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38 Cards in this Set
- Front
- Back
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The goal of CT is to overcome the limitations of radiography and tomography by achieving the following:
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1) Minimal superimposition
2) Improved image contrast 3) The recording of very small differences in tissue contrast |
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What are the two methods of data acquisition?
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slice-by-slice data acquisition
volume acquisition |
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In this type of data acquisition, a special beam geometry called spiral or helical geometry is used to scan a volume of tissue rather than one slice at a time. What is this called?
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Volume Data Acquisition
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In 2007, a MSCT scanner was created that can produce _____ slices per revolution of the x-ray tube.
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320
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What is the first step in data acquisition?
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Scanning
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What is attenuation?
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It is the reduction of the intensity of a beam of radiation as it passes through an object. Some photons are absorbed, while others are scattered.
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What does attenuation depend on?
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electrons per gram
atomic number tissue density radiation energy used |
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What is a homogeneous beam? pg 90
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A beam of radiation where all the photons have the same energy.
It is also called a monochromatic or monoenergetic beam. |
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What is a heterogeneous beam? pg 90
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All the photons have different energies.
Also called a polychromatic beam. |
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When Hounsfield invented the CT scanner, he used what kind of beam in his initial experiments?
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homogeneous beam
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X-rays can be attenuated because of the ___________ effect, or they can be attenuated and scattered by the ___________ effect.
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photoelectric effect
Compton effect |
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The photoelectric effect occurs mainly in tissues of high or low atomic number (Z)?
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high atomic number
The photoelectric effect occurs minimally in some soft tissues and substances with a lower Z. |
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The Compton effect occurs in soft or hard tissues?
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soft tissues
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The _________ effect depends on the beam energy.
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photoelectric effect
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The _________ effect is likely to dominate as beam energy increases.
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Compton effect
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What is data processing?
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This constitutes the mathematical principles involved in CT.
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What are the two steps in data processing?
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Raw data undergo some form of processing where corrections are made and some reformatting of the data occurs.
Then image reconstruction occurs. The scan data (represents attenuation readings) are converted into a digital image characterized by CT numbers. |
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Conversion of the attenuation readings into a CT image is accomplished by mathematical procedures called what?
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reconstruction algorithms
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CT numbers are established on a relative basis with the attenuation of ______ as a reference.
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water
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The CT number for water is?
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0
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The CT number for bone is?
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+1000
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The CT number for air is?
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-1000
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The CT number for lung is?
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-950 to -550
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The CT number for fat is?
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-100 to -80
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The CT number for a kidney is?
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20 to 40
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The CT number for the pancreas is?
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+30 to +50
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The CT number for the liver is?
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+50 to +70
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The CT number for blood is?
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+50 to +60
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In CT, high kV techniques are used because?
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1) To reduce the dependence of attenuation coefficients on photon energy.
2) To reduce the contrast of bone relative to soft tissues 3) To produce a high radiation flux at the detector |
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After the CT image has been reconstructed, it exits the computer in digital or analog form?
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digital
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The range of numbers in a CT image is referred to as?
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Window Width (WW)
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The center of the range of number is a CT image is referred to as?
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Window Level (WL)
or Window Center (C) |
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The tech must select the FOV or reconstruction circle, which is a circular region from which the transmission measurements are recorded during scanning. The region is specifically called the _______FOV?
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Scan FOV
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The display FOV can be equal to and less than the _____ FOV.
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Scan FOV
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The pixel size ranges from 1 - ___ mm on most scanners.
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1 - 10mm
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The voxel size depends not only on the thickness of the slice, but also on the ______size and the _____.
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Matrix size
FOV |
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When the voxel dimensions are all equal (perfect cube), the imaging process is referred to as what?
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Isotropic Imaging
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CT images can have 8, 9, 10, 11, or 12, bits per what?
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pixel
The image therefore consists of a series of bit planes referred to as the bit depth. |
