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what is interpolation?
a mathematical method of estimating an unknown value using the know values on either side of the unknown
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increasing the scan field of veiw __________
increases the number of detector cellls collecting data
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what is the function of the digital-to-analong converters?
converts the signal from the computer into an analog signal for the display monitor
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what hounsfield values are in the naturally occurring range (i.e., not manmade objects like surgical clips)?
-1000 to 1000
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decreasing the window width in an image decreases ____
the anatomic diversity displayed
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the window width of a ct image is set at 300, and the leve (or center) is set at 100. how is a structure with a measurement of 280 HU displayed?
white
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what is included in the scan parameters?
- mA
- scan time
- slice thickness
- FOV
- scan algorithm
- kVp
- pitch
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what is the uncoupling effect?
even when mA or kVp setting that is too high is used a good image results, making it difficult to determine when a dose is higher than necessary.
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what is a disadvantage of a bone algorithm?
it reduces the visibibility of soft tissue structures
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what is a partial scan?
a scan that collects data from 180 degree tube arc, plus the degree of arc of the fan angle
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what is the true test of the quality of a specific image?
spatial resolution and contrast resolution
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how often is the accuracy of a scanner's slice thickness tested?
daily
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what can be done to improve the linearity of a ct system?
daily calibrations
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what quality control test must be preformed by a medical physicist?
radiation dose measurements
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artifacts that reult from undersampling are called ___
aliasing artifacts
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raw data that results from an MDCT scan is used so that the 1-mm slices are combined to produce thicher slices for viewing. this is called ___
image reconstruction
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True or False
MPRs can be created either at the operator's console or at a separate workstation
True
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what is a limitation of scanner-created MPRs?
in most cases, only straight sagittal and coronal planes can be automatically generated
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in creating a surface rendered image, what can happen if the threshold set is too narrow?
actual protruding structures can be imperceptible
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True or False
a MIP is a good method to display contrast filled arteries
true
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True or False
a MIP is a good method to display the bronchial tree
False
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what has become the favored 3D imaging technique in CT?
volume rendering
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what type of reconstruction is used in a 1st generation CT Scanner?
iterative
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what is the largest deterrent of a scanner's contrast resolution?
noise
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how do you determine the minimum oject size that a scanner can resolve?
taking the reciprocal value of the scanner's limiting resolution.
explanation: a line pair is a line and the space so this value can be divided in half
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any ___ in the voxel dimension increases nois, or decreases the signal to noise ratio.
decrease
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what is used for current MDCT image reconstruction?
180 interpoltion
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how many HU can there be for any given pixel?
4096
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how do you calculate the display field of view?
pixel dimension multiplied by the matrix size
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HU with less attenuation than water are ___
negative
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HU with more attenuation than water are ___
positive
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pixels that differ by 1 HU represnt a ___ difference in attenuation.
0.1%
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explain a hybrid array.
detector array that contains mid line narrow elements flanked by wider detectors
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what does the spatial resolution of a scanner control?
its ability to image small structures
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