Chapt. 25 Computed Radiography

Digital terms are NOT uniform, but vary from one brand to the next!

DR introduced in 1981

• Fuji introduced “digital imaging” in 1981– Many forms of digital

• CR• DR

CR Terms

• PSL= photostimulable luminescence• PSP=photostimulable phosphore• SPS=storage phosphor screen• IP=imaging plate• SP=storage phosphore• PMT=photomultiplier tube• PD=photodiode

PSL & PSP

• Photostimulable luminescence (PSL)– Emit light promptly AND at a later time!

• Photostimulable phosphor (PSP) – barium fluorohalide looks physically like an intensifying screen BUT stores the latent image via metastable electrons (STORAGE PHOSPHOR SCREENS) SPSs

The “imaging plate”

• The PSP screen is inside a “rigid” (not really as they are brittle and do break easily – much more so than film/screen cassettes) cassette

• The actual imaging receptor is called the IMAGING PLATE

Light Stimulation Emission

• Process similar to Thermosluminescent dosimetry (TLD – film type badges for dosimetry)

• They are either “powder” (like an intensifying screen crystal) OR arranged like “needles”

• i.e. Powder PSP or Needle PSP

Thoughts on PSP’s

• Powder vs “needle”

• Highly sensitive to RADIATION = easily fogged!!!

CR Reader

• The physical receptor - cassette is “loaded” into the reader – IP is removed and moves along the LONG

AXIS of the IP!!! (slow scan)– As the IP is moved, a deflection device is

used to deflect the LASER beam across the IP (fast scan)

• IP doesn’t really separate from the cassette and doesn’t go around “rollers” etc.

CR Optical feature

• To properly and precisely translate EACH metastable electron (latent image forming charge) in a PRECISE fashion to provide for great SPATIAL RESOLUTION

Compare and contrast film-screen to CR techniques

• Film- screen has much LOWER latitude than CR!!!– As such – a WIDE variety of techniques can

be used = Burn em up BURNIE!!!! If in doubt – “BURN IT OUT”

• Compare Characteristic curves for each:

Image noise

• Box 25-1 SAME as screen-film noise

• PLUS Box 25-2– Mechanical defects – due to the scan drivers– Optical defects – laser, stimulating beam, light

quanta emission and collected– Computer defects – electronic noise; too few

sampling; too little quantization