Digital Radiography

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Digital Radiography

• DMI 50B
• Kyle Thornton

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What Does That Mean?

• Digital has a higher dynamic range than film
• The response is linear v. sigmoidal
• It provides more information at the low and high
  exposure levels

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Digital v. Computed v. Direct Capture Radiography

• Computed Radiography
• Uses a cassette with an imaging plate
• Digital Radiography
• Uses a linear array of radiation detectors
• Similar to Computed Tomography
• Direct Capture
• Uses a solid-state radiation detector device

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Computed Radiography

• A cassette is used
• Contains a photostimulable phosphor imaging plate
• Looks like an intensifying screen
• Contains a layer of europium-doped barium
  fluorohalide crystals
• When struck with x-rays, a new semistable state
  is produced
• The latent image is stored on this plate

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The Imaging Plate

• Made up of multiple layers
• A polyester support layer
• The phosphor layer
• A clear protective layer
• Reflective layer
• Conductive layer
• Support layer
• Backing layer
• Bar code label on the bottom
• Contains a number assigned to the image plate

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The Imaging Plate

• Very flexible
• Can maintain a latent image for about 24 hours
• Can be exposed to light
• Some image degradation may occur with extended
  storage time and light exposure

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The Image Reader

• Converts the analog information from the latent
  image to a digital format
• The image plate is scanned by laser
• The portion of the plate struck by laser emits
  light
• The light is directed to photomultiplier tubes
  that convert it to digital electric signals
• Most modern readers can process 110 140 plates
  per hour

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Terminology

• Rotation/inversion
• The ability to change the image presentation, or
  turn the image from a negative to positive
• Anatomic measurements
• The ability to measure certain areas of interest
• Short-term database functions
• Allows user to locate images, create lists of
  images, image interpretation, and maintain
  teaching files

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More Terminology

• Gradation Processing
• Controls the range of densities used to display
  structures on the image
• Spatial Frequency
• Controls the sharpness of boundaries between two
  structures of different densities
• Dynamic Range Control
• Provides a wide diagnostic field
• Allows for visualization of bone and soft tissue
  in a single image display

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Even More Terminology

• Magnification
• Allows enlargement of a specific area to enhance
  viewing and diagnosis
• Subtraction
• Allows the user to enhance certain areas of
  interest while fading other areas from view
• Enhancing from vascular detail while fading
  superimposing bony detail

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Image Storage

• Magnetic tape and optical disk
• Each CR image contains app. 8 megabytes of data

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CR Image Characteristics

• Image resolution
• Dimension of the crystals in the imaging plate
• Size of the laser beam in the reader
• The image reading matrix
• CR images average 2 5 lp/mm
• Standard film demonstrates 3 6 lp/mm

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Digital Radiography

• X-ray tubes for DR have a high heat capacity
• In excess of 1 MHU
• DR does not use cassettes
• A radiation detector array is used
• These detectors are gas-filled
• The more detectors, the better the spatial
  resolution

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Direct Capture Radiography

• Image forming x-rays interact with cesium iodide
  phosphors
• Patient dose is less with this approach
• Or
• They interact with a thin layer of selenium
• There is no spreading of light and spatial
  resolution is improved

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Digital Fluoroscopy

• A computer and two video monitors are required
• One video monitor is used to edit patient data
• One monitor displays the image
• The operating console is more complex
• Allows for the input of patient data

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Digital Fluoroscopy

• The video monitor is often a 1000 line system as
  opposed to 525
• The video signal is read in a progressive mode
• The electron beam of the of the TV camera tube
  sweeps the target assembly continuously from top
  to bottom in 33 ms.
• There is no interlace of fields
• The image is sharper with less flicker

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Digital Fluoroscopy and Dose

• Static images are made with a lower dose rate
  than with 105mm spot film cameras
• Most DF x-ray beams are pulsed
• The dose to the patient is about half that of
  conventional image intensified fluoroscopy

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PACS

• Picture Archive and Communication System
• Allows for the digitization of conventional
  radiographs
• Allows for acquisition, interpretation, and
  storage of images

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The Three Components of a PACS

• Display System
• Network
• Storage System

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Display System

• This is a cathode ray tube monitor
• Provides a workstation for the operator
• Must be very high resolution
• Ranges from 256 X 256 1024 X 1024
• This is lower than the spatial resolution of film
• The operator is able to subtract, use edge
  enhancement, window-level, highlight, pan, scroll
  and zoom on the monitor

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The Image Matrix

• A layout of cells in rows and columns
• Each cell is a specific location in the matrix
• Each cell is called a pixel
• Each digital image consists of a matrix of cells
• The matrix has various brightness levels
• The level of brightness depends upon the atomic
  number and mass density of the tissue that has
  received x-rays

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Network

• A number of computers connected to one another
• In a PACS, many people have access to an image
  for different purposes
• Data from one unit to another is first digitized
• Images can be transferred to another workstation
  for interpretation
• This is known as teleradiology

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Storage System

• PACS is able to archive
• Images are not lost
• The file room is replaced by a magnetic or
  optical memory device
• Electronically, images can be recalled to any
  workstation in seconds

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Pitfalls and Acceptance of CR and DR

• The acceptance of the system is increasing
• This is the future and in many imaging
  departments, the future is now!
• Department efficiency is increased
• It does not provide a traditional format
• Some resolution is lost
• Edge enhancement artifacts may actually create
  pathology
• Due to the increased latitude in image
  production, the patient may be exposed to too
  much radiation

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CR DR and Patient Dose

• If used correctly patient dose can be decreased
• CXR 5 decrease
• UGI 5 decrease
• IVP 10 decrease
• Pediatric examinations 15 30 decrease