Fill in the following logic flow.
Projections --> _______ --> Recon Slice
What are some of the key advantages of CT (3 and a half of them)
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
(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?
(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?
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?
Where is beam width defined at?
What is the slice width determined by?
Where is the slice width measured?
For the same scan parameters (same dose), what is the change in resolution and noise when using thinner slice?
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)
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
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
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?
(You're literally working backwards)
What is the main issue of simple back projection?
1/r image blurring from geometry of the back projection
What is the difference between filtered back projection and simple back projection?
Filtered removes the blurring using convolution before back projecting them
What are the pros and cons to Model based iterative reconstruction?
Enhanced image quality, reduced dose
Longer computational time
Why is 90 kVp used when contrasting vessels using an iodine solution?
Iodine has a photopeak of about 30 keV. Average keV in a 90 kVp beam is about 33 keV.
What affect does dual source have on
- Acquisition time
- Scatter cross talk
- SFOV of tubes A and B