Ch. 10 Axial and Lateral Resolution Flashcards
Resolution
is the ability to create accurate images.
(image accuracy is the essence of ultrasound)
Axial Resolution
describes one measure of the detail found in an image. It measures the ability of a system to display two structures that are very close together when the structures are parallel to the sound beam’s main axis.
Axial Resolution units
measured in mm or any unit of distance
Axial Resolution determined by
both the sound source and medium
Axial Resolution is related to
Spatial Pulse Length
(both sound & medium)
Pulse Duration
Shorter Pulses improve
axial resolution
In a particular medium, short duration pulses also have a
a short length
Axial Resolution synonyms (4)
Longitudinal
Axial
Range
Radial
Depth resolution
(LARRD)
Axial Resolution adjustable
No, since the spatial pulse length for a transducer is fixed.
Axial Resolution typical values
ranges from 0.1 to 1.0 mm
(lower numerical values indicate shorter pulses and improved image accuracy)
What is the relationship btw the numerical value of the axial resolution and the image quality?
directly related
lower numerical values of axial resolution indicate a shorter pulse
Shorter pulses create
more accurate images
Axial Resolution formula
Axial (mm) = spatial pulse length (mm) / 2
or
Axial (mm) = wavelength (mm) x # cycles in pulse / frequency (MHz)
What allows some transducers to have better axial resolution than others?
determined by the pulse length, with shorter pulses yielding improved axial resolution
A short pulse is created in two ways:
1) less ringing
2) higher frequency
Less Ringing
A pulse is short if there are few cycles in the pulse
One way to reduce ringing is to
dampen the crystal after it has been excited by an electrical signal from the system.
Higher Frequencies
a pulse is short if each cycle in the pulse has a short wavelength
Shorter wavelengths are characteristics of
higher frequency sound
Pulses made of higher frequency cycles have
superior axial resolution
Better axial resolution is associated with (5)
1) Shorter spatial pulse length
2) Shorter pulse duration
3) Higher frequencies (shorter wavelength)
4) Fewer cycles per pulse (less ringing)
5) Lower numerical values
Lateral Resolution
is the ability to distinctly identify two structures that are very close together when they are side by side, or perpendicular, to the sound beam’s main axis.
Lateral Resolution units
mm, cm, or any unit of distance
(Smaller # are preferable since they indicate more accurate images)
Lateral Resolution determined by
width of the sound beam; narrower beams have better resolution
Lateral Resolution synonyms
Lateral
Angular
Transverse
Azimuthal
(LATA)
Lateral Resolution is best at the
focus (at the end of the near zone) where the beam is narrowest. Lateral Resolution is good within the focal zone
Lateral Resolution (mm) =
Beam diameter (mm)
Which type of resolution is better in clinical imaging systems: lateral resolution or axial resolution ?
axial resolution is better bc pulses are shorter than they are wide. The numerical value is less too.
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 resolution is improved in the
entire image bc shorter pulses are associated with high frequency sound
Lateral resolution is improved in the
far field only bc high frequency pulses diverge less in the far field than low frequency pulses. Higher frequency sound beams are narrower than lower frequency sound beams.
Axial:
Orientation
Mnemonic
Determined by
Best with
Does it change?
In Near field, best with
In Far field, best with
1) front-back, parallel to beam
2) LARRD
3) Pulse length
4) Shortest pulse, highest frequency & fewest cycles
5) same at all depths, does not change
6) shortest pulse
7) shortest pulse
Lateral:
Orientation
Mnemonic
Determined by
Best with
Does it change
In Near field, best with
In Far field, best with
1) side by side, perpendicular to beam
2) LATA
3) Beam width
4) Narrowest beam
5) Changes with depth, best at focus
6) smallest diameter crystal
7) largest diameter & highest frequency (least divergence)
Focusing
concentrates the sound energy into a narrower beam and thus improves lateral resolution
What are the three methods of focusing?
1) External focusing - with lens
2) Internal focusing - with curved active element
3) Phased array focusing - with the electronics of ultrasound system
Internal and external focusing may be used with
single element transducers
Phased array focusing is reserved specifically for
array tranducers those with multiple active elements
Method Name. Type
1) Lens
2) Curved active element
3) Electronic
1) External: fixed, conventional, mechanical
2) Internal: fixed, conventional, mechanical
3) phased array adjustable type
Fixed Focusing (Conventional or mechanical focusing) includes
both internal and external techniques
With fixed focusing, the focal depth and the extent of focusing are determined when the transducer is
fabricated, and cannot be changed
External focusing
a lens is placed in front of the piezoelectric material.
Ex: glasses
External focusing:
As the arc of the lens becomes more _________, the degree of focusing ________ and the beam ________ in the focal zone.
prominent
increases
narrows
Internal focusing
a curved piezoelectric crystal concentrates the sound energy into a narrower or tighter beam.
NO lens
Internal focusing:
As the curvature of the PZT becomes more __________, the degree of focusing ________ .
pronounced
increases
Internal focusing is the most _______ form of _____ _______
common
fixed
focusing
Electric focusing
the system’s electronics focus the sound beam.
Electric focusing:
With phased arrays, the sonographer can _____ the focusing characteristics of a beam.
adjust
Electronic Focusing:
This technique may be used only on ___-___ _______ , never on _____ ______ ______.
multi-element transducers
single crystal transducers
Phased array technology is _____ _____ than fixed focusing techniques.
more versatile
Phased array means
adjustable or multiple focusing
What happens to a beam when it is focused?
a sound beam undergoes four distinct modifications when focused
4 Modifications of Focusing
1) Beam diameter in near field and focal zone is reduced
2) Focal depth is shallower
3) Beam diameter in the far zone increases
4) Focal zone is smaller
Determinants of sound beams:
Characteristics Determined
Frequency-Continuous Wave
frequency of electrical signal from ultrasound system
Determinants of sound beams:
Characteristics Determined
Frequency-Pulsed Wave
Thickness of ceramic and speed of sound in ceramic
Determinants of sound beams:
Characteristics Determined
Focal length
Diameter of ceramic and frequency of sound
Determinants of sound beams:
Characteristics Determined
Beam Divergence
diameter of ceramic and frequency of sound
Determinants of sound beams:
Characteristics Determined
Lateral Resolution
Beam width
