Computed Tomography (Diagnostic Radiology) Flashcards
Fill in the following logic flow.
Projections –> _______ –> Recon Slice
Sinogram
(raw data)
What are some of the key advantages of CT (3 and a half of them)
Eliminates superposition
Superior contrast
Fast image acquisition
Various acquisition modes
Which of these is not a way to increase the scan FOV?
Larger fan beam angle
Larger cone angle
More detectors
(not a trick question. One of them is right)
Larger cone angle
Display FOV (DFOV) is the reconstructed FOV with a mixed matrix size. That being said, what is the maximum DFOV possible?
The SFOV
(You can’t reconstruct what you never scanned!)
What is the moving component of the CT that enables helical CT scan mode, thus reducing the scan time?
Slip Rings
Main benefit to Z-dimension oversampling
Improves spatial resolution in z-dimension
What effect does the bow tie filter have on the x-ray fluence profile along the fan angle?
Reduces fluence at the ends, does not affect the middle. Makes things closer to uniform
Why does removing peripheral x-ray intensity not result in poor image quality?
Overall noise is unaffected because it is dominated by the central intensity anyway (minumum N)
Why do we need a uniform X-ray fluence/intensity at the detector?
For uniform image quality (N)
What area of the beam is noise dominated by? Central or peripheral?
Central
Where is beam width defined at?
Isocenter
What is the slice width determined by?
Detector binning
Where is the slice width measured?
Isocenter
For the same scan parameters (same dose), what is the change in resolution and noise when using thinner slice?
Better resolution
higher noise level
Mathematical equation for CT number in hounsfield units.
HU(x,y,z) = 1000 (µ(x,y,z) - µw) / µw
What is the dominant photon interaction in the energy range of CT (80-140 kVp)
Compton
What is the use of the localizer/scout? What is it?
A digital radiographic image used to set up the scan region, check patient centering and decide other scan parameters
Why is PA preferred over AP?
Reduced breast dose
What is the purpose of mA modulation?
Use lowest mA to get a present SNR
Reduces dose
What is the main purpose of the helical scan?
To increase the scan speed
Equation for pitch during a helical scan
Pitch = Ftable/nT
Where Ftable is the table feed distance per rotation of gantry (how much the table travels in 1 gantry rotation)
nT is the collimated beam width
What is the benefit of having a higher pitch? (two answers)
Shorter scan times
Lower dose
What is the main drawback of having higher pitch (two answers)?
Longer distance between interpolation points
Worse image resolution in z-dimension
What is the most commonly used type of reconstruction algorithm?
Filtered back projection
What is the method in which intensity values in each projection are smeared over the image matrix along the direction of the original rays to which they respond, then summed to produce the image?
Back projection
(You’re literally working backwards)
What is the main issue of simple back projection?
1/r image blurring from geometry of the back projection