chaoter 21 Computed radiography

Computed radiography (CR) systems

Two step process

Acquisition

Image processing and display

Phosphor layer

BaFBr:Eu

BaFl:E

Reflective layer

Sends light up toward reader

Conductive layer

Eliminates electrostatic problems

Absorbs light to increase sharpness

Color layer

In some newer plates

Stimulates and absorbs light

Europium (Eu)

Is considered and activator as it maintains the presence of electron holes in the PSP

K-edge attenuation

Is a sudden increase in x-ray absorption when the energy of the x-rays is just above the binding energy of the innermost electron shell of the atoms interacting with the photons

Best between 35 and 50 KeV

Absorb more low-energy radiation than radiographic film

More exposure needed if applied kVp is outside of this range

PSPs

More sensitive to scatter both before and after exposure, than radiographic film

Sensitive to low levels of radiation energy

IP

X rays exposure cassettes and latent image is stored here

Electron pattern store in active layer of exposed —-

Fluorohalides

Absorb beam through photoelectric interactions

Trap electrons to create holes at Eu sites

Latent image

Is millions of electron holes trapped in an energy layer of atom

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Latent image will lose approximately— % of its energy in 8 hours

Trapped electrons freed

IP scanned by finely focused neon-helium laser beam in raster pattern ( red laser)

Electrons return to lower energy state

Emit blue-purple light through photostimulable luminescence

Light captured by photomultiplier (PM ) tubes

The light is proportional to amount of light received

The intensity (brightness) of the light -correlates to the density on the image

IP erased via

Exposure to intense white light

Point-by-point readout

Laser moves from side to side

Line-by-line readout

Newer reader has reduced scan time

sampling frequency

Determines pixel pitch

NyQuils t frequency

Is the minimum rate at which a finite bandwidth signal needs to be sampled to retain all of the information