image receptors and digital images Flashcards
principles of computed radiography
- cassette based
- phosphor stores xray energy
- red plate by scanning laser
- digital image produced on monitor
structure of photostimulable phosphor
- protective layer
- phosphor layer
- reflective layer
- conductive layer
- support
- light shielding layer
- backing
storage of the latent image
- attenuated beam incident on PSP plate
- incident photons excite some e-s in phosphor layer from the valence band to conduction band
- around 50% return to valence band emitting energy as light
- remainder are ‘trapped’ in an excited state within the crystallised molecular structure of the PSP plate
what is the valence and conduction band
VALENCE:
- highest occupied energy level
- e-s are tightly bound to the atoms
- valence band is the outermost electron orbital of an atom
CONDUCTION:
- lowest unoccupied energy level
- e-s are loosely bound to the atoms = produce electrical current
- conductive band is the shell that is not occupied by any e-s
reading of phosphor plate
- laser light ‘reads’ PSP plate, line by line
- energy of laser gives ‘trapped’ e-s enough energy to become ‘untrapped’, emitting their energy as fluorescent light
- amount of light emitted will be proportional to the x-rays incident on the PSP
reading of phosphor plate (storage plate)
- processor has light sensitive detectors that can record how much light is emitted for each part of the cassette
- this is transferred into a digital signal by an ADC
- resolution of the system is dependent on scanning freq
- this determines pixel size and image resolution
unsharpness and CR
- plate made of phosphor crystals (internal scatter of light)
- similar problem with film screen
principles of digital radiography
- cassette less imaging
- x-ray photons are converted directly into electronical signal
indirect digital radiography
- uses scintillator to turn xray photons into light (CsI)
- this fluoresces light which is recorded by light sensitive photodiode or CCD
- photodiode: absorbs light photons and produces a current
- amorphous silicon works the same way as DDR to produce charge read by TFT
CCD- capacitors register light and convert it o a current of proportional strength. current is digitised as a pixel value
direct digital radiography
flat panel detector
- crystal (amorphous selenium)
- x-ray photons cause ionisation and energy is converted into electrical charge
- charge collects at capacitor
- switched by TFT
CR vs DR
CR:
max kvp is 90 which is more sensitive to compton s = lower contrast image
DR:
- instant image review and more sensitive at detecting photons
-higher kvp used=
lower patient dosage=
ensure adequate penetration=
post processing can adapt low contrast images
- higher kvp generally lower patient dose with adjusted mAs and give DR greater chance to display required contrast
detective quantum efficiency (DQE)
- DQE is a measure of how efficiently a detector uses the xray beam
- is the relationship between the density of useful quanta and density of xray quanta actually incident on the detector
- the more x-rays used to from the image, the lower the relative nose level- better SNR
- for a given level of xray exposure a detector with higher DQE will give an image with better SNR (better image quality)
HIGHER DQE= OBTAIN SAME IMAGE QUALITY= LOWER XRAY EXPOSURE
digital vs film- pros
pros:
- wider exposure latitude reduces repeats
- lower patient doses with DR (better DQE)
- digital instant transfer of images = broader comms
- reduced lost films
- no film costs, faster processing and fewer xray rooms required
digital vs films- cons
- initial start up costs, training n resources
- error in post processing may mask pathology
- comparison of images harder by post processing
- exposure creep
= ALL RESULT IN INCREASED PATIENT DOSE
DICOM images
digital imaging and communications in medicine
- ensures quality standards and uniformity across centres
- patient and exam info embedded to ensure no mix ups
