Physics Ch 8 US Flashcards
loss of 3 dB (-3 dB) –> signal intensity (power) –> dec/inc/no change? by how much?
dec by 50%
what is “half-value” thickness (HVT)?
tissue thickness that reduce/attenuate US intensity by 3 dB
US –> higher freq –> scatter –> dec/inc/no change?
inc
higher freq –> half value thickness (HVT) –> dec/inc/no change?
dec
attenuation –> loss of intensity of US beam (d/t absorption/scattering) –> what is rate of attenuation in soft tissue?
0.5 (dB/cm)/MHz
what factors determine strength of echoes? (2)
- angle
- impedance
higher freq US probe –> has thinner or thicker xl?
thinner
which component of the US probe contributes to spatial resolution (axial resolution)?
dampening block
thin dampening block:
- light vs heavy damping?
- low vs high Q?
- short vs long spatial pulse length?
- narrow vs broad bandwith?
- light damping
- high Q
- long spatial pulse length
- narrow bandwidth
thick dampening block:
- light vs heavy damping?
- low vs high Q?
- short vs long spatial pulse length?
- narrow vs broad bandwith?
- heavy damping
- low Q
- short spatial pulse length
- broad bandwidth
light damping (high Q) –> used for what kind of US study? why?
Doppler –> preserve velocity data
heavy damping (low Q) –> pro? (1)
high spatial resolution (axial resolution)
US probe –> matching layer –> fx?
minimize acoustic impedance differences bw transducer & pt
US probe –> matching layer –> optimal thickness (relative to wavelength)?
1/4 wavelength
higher freq –> near field (Fresnel zone) –> inc/dec/no change?
inc
larger US probe –> near field (Fresnel zone) –> inc/dec/no change?
inc
higher freq –> far field (Fraunhofer zone) –> divergence –> inc/dec/no change?
dec
US beam –> what is focal zone (focal depth)? what is its significance?
spot bw converging & diverging beams:
- narrowest beam
- area of max intensity
==> best lat resolution
what are the 3 dimensions in US?
- axial
- lat
- elevation (slice thickness)
2 objects need to be what distance apart to be seen as 2 separate objects?
1/2 spatial pulse length
US –> what is axial resolution?
ability to tell apart 2 closely spaced objects in the direction of the beam
US –> what is lateral resolution?
ability to resolve objects in a direction perpendicular to beam direction
axial resolution –> constant at different depths –> T/F?
T
higher freq –> lat resolution –> dec/inc/no change?
inc
US probe –> larger diameter –> lat resolution –> dec/inc/no change?
inc
US probe –> higher scan line density –> lat resolution –> dec/inc/no change?
inc
side lobe artifact –> occur more w linear vs phased array transducer vs no effect?
linear
power doppler –> Doppler angle does not matter –> T/F?
T
Power doppler –> produces aliasing artifact –> T/F?
F (color & spectral doppler do produce aliasing artifact. Power does not)
doppler –> aliasing –> occur when velocity is higher than what?
Nyquist freq
doppler –> aliasing artifact –> remedy? (2)
- inc pulse repetition freq (PRF)
- inc scale
can adjust what control to brighten or darken image? (2)
- output power (transmit gain)
- receiver gain
dark image –> want to brighten it up –> how do the following brighten image?
- output power (transmit gain)
- receiver gain
- output power (transmit gain): inc power of US beam
- receiver gain: inc strength of sound that returned to transducer
dark image –> want to brighten it up –> adjust output power (transmit gain) vs receiver gain –> which is optimal? why?
receiver gain
inc output power (transmit gain) –> degrade lat resolution
time gain compensation –> fx?
compensate for loss of echo strength caused by depth of reflector –> uniform brightness from top to bottom
how does pulse length relate to bandwidth?
pulse length = 1/Bw
harmonics –> pros? (3)
- improve lat resolution
- dec side lobe artifact (grating lobe artifact)
- dec superficial reverberation artifact
harmonics –> generated in superficial or deep tissue?
deep
what is compound imaging? clinical significance?
transducer –> US beams in mult directions –> sharpen edges of object
compound imaging –> cyst –> post shadowing –> dec/inc/no change?
no more post shadowing
harmonic vs normal US –> what happens to…
- comet tail
- reverberation
- through transmission
- acoustic shadowing
- speckle noise
- side lobe/grating
- comet tail: inc
- reverberation: dec
- through transmission: inc
- acoustic shadowing: inc
- speckle noise: dec
- side lobe/grating: dec
what is “intensity”?
power/area
amplitude –> squared –> what happen to intensity?
square amplitude –> 4x intensity
cavitation –> more likely to occur with…
- low vs high freq
- low vs high pressure
- low freq
- high pressure
NCRP –> risk-benefit decision at?
- thermal index (TI) of??
- mechanical index (MI) of??
- TI >1.0
- MI >0.5
pulse repetition freq (PRF) —> formula?
PRF = c/2D
c = 1540 m/s D = depth
soft tissue –> attenuation coefficient?
0.5 dB/cm/MHz
How does mechanical index depend on transducer frequency?
Inversely proportional to the square root
What is the wavelength of a 1.5 MHz wave?
~1mm
1.5 MHz / 1540 m/s = 0.00097 m = 0.001 m = 1mm
