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Sonographic physics

Chapter 12: Two Dimensional Imaging

QuestionAnswer
What two characteristics of sound must be overcome to create high quality 2D images? Sound only travels in a straight line; Beam must be narrow for best lateral resolution
What solves both difficulties with sound so 2D images can be created? Multiple pulses
What is a single, circular, disc-shaped element that is physically moved to create scan planes? Mechanical Transducer
How many active elements does mechanical transducers have and what does it resemble? one resembles a coin
What is the image shape of a mechanical transducer? fan or sector-shaped
How is the image shape created with mechanical transducers? Pulses radiate out like bicycle spokes from PZT; "Wiper blade on windshield" Crystal rotates around a single point
How is the beam steered with a mechanical transducer? It mechanically steered the 2D image is built by rotating the crystal with a motor
How is the beam focused with a mechanical transducer? Has fixed focal depth Conventional, mechanical or fixed focusing with internal or external Occurs in horizontal and vertical plane-hourglass beam shape
With a mechanical transducer, what happens if the PZT crystal is damaged? the entire image is lost because it only has one crystal
With modern TDs what is the single slab of PZT cut into separate pieces called? elements
What is the combination of the active element, wire and system electronics in modern transducers? channel
What are some different types of array transducers? Linear - arranged in a straight line Annular - circular ring (concentric) with common center Convex - arranged in arched line; can be curved or curvilinear
The linear array transducer is often called what? Phased array
what is it called when a beam is focused or steered electronically? phased
What are some characteristics of a linear phased array transducer? small surface area that touches skin (compact footprint, square shape about 1cm on each side); no moving parts and electronically steered
Describe the active elements in (linear) phased array TD. many (100-300) elements along the probe face side by side shaped rectangular and narrow; element's width is about 1/4 to 1/2 of the sounds wavelength
What is the image shape of the (linear) phased array transducer? fan or sector-shaped
How is the beam steered with a linear phased array? phased; electronically transmitted in different directions with no moving parts; "wiper blade" or "bicycle spokes"
What always means adjustable or multi-focus? Phased array
How are linear phased array TDs beams focused? electronically; control can be adjusted by the sonographer (depth and amount); multi-focusing
What is multi-focusing? transmit multiple beams down single scan line; each with a different focal depth
What happens with a linear phased array transducer if their is a damaged PZT? inconsistent/erratic beam steering and focusing; effect of damage is not precise due to complex sound pulse interactions from the many elements
How many elements are excited to create sound beam in phased array? all elements are fired to create sound beam
What determines the beam's direction and focus in the linear phased array? pattern of electrical signals
How is the beam direction determined with linear phased array? draw a line connecting spikes; draw another line perpendicular to original line extending out of TD; the direction of this second line indicates sound beam direction
In what direction will the sound beam go if the upper element is excited first? sound beam directed downward
In what direction will the sound beam go if the lower element is excited first? sound beam directed upward
What creates the sector shaped image in the linear phased array? variations in slope
In linear phased array, a straight electrical spike line is what kind of beam? unfocused sound beam
With linear phased array, when a electrical pattern has a slope, the beam is what? the beam is steered
With linear phased array, when the electrical pattern is curved, the beam is what? the beam is focused
With linear phased array, a more pronounced curve means what? a shallower focus
How is multi-focusing done with phased array? multiple sound beams sent down each scan line; distinct pulses are transmitted with differing patterns; each electrical spike pattern will have different degree of curvature; least curve produces deepest focus
What is time delays in the reception phases? Dynamic Receive Focusing
What is annular array transducers? elements are disc-shaped concentric rings cut from the same circular slab of PZT
How are the active elements arranged in a annular array? multiple ring-shaped elements with common center similar to "bulls eye target"
What is the image shape for annular array? fan or sector-shaped
How is the beam steered with annular array? mechanical steering; physically rotates the ringed-element to transmit sound in different directions
How is the annular array focused? multiple transmit focal zones; "multi-focus" Smaller diameter ring-shallower focus, diverge rapidly; collects data from the shallowest depth; only information from the focal zone stored; this repeated for each ring; focusing on all planes at all depths;
What happens with a damaged PZT with annular array? horizontan band dropout; only portion of image lost side to side; If innermost ring lost-lose superficial data; if outer ring lost-lose deeper data
What is Linear Sequential Array also called? Linear Switched Array
What isa linear Sequential Array? elements arranged in a line with a larger acoustic footprint
How are the elements arranged in a linear sequential array? elements are rectangular; 120-250 elements; larger elements than linear phased array; size is about 1 wavelength
How is the beam steered with Linear Switched Array? some but not all crystals are fired simultaneously to create beam; beams are parallel to each other-directed straight ahead; first group of crystals fired, then another
How is the beam focused with linear sequential array? early models had fixed conventional focus using internal or external technique; modern models use electronic focusing using electronic time delays; transmit focusing by curved phased crystal excitation, receive focusing by electrical delays in signals
What is the image shape of a linear sequential array? rectangular image; image never wider than TD; can produce smaller width image; sound beams are emitted from small groups of crystals at different locations along face of TD
What happens with linear sequential array if there is a damaged PZT? vertical dropout directly below element; only portion is affected; extends from top to bottom of image (superficial to deep)
Why are groups of elements used instead of single element with linear sequential array? to overcome divergence and produce well-defined beam
What occurs with Phased Linear Sequential Array? beam is electronically steered creating a parallelogram shaped image that is steered by slope delay in electrical excitation of elements; steered left or right; Grayscale anatomic image no slope, rectangular straight down; Doppler portion have slope
What are Curved Array Transducers? convex, curved or curvilinear array; bowed, arched shaped; similar operational characteristics to linear array
How are the active elements arrange with curved array TDs? rectangular shaped strips of PZT arranged side by side in bowed line; 120-250 elements; large footprint array; elements about 1 wavelength in width
How is the beam steered with curved array? some but not all crystals fired at once to create sound beam; beam directed straight out of crystal; travel in different directions due to arc shape; sound beams are not parallel to each other
How is the beam focused with curved array? electronic focusing with delays in curved spike line same as linear array and dynamic receive focusing
What is the image shape of a curved array? blunted sector shape with arc at top of image corresponds to directly curved array footprint
What happens in a curved array whenthere is a damaged PZT? dropout directly below element; portion affected from top to bottom; only scan lines from defective crystal are affected
Created by: quietone055 on 2010-03-30



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