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Sonographic physics
Chapter 12: Two Dimensional Imaging
Question | Answer |
---|---|
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 |