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Chapter 12: Two Dimensional Imaging
|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?
|What is a single, circular, disc-shaped element that is physically moved to create scan planes?
|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?
|What is the combination of the active element, wire and system electronics in modern transducers?
|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?
|what is it called when a beam is focused or steered electronically?
|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?
|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