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Exposure CR/DR
| Question | Answer |
|---|---|
| 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) |
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