Ch.12 Two-Dimensional Imaging Flashcards
Mechanical Transducer
contains a single, circular, disc-shaped active element that is physically moved. This movement creates a scan plane
Mechanical Transducer
Crystals: # and shape
can have only 1 active element ; coin shape
What image shape do mechanical transducers make?
fan or sector shaped; crystal is physically moved. (Car wind shield wiper sweep)
Mechanical Transducer:
Beam steering
Beam focusing
Mechanical steering
-2D image is built up by rotating the crystal with a motor.
Fixed focal depth called:
conventional, mechanical, or fixed focusing
What are the two methods of fixed focusing?
Internal focusing: the use of a curved active element.
External focusing: the use of an acoustic lens.
Focusing occurs in what planes?
Both the horizontal and vertical planes bc of the hour glass shape of the sound beam.
Whenever the crystal in any single crystal transducer breaks, the ______ image is ______ .
entire
lost
Transducer Arrays
contain multiple active elements.
-comprise a single slab of PZT cut into a collection of separate pieces called elements
Each active element is connected by a _____ to its own _______ _______ in the ultrasound system.
wire
electric circuitry
Channel
is the combination of the active element, wire, and system electronics
Each element has its own electric connection, the system can excite a ______ or _______ _________ in various ways during ___________ .
single or multiple elements
transmission
During reception, each individual crystal produces a ________ ________ ________ that returns to the system’s receiver.
small electronic signal
What are the types of array transducers?
Linear
Annular
Convex
Linear transducer arrangement
active elements are arranged in a straight line
Annular transducer arrangement
elements are arranged as circular rings with a common center
Convex transducer arrangement
Active elements are arranged in a bowed or arched line. The transducer may also be called a curved, or curvilinear array.
Phasing
is when the beam is steered and focused using an electrical technique
Linear Phased Array Transducers is a
small square or rectangular transducer about 1 cm on each side that has no moving parts. (carotid one)
Linear Phased Array:
Crystal # & Shape
as many as 100 to 300 elements arranged along the face of the probe.
Placed side by side, each element is rectangular and narrow with a width of 1/4th to 1/2 of the sound’s wavelength.
Linear Phased Array:
image shape
fan or sector-shaped similar to mechanical transducer
Linear Phased Array:
Beam Steering
2D image is built up with an electronic steering process called phasing ; no moving parts
(wind shield wiper sweep motion)
Phased array ALWAYS means:
adjustable or multi-focus
Linear Phased Array:
Beam Focusing
focused electronically
Unlike mechanical transducers with a single fixed focus, the phased array system has controls that allow the sonographer to ______ the ______ and amount of ________ of the sound beam.
modify
depth
focusing
What is the advantage of phased array systems?
is the availability of electronically controlled, adjustable focusing.
Multi-focusing
phased array systems can transmit multiple beams down the same scan line, each with a different focal depth
Damage to one of the elements of a phased array transducer results in
inconsistent or erratic beam steering and focusing
How many elements in a phased array probe are used to create a single sound pulse?
all the active elements in a linear phased array probe are fired to create each sound pulse.
Ex: if 128 elements are present then 128 wavelets are created
Phased array wavelets interfere
constructively and destructively creating a single sound pulse.
The overall pattern of the signals determine the sound beam’s direction and focus
The sound beam is steered if the
spike line has a slope
Beam former
the electronics within the ultrasound system that create the electrical patterns of travel
When a spike pattern is a straight line, an
unfocused sound beam is created
How does the pattern of the electrical spikes from the beam former focus the sound beam during transmission?
curved narrow pattern creates a focused sound beam during transmission.
outer crystals are excited earlier than inner
The sound beam is focused if the spike pattern is _________ .
curved
The sound beam created by a more curved electrical pattern will have a
shallower focus; however, the sound beam will still be directed straight down
Linear phased array:
Electronic Pattern -> Sound beam
Slope
Curvature
Sound beam:
steering
transmit focusing
How does a phased array system create images with multiple transmit foci?
to create an image with multiple transmit foci at different depths, the ultrasound system must send multiple sound beams down each scan line
Annular Phased Arrays
active elements of annular array transducers appear disc-like. The disc comprises multiple ring-shaped elements with a common center.
Appears like a rings on a bull’s eye
Annular Phased Array:
Beam Steering
Mechanical steering
(electrical not possible with annular arrays)
What is mechanical steering?
A motor physically moves the ringed-element array so that the sound beams are transmitted in different directions.
Annular Phased Array:
Beam Focusing
Multiple Transmit focal zones (Advantage)
How is data collected with annular phased array transducers?
Information is gathered and stored from a series of 4 different rings. The inner rings collect data from shallow depths. The outer rings collect data from deeper regions. Info is stored only from the respective focal zone’s of each ring.
Annular Phased Array transducers:
Image shape
fan or sector shaped
What happens when one ring of an annular array transducer malfunctions?
Only a portion of the image is lost. This will appear as a horizontal or side-to-side band of dropout at a particular depth depending on which ring.
What kind of images and acoustic footprint do linear sequential arrays create?
large acoustic footprints and rectangular images
Linear sequential array transducer:
Crystal number and shape
120-250 rectangular shaped stripes of PZT arranged side by side in a line
The active elements in linear sequential arrays are much _______ than those found in linear _____ arrays.
larger
phased
Linear Sequential Array:
Beam Steering
a small group of the crystals are fired simultaneously to create each sound beam. They are parallel to each other and are typically directed straight ahead.
Linear Sequential Arrays:
Beam focusing
in modern linear s. arrays focusing is achieved electronically. They perform both transmit and receive focusing using electronic phase delays.
Electronic transmit focusing is achieved with a ?
curved excitation pattern of the active elements. The outer elements in the group of PZT crystals are fired earlier than the inner elements
Receive focusing is achieved dynamically by
introducing electrical delays in the signals returning from the transducer. The phase delays change continuously during reception, thereby focusing the beam at many depths.
Linear S. Array transducers:
image shape
rectangular image
(image is never wider than the transducer)
When on element in a linear sequential array is damaged, only the
portion of the image extending directly below the damaged element is affected
While using linear array transducers, why is it necessary to fire a group of crystals to produce a sound beam?
A sound beam created by the combination of wavelets from a small group of active elements is directional and well defined
Why can’t a single crystal be used in a linear array?
a sound wave fired from a single element of a linear sequential array would have a Huygen’s V shaped wavelet. The beam would quickly diverge and result in poor lateral resolution.
Can linear sequential array transducers steer sound beams electronically?
Yes, this form of steering creates a parallelogram shaped image rather than rectangular.
_____-______ phase delays in the electrical excitation spikes of the elements steer the beam in linear sequential arrays.
Sloped-patterned.
(color doppler has no slope so it is a rectangular shaped image made by linear array, but the color doppler portion is a parallelogram bc electrical spike patterns create sound beams slopes
To increase clinical utility of linear array probes, manufacturers have designed arrays with a
bowed or curved shape called convex, curved, or curvilinear
Convex arrays;
number and shape
has 120 to 250 rectangular shaped strips of piezoelectric material arranged side by side in a bowed line (large array)
The acoustic footprint may be as long as _ _ cm.
The active elements in a convex sequential array are ________ ___ ______ to a linear array, and much larger than those found in a _______ _______.
10
similar in size
phased array
Convex transducers:
Beam Steering
some, but not all of the crystals are fired simultaneously to create each sound beam
The sound beams transmitted from a convex array, unlike linear sequential arrays, are _____ _______ to each other.
not parallel
Convex transducers:
Beam focusing
electronically
Just like linear array probes, transmit focusing in convex elements in the fired group with a _______ ______ ______ pattern.
curved spike line
Dynamic receive focusing
is achieved by introducing varying phase delays in the signals returning from the transducer to the ultrasound system during echo reception.
Convex transducer:
image shape
blunted sector-shaped images
Convex transducers rather than coming to a point in the ____ ____, the image appear to have a “bite” taken out of it.
near field
The curvature at the top of the image of a convex transducer corresponds _______ to the ____of the convex array transducer.
directly
arc
When one active element in a convex array is damaged, only the portion of the image ________ _______ the damaged element is affected.
directly below
Vector array is
a combination of linear sequential and linear phased array technologies.
Sloped phase delay patterns can electronically steer the sound beams in ________ ________.
different directions
Vector arrays:
Number and shape of crystals
120 to 250 rectangular shaped strips of pzt elements arranged side by side in a line
The footprint of the vector array may be ________, often only a few ____
small
cm
Vector arrays:
Beam steering
vector arrays combines linear and phased array technology. Crystals are fired simultaneously to create a single sound beam. (like linear sequential)
The electrical signals from the beam former are delayed in a _____ spike-line pattern as they excite a group of elements in a vector array.
sloped
The beams radiate out in ________ ________ from the face of the transducer, this is similar to a _____ _____ transducers.
different directions
phased array
Vector array:
Beam focusing
electronic focusing
Vector array:
Similar to the phased array probes, both _______ focusing and ______ _______ focusing improve ________ resolution over a great range of depths.
transmit
dynamic receive
lateral
Vector image shape
Trapezoidal
(Unlike phased or convex array images, the top of the vector image is flat
Active Element Malfunction
Transducer type: Effect on image:
Mechanical
loss of entire image
Active Element Malfunction
Transducer type: Effect on image:
Linear and Convex
dropout of image info from the top to the bottom of the image. The location of the line corresponds to the broken crystal
Active Element Malfunction
Transducer type: Effect on image:
Phased array
Erratic steering and focusing. The extent to which the image is affected is variable
Active Element Malfunction
Transducer type: Effect on image:
Annular Phased array
a horizontal or side-to-side band of dropout at a particular depth
Mechanical:
image shape
steering technique
focusing technique
sector
mechanical
fixed
Linear sequential
image shape
steering technique
focusing technique
rectangular
electronic
electronic
Phased Array:
image shape
steering technique
focusing technique
sector
electronic
electronic
Annular Phased:
image shape
steering technique
focusing technique
sector
mechanical
electronic
Convex:
image shape
steering technique
focusing technique
blunted sector
electronic
electronic
Vector:
image shape
steering technique
focusing technique
trapezoidal
electronic
electronic
Slice Thickness or Elevational Resolution answers the question
are the reflections on the image created from structures directly in the imaging plane or from structures that lie above or beneath the imaging plane?
Slice thickness is measured in
a direction perpendicular to the imaging plane or above-to-below the imaging plane
The beam has a measurable thickness that can ______ with _______ .
vary
depth
When the slice is thick, _____________ above and beneath the “assumed razor thin plane” create ______________ that appear in the image.
structures
reflections
What shape of active element creates the best elevational resolution? Which transducer types have the best elevational resolution?
Disc-shaped crystals
mechanic and annular phased array transducers
Active elements that are disc-shaped provide the __________ ultrasound ______ and the _____ elevational resolution within the ______ _____.
thinnest
slices
best
focal zone
The width of the beam’s circular cross sectional area in one direction indicates the _________ (determining lateral resolution) and the height of the circle indicates the beam _________ (determining elevational resolution).
diameter
thickness
What determines elevational resolution in phased array, linear array, and convex array transducers?
These array probes have rectangular shaped active elements placed side by side. With phasing, the beam is narrowed only side to side, but not above or below. This narrowing improves lateral resolution but does not affect slice thickness
2-D array
contains thousands of elements arranged in a checkerboard pattern, creates 3D images
4-D imaging
is real-time 3D imaging also uses 2-D array probes
Rendering
creates an image from 3D data, constructs photo-like pictures with color, shadows, texture etc.
Side lobes
additional areas of sound energy that extend outside of the main beam created by a single element transducer in the far zone, off-axis sound beams
If side lobes are of sufficient intensity, structures within them can generate reflections that create _______ .
artifacts
Reflections arising from within the side lobes degrade _________ __________ .
lateral resolution
Grating lobes
similar to side lobes, but these are created by array transducers, beam’s main axis is stronger than these
Why are grating lobes undesirable ?
they degrade lateral resolution
reduce image quality
create artifacts
Transducers are designed to reduce the strength of side and grating lobes. How is this accomplished?
apodization
What does the apodization process do?
alters the electrical spike voltages and reduce lobe strength
Explain apodization
stronger electrical signals are used to excite the inner crystals, and progressively weaker electrical spikes excite the outer crystals. Lobes are diminished.
What is Subdicing?
a crystal is divided into a group of smaller crystals called sub-elements. These are electronically joined and act as if they are a single crystal, which reduces grating lobes.
Transducers are designed to keep the sound beam narrow over a substantial depth range. How is this accomplished ?
Dynamic aperture
Dynamic aperture
can be used to make a sound beam narrow which improves lateral resolution at a wide range of depths
Aperture (variable aperture or dynamic aperature)
is a process the ultrasound system does by changing the # of crystals along the face of the probe used to transmit pulses and receive reflections.
