Alot Of CT Flashcards
What are the factors that affect quality of a CT image
- mA
- scan time
- FOV
- reconstruction algorithm
- kV
- pitch
What are image quality metrics
High contrast (detail) spatial resolution
Low contrast resolution (contrast detectability)
Temporal resolution
CT number uniformity
Accuracy
Noise
Artifacts
What is spatial resolution
Ability to resolve small, high contrast objects that are very close together
What is spatial resolution
Ability to resolve small, high contrast objects that are very close together
How do we measure spatial resolution
Directly (line pairs)
MTF modulation transfer function
What factors affect spatial resolution
Matrix size
Pixel size
DFOV
Slice thickness
Reconstruction algorithm
Interactive reconstruction
Focal spot size
Pitch
patient motion
Pixel size formula
DFOV / matrix size
What is sampling frequency
Directly related to spatial resolution
More sampling = more image detail
How do we increase sampling frequency
- increase sampling rate
- increase speed at which detectors are read
- increase time during which PET radio tracer emissions are read
Image noise increases with
- increased spatial frequency
- ## increased image detail
How does FBP work in relation to image resolution
- increases noise and dose
- can offer high spatial resolution or high contrast resolution but not at the same time
How does iterative reconstruction affect image resolution
Enhances image quality with lower dose
What are the drawbacks for iterative reconstruction
Image artifacts
Artificially enhanced edges (becomes sharper, soft tissue becomes smoother)
How does focal spot size effect resolution
Large focal spots cause more geometric unsharpness which reduces spatial resolution. (Release of heat and photons)
What is low contrast resolution
Ability to differentiate objects with slightly different densities
Between film/screen and CT, which is better in spatial resolution and resolving differences in tissue density (LCD)
Film/screen - superior to CT in spatial resolution
CT is superior - in resolving small differences in tissue density
When is LCD used
Discussing the ability to see an object that is nearly the same density as its background
What 3 factors does visible object size depend on
Level of contrast in object
Image noise
Window setting
How would someone decrease noise by 1/2? How does this effect dose
- mAs is increase by 4x
- dose is increased by 4x
How is slice thickness related to noise
- as thickness decreases, noise increases
- thicker slice, more photons hits detector but you cannot see smaller objects as well anymore
If window level is the same but window width is increased, how does the image change
- less black and whites
- more greys
How do bone algorithm and soft tissue algorithm differ
Bone - produces lower contrast resolution but better spatial resolution
Soft tissue - produces better contrast resolution but lower spatial resolution
How do larger patient effect contrast resolution
more photons attenuated
Fewer will reach detectors
Higher noise
Lower contrast resolution
What is temporal resolution
Indication of the CT scanner’s ability to freeze motion of an object
How is temporal resolution controlled
gantry rotation speed
# of detector channels in the system
How is temporal resolution achieved
Prospective gating (x ray tube is turned off during acquisition times)
Balance between dose and image quality
Retrospective gating (beam on entire time)
What is the advantage and disadvantage of prospective gating
- reduce dose to patient
- reliance of regular heart beat
What is the fastest rotation time available in CT scans
0.275s (275ms)
What is the CT number of water and air
Water 0
Air -1000
What is CT number uniformity
- ability of scanner to have the same CT number regardless of where it is in an object
What are the sources of image noise
- quantum noise / mottle
- inherent physical limitations
- reconstruction parameters
What is quantume noise / mottle and how is it influenced
- x ray flux (number of detected photons)
scanning parameters
Scanner efficiency
Patient size
What are some physical limitations that cause electronic noise
- detector in photodiode
- data aquisition system
- scatter radiation
What are high resolution kernels
- increase noise level
- increase high frequency signals `
How do we verify external to internal lasers and ISO
Option 1 - incorporate into QA phantom (take and check image)
Option 2 - use separate phantom
How is CT pet done
Separately then together , CT first
- warm up
- air calibration
- phantom checks
How is PET QA done
- use water phatom mixed and calibrated with solid geranium (68 phatom)
- check coincidence
What is a Jaszcak / ACR phatom
- water phatom with sphere that can be filled with contrast or F-18 solution mix
How is MRI QA done
- phantom with anterior receiver coil
- slices taken through phatom for noise, spatial linearity, flood field uniformity, slice profile and spatial resolution tests
How is MV and MR iso checked
- zirconium ceramic balls are surrounded by an MR visible fluid separated by a thin layer of phantom plastic
Why are tests from MRI to CT different
- effect of machine geometry and magnetic field on radiation beam measurements
- effect of magnetic field on measuring tools and devices
- consortium has put out some recommendations
