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38 Cards in this Set
- Front
- Back
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Conventual x-ray suffers from the collapsing of 3D structures into
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2D image
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Spatial resolution is low in CT but......
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it has very good low contrast resolution, which make sit good for very small changes in tissue type.
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CT good give accurate diagnostic information about the distribution of
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structures inside the body.
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Construction of a CT scanner
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Aperture = bore
tube detectors |
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cathode
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- negative
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anode
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+ positive
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X - Ray uses
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bremsstrahlung radiation which means breaking radiation
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Anode/ tungsten target
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is where the beam of electrons hit the rotating wheel and is produced into x-rays (negative)
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The tungsten target need to rotate because
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the wheel will burn a hole into it if the same spot was getting hit with electrons of energy
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X rays produced by the x ray tube are
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polychromatic
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The effective energy used in CT is less than the gamma rays we use in nuclear medicine.
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120 kVp has an effective energy of about 50 keV.
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Tc99m
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140 keV
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F-18/FDG
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511 keV
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Co-57
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122 keV
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I 123
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153 keV
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What are we measuring?
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the average linear attenuation coefficient between tube and detectors
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Attenuation coefficient reflects the degree to wich the x-ray intensity is ________ by a material
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reduced
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u values are scaled to that of water to give CT number
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u tissue- u water x 1000 = CT #
u water |
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water =
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0
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air =
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1000
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bone =
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1000
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slip rings
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allow for continuos rotation
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Helical Scanning
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combines continuos gantry rotation with table movement
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Path of x-ray beam around the patient follows a
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helical path
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Multi-slice helical reconstruction
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produces a set of interleaved helices
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Advantages of Multi-slice
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Same acquisition in shorter time or thin slices give better z axis resolution or scan larger volumes in same time
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Multi-slice scanners have scan times of
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0.5 seconds or 0.7 to 0.8 for a single slice
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Pitch
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is the table movement per width of the collimated scan beam
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Pitch =
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table movement per rotation
------------------------------------------ (# of slices per rotation)(slice width) all values in mm |
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Thus, with a pitch greater than 1
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WE HAVE LOWERED THE PT RADIATION DOSE...the table is moving faster than the tube is rotating, spreading out the exposure
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Raising the pitch to high will.....
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degrade the image quality..... a certain amount of radiation is needed to create a good picture.
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Therefore, with a pitch less than 1
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WE HAVE RAISED THE PT RADIATION DOSE....we are over-scanning parts of the body....some areas are being exposed more than once....small parts in the body such as the sinuses need overexposure for a good picture.
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Streak artifacts caused by metallic materials
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- dental fillings
- prosthetic devices -surgical clips -electrodes |
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Isotropic resolution
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ideal case for imaging three dimensional structures is to have resolution identical in all dimensions
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In PET/CT scans the ________ ___________ co-efficient numbers are used for attenuation correction of the PET portion of the scan
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linear attenuation
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1.)This provides a _________ attenuation map because of the high photon portion of the scan
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noiseless
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2).Using a CT..... we do not have to replace radioactive sources in the PET machine
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3.) It is faster than acquiring correction info from external radioactive sources.
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However, the energy of the CT scan must be corrected to match the 511 keV from PET because their penetrating ability varies.
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Using CT for AC does not work well with filtered backprojection.......PET/CT scanners use iterative reconstruction methods.
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