Digital Radiography Flashcards
What are some advantages to digital radiography?
Better image quality
Post processing simpler- more efficient
Easier to store and transport
More cost effective
What are the basic characteristics of a digital image?
Recorded as multiple numeric values
Divided into small elements
Begin analogue, turned into electrical signals then digitised
What are the basic characteristics of an analogue image?
X-ray is converted to light, recorded on film
Capture involves measurement of continuously changing signal
What are some key features of a digital image?
Spatial resolution
Contrast resolution
Noise
Dose efficiency
Describe pixel bit depth
Determined by number of bits used to define each pixel
Number of grey tones produced = bit depth to the power of 2
Describe matrix
Matrix - square arrangement of numbers and columns
Pixel size = image size (mm)/ matrix size (pixels)
Describe image display
When an image is processed it can be:
- Displayed on a monitor
- Printed on film
- Sent to PACS
What are the basic parts of computerised radiography?
Cassette, photostimulable phosphor plate, computer workstation
Describe the process of computerised radiography
Uses an imaging plate made of phosphor
Catches x-ray energy
Scanned with laser, emits light
Captured by plate reader to build image
Describe how we use photostimulable phosphor
Creates and stores latent image
Scanned by laser of specific wavelength to view image
Scanning produces light proportional to image - photo stimulated luminescence
What characteristics are required for photostimulable phosphor?
Good absorption
Capable of being laser stimulated
Luminescence compatible with photomultiplier tube
Luminescence time shorter than 1μs
Should be able to store latent image
What is contained in phosphors?
Barium fluorohalide: europium
Halide can be chlorine, bromine, iodine or mix
Describe the process of how the PSP responds to x-rays
Phosphor atoms ionised
~1/2 removed electrons trapped in conduction band
Quantity and distribution proportional to exposure
Represents latent image
Describe the latent image sequence
Radiation interacts with imaging plate
Some absorbed energy released as light
Most energy absorbed, electrons trapped in colour holes
Number caught proportional to exposure
Describe the function of a charged coupled device
Indirect conversion of x-ray photons into electric charge
Photons first converted to light via scintillating screen
Describe Scintillators
Caesium iodide - coupled to CCD using fibreoptic bundle.
Light energy transmitted to CCD
Converted to electronic signal
Can absorb moisture
Describe the structure of a charged coupling device
An array of CCDs
Arranged in tiling pattern, resulting in seams
Detectors are sealed to avoid this
Describe the characteristics of a charged coupling device
Light sensitive
Wide dynamic range - wide range of light intensities
Describe thin film transistors
Capture - scintillator
- caesium iodide
Coupling - contact layer
- amorphous silicon photodiode
Collection - Thin film transistors
- DELs
Describe direct capture
No scintillator - no loss of resolution
Capture - Photoconductor
- Amorphous selenium
Collection - TFT
Describe aquisition
Computer creates histogram
Digital pixel values vs. relative prevalence of values on image
Computer analyses and compares
Describe exposure latitude
Range of exposure values detector can produce
Images require sufficient contrast resolution
Film has limited latitude
Describe brightness
Amount of luminance on display monitor
Describe contrast resolution
Systems ability to display subtle changes in grey tones
