Physics Flashcards
Doppler equation
Doppler shift = V cos theta 2Ft/c
Five functions of US receiver
Amplification Compensation Compression Demodulation Rejection
Amplification
enlargement of returning signal (“gain), makes returning signals brighter. No change or increase in energy.
Compensation (aka time gain compensation)
makes echos from similar structures appear with similar brightness.
deeper structures = more attenuation = less bright
Compensation will selectively amplify signals coming from deeper structures
Compression
reduces total range of signals
“compresses” the size of the difference b/t signals
More compression = less dynamic range
Characteristics of low compression
Higher dynamic range
more shades of gray
fewer bright brights and dark darks
What are the two main types of resolution?
Spatial & temporal
Spatial → discern two objects as separate.
Temporal → ability to locate moving structures
Three types of spatial resolution and what determines each?
Axial → spatial pulse length
Longitudinal → beam width
Elevational → beam height or “slice thickness”
Equation for axial resolution.
Relationship b/t axial resolution and SPL?
Axial resolution = SPL/2
Shorter pulse length = higher frequency = increased axial resolution
What determines temporal resolution?
Pulse repetition frequency (PRF)
Temporal resolution increases with higher frame rate (increased PRF) and decreased depth
Equations for Nyquist limit, PRF, and frame rate
Nyquist limit = PRF/2
PRF = Nyquist limit x2
PRF = 1/PRP
PRF = c/(depth)(2)
Frame rate = PRF/#lines
Synonyms for lateral resolution?
LATA.
Lateral, angular, transverse, azimuthal
What variables affect frame rate?
pulses per scan line
line density (# lines/image)
sector width
depth (listening time)
How to optimize images?
Decrease depth
Narrow sector width
Focal point at ROI → narrowest beam width → highest lateral resolution
What is TGC and LGC used for?
Adjusts gains in horizontal and vertical plane. Both used to compensate for attenuation.
LGC → compensates for enhancement artifact.
What is compression?
Post-processing decrease in returning amplitudes without changing relative difference b/t them. Increased compression = decreased dynamic range.
What is the relationship between compression and dynamic range? What are the features of low and high compression?
Inversely related.
Low compression → large dynamic range, less bright brights and dark darks, smoother, more shades of gray, less contrasted
More compression → less dynamic range, brighter brights/darks, more contrast, less shades of gray, harsher image
Why are low frequency transducers used to image deeper structures?
Less attenuation than higher frequency probes, at expense of lower spatial resolution.
