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