lecture 5 ( CR and DR) Flashcards
Computed radiography ?
cassette-based system
uses a special solid-state detector plate instead of a film inside a cassette
CR cassette ?
contains a solid-state plate called a photostimulable storage phosphor or imaging plate (PSP) or (IP)
Plate is made of a thin, plastic material and is extremely fragile
CR plates and cassettes can be reused many thousands of time
Imaging plate consists of ?
Protective layer:
very thin, tough, clear plastic that protects the phosphor layer from trauma
Phosphor layer:
- active layer, photostimulable phosphor, traps electrons during exposure.
- made of barium fluoro-halide phosphors
Conductive layer:
This layer grounds the plate to reduce static electricity problems and to absorb light to increase sharpness
Support layer:
This is a semi-rigid material, provides strength and is a base for coating the other layers.
Light shield layer:
This prevents light from erasing data on the imaging plate or striking through the backing layer
Backing layer:
This is a soft polymer that protects the back of the cassette.
Image formation ?
The trapped signal lasts for hours or days but starts to fade right away, so it’s important to process the plate quickly after exposure
How do we create a latent image ?
When X-rays hit the photostimulable phosphors in the imaging plate, they stimulate electrons, which get trapped in the phosphor center
Reading the imaging plate ?
The CR cassette is placed in a processing reader to produce visible image , which opens it and scans the detector plate with a laser beam
How to we create visible image ?
By red laser light which energizes the trapped electrons, causing them to move and release blue light as they return to a lower energy state
The imaging plate ?
As the laser scans the imaging plate, the photomultiplier tube detects the light intensity and converts it into an analog electronic signal
This signal is then turned into a digital signal using an analog-to-digital converter (algorithmic formulas ) which determines the gray scale
Which detects the light intensity and converts into an analog electronic signal ?
photomultiplier tube
The imaging plate ?
After scanning, a high-intensity light releases any remaining trapped energy from the plate, preparing it for reuse.
The cassette is then closed and returned to the ready bin , the entire process takes about 60 seconds.
Benefits of CR ?
Utilizing the existing equipment
Transmit images to remote sites
quick access to previous CR images for comparison
This makes diagnosis of any abnormalities more accurate
The image is directly transferred to the computer using solid-state detectors inside a protective housing, which convert X-ray photons into a digital image is ?
Direct/Digital Radiography
Direct/Digital Radiography ?
Which there are no cassettes
Both CR and DR images can be electronically sent to a Picture Archiving and Communication System (PACS)
Image formation ?
In flat panel detectors, image data is transferred directly to the computer.
A high voltage charge is applied to the selenium layer just before the X-ray exposure
When X-ray photons ionize the selenium, it releases electrons that are absorbed by electrodes at the bottom of the selenium layer
These electrons are stored in the TFT (thin-film transistor) detectors, which consist of small pixels
Which separates each pixel and sends the charges to the image processor
( photosensitive ) ?
A silicon ( TFT)
