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Computed Radiography

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Wendy Ross

on 17 September 2013

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Transcript of Computed Radiography

Computed Radiography
Bushong Chapter 15
The CR image receptor
Can CR cassettes and film/screen cassettes can be used interchangeably with any x-ray imaging system?
In CR, the response to
x-ray interaction is seen as:

a. a scintillator that emits light in response to the x-ray interaction
b. trapped electrons in a higher energy metastable state
When does a PSP emit light?
a. immediately after exposure to x-rays
b. when stimulated by light
c. over time
Storage Phosphor Screen
also called PSP screen
Digital Radiography was first introduced
by Fuji in 1981 with a CR imaging

x-ray interaction with a psp results in excitation of electrons into a metastable state
CR terms
IP a. storage phospohor
PD b. photostimulable phosphor
PMT c. photostimulable luminescence
PSL d. imaging plate
PSP e. storage phosphor screen
SP f. photodiode
SPS g. photomultiplier tube
What is this PSP you speak of?
barium fluorohalide with europium
BaFBr:Eu or BaFl:Eu
Europium is the activator and is responsible for the storage property
barium fluorohalide emits light when exposed to light from a laser
What is that called?
Photostimulable luminescence
The electrons from before-that were in a metastable state after interaction with an x-ray--remember them?
When exposed to laser light, they return to their ground state and visible light is emitted.
all of the above--some light is emitted when the x-ray interaction occurs. If left alone, the electrons will return to a ground state on their own and in the process emit light. When exposed to the intense infrared light of a laser, the electrons return to a ground state and light is emitted.
SPSs are mechanically stable
electrostatically protected,
and optimize the intensity
of stimulated light
The PSP screen is housed in a rugged cassette similar to a screen-film cassette called an imaging plate.
A darkroom is not needed.
Imaging Plate
The IP has lead backing to reduce backscatter.
This improves contrast resolution.
Light Stimulation-Emission
Sequence of events
x-ray beam exposes PSP
electrons are excited to metastable state
50% return to ground state immediately, emitting light
what happens to the rest?
Latent image fade
remaining metastable electrons will return to ground state over time
CR signal loss is objectionable after how many hours?
Sequence of events
IP is placed in the reader
Laser is directed at PSP
What is beam diameter?
Does the beam spread as it penetrates the PSP?
Sequence of events
Blue light is emitted and detected and measured.
Light detectors
some signal is lost
scattering of emitted light
collection efficiency of the photodetector
PD = photodiode
PDs are the light detectors of choice for CR.
Sequence of Events
not all the electrons are returned to ground state by laser stimulation
The phosphor is flooded with very intense white light to remove residual latent image.
How do the x-rays become the latent image in Computed Radiography?
How do the x-rays form the latent image?
Sequence of Events
1. Expose (also called metastable state)
2. Stimulate
3. Read
4. Erase
After the IP is stimulated by the laser light, what state are most of the electrons in?

After stimulation, the electrons are mostly stabilized.
The IP is very sensitive to background radiation.
Use it soon after erasing to prevent fogging.
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