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Exposure CR/DR

QuestionAnswer
PSL photostimulable luminescence
PSP photostimulable phosphor
SPS storage phosphor screen
IP imaging plate
SP storage phosphor
PMT photomultiplier tube
ADC analog digital converter
TFT thin film transistor
Advantages to digital imaging increased visualization increase in image thoughput post-imaging capability rapid storage retrieval of images PACS/teleradiography
Similarities between screen film and digital tube xray production xray interaction with patient
differences between screen film and digital ir image processing recording of image technical factors
Review of conventional or analog film based systems film based uses intensifying screens film placed between two screens screens emit light when xrays strike them film is processed chemically processed film is viewed on lightbox
Computed Radiography been around since 1980s uses an imaging plate need cassette reader images can be sent to PACS
Computer Radiography Indirect CR cassette or imaging plate durable, light weight plastic, backed by Al or Pb sheet
PSP make up a rigid sheet with several layers
Layers of IP protective layer phosphor layer conductor layer support layer light shield layer
Protective layer insulates the plate from rough handling
phosphor layer active layer. holds the photostimulable phosphors. Phosphors are composed of fluorohalides contain europium activator.
Europium sensitivity center. Very sensitive to scatter
most common is barium fluorobromide and barium fluoroiodide phosphors
conductor layer grounds the plate, helps absorb and eliminate static
support layer base on which other layers are coated
light shield layer prevent light from erasing data on plate
This can fog the Ir so it is cleaned and erased daily white light
Image Acquisition ip is loaded into cassette cassette is placed either table top or bucky patient positioned technique is set image is taken IP is processed
Latent Image production in CR remnant radiation exits the patient, strikes the IP. Gives energy to the electrons in the fluorohalide crystals these electrons then become excited They either fluoresce and return to normal energy state OR they retain the energy.
Latent image production in CR continued those that retain their energy become trapped at europium site
The number of trapped electrons is equal to the intensity of the xray at that location
Latent Image to Manifest image made by the image reader device (IRD) cassette is place in IRD and IP is extracted IP is scanned by a helium-neon laser (freezes the electrons)
Latent to manifest image continued laser causes the phosphors to emit the latent image in the form of violet light. light emitted = energy of what they received. The light is picked up by a PMT. PMT converts light into electric signal. Analog signal is sent to analog digital converter.
Latent to manifest image continued 2 ADC converted the analog signal to a digital signal digital signal is sent ot computer for display
What will lose 25% of its energy in 8 hours IP
Flat Panel Radiography directly converts the incoming xray photons to an electronic digital signal.
amorphorous selenium xray photons strike selenium atoms atoms free their electrons electrons are then collected by an electrode the charge colelcted then transmits through TFT to the computer for processing. process direct flat panel radiography
xray photons strike selenium atoms interaction emits light light strikes the silicon silicon converts light to an electronic chage charge is then transmitted thru TFT for processing process of indirect flat panel imaging
charged couple device converts xrays to visible light then the light to an electronic signal
charged couple device process xrays strike scintillator interaction emits light light is then coupled to CCD by fiberoptics produces electronic signal that is sent to an ADC ADC then sends the digital signal to the computer for processing
Raster pattern when the image is scanned the beam reads it in a line by line fashion
digital image rows/columns matrix
pixels picture elements of digital image
how many shades of grey 256
3 dimensional volume of tissue voxel
what creates a histogram processing. representation of imaged area broken down into pixels.
what does the histogram represent exposure index (EI) controlled by mAs IRD will provide proper shades of gray scale regardless of kVp or mAs variations
mAs is selected based on: number of photons needed for a particular part. Low mAs will lack sufficient phosphor stimulation and equal noise/quantum mottle
kVp selected based on the penetration of the part. Low kVp or high kVp cannot be compensated for
Density controlled by window level (brightness). Direct relationship
contrast controlled by window width (grey scale) inverse relationship. controls visibility of detail
resolution controlled by pixel size, matrix size, laser size
increase in matrix size= increase in resolution. direct relationship
smaller pixels= increase in resolution. indirect relationship
noise/quantum mottle or graininess caused by low mAs affected by SNR
Low s/n high noise low contrast
high s/n low noise high contrast
Agfa- lGm 2.1 - 2.3 direct inc of change .3 = double density dec. of .3 = 1/2 the density
Kodak carestream EiI EI 1800-2200 direct inc of 300 = 2x mAs dec. of 300 = 1/2 mAs
fuji/philips/konica S# 150-250 inverse inc of 200 = 2x mAs dec of 200 = 1/2 mAs # inc = light # dec. = too dark
CR systems cant compensate for insufficient mAs insufficient or excessive kVp excessive part size inadequate grid use
dose creeping setting higher techniques (thus increasing patient dose)
Created by: 671470335