7. Digital Imaging: Image Receptors Flashcards
what are the 2 types of digital IRs
what are their differences and similarities
computed and direct
different construction techniques and acquire latent images differently however how this digitized and processed is the same regardless of technique
what is the latent image
image on an exposed film or print that has nor yet been made visible by developing
for CR what is done to the exposed image plate and processed
placed/sent to a reader and converted from analog plate to digital data for processing
for CR the reader units have drive mechanisms what does these do
move the plates through the scanner
for CR the reader units have an optical system
what components does it consist of
what do these do
laser, laser scanner and erasure unit
laser scans the exposed image plate to release the stored energy via visible light
for CR the reader units have photomultiplier tube what does these do
collect, amplifies and converts the light to a electrical signal
for CR reader units the readers have analog to digital converter what does these do
what 2 characteristic of the ADC?
convert analog to digital data
sampling frequency and pitch
in a CR reader units what are the 4 main components
drive mechanisms
optical system
photomultiplier tube
analog to digital converter
in CR the cassette holds what
the image plate
the image plate of CR what interactions occur at the Image plate
exiting radiation interacts with the IP
photon intensities are absorbed by the phosphor
what is luminesence and what does it create
emission of light when stimulated by radiation
creates latent image
what 3 components make up the image plate in CR
what are the 2 types of phosphor
phospor, protective layers and support
photostimulatble phosphor
photostimulatable luminescence
what do the image plates do in terms of the radiation and photon absorption
stores images from exit radiation via photon intensities absorbed by the phosphor layer
how are images erased off the CR image plate
expose to bright white light
what is the photostimulatable phosphor made of
what is its important feature
It is composed of barium flourohalide crystals doped with europium
it’s luminescent so when stimulated by a high-intensity laser beam it emits a visible light. (absorbed energy released as light)
in CR imaging what are the 2 steps to creating the image
1/ Image capture
2/ image readout
in CR what happens in the 1st step of creating an image when the image is captured
what happens in terms of exit x-rays and atom excitation
latent image is formed in the photostimulable phosphor when the exit x-ray is obsorbed.
exit x-ray is energy which excites the atoms in the phosphor layer
When an atom is excited it gets a higher energy level and that allows it to move. Spme atoms retun to normal state but some atoms get trapped in their new place until released by the processing laser and this is what gives us the tissue differential from the x-ray absorption.
the image plates in the CR imaging should be processed quickly why
(within an hour) as latent images dissipate over time.
what is exit radiation
photons that made it through tissue
what are the 3 stages when digitizing the CR latent image
scanning
sampling
quantization
what is the purpose of the scanning stage when digitizing the CR latent image
convert latent image into an electrical signal - voltage - that can be digitized and displayed as an image
once in the reader unit what has happened to the IP
IP removed from cassette and scanned with a He neon laser beam to release the stored energy as visible light
what does absorption of the laser beam for a CR digitization of image do to the e- in the IP
releases trapped e- and they return to a lower energy state
the scanning of the IP in CR digitization of the image results in what happening to the light intensities and where is it sent to
continuous pattern of light intensities being sent to the PMT which is then directed to the ADC for sampling and quantization
what does the PMT do in terms of CR digitization of an image
collects, amplifies and converts the visible light to an electrical signal proportional to the energies stored in the IP
why is sampling needed in the digitization of the image in CR
to digitize the analog signal from the PMT
what is the sampling frequency and related to in the process of image digitization in CR
ADC
what is the sampling frequency and what does it determine
determines how often the analog signal is reproduced in its discrete digitized form
increasing the sampling frequency of an analog signal has what effect on the pixel density and spatial resolution
increases pixel density and improves the spatial resolution
what is the sampling pitch
the distance between 2 sampling points
increasing the sampling frequency will have what effect on the sampling pitch
what relationship do they have with eachother
increase sf = decrease sp and results in smaller sized pixels
inverse relationship
the larger the sampling pitch the ___ the pixel size
larger
what is the pixel pitch
the distance between the midpoint of one pixel to the midpoint of an adjacent pixel
in terms of the size of pixels and the number of pixels what improves the spatial resolution
increasing number and smaller sized pixels
increasing the sampling frequency results in what effect on the sampling pitch and pixel pitch and spatial resolution
the smaller the gap between pixels so smaller for both pitches
improves spatial resolution
why would fixed matrix size system improve the spatial resolution
to maintain the same matrix size and number of pixel the pixels must be smaller in size
what does it mean when the spatial resolution for an IP is fixed in terms of matrix and IP size
what is fixed
matrix size is proportional to the IP size (larger IP size means it will have a larger matrix to maintain the same pixel size)
sampling frequency is fixed
Fixed sampling keep pixels the same size
what does it mean when the matrix size for an IP is fixed
change the sampling frequency to maintain the matrix size
changing the size of the IP would affect the spatial resolution of the digital image as the pixels in a smaller image plate will be diff size/smaller than those in a large one.
a larger IP size will result in a larger pixel size and decreased spatial resolution
what is a matrix
Number of pixels = matrix
for a fixed matrix size CR system using a smaller IP (decreasing the IP) for a given field of view has what effect on the spatial resolution
improves the spatial resolution of the digital image
what does it mean when the degree of CR quantization is mentioned
the pixel bit depth
what does CR quantization control
number of grey shades/contrast of the image
the bigger the pixel bit depth = more values of grey
what does CR quantization reflect
precision at which each sampled point is recorded
before CR plates are reused/returned to service what must be done and why
exposed to intense white light to release residual energy that could affect future exposures
what is the DR’s scanning system like
self-scanning
DR systems convert x-ray intensities into proportional ______
electronic signals for digitization
what does DR allow in terms of image acquisition and readout
reduces the delay between the 2 processes
what is the difference between DR and CR in terms of the image acquisition process
CR requires 2 step process and results in a delay between image acquisition and readout
DR combines the 2 processes so images are available directly after exposure
what are flat panel detectors and which type of radiography are they used in
DR
solid state IRs using a large area active matrix array of electronic components
what are the 3 processes integrated into the flat panel detector
signal storage, readout and digitizing
in a DR flat panel detector do the first, second and third layers composed of
1st layer = x-ray converter
2nd layer = thin film transistor array
3rd layer = glass substrate
what is the thin film transistor in the flat panel detector divided into
what do these divisions do
square detector elements (DEL)
they store electrical charges and a switching transistor for readout
what does the electronic signal get sent to after being processed by the flat panel detector in DR
ADC for digitization
what is the limitation of the DEL in the flat panel detector in DR
in terms of the pixel size
the DEL has a x-ray sensitive area representing each pixel in the image matrix
the pixel is therefore smaller than the DEL and can only capture a % of the x-rays reaching the detector
what is the fill factors for DELs of DR
what is its value normally
the percentage of x-ray capture by the DEL
normally around 80%
flat panel detectors with smaller DELs have what pro and con
pro = smaller pixel size so improved spatial resolution con = fill factor decreased
Efforts to further decrease the DEL for improved spatial resolution would require more radiation exposure to reach the IR to create the digital image as smaller pixel size means more radiation is needed as there is more sampling
what are 3 advantages of using flat panel detectors DR in terms of dose, spatial resolution and pixel size/pitch
highly dose efficient
better spatial resolution than CR
fixed pixel size and pitch so limits spatial resolution to square detector elements
what are the 2 ways that flat panel detectors create electrical charges proportional to the x-ray exposure
indirect and direct
indirect conversion detectors use a ___ to convert ___ radiation into ___ ___
scintillator
exit radiation into visible light
in indirect conversion detectors the visible light is converted into electrical charges by ____ and the electrical charge is stored by __ in the __ ___ before being digitized and processed in the computer
converted to electrical charges by photodetectors
stored in the capacitor in the TFT array
what are the 3 steps in indirect conversion detectors
scintillators detectors converts exit radiation into visible light
photodetector converts visible light to proportional electrical charges
electrical signals sent to the ADC for digitization
what are the 2 stage processes in indirect conversion detectors
convert x-ray intensities first to visible light
and then to electrical charges during image acquisition
are the 2 steps in the direct conversion detectors
amorphous selenium coated detector converts exit radiation directly into electrical charges
electrical signals are sent to the ADC for digitization
why is an electrical field applied across the selenium later of a direct conversion detector
limit the lateral diffusion of e- as they migrate towards the thin film transistor array
what are the similarities between the indirect and direct conversion detectors in the process of charge storage
electrical charge is stored in a TFT array before its amplified, digitized and processed in the computer
what stage does the direct conversion detector skip when compared to the indirect
skips the stage of needing to create physical light
what is the dynamic range
ability of a detector to accurately capture the range of photon intensities that exit the patient
what does a wider dynamic range mean for the contrast and exposure
more contrast and risk over exposure
what is the detective quantum efficiency
measurement of the efficiency of an image receptor in converting the x-ray exposure it receives to a quality radiograph image
what does 100% detective quantum efficiency mean
no info is lost
the higher the detective quantum efficiency what happens to the radiation exposure to produce a quality image and patient exposure
lowers the radiation exposure required to produce a quality image
decreases the patient exposure
which 2 factors is the detective quantum efficiency affected by
the type of material used in the IR to capture the exit radiation
the energy of the x-ray
what is the signal to noise ratio
describes the strength of the radiation exposure compared with the amount of noise apparent in a digital image
what is the signal in the signal to noise ratio
the signal is the strength of the radiation exposure
what is the noise in a digital image from
sources include e- that capture, process and display the image
what is quantum noise
too few x-ray photons captured by the image receptor to create a latent image
what does increasing the signal to noise ratio do for the quality of the digital image
improve the quality by improving the visibility to anatomical details
what is the contrast to noise ratio
describes the contrast resolution compared to the amount of noise apparent in a digital image
increasing the contrast to noise ratio does what to the visibility of anatomic details
increases visibility
