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What is a hydrophone? What does it measure?
A small needle with piezoelectric crystal at the end. It measures pressure in a sound beam.
What is a schlieren?
A machine that uses acousti-optics to measure sound and light in beam profiles.
What is a calorimeter?
A machine that can calculate total power of the beam measuring heat gain and the time itakes to obtain the heat.
What is the AIUM and FDA bioeffects intensity limit?
SPTA
100mW/cm2 (unfocused)
1 W/cm2 or 1,000mW/cm2 (focused)
Highest and lowest output intensity
Highest- Pulsed doppler
Lowest- gray scale imaging
Are the strongest conclusions made when mechanistic and empiracle approaches agree?
Yes
Mechanistic- “Cause-effect”
Empiraical- “Exposure- Response”
Where is temperature elevation at a tissue-bone interference most likely?
Bone since bone is a sound absorber.
Which intensity is related to heating of tissue?
SPTA
exams that cause temps over 41C is considered harmful to a fetus.
Name the three thermal indices
TIS-Thermal index in soft tissue
TIB- Thermal index in bone
TIC- Thermal index at cranial bone or near skin surface
TI of 3 means that a temperature elevation of 3*C may occur.
Intensity of focused and unfocused beams
Unfocused- 100mW/cm2
Focused- 1,000mW/cm2 or 1 W/cm2
Mechanical index equation
MI= Peak-Pressure / square root of frequency
Mechanical index is higher (more likely to produce cavitation) with:
Higher pressure, lower frequency
Stable cavitation
Bubbles do not burst
Transient Cavitation
Bubbles burst
*also called normal or inertial cavitation.
In Vitro
“Outside of body”
Epidemiological studies
A large number of patients is required.
Limitations of epidemiologic studies
often retrospective
amiguities
other risk factors
Which beam is more dangerous, focused or unfocused?
Unfocused
What is the importance of quality assurance?
Routinely evaluate the ultrasound system to obtain optimal images.
Is a phantom an objective or subjective standard?
Objective
AIUM 100mm Test Object
Propagation speed of 1,540m/s but does not have attenuation properties of soft tissue and cannot evaluate gray scale.
Tissue Equivalent Phantom
Sound has a velosity of 1,540m/s. Attenuates like soft tissue so it can evaluate gray scale.
Doppler phantom
pump forces echogenic fluid through the vessels at known velocities. Used to assess accuracy of CW doppler.
What is ultrasound’s gold standard?
angiography
Hyperechoic
Portions of an image that are brighter than surrounding tissues
Hypoechoic
Portions of an image that are not as bright as surrounding tissues, or tissues that appear less bright than normal
Isoechoic
Structures with equal echo brightness
Homogeneous
A protion of tissue or a structure that has similar echo characteristics throughout.
Heterogeneous
Displaying a variety of different echo characteristics within the tissue.
Six basic assumptions of imaging systems
1. Sound travels in a straight line
2. Sound travels directly to a reflector and back
3. Sound travels exactly 1,540m/s
4. reflections arise from structures positioned along the beam’s main axis.
5. intensity of the of the reflections is related to the scattering characteristics of the tissue.
6. The imaging plane is extremely thin.
Reverberations
Multiple echoes appearing on the display as a result of US “pin-ponging” between two reflectors. Looks like a ladder.
*will always be mutliple, equally spaced, parallel to sound beam , along a straight line.
Comet Tail or Ring Down artifact
Appears as a solid line directed downward.
*single solid hyperechoic line, long echo, parallel to sound beam
Shadowing artifact
US beam is unable to pass through a structure because the structure has a haigher than usual attenuation. An anechoic shadow occurs under structure covering all structures deeper.
Edge Shadow or Shadowing by Refraction
Frefraction at the edge of a circular structure. Also called shadowing by refraction or edge shadow.
Enchancement artifact
Occurs when the medium the sound travels through has a lower attenuation rate than soft tissue.
*parallel to sound beam
Focal Enhancement or Banding artifact
Results from increase in beam intensity found with the focal zone of a sound beam.
*side by side band
Mirror Image artifact
sound may bounce off of a strong reflector, called a mirror, in it’s path and may be redirected.
*artifact deeper than true reflector
True reflector and artifact are equal distances from mirror.
Propagation Speed Errors
Results in correct number of reflections on scan and improper depths.
Appear as a step-off, split or cut.
Refraction artifact
Sound changes direction striking a boundary: obliquely, and when the media have different propagation speeds.
How to fix? Change your view.
Lobe artifacts degrade which resolution?
Lateral
How are side lobes created?
Mechanical or single crystal transducers create side lobes
Grating lobes
Arrays create grating lobes
Apodization
Subelements closer to the center of the sound beam are excited with higher voltages while the outermost sub elements, further away from the center of the beam, are excited with lower voltages.
Subdicing
Grating lobe artifact can be reduced or cured by dividing each element into even smaller, miniature pices.
Elevational resolution
Slice thickness artifact occurs when beam has a greater width than the reflector.
*We assume the imaging plane is razor thin.
Partial volume or section thickness artifact
Fill-in of an anechoic structure by structures above or below the ideal imaging.
Which transducer has the poorest elevational resolution?
Linear array
Speckle
grainy appearance, interference effects
Range ambiguity artifact
can be cured by lowering the PRF because the PRF is set too high.
General concept of artifact
“Small sound is good.”
Artifacts are created when the sound beam is larger than the reflector.
Ghosting (Doppler artifact)
Small doppler shifts that “bleed” into anatomy. Wall filters are used as a reject for doppler.
Cross talk
found in doppler only
*”doppler mirror”
Caused by doppler gain set too high and incident angle near 90* when flow is at focus.
Which method does CW Doppler use?
Fast Fourier Transform (FFT)
Which method does color doppler use?
Autocorrelation
Sound waves are:
mechanical, longitudinal waves, that travel in a straight line.
What are the three acoustic variables?
pressure, density, distance
What are the seven acoustic parameters?
period, frequency, amplitude, power, intensity, wavelength, propagation speed
Pressure units:
Pascals
Period
time it takes a wave to vibrate a single cycle, or time from the start of a cycle to the start of next cycle.
Frequency
number of particular events that occurs in specific duration of time.
Frequency of ultrasound
greater than 20kHz
Parameters to describe size or strength of a sound wave
Amplitude, power, intensity
Intensity
concentration of energy
Wavelength
distance or length of one complete cycle
What is the relationship between wavelength and freqency?
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Steve Ramsey, Alberta – Canada