S2 L8 Flashcards
How does a grid work
Transmit useful xray photon
Reject (absorb) scattered photons
Structure of a grid
Lead strips very thin and interspace material
Lead strips absorb more secondary photons due to their path
Define bit depth
How many levels of grey can be represented
What is quantisation error
Discrepancy between true value and value represented by digital value
Caused contouring in smooth areas and increased noise
What does window width do
Increase contrast of structures that have detector pixel values within window
Change level means different range of dpv displayed
Factors that affect displayed contrast
Image processing
Quality of monitor
Maximum brightness, number of grey levels, calibration
Sources of unsharpness
Geometric
Detector layer
Pixel size
Movement
Movement unsharpness
During exposure
Relative to source and detector
Decrease:
Lower exposure time
Hold breath
Immobilise
Pixel size unsharpness
Pixel pitch: distance between 2 adjacent pixels
Lower it decrease unsharpness if fine details not lost in detection layer
Need to increase exposure
Increase storage requirements
Geometric unsharpness
Object edges blurred (penumbra)
Shadow under object (umbra)
Decrease:
Move object closer to detector
Move focal spot away from object (but need to increase exposure for this )
Decrease focal spot size (but decrease max exposure)
Detector layer unsharpness
Light scatter in p detectors
Thicker increases it
Thinner increases noise so need to increase exposure
What is noise
Superimpose pattern onto image
Can obscure details
Sources of noise
Pattern
System
Quantum
Pattern noise
Can come from:
Faulty detector element
Unmatched line amplifiers
System noise
Can come from electronics
Eg ADC, radio interference
Quantum noise
From using finite number of xray photons to make an image
Beam randomly distributed in space so the intensity arriving at detector is not equal
Fluctuates about a mean
Decrease: increase beam intensity
Which type of noise should be dominant
Quantum
Called quantum limited system
