C4: Resolution Flashcards
resolution can be broken down into what 3 main groups/aspects?
detail/spatial
contrast
temporal
what are the 3 main groups of detailed/spatial resolution?
axial
lateral
elevational/z-axis
if structures are closer to one another than 1/2(SPL) will they be resolved?
No
formula for axial resolution
AR=1/2(SPL)
does axial resolution get better with increased frequency?
yes…. because as F increases, lamda decreases which shortens the SPL
list the 5 items that improve axial resolution
higher frequency shorter wavelength increased damping (because it reduces RD and SPL) wider bandwidth lower Q factor
what is LARD for axial resolution?
Longitudinal
Axial
Range
Depth
is lateral resolution constant with depth?
no
which type of resolution is typically the best?
axial
list the 5 items that improve lateral resolution
- distance from the probe (reflector and focus)
- amount of focusing (increased focusing gives us better lateral resolution
- size of aperture
- higher frequency
- NZL
what is LATA for lateral resolution?
Lateral
Angular
Transverse
Azimuthal
what is slice thickness
that ability to separate reflects perpendicular to the image plane of the beam
how do we focus in the z axis
mechanically with a lens or acoustic mirror unless you have a 1.5D probe
if a structure is smaller than the beam thickness, will it be resolved well?
No. you will get a slice thickness artifact that will put fake echos into anechoic structures
list the 4 items that improve elevational resolution
- distance from the probe
- higher frequency
- NZL
- amount of focusing in Z axis
another term for the elevation plane
slick thickness
define contrast resolution
what does it depend on?
the ability to distinguish between like shades of grey
it depends on the systems bit depth which determines the number of shades of grey you can produce
what happens when you have too little contrast?
too much?
too little: the shades of grey are too similar to one another so you have no tissue differentiation
too much: similar shades of grey are grouped together and you can cant tell them apart
can you have good contrast but poor contrast resolution? give and example
yes
bistable
whats the relationship between bit depth and the number of shades of grey
higher the bit depth the greater the shades of grey
whats another term for dynamic range?
compression
how can you change the number of shades of grey shown on the monitor?
increase or decrease the dynamic range
what happens when we decrease the dynamic range?
fewer shades of grey are displayed which can improve out ability to distinguish between them
whats temporal resolution (TR)
ability to see moving structures in real time
another word for TR
frame rate (FR)
what must the FR be > or = to?
20 fps
whats the relationship between PRF and temporal resolution?
increase in PRF will improve TR
directly related
whats the formula for TR?
PRF (in Hz) = n x LPF x FR
n= # of foci LPF= lines per frame FR= frame rate (in Hz)
whats the range ambiguity formula?
what should this value be and why is this number used?
penetration x n x LPF x FR
should be < or = 77,000 cm/s
-number is used because its half the avg. speed of sound in soft tissue
what limits the PRF?
depth or penetration of sound
list the 5 items that improve temporal resolution
- depth of penetration (deeper = lower TR, b/c it takes longer for sound to return to the probe)
- # of foci (more foci decreased FR because you must use more pulses per scan lines to create multiple areas of focus)
- lines per frame (fewer lines = better TR)
- sector width (lower sector means fewer scan lines for improves TR)
- FR
Does a fixed focus mean that you have a fixed NZL?
yes
where r echo delays located?
beam former
which type of focusing provides uniformity through out the image?
receive focus
How do we reduce the number of cycles in the pulse?
Damping
Is the axial resolution equal throughout the beam?
Yes
What’s the minimum number of rows of crystals you need to focus in the elevational plane
3
The additional 2 rows on the edge should be half the length of the central row of crystals
If you want to double your FR what can you change?
1/2 the depth of interest Or 1/2 the number of scanlines Or remove one foci
