Transcript Digital Radiography Chapter 25

Unit III
Creating the Image
Chapter 25
Digital Radiography
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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4
Objectives
• Describe how the digital radiography
histogram is acquired
• Describe how the display algorithm is
applied to collected data
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5
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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Historical Development
• Fuji Systems
– 1980s
• Today’s Systems
– Several manufacturers
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Indirect Photostimulable
Phosphor Imaging Plate Systems
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Photostimulable imaging plates
Latent image production
Image acquisition
Reading digital radiography data
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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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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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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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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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Reading Digital Radiography
Data
• Trapped electrons are freed
– IP is scanned by finely focused neonhelium 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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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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Reading Digital Radiography
Data
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Reading Digital Radiography
Data
• Two types of IP processing
– Point by point readout
– Line by line readout
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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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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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Analog to Digital Conversion
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System locates raw data
Samples
Quantitize
Determine average value
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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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Exposure Data Recognition
(EDR)
• Fuji systems’ method of locating the raw
data
– Fixed
• Does not adjust sensitivity or latitude
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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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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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Multiple Projections on One IP
• Semiautomatic mode
– Can be used when collimation is not
parallel/equidistant
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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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Spatial Frequency Processing
• Affects image sharpness
– Edge enhancement
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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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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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Computed Radiography Image
Quality– Fuji System
• Each manufacturer has their own
system
• Basic concepts are similar
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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 150-250
– S number 200 ~ 1mR exposure
• Higher S number indicates overexposure
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CR Image Quality—Fuji System
• Increased latitude
compared to
film/screen
radiography
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CR Image Quality—Fuji System
• Linear response
– No Dmax
– Computer can bring densities into visual range
despite overexposure
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Toleration of Overexposure
• Radiographers professional and ethical
responsibility
– Minimize patient dose
– ALARA concept
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Image Acquisition Elements
• Sensitivity
• Data clipping
• Spatial frequency processing
– Edge enhancement
– Image blurring
• Look up table adjustments
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Image Acquisition Elements
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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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Spatial Frequency Processing
• Edge enhancement
• Image blurring
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Look-up Table Adjustments
• Adjustment similar to changing DlogE
curve of the image receptor
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Histogram Equalization
• Example
– Normal chest x-ray
– Bone enhanced histogram image
– Soft tissue histogram image
• Possibilities endless
– ACR standard procedure
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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
– > 24-26 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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Direct Exposure Imaging
Systems
• Direct selenium flat panel imaging plate
systems
• Indirect silicon flat panel imaging plate
systems
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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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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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Thin Film Transistors (TFTs)
• Array or matrix of pixel detectors
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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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Computed Radiography Artifacts
• Acquisition artifacts
• Post acquisition artifacts
• Display artifacts
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Acquisition Artifacts
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Phantom images
Scratches
Light spots
Dropout
Fogging
Quantum mottle (reticulation)
Heat blur
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Heat Blur
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Post Acquisition Artifacts
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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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