unit iii creating the image chapter 25 digital radiography
TRANSCRIPT
Unit III
Creating the Image
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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Historical Development
• Fuji Systems– 1980s
• Today’s Systems– Several manufacturers
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Indirect Photostimulable Phosphor Imaging Plate Systems
• 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 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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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
• 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
• 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
• 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
• 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
• 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