Digital Radiography part 1 (CR and DR)
charge coupling device (CCD) array or reader
-another form of IP reader -able to both collect the admitted light photons from the IP and convert them to digital information -doesn't have the PM tube or the analog-to-digital converter
which is preferred: cesium iodide or gadolinium (scintillation layer for TFT)
-cesium iodide because it can be formed into very small rods which are able to produce a smaller, more focused light beam, which in turn increases spatial resolution
bromide phosphors within the active layer
-comprised of 85% barium fluorohalides and 15% Europium activated iodides
DELs (indirect TFT; DR)
-comprised of a capture element or pixel detector, (active element within each DEL), a storage capacitor (stores electric charge produced by capture element), and a TFT or switch (acts to open and close the release of electric charge leaving each DEL, thus producing digital image)
TFT array layer (indirect TFT; DR)
-comprised of array or matrix of very small detector elements known as DELs -electrical signal or electrons produced by photodiode are transferred here
photodiode layer (indirect TFTs; DR)
-comprised of semiconducting material amorphous silicon -converts incoming light photons from scintillation layer into an electric charge
DR stands for
-digital radiography
the relationship between spatial resolution and the fill factor is
-direct -as active area increases (takes up a greater percentage of space within the DEL), spatial resolution increases
what are the 2 types of DR
-direct -indirect
light shield layer (CR)
-directly under conductor layer -also known as reflective layer -prevents unwanted light and background radiation from affecting the unexposed latent image -adds another layer of protecting to the IP
what is the purpose of exposing the IP to a bright, photo-stimulating fluorescent light at the end of the process
-exposure causes the release and any remaining latent image on the IP -erases the plate
bromides
-have ability to both store and release energy
how is the latent image released
-it is exposed to a laser light within the IP digitizer
conductor layer (CR)
-located directly under phosphor layer -serves to ground imaging plate from electrostatic charge -helps increase spatial resolution by absorbing light
phosphor layer (CR)
-most important layer within the IP -comprised of extremely small particles known as bromides -material that actually absorbs and stores the x-ray energy
top protective layer (CR)
-no image producing qualities -serves as a way to protect active phosphor layer
direct DR
-non-scintillating -comprised of amorphous selenium-based semiconductor and a TFT -converts incident x-rays directly into electrical signal which are then collected by TFT array
CR radiography systems are comprised of
-outer protective housing (cassette) -imaging plate -image plate reader (digitizer)
what happens when incident photon interacts with bromide phosphors
-phosphors become stimulated and luminesce (release light) -stimulated luminesced phosphors have the ability to store some of the light energy produced
-backing layer (CR)
-prevents unwanted light and background radiation from affecting the unexposed latent image -adds another layer of protecting to the IP
bad quality of scintillation
-produces light isotropically which decreases spatial resolution -converted light photons are of a greater size than the incoming x-ray photons -scintillation layer creates small amount of blur on image
outer housing/cassette (CR)
-protects imaging plate from damage -come in many different sizes -typically constructed of hard plastics, light metals, and carbon fibers -only serves as a protective barrier; no bearing on image production
what happens after exposure is taken (CR)
-recorded detail is stored within IP as latent image
how does direct DR work
--high voltage charge is applied to top surface nanoseconds before x-ray exposure is made -interaction between incident photons and high-V charge causes selenium atoms to release their electrons, which are then able to be collected and processed by TFT layer
digitizer
-IP reader -device used to convert latent image into a digital radiograph -most common types are comprised of helium neon laser, laser separator or splitter, focusing lens, reflecting mirror, photo-multiplier (PM) tube, and analog-to-digital converter
top scintillation layer (DR; indirect TFT)
-aka light layer -where incoming x-ray photons are converted into light photons -scintillation means "burst of light"; as photon hits scintillation layer, it creates small burst of light -comprised of ionic compound cesium iodide or chemical element gadolinium
imaging plate (IP) (CR)
-also known as photostimulable phosphor plate (PSP) -thin, multi-layer plate -consists of top protective layer, active phosphor layer, conductor layer, support layer, light shield layer, bottom protective backing
what happens after CR cassette is inserted into digitizer
-set of rollers extract IP from protective cassette -while IP is being extracted, laser is projected through the laser splitter, focusing lens and reflective mirror in a quick back-and-forth motion (raster pattern- similar to how we read text, starting at top left and moving to the right and down) -when helium neon laser interacts with individual phosphors of IP, the stored energy within phosphors are released in the form of light photons -released light photons are then collected and amplified by the PM tube, which acts to multiply light photons into larger, brighter light photons -analog light leaving leaving PM tube is sent to analog-to-digital converter to be converted into the digital signal needed for computer processing (converts analog electrical signals into digital signals, or binary signals) -final stage is to expose the entire IP to a bright, white photostimulating fluorescent light
latent image (CR)
-stored x-ray image within the phosphor layer -made up of the stored information housed within the phosphors -not visible until processed
how does the CCD system differ from the TFT system (indirect DR)
-the CCD system does not use a photodiode or TFT layer -scintillation layer within CCD systems are optically coupled to each CCD sensor chip by lenses or fiber optics -CCDs can convert light photons into electrical signals and send electrical signals to computer for processing on its own
fill factor (indirect TFT; DR)
-the percentage of DEL to active pixel -aka active portion because its the only part of the detector that can collect x-rays for processing -most detectors have fill factor of 80%
the ability of each DEL to produce a high spatial resolution image is designated by
-the percentage of the active pixel area within each DEL
defining characteristic of CR
-the temporary storage of the x-ray information on the phosphor plate that must be processed through a digitizer
types of indirect DR
-thin flat-panel transistors (TFTs) -charged-coupled device (CCDs)
three main layers of indirect TFTs
-top scintillation layer -middle photodiode layer -bottom TFT array
defining characteristic of DR
-uses equipment that immediately produces the x-ray image without temporarily storing the x-ray information on an IP -no cassette or IP reader
CR stands for
computed radiography