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101 Cards in this Set
- Front
- Back
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What is focal depth proportional to?
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the diameter of the crystal squared
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What is the equation for focus depth?
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Diameter squared over four times the wavelength
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Where is the beam diameter smallest at?
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the end of the near zone, fresnel zone, focal zone
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What is beam divergence?
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the gradual spreading of the beam past the focus, in the fraunhofer zone
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When the cross sectional ares is ______ there is less intensity.
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greater
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what are the two factors governing beam divergence?
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Transducer diameter and frequency of sound.
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The smaller the crystal diameter the _____ the divergence in the fraunhofer zone.
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greater
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the larger the crystal diameter the _____ the divergence in the fraunhofer zone.
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smaller
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How are crystal diameter and beam divergence related?
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inversely
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Because larger crystals diverge less in the far field they will have.....
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better lateral resolution
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frequency and divergence have a(n) ______ relationship.
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inverse
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lower frequency sounds more or less in the fraunhofer zone
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more
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higher frequency sound will create better .......
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lateral resolution in the far field.
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larger diameter crystals and higher frequency will create less or more divergence?
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less
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smaller diameter crystals and lower frequency will create more or less divergence?
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more
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Which is the most divergent:
a) 6cm and 4mHz b) 2cm and 3mHz c) 12 cm and 8mHz |
b
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Which is the least divergent:
a) 6cm and 4mHz b) 2cm and 3mHz c) 12 cm and 8mHz |
c
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What shape are sound waves that are produced by tiny PZT diverge in?
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V shape
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When do spherical waves occur?
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when the source is about the size of a wavelength
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What are V shaped waves known as?
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Spherical waves, diffraction patterns, or Huygens wavelets
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The tiny source that produces wavelets are known as?
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huygens sources
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According to Huygens' principle a large active element may be thought of as .....
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millions of tiny distinct sound sources
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A huygens source creates....
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a huygens wavelet with a v shape
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Why is Huygens' Principle important?
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It helps us understand why a circular active element is going to form and hour glass shape.
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The hour glass shaped main beam is formed by
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wavelets overlapping and interfering constructively
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In the area where the sound beam is absent ____ interference occurs
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destructive
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When stated simply what does Huygens' principle explain?
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Why the sound doesn't just go out straight and come back in straight.
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What is sequencing?
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The excitation of elements in a specific pattern to linearly scan a region.
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How are groups of elements switched on or off in a linear transducer?
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electronically
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About how many elements are in a linear switched array transducer
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200+
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Describe a linear switched array transducer
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the beam is asymmetric in the lateral and elevation planes (its wider than thick), the large lateral dimension is created by sequencing
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What is a linear switched array transducer typically used for?
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Used for vascular 2D, color and Doppler
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What shape is the image produced by a linear switched array transducer?
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rectangular
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What are the disadvantages of a linear switched array transducer?
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Fixed focus, no steering, and expensive in comparison to a single element mechanical transducer
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What was the advantage of a linear switched array transducer?
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it allowed for a wide linear image in the near field, but it couldn't steer
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Why was a mechanically steered transducer developed?
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For cardiac imaging to create an automated image and get in between rib spaces.
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Describe how a mechanically steered transducer worked
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an element was mounted on a motor head which swept from one point to another acquiring scan lines from multiple positions over time
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What is a mechanically steered transducer also known as and why
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a mechanical sector transducer, because the beam is sector shaped
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What was a mechanically steered transducer used for?
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2D imaging, M mode, Doppler, color Doppler
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Other than being sector shaped, what other qualities did a mechanical steered transducer beam have?
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it was symmetrical in elevation and lateral planes with a broad depth of field and deep focus
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What are the disadvantages of a mechanical steered transducer?
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fixed focus, parts wearing out, motion artifact, little imaging flexibility, air pockets in the gel inside the transducer head
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What are the advantages mechanical steered transducers had over phased array transducers?
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they were less expensive to make, less expensive to run, and ran on a single channel
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What is the difference between a mechanical annular array and a mechanical steered transducer
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annular array crystals are cut into concentric circular rings
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How did cutting the crystal into circles solve the fixed focus problem?
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the varying diameter of the circles, and adjusting which ones were on at any given time.
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What was a mechanical annular array used for
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2D, M-mode, Doppler, and Color
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What did a mechanical annular array image look like
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A sector image with a curved top
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How was a mechanical annular array steered?
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by the elements being mounted on a wobbling motor
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What were the advantages of a mechanical annular array transducer?
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it had variable focus in the lateral and elevation dimension, and variable depth of field
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What were the disadvantages of a mechanical annular array?
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Limited temporal resolution, excessive grating lobe artifact, more expensive to manufacture due to cut elements, and more expensive to run electronically because of multiple channels
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Phasing is a term used to determine a ___ ___
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time reference
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For a wave, a phase difference is?
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the amount of time shift necessary to make two waves align
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in order to create constructive and destructive interference we need....
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multiple waves
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Each transducer element acts as an individual _____ _____, so many elements collected together can create _____ _____ simultaneously.
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wave source, multiple waves
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What is a collection of elements known as?
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an array
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What is an array
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a group of small transducers which can be used together to form a larger more flexible transducer
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an array is really a group of small transducers which can be used together to form....
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a larger more flexible transducer
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what is electronic steering
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steering achieved by using a small phase delay between excitation pulses to each of the elements in an array
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What are the advantages of electronic steering?
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varying focus, phasing, parallel processing
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What are the biggest disadvantages of electronic steering?
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cost of production and complexity of the design
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What is electronic steering for receive?
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listening with the same phasing we transmitted
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What are phased delays also used for?
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focus transmit and receive beams
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True of False. You can ONLY focus shallower or at the natural focus of the transducer
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true
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For deeper focusing you use ____ extreme phasing
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less
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for shallower focusing you use ____ extreme phasing
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more
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The receive phase delay must match the....
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transmit phase delay
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True or false there will be many instances when we will want to focus and steer at the same time
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true
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How do we focus and steer together?
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by adding the phase profile for steering to the focus profile
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What was a phased array sector transducer designed for?
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scanning between ribs
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How large is the footpring of a phased array sector transducer?
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small, 64-128 elements
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What is the frequency ranges of a phased array sector transducer?
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2-4Mhz for adults and 6-10Mhz in children
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What is the phased array sector transducer's doppler frequency?
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1.8-4 Mhz
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What does a phased array sector do?
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2D imaging, M-mode, Doppler, Color
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What shaped image does a phased array sector transducer produce?
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a sector shaped image
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Describe the element(s) of a phased array sector transducer
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multiple square or rectangular elements
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a phased array sector transducer uses phasing in the lateral dimension to achieve
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electronic steering and variable transmit and receive focus
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Does a phased array sector use a lens, if so what for
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Yes, to create fixed elevation focus
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Is the beam from a phased array sector transducer symmetrical?
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no its not symmetrical in elevation and lateral plane
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What are the disadvantages of a phased array sector transducer
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more expensive than a single element transducer, it has more expensive electronics, and it has a fixed elevation focus
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What are the advantages of a phased array sector transducer
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variable focus in the lateral dimension, no motion artifact, and flexibility to perform parallel processing and other advanced techniques
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A sound beam has only one ___ or "___"
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focus, waist
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how does a machine create multiple foci?
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by transmitting multiple sound beams down each scan line, each impulse having a different phase profile creating a focus and more than one depth
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As the returning sound waves arrive at the transducer....
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elements on the front of the probe are excited
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In dynamic receive focusing when the elements are excited it
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creates an electrical impulse that returns through multiple channels to the machines receiver
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Why does the delay pattern change continuously?
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because the transducer is listening for reflections from different depths
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The deeper the depth of return signal the ____ the time delay is applied to that signal
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greater
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Can the dynamic receive focusing be changed by the technologist?
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no, it is internally regulated
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Why was the linear phased array transducer created?
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for scanning vascular and small parts, with wide field of view in the fresnel zone
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What does the linear phased array transducer replace?
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the linear switched array transducer
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What is the difference between the linear phased and the linear switched in terms of switching and sequencing?
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the linear phased can be steered and focused when desired
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About how many elements are in a linear phased array transducer head
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200-300+ elements
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What are the advantages of the linear phased array transducer?
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in has variable focus in the lateral dimension, in allows for creation of wider linear image in the near field, and it has flexibility for advanced techniques and parallel processing
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What are the disadvantages of the linear phased array transducer?
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its more expensive than a single element mechanical, it has more expensive electronics, it has a fixed elevation focus, and it has no elevation steering.
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Are the lateral and elevation planes of linear phased array transducer symmetrical?
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no
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Does a linear phased array use a lens? if so why
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yes, for elevation focus
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What kind of control does a linear phased array have over the lateral dimension?
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It can electronically steer, it has variable received electronic focus, and it has dynamic receive focus
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What shaped image does a linear phased array create?
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rectangular imaged through sequencing and parallelogram image by phasing each group of sequenced elements
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What can a linear phased array transducer do?
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2D, Doppler, Color
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Describe the characteristics of a Curved Linear Phased Array transducer
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Basically the same as linear with Curved, convex face creating a wider image in the near and far field, it needs steering as the scan head created the desired image geometry
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A lack of elevational focus is a result of only 1D array (one row of crystals). Describe what 1.5D and 2D transducers are
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1.5D adds a small row of crystals to either side of the main element allowing for limited control over elevational focus, 2D has multiple elements in lateral and elevation planes so that both can be focused
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1.5D added a small row of crystals to either side of the main element which allowed...
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limited control over elevation focusing
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2D has multiple elements in both lateral and elevation so beams can...
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focus and steer in both lateral and elevation directions
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