2. Unit III
Creating the Image

3. Chapter 25
Digital Radiography

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Objectives
Describe various digital radiography image receptor and detector systems
Explain critical elements used in the different digital radiography systems
Discuss limitations inherent in currently available digital radiography systems
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Objectives
Describe how the digital radiography histogram is acquired
Describe how the display algorithm is applied to collected data
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Objectives
Explain why digital radiography systems have greater latitude than conventional film-screen radiography systems
Analyze elements of digital radiography systems
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Objectives
Discuss what makes them prone to violation of ALARA radiation protection concepts
Explain the causes of sever digital radiography artifact problems
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8. Historical Development
Fuji Systems
1980s
Today’s Systems
Several manufacturers
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9. Indirect Photostimulable Phosphor Imaging Plate Systems
Photostimulable imaging plates
Latent image production
Image acquisition
Reading digital radiography data
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10. Photostimulable Imaging Plates
Photostimulable phosphor
PSP
Imaging plate IP
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Common Phosphors
Europium activated barium fluorohalides
Chemical formulas
BaFBr:Eu
BaFI:Eu
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K-edge attenuation
Best between 35 – 50 keV
35 keV: average energy of 80 kVp beam
More exposure needed if applied kVp is outside of this range
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Scatter Radiation
PSPs absorb more low energy radiation than radiographic film
More sensitive to scatter both before and after exposure than radiographic film
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14. Latent Image Production
Electron pattern is stored in active layer of exposed IP
Fluorohalides absorb beam through photoelectric interactions
Energy transferred to photoelectrons
Several photoelectrons liberated
More electrons freed by photoelectrons
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15. Latent Image Production
Liberated electrons have extra energy
Fluoresce - or- get trapped by fluorohalide to create holes
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Hole Formation
Fluorohalide crystals trap half of the liberated electrons
Europium sites contain electron holes
This is the actual latent image
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Important Note!
The latent image will lose about 25 percent of its energy in 8 hours, so it is important to process the cassette shortly after exposure
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Image Acquisition
IP cassettes
Also know as filmless cassettes
Can be used tabletop or with a grid
Rules of positioning remain the same
Wider latitude when compared to film/screen radiography
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19. Radiographic Technical Factor Selection
“It is the responsibility of the radiographer to select proper technique; chronic overexposure should be avoided.”
Ethical principles
ALARA concept
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20. Reading Digital Radiography Data
Trapped electrons are freed
IP is scanned by finely focused neon-helium laser beam in a raster pattern
Electrons return to lower energy state
Emit blue-purple light
Light captured by Photomultiplier (PM) tubes
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21. Reading Digital Radiography Data
PM tubes convert light to analog electronic signal
Analog electronic signal sent to analog to digital converter (ADC)
ADC sends digital data to computer for additional processing
IP erased via exposure to intense light
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22. Reading Digital Radiography Data
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23. Reading Digital Radiography Data
Two types of IP processing
Point by point readout
Line by line readout
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24. Reading Digital Radiography Data
Plate throughput
30 – 200 plates per hour
Throughput and spatial resolution can be improved by using dual-sided PSP
Self contained units
House plates and reader within upright bucky or table
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25. Reading Digital Radiography Data
PM tubes output signal
Infinite range of values must be digitized
Converted to limited, discrete values
Automatically adjusted
Optimizes handling during digitization
Pixel depth
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Pixel Depth
Determines number of density values
Affects density and contrast of system
Controlled by ADC
10 bit (210 = 1024)
12 bit (212 = 4096)
16 bit (216 = 65,536)
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Pixel Size
Inversely related to spatial resolution
Sampling frequency
Expressed as pixels/mm
Dependent on:
Matrix
Image receptor size
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Image File Size
Affected by:
Pixel size
Matrix
Bit depth
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Preprocessing
Communicates to the system:
What part
Orientation of the part
Number of projections per plate
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30. Analog to Digital Conversion
System locates raw data
Samples
Quantitize
Determine average value
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31. Exposure Data Recognition (EDR)
Fuji systems’ method of locating the raw data
Automatic
Adjusts the latitude and sensitivity for the image
Semiautomatic
Adjusts the sensitivity, but not the latitude
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32. Exposure Data Recognition (EDR)
Fuji systems’ method of locating the raw data
Fixed
Does not adjust sensitivity or latitude
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33. Multiple Projections on One IP
Scanning projection pattern
“The beam and part should be centered within each pattern, and collimation should be parallel and equidistant from the edges of the imaging plate.”
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34. Multiple Projections on One IP
Automatic mode
Used when collimation is parallel/equidistant and the central ray and part are centered
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35. Multiple Projections on One IP
Semiautomatic mode
Can be used when collimation is not parallel/equidistant
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36. Multiple Projections on One IP
Fixed mode
Requires use of proper technical factors
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Histogram
Graphic representation of pixels and signal intensities present in image
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Look Up Table Data
Contains standard contrast, speed and latitude for given exam
Appropriate part and projection selected by radiographer prior to processing
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Look Up Table Data
True patient image information is determined
Automatically rescaled
Algorithms used for processing
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Histogram Adjustment
Image processing in proper range of exposure
Yields consistent gray scale regardless of technique
Outside of appropriate range
System cannot compensate
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Image Reprocessing
Raw data
Stored by CR system workstation
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Gradation Curves
Contrast requirements
Similar to DlogE curves of different types of radiographic film
Scale of contrast or the slope of the DlogE curve can be adjusted
Window width
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43. Spatial Frequency Processing
Affects image sharpness
Edge enhancement
Unsharp mask technique
Low-pass filter
High spatial frequency signal remains
High spatial frequency signal is amplified and added back into the image
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44. Spatial Frequency Processing
Affects image sharpness
Edge enhancement
Increases noise resulting in lower quality images
Lower contrast and higher base fog levels
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45. Computed Radiography Image Quality– Fuji System
Each manufacturer has their own system
Basic concepts are similar
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46. CR Image Quality—Fuji System
S number
Inversely related to the amount of exposure to the image receptor
Properly exposed IP should have S number of
S number 200 ~ 1mR exposure
Higher S number indicates overexposure
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47. CR Image Quality—Fuji System
Increased latitude compared to film/screen radiography
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48. CR Image Quality—Fuji System
Linear response
No Dmax
Computer can bring densities into visual range despite overexposure
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49. Toleration of Overexposure
Radiographers professional and ethical responsibility
Minimize patient dose
ALARA concept
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50. Image Acquisition Elements
Sensitivity
Data clipping
Spatial frequency processing
Edge enhancement
Image blurring
Look up table adjustments
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51. Image Acquisition Elements
Histogram equalization
Collimator edge identification
Image stitching
Grid use
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Data Clipping
Clinically irrelevant data is not included in image display
Dependent upon the part and projection
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53. Spatial Frequency Processing
Edge enhancement
Image blurring
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54. Look-up Table Adjustments
Adjustment similar to changing DlogE curve of the image receptor
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55. Histogram Equalization
Example
Normal chest x-ray
Bone enhanced histogram image
Soft tissue histogram image
Possibilities endless
ACR standard procedure
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56. Collimator Edge Identification
Algorithm that detects edges of exposure vs. nonexposure
Can sometimes be triggered by prosthetics or implants
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Image Stitching
Overlapping exposures
Verified registration marks
Combine several images into one
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Grid Use
Digital systems are more sensitive to scatter radiation
Grids should be used more often
Radiography of the chest
> cm should use grid
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Overexposure
Overexposure > 2X
Results in enough scatter to degrade image
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Underexposure
Quantum mottle/reticulation
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61. Direct Exposure Imaging Systems
Direct selenium flat panel imaging plate systems
Indirect silicon flat panel imaging plate systems
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62. Direct Selenium Flat Panel Imaging Plate Systems
Amorphous selenium directly converts ionization from x-rays into electronic signal
Electronic signal received by thin film transistors (TFTs) and sent to computer
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63. Indirect Silicon Flat Panel Imaging Plate Systems
Amorphous silicon combined with scintillator
Scintillator or intensifying screen converts x-rays to light
Amorphous silicon acts as photodiode
Converts light to electronic signal
TFTs send signal to computer
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64. Thin Film Transistors (TFTs)
Array or matrix of pixel detectors
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65. Charged Coupled Devices (CCD)
Photodetector typically used with a screen scintillator
Requires optical coupling by lenses or fiber optics
Electric signal from CCD sent to computer
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DICOM Standard
System of computer software standards
Allows different digital imaging software to understand each other
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67. Computed Radiography Artifacts
Acquisition artifacts
Post acquisition artifacts
Display artifacts
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68. Acquisition Artifacts
Phantom images
Scratches
Light spots
Dropout
Fogging
Quantum mottle (reticulation)
Heat blur
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Heat Blur
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70. Post Acquisition Artifacts
Algorithm artifacts
Dropout artifacts
Laser film transport artifacts
Histogram error
Nonparallel collimation
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Display Artifacts
Density/brightness window level adjustments
Contrast window width adjustments
Image enhancement artifacts
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