DR: Image Quality Flashcards
What is contrast?
Difference in attenuation (subject contrast) or displayed (image contrast) of an image
How can subject contrast be improved?
- Thicker object
- Greater attenuation between objects
- Decreasing kV
- Increasing Z (atomic number) differences in objects
- Increasing difference in density of objects
What can reduce subject contrast?
Noise (no anti-scatter grid, smaller air-gap used)
What is spatial resolution?
Measure of how far apart two objects must be until they can be seen as seperate details in an image
How can spatial resolution be measured?
- Line spread function (how spread an image of a sharp object becomes. Difficult to measure)
- Line pairs per mm (lp/mm)
What are the factors affecting spatial resolution?
Object properties
- Object spatial frequency: if it is too high for the system it will not be displayed accurately
- Object low contrast: lower contrast objects reach an MTF (modulation transfer function) of 0 at lower spatial frequencies
Computed/digital radiography
- Detector element size: smaller element = higher spatial resolution
- Distance between detector elements: smaller distance = higher spatial resolution
Others: anything that increases unsharpness
What is noise?
Noise is inversely proportional to SQRT(photons)
How can noise be reduced?
Anything that increased number of x-ray photons (x-ray beam) produced and absorbed and the number of light photons (at the image receptor) produced:
- Increasing dose (mA)
- Using an image receptor with a greater attenuation coefficient
- Making the image receptor thicker
- Using larger detector elements
How can magnification affect image quality?
Greater magnification can be achieved by moving object further from detects
What causes distortion?
Due to the finite size of the focal spot an image may be distorted depending on the angle at which it is imaged
