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70 Cards in this Set
- Front
- Back
what is housing?
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surround structures except the connector and is usually constructed of plastic. It holds the transucer together and provides something for sonographers to grip.
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connector
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electronic link btw transducer and ultrasound unit.
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tuning wires
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allow the maunfacture to fine tune the transducer so that the crystal more closely matches the electronics of the scanner. This allows for optimal imaging.
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electrical wires
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electrical signal must be transmitted from connector through tuning wires and then to the crystal. that's the job of the electrical wires.
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crystal, transducer, element
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generates the ultrasound wave which will be transmitted to the patient.
-electrical energy is converted into mechanical energy and visa versa. |
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what determines the crystal thickness?
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crystal thickness is about 1/2 the wavelength of sound emitted by the crystal.
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what are some naturally occuring peizoelectric materials?
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quartz
lithium sulphate rochelle salts tourmaline |
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what are some synthetic peizoelectric materials?
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PZT(most common)
barium lead titanate lead metianobate PVF |
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what material decreases or eliminates the need for matching layers? why?
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PVF(polyvinylidene flouride)-it has an acoustic impedence closer to that of human soft tissue.
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what are piezoelectric materials?
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material that produce a voltage when strain or pressure is applied to them.
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what is the reverse peizoelectric effect?
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when a voltage is applied to peizoelectric material, it is deformed.
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What material contain dipoles?
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ferroelectric material
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T/F all piezoelectric material is ferroelectric
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true
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what are ferroelectric materials?
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materials that are able to retain their dipole arrangementa after electrical field is removed.
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Why are transducers not autoclaved?
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the process will excede the curie temperature causing the crystal to lose it's piezoelectric qualities
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what is a composite element? what are it's qualities?
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when piezoelectric materials are combined with other materials. THis results in a lower impedence, and improved sensitivity, bandwidth, and resolution
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Insulators
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crystal has a tendancy to vibrate up and down, so the insulating rings damp this vibration.
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damping(backing)material
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This dampens or reduces the number of cycles of the ultrasound wave.
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WHat is mechanical damping?
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the energy that is lost by reducing the number of cycles absorbed by the damping material.
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Electrical shield.
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-lines the inside of the housing
-provides a shield from external electrical interference(ie. radio frequencies,etc) This increases the signal to noise ratio. |
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Matching layer; what is the optimal thickness of a matching layer?
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minimizes the acoustic impedence difference at the transducer patient interface. The optimal thickness of the matching layer is 1/4 of the wavelength. Use of a matching layer causes the beam to have a wider bandwidth which allows better matching of the crystal to electronics of the ultrasound system.
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What is the main bang?
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in the old days, this was the result of a large acoustic impedence difference at the transducer patient interface.
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What are the 2 ways in which ultrasound crystals can be activated?
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-shock excited mode-single burst of non-alternating voltage
-burst excited mode-crystals are activated by one or two cycles of an alternating current. This allows the sonographer to select the frequency operated by the transducer. |
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what is bandwidth?
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range at which the transducer frequencies operate
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what is the formula to determine the frequency at which a crystal will vibrate?
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propegation speed/2xthickness(mm)
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as the thickness of a crystal decreases, what happens to the transducer frequency?
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it increases
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what is bandwith? What is a synonymn for it?
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the difference btw the highest and lowest frequency emitted. Synonymn is transducer pass band
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how does SPL affect the bandwidth?
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the shorter the SPL, the wider the bandwidth.
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what are some advantages of wide bandwidth tranducers?
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-are necessary for multihertz transducers
-impove sensitivity -improve image quality -better axial resolution -crystal more easily matched to electronics -low frequencies in the beam can be controlled to decrease beam divergnece in the far field. |
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what happens to the bandwidth as spacial pulse length increases?
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bandwidth decreases.
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what are 1/4 and 1/2-power bandwidths?
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pass bands that eliminate the weaker 1/2 or 1/4 of the frequencies.
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which is narrower, a half power bandwidth, or a quarter power bandwidth?
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a half hower.
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what is fractional bandwidth, what is the formula?
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fractional bandwidth=bandwidth/frequency
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what(with regards to the fractional bandwidth) is considered a narrow bandwidth?
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bandwidth is considered narrow if the fractional bandwidth is less than or equal to 15 percent.
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what is Q factor?
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a measure of how efficiently the transducer changes electrical voltage to a sound wave. Also indicates how good of a transmitter a crystal is.
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What are the formulas for Q factor?
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Q=operating frequency/bandidth
Q=energy stored per cycle/energy lost per cycle Q=1/fractional bandwidth. |
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T/F Q factor is approximately equal to the number of cycles in a pulse
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True
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as the Q increases, what happens to the bandwidth?
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it decreases
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as the Q increases, what happens to axial resolution?
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it degrades
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what 3 things happen as the SPL becomes shorter?
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-axial resolution improves
-bandwidth increases -Q factor will decrease |
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what is transmission co-efficient?
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fraction of electrical energy converted into acoustic energy.
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reception co-efficient?
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fraction of returning energy converted into electrical energy
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electromechanical coupling co-efficient
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how efficiently the transducer converts electrical stimuli into ultrasound energy and recieved ultrasound energy back into electrical energy.
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dielectric constant
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characterises mechanical and electrical properties of a crystal. Describes movement of a crystal when voltage is applied ot it,and to voltage induced when a strain is applied to it.
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acoustic impedence
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a measure of the resistance of a medium to the transmission of sound.
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state 3 synonymns for transducer frequency?
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preffered, operating, resonate, central
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what are the consequences of not including backing material in transducers?
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decreased axial resolution, decreased sensitivity
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why is damping material not inculded in trasducers used specifically for continuous wave applications.
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because damping material causes SPL to be shorter, but since continuous waves don't have SPL, it doesn't matter. Also, using damping material causes a loss in energy, so it is better not to use it.
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why is the inculsion of backing material considered mechanical damping?
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because the pulse is dampened by physical means
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what are the implications of changing the thickness of the crystal?
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change the central frequency of the beam
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what is pass band?
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bandwidth
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why are wide band transducers typically preferred over narrower band transducers
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because they improve axial resolution, increase sensitivity, multihertzing, increased image quality, crystal more easily matched to system
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a transducer which is .22 mm thick, will produce what frequency?
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element thickness is half the wavelength, so a transducer with .22 with produce a wavelength of .44. This goes into the c=f/wavelength to get 3.5 mHZ
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For a given frequency, what will happen to the fractional bandwidth as the bandwith increases?
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decreases
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what would happen to the ultrasound image is hte matching layers are removed?
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overall image quality would degrade. Decreased axial resolution, decreased sensitivity, decreased penetrating ability
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how thick is the typical matching layer?
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1/4 wavelength
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why are composite crystal the rule rather than the exeption?
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composities produce a beam with lower impedence, wider bandwidth, greater sensitivity, and better axial resolution
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why should transducer elements never be subjected to a temperature which exceeds the curie temperature?
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loss of piezoelectric qualities
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what measures can ve taken to improve sensitivity?
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adding matching layers, increased output or gain
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what benefit does the presence of the electrical field bestow?
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increased image quality, prevent radio frequencies from reaching the transducer, produce artifacts
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what would be the consequence of not having amatching layer attached to the face of the crystal?
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the acoustic impedence remains high which causes a large reflection of energy
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the lack of which transducer part could result in unacceptable side lobes.
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insulator
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for a given transducer material, what happens to the frequency when crystal thickness is reduced by half?
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frequency is doubled
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as the transducer increases, what happens to the bandwidth?
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it becomes narrower
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what is the advantage of a 2-d transducer over a 1.5D transducer?
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can focus in more dimentions. THere is an increase n the # of rows increasing the possiblity for timing delays.
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what are vaxels?
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3D pixels
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what are some free hand scanning methods with sensors?
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-articulated arm-gantry system
-magnetic field-transmitter transmitts several different magnetic fields so the machine can tell the orientation of the transducer -acoustic ranging-have recievers in the ceiling -imaged base corrilation-camera looks for similar traits in am=n imge and it matches them up |
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what are grating lobes?
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beams of energy not in much of the ultrasound beam-angle depends on wavelength and element spaceing
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what is subdicing?
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choping a crystal into 4
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which part of the transducer is responsible for improving the signal to noise ratio?
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electrical field
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