CT Image Quality Flashcards
Define spatial resolution.
the ability of a CT system to display fine details separately; the degree to which a CT system can distinguish very small parts
Define MTF. What is its range?
modular transfer function. used to show accurately a CT system can display an object. an MTF of 1.0 is 100% to scale, while an MTF of 0.0 does not show the object
A small focal spot ________ spacial resolution.
increases/improves
How can detector size and geometry affect spacial resolution?
smaller, closely spaced detectors IMPROVE spacial resolution
High frequency reconstruction algorithms (kernels) are used for what type of anatomy? How would this kernel affect spacial resolution?
- bones, or edges (of anatomy)
2. improve spacial resolution
What combination of large/small matrix size and large/small pixels/DFOV gives the best spacial resolution?
large matrix size and small pixels (small DFOV)
What element affects longitudinal spacial resolution? How is LS resolution measured?
how quickly the patient moves through the gantry (pitch). it is measured by the slice sensitivity profile
By increasing pitch, you ______ longitudinal spacial resolution.
decrease
Define contrast resolution.
the ability of a CT system to differentiate two objects with very similar densities, or linear attenuation coefficients
What is the smallest difference, in HU, a modern CT system can differentiate?
3 HU
A decrease in noise results in a(n) _______ of contrast resolution.
increase
What algorithm results in increased contrast resolution?
low frequency
Increasing detector collimation _______ contrast resolution.
decreases: this will increase noise which reduces contrast
Define temporal resolution.
the ability of a CT system to “freeze” motion
What is the main factor controlling temporal resolution?
the speed of the gantry rotation
What type of artifact is common in CT systems with low temporal resolution?
misregistration.
An increase in x-ray photon flux causes a(n) ______ in image noise.
decrease
How does voxel dimension affect image noise?
by increasing voxel dimension, there is more space for radiation to interact therefore reducing noise.
What does a CT system’s Uniformity measure?
uniformity measures the system’s ability to maintain relatively consistent CT numbers of an entire object of equal density
What does a CT system’s Linearity measure?
how well the system displays CT numbers based on known and accepted HU for certain objects, i.e. -1000 for air and 0 for water. when scanning water, the HU should be as close to 0 as possible
