CR & DR Flashcards
Indirect DR
Scintillation layer?
Absorption efficiency?
Thickness of scintillator layer?
85% absorption efficiency of Caesium iodide crystals
Thickness is 500micrometres (µm)
- Caesium iodide crystal internally reflect light thus reducing spread of light in the phosphor layer
- Caesium iodide crystals are grown PERPENDICULAR to the detector surface
Gadolinium oxysulphide is an alternative to caesium iodide
Indirect DR
Photodiode & Active Matrix?
Amorphous Silicon
- has 100% efficiency
- Converts light into electrical charge
- 1 photon produces = 1500 charge storage carriers
- Cheap
Active matrix:
- Photodiode
- TFT
- Capacitor (stores charge)
Fill factor = sensitive area/whole area
(Increasing fill factor increases sensitivity)
Direct DR
Direct conversion of x-ray photons to electrical charge
?Photoconductor
Amorphous selenium
- 500µm thick (0.5mm)
- Absorption of x-ray photon releases positive and negative charge carrier pairs
Positive holes carry the image information
- negative electrons are attracted away
There is NO potential applied to increase the charge
Indirect DR Spatial resolution and DQE?
Indirect:
- better DQE than CR and direct DR (caesium iodide has higher K edge than selenium so can absorb more photons)
- lower dose for same image quality
- higher SNR
- worse spatial resolution due to scattering of light in scintillator)
Direct:
- lower SNR
- better spatial resolution (no scattering)
- higher dose needed than indirect
- better pixel fill factor
DQE and Spatial resolution???
DR
vs
Computed Radiography
DQE:
- 60% for DR
- 40% for CR
Spatial Resolution
- DR is 3-4lp/mm
- CR is 5lp/mm approx
- Film screen higher again
Noise can be reduced by low pass filtering
Mammography
- Angled tube
- Fixed focus to detector distance
- Fixed field size - 65cm
- Compression device (reduces dose and geometrical unsharpness)
- Moving grid is used
- Magnification can be achieved by increasing the object to detector distance*
- Exposures are LONG - up to 2 seconds
Dosage is 1 - 1.3mGy per breast
Beryllium can be used where inherent filtration needs to be minimised (Atomic number 4)
Mammography - Spatial Resolution
Digital mammography - 7-10lp/mm
Film screen - 15lp/mm
How does sampling frequency affect spatial resolution and sensitivity??
Increasing sampling frequency increases spatial resolution (think of it as increasing number of pixels)
Increasing sampling frequency DECREASES sensitivity
Digital Image
- 12 bits usually required
- Image displayed as DISCRETE grayscale values
- Number of bytes depends on the
- number of pixels
- matrix size
- (NOT PIXEL SIZE)
A wide range of data is obtained during exposure which needs to be processed. This is done by Autoranging
Autoranging is a process where only the most important parts of the data range are displayed. This is done automatically when the type of examination is chosen e.g CXR or AXR
- automatic process
- does NOT affect spatial resolution
Images of a larger MATRIX will tolerate compression better than a small matrix
Exposure Index
A measure of the dose incident on the image plate
- The relationship of dose and exposure index is not linear
- Can vary between manufacturers
*
Absorbed Dose
A measure of energy absorbed per unit mass of medium
- Measured in grays
- Measured in joules/kg
Radiographic Image
- Human eye can differentiate 15-28 shades of grey
- Can detect min optical density of 0.04
Nyquist frequency
Is the max frequency that can be sampled
- It is equal to half the sampling frequency
MTF
MTF goes down as spatial resolution (spatial frequency goes up)
Computed Radiography plate
- 0.3mm thick
- Barium fluorohalide doped with europium is 85% bromide and 15% iodine
- CR reader uses a ROTATING mirror
- Plate is wiped before use with bright light
Pixel size calculation
Pixel size = FOV/number of pixels
