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
File compression techniques
Can be:
- Lossy
- Lossless
Not all result in loss of data
PACS
DICOM
?
Picture archiving and communications system
Digital imaging and communications in medicine services
Pixel Sampling interval and Fill Factor
Fill factor (sensitive area) increases as pixel sampling interval increases
Sampling interval is number of pixels and space between them
DQE
Is ratio of SNR (output:input)2
Dependent on:
- radiation exposure
- MTF
- Detector material
DR Detector
- Amorphous SELEnium when irradiatied produces an electrical charge
- Upper electrode is POSITIVE (high positive potential) and attracts all negative electrons to waste them
- Positive charges are then drawn to the TFT array
*
Is geometric unsharpness less towards anode side of image?
Yes
Is movement unsharpness an issue in Mammography?
Yes due to long exposure times.
Can be greater than 1 second
Normal anode angle?
7 - 20 degrees
