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
PACS
picture arching and communication systems
- intranet software for storing, retrieving and distributing digital MI within radiology dept and hospitals
- accompanied by radiology info systems used for apps, record and dose keeping
digital images
- CR and DR image is a matrix of pixels
- each pixel reps area of anatomy
- more pixels means more res as pixels are smaller
- each detector gives a digital signal to assign a grey scale to the corresponding pixel
SNR
SIGNAL: useful info contributing to image i.e unattenuated primary beam
dependent on
- exposure factors and incident remnant beam
- patient characteristics
- receptor DQE
NOISE: random info not resembling image
sources
- scattered rad
- detectors
- electronics
characteristic curve
shows the relationship between exposure of a film and the resulting density after processing of the image
FILM:
- if exposure way off mark, repeat required
COMPUTED:
- produces linear graph so under and overexposure can be compensated
dynamic range
- useful range of exposures that can be recorded and displayed on an image
- far greater than film
great advantage of digital:
- can assign pixel values in a linear fashion across a much wider range of exposures then post process a characteristic type curve
image processing
- software must decide how to present pixel values
- does this thru a histogram, a graph of all pixel values
12 bit pixel value= 4096 diff grey scales
8 bit monitor= 256 diff greyscales
human eye= around 80 greyscales
post processing stretches, condenses or amplifies the histogram to match an ideal reference histogram for specific projection taken
positioning of data on histogram
too dark= histogram shift right
too light= histogram shift left
brightness affects the position
contrast is spread of histogram across all range of values
histogram analysis
- software can trim lower scatter/noise/ unexposed values
- remaining histogram is compared to a look up table (ref histogram for each examination)
- image histogram is manipulated to try and match look up
- look up also manipulate pixel values to apply contrast, edge enhancement and noise reduction required
windowing
adjusts the greyscale of the image to highlight specific structures and make it easier to view
large dynamic range but human eye can only perceive 80 levels
window level (WL) is central digital number
window width (WW) is range of digital numbers
WW = contrast
increased ww means contrast increases and vice versa
WL = brightness
increased wl means image is darker and vice versa
exposure
important despite flexibility of CR/DR post processing
- image quality vs patient dose
- kvp= sets image contrast and penetration
- mas= ensures enough photons each image receptor to from image
dose creep
due to risk management, patients can be overdosed to ensure the receiver has enough photons to produce the image.
over time this can lead to overexposing patients
digital techniques have a clearer image produced irrespective of dose since there’s a better dynamic range compared to film- screen
exposure indices
indicates amount of radiation received by IR for an exposure
digital number:
KONICA- sensitivity index, S (ideally 150 - 250)
AGFA- LgM; IEC EI
Siemens- Exposure index, EXI
auto collimation affects image processing and EI values
artefacts
any unwanted info contributing to an image
its avoidable and unavoidable
e.g:
ghost images (previous overexposure)
scratches on CR plate
defective DR detectors
dead pixels on image (fail 2 receive signal and appear as bright white dot)
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