Ch. 12 (2D imaging) Flashcards
two characteristics of sound make 2D images challenging to make… what are these 2 characteristics
sound travels in a straight line
sound beams must be narrow to optimize lateral resolution
how do we overcome these difficulties?
the use of multiple pulses to create a 2D image
why does the US system send multiple pulses?
each pulse sends back a line of reflections to eventually gather all stored pulses to create a 2D image
sonographers are usually unaware of this because it happens so rapidly
Mechanical Transducer characteristics
one coin/disc shaped crystal (curved) sector shaped image mechanical steering fixed focusing loss of entire image if crystal malfunctions
linear sequential array
rectangular strips of crystals in a straight line [][][][][][][]
element groups are fired in a sequence to create an image
parallel scan lines
rectangular image shape - no wider than transducer
electronic steering
electronic focusing (transmit - curved pattern of crystals excitation, receive - electrical delays in returning signal)
vertical dropout
linear phased array
arrangement of elements - line sector image shape electronic steering electronic focusing extent of image distortion is variable small acoustic footprint crystals fired at same time to create one sound pulse (phasing) small time delays to beam steer and focus
annular phased array
crystal rings (inner crystals - shallow scanning, outer crystals - deeper scanning) creates multiple focal zones sector image mechanical steering electronic focusing horizontal dropout
convex / curvilinear transducers
elements arranged in an arc blunted sector electronic steering electronic focusing vertical dropouts
vector transducers
combination of linear sequential and linear phased array
trapezoid image shape (flat topped sector)
electronic steering
electronic focusing
arrangement of crystals in linear array
straight line
arrangement of crystals in annular array
circular rings
arrangement of crystals in convex array
bowed / arched line
transducer may be called curved or curvilinear array
what does phased array mean
adjustable or multi focus
if there is a slope, there is __
steering
if there is a curve, there is __
focusing
what does resolution mean in imaging?
accuracy
image resolution deals with 3D space. name the 3 types of space
shallow to deep
side to side
above to below
slice thickness resolution is also called
elevational resolution
how to measure slice thickness
perpendicular to imaging plane or above to below the imaging plane
T or F
Slice thickness can vary with depth.
true
active element shape that creates the best (thinnest) elevational resolution
disc shaped
disc shaped crystals are found in what type of transducers
mechanical and annular phased array
the width of the circle (in a disc shaped element) indicates the
beam diameter (lateral resolution)
the height of the circle indicates
beam thickness (elevational resolution)
the elevational resolution is identical to the ___ resolution with disc shaped elements
lateral
what determines elevational resolution in phased array, linear array, and convex array transducers?
these array probes have rectangular elements placed side by side [] [] [] [] [] []
with phasing, the beam is focused / narrowed only side to side in the imaging plane
this narrowing improves lateral resolution but does not affect slice thickness
Do all lenses have a fixed focal depth?
yes
beam thickness is equal to the height of the _____
active element
a limitation of linear arrays
inability to create thin slices
newer types of transducers have improved slice thickness resolution, name one
1 1/2 dimension array transducer
checkerboard appearance
multiple crystals
the 1 1/2 D array improves what type of resolution
elevational
resolution from best to worst
axial, lateral, elevational
what is rendering
creating an image from 3D data and providing a realistic view of the image
sound beams created by single element transducers that are hourglass shaped
side lobes
side lobes degrade what type of resolution
lateral
side lobes are reflections arising outside of the main beam in the far zone, but show up where on images (when shown as artifact)
middle of image
similar to side lobes but are created by array transducers
grating lobes
grating lobes degrade ___ resolution
lateral
Do grating lobes create artifact that reduce image quality?
yes
how do we fix grading lobes
apodization
subdicing
aperture
what is apodization
hitting inner crystals with stronger intensity than outer crystals reduces intensity of reflection and side lobes
what is subdicing
dividing crystals into groups of smaller crystals, sub elements, not isolated
what is aperture
diameter of beam
frequency of an imaging transducer is based on what 2 things
thickness and speed
what is a 3D pixel called
voxel
no slope, but there is a curve
focused
straight imaging line is focused or unfocused
unfocused
time differences between many wavelets firing
phase delays
electronics within ultrasound system that create patterns
beam former
what is an array
single slab of crystal cut into a collection of elements
can ring a single element or group of elements in various ways
what is a channel
the combination of the active element, wire, and system electronics
can an US system excite a single or multiple elements in various ways during transmission?
yes
In a linear array transducer (cardiac) returning echoes have time delays… this creates receive zone focus with optimal image quality. What is this termed?
dynamic receive focusing or
dynamic focusing
axial resolution
top to bottom
lateral resolution
side to side
elevational resolution
front to back
beam thickness varies with depth
which shape of crystals create best elevational resolution
disc shaped crystals
how can we create a narrow sound beam over greater depths?
aperture
how? change # of crystals used to transmit & receive
aperture is an opening or hole
improves lateral resolution at a wide range of depths
diameter of beam is referred to as ___ on ARDMS board exam
aperture
