
- frequency decreased
- positive / blue shift
- object moving toward transducer
- frequency incerased
- wavelength decreased
- negative / red shift
Define doppler angle?
- angle between:
- the direction of flow
- ultrasound beam
With time gain compensation, the machine is generally preset to perform this action?
- Decrease signal in near field, increase signal in far field
- increasing depth = increased attenuation
- TGC accounts for this signal loss by effectively incresing gain in parallel with increase in depth
- many modern systems automatically account for this so the knobs should be left in neutral position to start with then adjusted as needed
Define Harmonic imaging?
Echocardiogaphic images are formed from returning echoes at twice the insonifying frequency (second harmonic)
When does aliasing occur?
when doppler shift of high-velocity flow exceeds the Nyquist limit (1/2 the Pulse Repetition Frequency)
What are the two forms of “harmonics” or image enhancement currently used in diagnostic ultrasound?
- native tissue harmonic imaging
- contrast harmonics
Define contrast harmonics
- occurs when tiny bubles are insonified at a lower frequency than their natural frequency of vibration
- the vibrating bubbles then radiate back significant amounts of energy at the second harmonic
- requires a high-pass filter on the ultrasonic receiver to eliminate signal at the fundamental frequency
- the second harmonic image will show the echo contrast with significantly enhanced intensity relative to the surrounding tissue, which does not reflect the harmonic frequency
The energy of the transmitted ultrasound wave can be changed by adjusting which of the following?
- Power
- increasing the power increases the energy / heat delivered to the tissues
- increased gain will not do this
- visually this can create similar changes compared to adjusting the gain, which increases the amplitude of the signal
- increasing the power increases the energy / heat delivered to the tissues
Define spatial resolution of an ultrasound image?
- smallest distance between two objects that allows distinction between them
- also determines the size of the smallest object that can be visualized
Image resolution for a region of interest can be improved by this action?
Increasing the write zoom
What does time gain compensation do?
- part of postprocessing of the reflections designed to correct for beam attenuation as it travels through the body
- aims to provide a correction for the loss of intensity (or attenuation) by all these different mechanisms (scattering, absorption, reflection)
- based on assumption that acoustic properties of surrouding tissues are the same
The dynamic range of echoes displayed on the screen is adjusted by this?
- Compression control
- used to include or suppress weak echoes
When does time gain compensation become inaccurate?
- large differences in acoustic properties between adjacent tissues
- contrast agents that show much stronger attenuation
- This is the reason why acoustic shadowing artifacts are frequently seen distal to contrast filled blood pools, such as ventricles or atria
Attenuation is the combined result of these:
- scattering
- absorption
- reflection
The strength of the transmitted ultrasound wave is controlled by adjusting this?
Power control
What does Gain Control do?
determines to what extent the received signal is amplified
What is Range ambiguity artifact
- Deeper structures –> appear closer to the transducer than true location
- Occurs with high Pulse Repetition Frequency (PRF):
- when a second pulse is sent out, before the first signal along the same scan line is received
- Pulse repetition period determines maximum depth imaged
- US does not “disappear” beyond this depth and may be strong enough to return a signal
- Machine assumes all returning signals are result of most recent pulse
If a rapidly moving structure such as a cardiac valve appears to be moving in slow motion, what may be set too high?
- Persistence
- images can be averaged together to create a smoothing effect by increasing persistence
- lower persistence maintains temporal resolution and can keep a structure from appearing as though it were moving in slow motion
What does compression control do?
determines the dynamic range of received signals that are used to create the image
What is the difference between read and write zoom?
- Read zoom only magnifies the image without a change in resolution
- Write zoom increases line density and number of pixels in a given area
What does a positive doppler shift indicate?
indicates that the reflector is moving so that the angle between the transmitted beam and the direction of flow is > 90 degrees
What does a Doppler shift of zero indicate?
- indicates that the reflector is stationary or moving in a direction perpendicular to the beam
- Doppler angle = 90 degrees –> flow is neither toward nor away but perpendicular to the beam
Filtering eliminates “ghosting” artifact by removing this?
- Low-velocity signals
- when imaging higher-velocity regions, movement of cardiac structures produces low-velocity signals that appear on the screen as color and make it harder to interpret the area of interest
What is Pulsed repetition period (PRP)?
- time between pulses
- directly related to imaging depth
Increasing scan line density with a fixed sector width results in this?
Increase in spatial resolution
- increased resolution at the cost of decrease in frame rate and temporal resolution
The spatial resolution of an ultrasound image is equal to this?
- size of a pixel in the relevant direction
- also resolution is directly related to wavelength
Define temporal resolution
- shortest time between two events that allows distinction between them
- determines the shortest duration of an event that can be detected but “with confidence”
Define contrast resolution
- Minimal difference in the parameter displayed in the image as distinct pixel intensities
- reflection intensity in US images
What can be done to increase image resolution?
- increase write zoom
- reducing sector width (while maintaing scan lines –> increased spatial resolution)
- changing focal point on display
The temporal resolution of a sequence of ultrasound images is equal to this?
directly related to frame rate
- increased FR = improved TR
- decreased FR = decreased TR
What does compression control do?
- used to adjust the dynamic range of echoes displayed on the screen
- can be used to include or suppress weak echoes
Define duty factor
the fraction of time the transducer is sending compared to the time it is receiving
Describe how PW doppler works
- transducer sends out packets of US waves in a single pulse
- transducer waits for waves to interact with the subject and return to the transducer where frequency shift is measured
- transducer will only send out another pulse after it receives the preceding pulse
- PW doppler can determine the velocity of blood cells at a very specific spatial location
- because the time it takes for the beam to return is a known constant (velocity of US in tissue = 1540 msec)
What are the positives/negatives of PW doppler when compared to CW doppler?
- Positive
- spatial localization better
- Negative
- there is a maximum velocity that can be measured
- can only sample at a defined pulse rate frequency (PRF)
- PRF is the time it takes to send out and receive a signal pulse
What is the Nyquist Limit?
- Maximum velocity that can be sampled without aliasing
- equal to = PRF/2
- Synonymous with aliasing
What are ways to reduce aliasing (and thereby increase the Nyquist Limit)?
- change the baseline
- increase the PRF
- decrease the imaging depth
- decrease the frequency of the transducer
- decrease the size of the sampling volume
What are the positives/negatives of CW doppler when compared to PW doppler?
- Positive
- Can sample much higher velocities (because it is constantly sending/receiving pulses)
- like those seen in the LVOT / aorta
- not limited by PRF
- Can sample much higher velocities (because it is constantly sending/receiving pulses)
- Negative
- inability to localize velocity to a specific location
- samples all frequency shifts along a given sample line
Myocardial Speckle tracking
- speckles are formed from interface patterns between ultrasound and the myocardium
What are the 2 main assumptions when working with the Bernoulli equation?
- Linear acceleration
- Negligent viscous friction
What changes will result in improved near field spatial resolution?
- changing to a high frequency probe (shorter wavelength)
What are two ways to increase temporal resolution?
- decreasing depth
- narrowing the sector width
What is one way to avoid Range Ambiguity artifact?
decreasing PRF when scanning deeper
In a patient with A-fib and calcific aortic disease, what adjustment needs to be made when obtaining continuous wave (CW) Doppler in order to capture the best representation of the aortic velocity in one image?
- Lowering the sweep speed
- Patients with A-fib will have varying RR intervals resulting in variable peak velocities across a stenotic AV due to differing stroke volumes
- important to calculate an average of 10 cardiac cycles
- lower sweep speed allows you to see more cardiac cycles per frame, allowing for calculation of the average velocity