Flashcards in Axial and Lateral Resolution chapter 10 Deck (41):
What is resolution?
the ability to create accurate images
What is axial resolution?
one measure of detail found in an image
measures the ability of a system to display two structures that are very close together when the structures are PARALLEL TO THE SOUND BEAMS main axis
the reflectors are positioned one in front of the other
What are the units of axial resolution?
measured in mm or any other unit of distance
what is axial resolution related to?
spatial pulse length
determined by both the sound source and the medium
what improves axial resolution?
in a particular medium short duration pulses also have _______
What are the synonyms for axial resolution?
is axial resolution adjustable?
no...spatial pulse length for a transducer is fixed.
what are typical values for axial resolution?
0.1 to 1.0 mm
lower numerical values indicate shorter pulses and improved image accuracy
what is LARRD
all the synonyms for axial resolution
What is the relationship between the numerical value of the axial resolution and the image quality?
Lower numerical values of axial resolution indicate shorter pulses
shorter pulses create more accurate images
image quality is better when axial resolution has a lower numerical value
What is the formula for axial resolution?
axial resolution (mm) = Spatial pulse length (mm)/ 2
axial res (mm) = wavelength (mm)x #of cycles in pulse/2
what is the formula for axial resolution in soft tissue?
axial res (mm) = 0.77 x # cycles in pulse/frequency MHz
How are short pulses created?
What is less ringing?
a pulse is short if there are few cycles in the pulse
dampen the crystal after it has been excited by an electrical signal from the system.
a pulse is short if each cycle has a short wavelength
shorter wavelengths are characteristic of higher frequency sound
pulses made of higher frequency cycles have _______axial resoultion
Better axial resolution is associated with....
shorter spatial pulse length
shorter pulse duration
higher frequencies (shorter wavelength)
fewer cycles per pulse (less ringing)
lower numerical values
Axial resolution is _________ the spatial pulse length
What is lateral resolution?
the ability to distinctly identify two structures that are very close together when they are SIDE BY SIDE or PERPENDICULAR to the SOUND BEAM main axis
What are the units of lateral resoultion?
mm cm or any distance
smaller numbers are preferred since they indicate more accurate images
What is lateral resolution determined by?
the WIDTH OF THE SOUND BEAM
narrower beams have better resoultion
beam diameter varies with depth
later resolution changes with depth
what are the synonyms for lateral resolution?
What location is lateral resolution the BEST at? Near zone, focal zone, focus, or far zone?
at the focus where the beam is the narrowest
lateral res is GOOD within the focal zone
What is the mathematical relationship that describes lateral resolution and beam diameter?
lateral resolution (mm) = beam diameter (mm)
Which type of resolution is better in clinical imaging systems: lateral or axial resolution?
ultrasound pulses are shorter than they are wide
therefore the numerical value for axial res is less than the value for lateral res
what appears on an image when two reflectors positioned perpendicular to the sound beam are closer to each other than the beam width?
When two side-by-side reflectors are closer to each other than the width of the beam, only one reflection is observed on the image.
With regard to resolution, what are the advantages of using a high frequency transducer?
higher frequencies improve both axial and lateral resolution
axial: improved in the entire image because shorter pulses are assoc with high frequency sound
Lateral: is improved in the far field only because high frequency pluses diverge less in the far field than low frequency pulses (higher frequency sound beams are narrower!)
Axial resolution facts:
orientation: front-to-back parallel to beam
Best With: shortest pulse, highest frequency and fewest cycles
does it change: same at all depths, does not change
In Near field best with: shortest pulse
In the far field, best with: shortest pulse
Lateral resolution facts:
orientation: side-by-side perpendicular to beam
Determined by: beam width
Best with: narrowest beam
Does it change: changes with depth, best at focus
In the near field, best with: smallest diameter crystal
In the far field, best with: largest diameter and highest frequency (least divergence)
What is focusing?
concentrates the sound energy into a narrower beam and thus improves lateral resolution
What are the three methods of focusing?
External - with a lens
Internal - with a curved active element
Phased Array - with the electronics of the US system
what kind of transducer is phased array focusing reserved for?
array transducers ( those with multiple active elements)
What is fixed focusing?
aka: conventional or mechanical focusing
both external and internal techniques
with fixed focusing what is determined when the transducer is fabricated and cannot be changed?
extent of focusing
With external focusing, what is placed in front of the piezoelectric material?
When you use a lens for fixed focusing technique, what happens to the lens arc?
becomes more prominent
the degree of focusing increases
beam narrows in the focal zone
With internal focusing a curved piezoelectric crystal does what?
concentrates the sound energy into a narrower or tighter beam
what happens as the curvature of the PZT becomes more pronounced?
the degree of focusing increases
***internal focusing is the most common form of fixed focusing***
With phased array focusing what can the sonographer do?
adjust the focusing characteristics of a beam
only can be used on multi-element transducers
What does phased array mean?
adjustable or multiple focusing