C3: Ultrasonic Field Flashcards
what does the term uniformity refer to
uniformity of the sound waves, pressure and intensity
describe the uniformity of the near field
it has non uniform sound waves due to the interference of wavelets which have not yet formed a wavefront….
it has have higher intensity but non-uniform intensity and non uniform pressure
describe the uniformity of the far field
it has uniform sound waves as the wave fronts have been form by now and are mainly diverging…
it has uniform and weaker intensity and has uniform pressure
how do the many frequencies emited from a low Q probe effect the uniformity of the near field
makes it less uniform
2 alternative names for the near field and far field.
near: fresnel (also focal length)
far: fraunhofer
compare the beam width in the near and far field
near: nearly constant beam width
far: divergent beam width
when does the far field end?
once all the sound has attenuated
in old flat disc mechanical probes, where was the natural narrowing of the beam located
one near field length/near zone length (NZL)/focal length
whats another term for the focal point
transition zone
in old flat disc mechanical probes, what is the diameter of the beam at the transition zone
1/2 the crystal diameter
in old flat disc mechanical probes, when will the diameter of the beam in the far field be equal to the crystal diameter
2 NZL
in old flat disc mechanical probes, what is the usable beam length? why?
2 NZL
this distance is narrow enough to provide reasonable lateral resolution. beyond it, the resolution is too poor
what is the general formula for NZL
NZL = D^2 / 4 x lambda
what is the formula for NZL in soft tissue
NZL = D^2 (f) / 6
if you extend the NZL are you penetrating further?
no, you are just moving the focal zone deeper or shallower
in the NZL formula, which variable has the biggest impact
the diameter of the crystal b/c its ^2
if you increase frequency or diameter, what happens to the NZL
it increases
formula to calculate angle of divergence of the far field
sin of the angle = 1.22 (lambda) / D
if you increase the frequency or the diameter of the crystal, how does it effect the lateral resolution (this is for a single disc mechanical probe)
it will improve it, but only in the mid and far field because the beam will be skinnier and diverge less…
…however, it will make it slightly wore at the face of the probe (main bang)
how does focusing effect the angle of divergence
the more you focus, the more the beam diverges
the term for diameter in an array probe
aperture
in an array probe, as we increase the depth of the focus how do we maintain a relatively constant beam width at the focal point
you increase the aperture size
will an array probe with a small or large aperture have better lateral resolution
small, b/c the focal point can be the narrowest
how does beam width effect sensitivity
skinnier beam = better sensitivity
what are the 2 main types of focusing
- mechanical
2. electronic
2 types of mechanical focusing
- internal : focusing is achieved by applying a curve to the crystal itself
- external : focusing is achieved by applying a curved lens or mirror
what was the only way you could you change the focal point w/ an old single disc mechanical probe
change the probe
what are the 3 categories of focal lengths for mechanical probes
- short (strong)
- medium (med)
- long (weak)
why is a short focal length the strongest
it will provide a more intense focus b/c the beam and aperture size will be the skinniest
list the distances for short, med and long focal lengths for mechanical probes
S: 1-4 cm
M: 4-10 cm
L: 7-19 cm
what type of probes use electronic focusing
array transducers
another term for electronic focusing
transmit focusing
how is transmit focusing achieved
by phasing which involves delaying the pulses going to the crystal (pulse delays in the machine)
what happens if all elements in an array probe are excited at the same time
they act as a flat single disc mechanical probe and we couldn’t adjust the focus
what is it called when we apply delays to the crystals
phasing
how does a time delay/phasing allow us to focus the probe
we can excite certain groups of elements at one time, but not others, and excite them in such a way that the wavefronts converge towards a focal region
how is the amount of delay applied to the elements related to the strength of the focusing
the are directly related
what type of focal point would be produced with an increased time delay
a narrower, sharper focal point that is closer to the face of the probe
what type of focal point would be produced with an decreased time delay
a wider, less sharp focal point that is further away from the face of the probe
what are the disadvantages of focusing
there is greater divergence in the far field
and
the NZL will be greatly decreased
whats the relationship between the NZL and focusing
more focusing/narrower focal point results in a shorter NZL
how can the disadvantages of using transmit focusing be overcome
dynamic aperture and frequency
what is dual focusing
using both mechanical and electrical focusing in a probe
how do we focus in the Z axis
mechanical focusing only
what is multiple focusing
having more than one transmit focus on the image, doing this expands the focal regions of the probe creating a long focus
how do we have to change the way we pulse the US beam to achieve multiple foci
multiple foci requires multiple pulses per scan line, with each pulse focused at a different depth
advantages of multiple foci
improves resolution
what is receive focus, how does it work
focusing the returning echos by applying time delays…. this is done in order to allow for constructive interference and avoid destructive interference
does receive focus effect frame rate
no
is receive focus operator controlled
no
is receive focus a dynamic process
yes, because the aperture size depends on how many crystals the sound waves stimulate at one time (which depends on depth of the returning echos)
other terms for receive focus
dynamic aperture, echo delays or dynamic receive focus
do echos that are returning from reflectors that are close to the probe need more of less time delays
more time delays are needed because the echos have less time to diverge before they stimulate the crystal, so they will stimulate fewer elements at one time
do echos that are returning from reflectors that are far away from the probe need more of less time delays
less, because the echos have more time to diverge before they stimulate the crystals, so will stimulate more crystals at one time
what are all the terms that refer to slice thickness?
section thickness, volume averaging, partial volume, elevational plane, z axis
what does elevational resolution depend on
the width of the beam perpendicular to the image plane
what is a slice thickness artifact
this occurs when a cystic structure is smaller than the width of the beam.
it leads to false echos/debris being placed in the cyst due to off axis echos being picked up…. the cystic structure is poorly resolved
another term for effective beam shape
usable beam width
list the 4 controls that determine the sensitivity of the system and can change the effective beam shape
gain
power
suppression (rejection)
TGCs
