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21 Cards in this Set
- Front
- Back
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What variable describes the resolution properties of an imaging system in the spatial frequency domain?
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MTF- Mean transfer function
Rule of thumb: an object becomes unobservable/unresolvable when imaging MTF<10% MTF is the product of all the components |
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What is the relationship between focal spot size, magnification and spatial resolution?
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smaller focal spot size and less magnification favor improved spatial resolution
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What is the effect of detector aperture size on spatial resolution?
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detector aperture refers to detector size in the z axis. The smaller the aperture size, the higher the z spatial resolution. Aperture does NOT influence x and y resolution.
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How does spatial resolution vary with number of projections
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Proportionally
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What is the relationship between slice thickness and spatial resolution
How about reconstruction algorithm? |
Inversely proportional, thinner slices = better spatial resolution
bone algorithm gives better spatial resolution at expense of noise smooth algorithm minimizes noise at expense of spatial resolution |
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How do you calculate pixel size?
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FOV/matrix= p
Decreasing FOV or increasing matrix size= smaller pixel size = better spatial resolution. |
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how does pitch relate to spatial resolution
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Inversely related. High pitch = resolution degradation
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What is the relationship between photon flux and contrast resolution?
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The more photons used, the better the contrast resolution at the expense of greater patient dose.
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How do we define signal-to-noise ratio (SNR)
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SNR= N/o
as the number of photons are doubled the SNR increases by a factor of 2^1/2. Therefor increasing number of photons results in increased contrast resolution but at a lesser rate, but increases patient dose proportionally |
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how does image contrast and spatial resolution vary with slice thickness
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small slice thickness = greater spatial resolution and degraded contrast resolution
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How does pixel size relate to spatial resolution and contrast resolution?
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small pixel= better spatial resolution at expense of degrading contrast resolution
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How does reconstruction algorithm filter affect spatial and contrast resolution?
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sharpening algorithm improves spatial resolution at the expense of a noisier image which degrades contrast resolution.
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How does prospective ECG triggering work?
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The step and shoot method is triggered by the patient's R-R intervals, the tube is not on continuously so cannot give functional information.
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How does retrospective ECG gating work
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The scan is acquired helically while the tube is continuously on and projection data is selected retrospectively. This provides functional information at the expense of patient dose.
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Which reconstruction method offers better temporal resolution (partial or multiple-segment reconstruction)
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multiple segment reconstruction but does so with higher radiation dose to the patient
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How does ATCM work?
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The CT scanner calculates attenuation maps in the AP and lateral projections which drive the kVp and mAs during the scan resulting in more uniform scanning from patient to patient while minimizing dose to the patient and maintaining acceptable image quality.
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What is the timing for the following phases?
Pulmonary arterial Systemic arterial portal venous phase |
PA: 10-25
SA: 25-35 PV: 65-75 |
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What's the best contrast bolus timing for the following pathology?
HCC Hemangioma RCC TCC |
HCC 30 sec (late arterial) then 5-10 min (equilibration)
Hemangioma 65-75 sec (portal venous) then 5-10 min RCC (Nephrographic phase) 80 sec TCC (excretory phase) 3-10 min |
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What is the timing for the following renal phases:
Cortico-medullary nephrographic pyelographic |
Corticomedullary: 30 sec
Nephrographic: 80 sec Pyelographic: 4-10 min |
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How does DECT work?
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Attenuation coefficient is related to the property of the material and the photon energy used. The same material will have different attenuation coefficients when imaged using different photon energies. Therefor by acquiring CT's at different energies, we can characterize and isolate the composition of a material.
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Automatic exposure control in CT works by modulating (tube voltage or current)
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Tube current
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