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With CR systems, no chemical processor or darkroom is necessary.
Cassette is fed into a reader:
Removes the imaging plate
Scans it with a laser, releasing the stored electrons
Laser stands for light amplification by stimulated emission of radiation.
Laser creates and amplifies a narrow, intense beam of coherent light.
- Spatial resolution refers to the amount of detail present in any image.
- Phosphor layer thickness and pixel size determines resolution in CR.
- The thinner the phosphor layer is, the higher resolution.
- Film/screen radiography resolution at its best is limited to 10 line pairs per millimeter (lp/mm).
- CR resolution is 2.55 lp/mm to 5 lp/mm, resulting in less detail
- CR dynamic range, or the number of recorded densities, is much higher, and lack of detail is difficult to discern.
- More tissue densities on the digital radiograph are seen, giving the appearance of more detail.
Spatial Resolution
For example, an anteroposterior knee radiograph typically does not show soft tissue structures on the lateral aspects of the distal femur or proximal tibia or fibula.
An anteroposterior knee digital image shows not only the soft tissue but also the edge of the skin. This is due to the wider dynamic recording range and does not mean that there is additional detail.
Objectives
Cassette & Imaging Plate
* Describe the basic construction of a computed radiography cassette.
* Describe the construction of a computed radiography imaging plate.
* Identify the various layers of the imaging plate.
* Describe the purpose of each layer of the imaging plate.
Construction
Image recorded on a thin sheet of plastic known as the imaging plate
Consists of several layers:
Protective layer: a very thin, tough, clear plastic for protection of the phosphor layer
Phosphor, or active, layer
A layer of photostimulable phosphor that “traps” electrons during exposure
Cassette
Looks like a film/screen cassette
Durable, lightweight plastic
Backed by aluminum
No intensifying screens
Antistatic material
Explain the process of photostimulation in the imaging plate.
Describe the process of laser beam formation.
Explain the process of reading the imaging plate.
Compare conventional radiographic screen and film speed to computed radiography systems.
Discuss how an image is erased from the imaging plate.
Reflective layer: a layer that sends light in a forward direction when released in the cassette reader
May be black to reduce the spread of stimulating light and the escape of emitted light.
Conductive layer: a layer of material that will absorb and reduce static electricity
Color layer: Newer plates may contain a color layer, located between the active layer and the support that absorbs the stimulating light but reflects emitted light.
Support layer: a semirigid material that gives the imaging sheet some strength
Backing layer: a soft polymer that protects the back of the cassette