What is noise?
Any data that does not contribute to the image
What are 3 factors that affect scatter?
- KVP- Higher KVP means Compton Scatter predominates
- Thicker- Thicker Tissue= more Scatter
- Large FOV= More Scatter
How does Collimation affect noise?
Collimation decreases noise
What is the Grid Ratio?
Height of the Lead/Distance between lead bars
-Increased Grid Ratio means less scatter but more dose
What is the Bucky Factor?
Increase in Dose due to use a grid (due to the increased mA)
What is Grid Cutoff and how to you fix it?
Image becomes very noisy bc too many photons are blocked.
-Realign the Grid, this usually occurs bc the grid is aligned incorrectly.
Should you increase mA or kVp to decrease noise?
mA, because increasing kVp can also lead to more compton scatter
How can you decrease noise?
- Use a Grid (more Dose)
- Increase the mA
- Use an Air Gap
How does distance from the source affect image noise?
Increased distance between source and detector= increased noise
Inverse square law= Energy twice as far from the source is spread out over 4 times the area
What is spatial resolution?
The ability to tell two lines apart that are very close to each other.
What are the 3 types of unsharpness?
- Motion unsharpness (pt moves)
- System unsharpness (Detector issues)
- Geometric unsharpness (focal spot size and Source to object distance)
What limits spatial resolution in film, computed radiography and Digital radiography?
- Film: Limited by Grain size
- Computed Radiography: Limited by size of laser to read phosphor plate
- Digital Radiography: Limited by size of thermoluminecient transitor
What is the equation for Magnification?
Source to image distance/Source to object distance
What is the difference between sharpness and resolution?
- Sharpness: Ability to define an edge
- Resolution: Ability to tell 2 things apart
What is Modulation Transfer Function?
Output/Input
-It describes the resolution capabilities of the system
What is Detective quantum efficiency (DQE)?
- Efficiency of the Detector in converting x ray signal to image
- Estimates the require dose to create an optimal image
How does DQE effect dose?
- High DQE= Low Dose
- Low DQE= High Dose
How does DQE relate to MTF?
Directly proportional
How does DQE relate to SNR?
Inversely proportional
How dose DQE relate to Spatial resolution?
Inversely proportional
What is pixel pitch?
Distance from the center of one pixel to another pixel
-Decreased pixel pitch= better resolution
What is the primary factor influencing imaging contrast in Film versus digital?
- Film: KVP
- Digital: Look Up Table (LUT)
WHat is window length and window width?
- Length: Selects the Image Brightness
- Width: Selects the contrast
What is the 30% rule?
-Need to increase the mAs by at least 30% to see a change in film density (blackness)
