Digital Imaging Systems

1
Digital Imaging Systems

• Thanks to the work of Dr. Perry Sprawls of Emory
  University and the Sprawls Educational
  Foundation, this material is available on-line.

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Digital Radiography Systems
3
The Digital Radiography System

• Digital radiography is performed with a system of
  the following functional components
• A digital image receptor
• A digital image processing unit
• An image management system

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The Digital Radiography System

• A communications Interface to a patient
  information system
• network
• A display for viewer operated controls.
• At this time we will briefly introduce the
  various components.

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Image Receptor

• The image receptor intercepts the x-ray beam
  after it passes through the patients body.
• It produces an image in digital form, that is a
  matrix of pixels, each with a numerical value.

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Image Receptor

• It replaces the cassette with intensifying
  screens and film.
• It can be the entire receptor that looks like a
  Bucky as in Direct Digital.
• It can be a cassette the contains a Stimulable
  Phosphor as in Computed Radiography.

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Image Management System

• Image management is a function of a computer
  associated with the digital radiography process.
• Functions consist of
• Movement of the images among the other components
• Associating other data and information with the
  images.

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Image Management System

• Functions may be performed by the computer of the
  specific radiography device or by a more
  extensive Digital Image Management System (DIMS)
  that serves many imaging devices within the
  facility.
• Note that sometimes the DIMS is called by its
  old name PACS (Picture Archiving and
  Communication System.

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Patient Information System

• The Patient Information System , sometime the RIS
  (Radiology Information System) is an adjunct to
  the basic digital system.
• Through the system information such as the
  patient I.D., billing and scheduling is
  transferred.

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Image Processor

• One of the major advantages of digital imaging is
  the ability to process the image after they are
  recorded.
• Various forms of digital processing can be used
  the change the image.

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Image Processor

• The ability to change or optimize the density and
  contrast of the image is of great value.
• It is also possible to enhance visibility of
  detail in some radiographs with magnification or
  inversion.

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Digital Image Storage

• Digital radiographs and other information are
  stored as digital data.
• Advantages (compared to film) include
• Rapid storage and retrieval
• Less physical space required
• Ability to copy and duplicate without loss of
  image quality.

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Communications Network

• Another advantage of digital images is the
  ability to transfer them from one location to
  another rapidly.
• This can be
• Within the facility to storage and display
  devices
• To other locations around the world via the
  internet.

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Display Devices

• Digital images displayed on a monitor are
  referred to as softcopy.
• One major advantage is the ability of the viewer
  to adjust and optimize the image characteristics
  such as contrast and density.

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Display Devices

• Other advantages include
• Zoom
• Compare multiple images
• Perform analytical functions such as measure
  distances and angles accurately.

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Direct Digital Image Receptor

• With direct digital imaging, the phosphor is
  built into the Wall Bucky or Table Bucky.
• Almost as soon as the image is produced, it can
  be viewed.
• Great through-put but less flexible than CR.

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Direct Digital Image Receptor

• The direct digital receptor is basically a
  digital x-ray camera.
• The pixel area is exposed by the x-rays exiting
  the patient.
• The photons are absorbed and the energy produces
  an electrical signal.

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Direct Digital Image Receptor

• The electrical signal is a form of analog data
  that is converted into a digital number and
  stored as one pixel in the image.

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Computed Radiography Receptor

• CR uses a Stimualible Phosphor Receptor inside a
  cassette.
• It can be used with existing radiographic
  systems.
• The x-ray exposure produces an invisible latent
  image.

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Computed Radiography Receptor

• The difference between intensifying screens and
  computed radiography is that there is a delay
  between exposure and the production of the light.
• Here is how it works

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Computed Radiography Receptor

• First a receptor containing the phosphor plate is
  exposed to record an image.
• At this point it is an invisible latent image.
• The next step is the cassette is placed into
  reader and processor unit.
• The plate is scanned with a very small laser
  beam.

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Computed Radiography Receptor

• The laser beam stimulates the plate causing light
  to be produced.
• The light that is produced is proportional to the
  x-ray exposure to that specific spot.
• The result is an image formed by the light
  produced on surface of the plate or screen.

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Reading Phase

• A light detector measures the light and sends the
  data on to produce a digitized image.
• As the surface of the stimualible phosphor screen
  is scanned by the laser beam, the analog data
  representing the brightness of the light at each
  point is converted to digital values for each
  pixel.

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Reading Phase

• The digital data is stored as a digital image.
• The process takes about 50 seconds compared to
  two minutes or more with film.

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Image Receptor

• BaSrFBrlEu phosphors are used to produce the
  image.
• Its luminescence spectrum is at about 390 nm in
  lattices of the BaFBr-type.
• The top of the spectrum is shifted slightly to
  longer wavelengths due to the incorporation of
  iodide.
• The stimulation spectrum is much broader then
  pure BaFBr. The shift is the result of partial
  replacement of Ba by SR and by the iodide.
• The red shift of the stimulation spectrum assures
  maximum stimulability at 633 nm, the wavelength
  of the stimulating laser.

26
Agfa CR 35 Computed Radiography Unit

• This is the digital radiographic processor used
  in the clinic.
• We will go into detail on how to use it in 9th
  Quarter when you start taking radiographs on
  student patients.

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Digital Receptor Dynamic Range

• A wide dynamic range of exposure is a
  characteristic of many digital radiography
  systems.
• This means that the receptor respond to x-ray
  exposure and produce digital data over a wide
  range of exposure.

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Film Latitude or Dynamic Range

• Most film systems have a very limited dynamic
  range of exposure.
• Latitude is the range of exposure that forms an
  image.
• Latitude is associated with the slope part of the
  H D curve.

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Film Latitude or Dynamic Range

• The region of the toe of the curve has no
  significant contrast and it corresponds to the
  underexposed areas of the image.
• The shoulder region also has no significant
  contrast and corresponds to over exposure.

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Film Latitude or Dynamic Range

• The limited latitude is due to the way the image
  is formed with the silver halide crystals.
• Digital receptors do not have this limitation.

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The Exposure Histogram

• Before we go deeper into characteristics of
  digital receptors, lets develop the concept of
  the exposure histogram.
• The x-ray image and contrast are formed as the
  beam passes through the body and experiences
  different levels of attenuation through the
  anatomical regions.

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The Exposure Histogram

• In the chest, the low-density lung areas produce
  a relatively high exposure to the receptor and
  dark areas of the image.
• The more dense areas like the spine and below the
  diaphragm produce relatively low exposure and
  light areas of the image.

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The Exposure Histogram

• The histogram as we see it here, shows the amount
  of image area that receives the different levels
  of exposure that forms the image.
• At this time our primary interest is in the range
  of exposures.

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Imaging with Film

• The greatest challenge of film radiography is to
  get the range of exposure to fit into the dynamic
  range of the film.
• If the exposure falls outside the latitude, there
  will be little or no image contrast formed.

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Imaging with Film

• There are two conditions that contribute to
  receptor exposure outside the film latitude
• Error in setting the correct exposure factors.
• Some regions of the body have wide dynamic of
  exposure that exceeds the latitude of the film.
• Using a wider latitude film can correct this but
  results in lower image contrast.

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The Advantage of a Wide Dynamic Range

• Here we can see the advantages of a digital
  receptor that has a wide dynamic range.
• Even when there is a wide range of exposure
  coming from the body (wide histogram) and
  exposure at different levels (exposure errors)
  they still fit within the dynamic range.

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Digital Image Contrast

• In a digital image, contrast is represented by
  different pixel values.
• A typical digital receptor will have a linear
  relationship between exposure and resulting pixel
  value.

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Digital Image Contrast

• We have seen that this relationship extends over
  a relatively wide range of exposures.
• This is contrasted with the non-linear
  relationship seen with film.

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Optimum Exposure in Digital Radiography

• The wide dynamic range and linear response of
  digital receptors is like a double edged sword.
• The advantage is a wide range of exposures and
  exposure errors will still produce an image with
  good contrast.

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Optimum Exposure in Digital Radiography

• So, what is the problem? There are two that we
  can see here.
• Even though good contrast with low exposure is
  obtained. Due to the low exposure, we have high
  noise.

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Optimum Exposure in Digital Radiography

• With film the image would be light.
• The other problem is that exceptionally good
  images are obtained with high exposure ( very low
  noise).
• With film the image would be dark or over exposed

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Optimum Exposure in Digital Radiography

• In general, the challenge is to make sure the
  exposure factors are set for optimum image
  quality.

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Monitoring Exposure Levels

• One of the challenges is to know when the digital
  image is properly exposed since it is unlike film
  where under and over exposure is obvious.
• Each manufacturer of digital receptors will
  provide an approach to calculate the image
  exposure information.

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Monitoring Exposure Levels

• Some will display the S number. This displayed
  value generally indicates the speed of the
  receptor that would match the actual exposure.

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Monitoring Exposure Levels

• A low exposure would result in a high calculated
  S number (like S1000) and a high exposure would
  produce a low number (like s50).
• This is opposite of screen-film speed.

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Monitoring Exposure Levels

• The operator should determine the appropriate
  range of values for optimum exposure and monitor
  the values.
• The method varies by manufacturer and clinical
  procedure.

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ACR Exposure Factor Chart
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ACR Digital Radiography Guidelines

• The American College of Radiology also provides a
  chart to estimate what exposure change is needed
  to achieve optimum exposure.
• With the LGM system to go from 1.80 to 2.20, the
  mAs is doubled or kVp increased 50 or increase
  kVp by 15.

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Digital Exposure

• Proper exposure Over exposure

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Digital Radiography Image Quality

• Like all x-ray images, there are 5 specific
  quality characteristics.
• Spatial detail
• Detail
• Contrast
• Noise
• Artifacts
• We will now see how 3 of these, contrast, detail
  and noise are effected by the operation of
  digital radiography.

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Digital Radiograph Contrast

• Contrast sensitivity of digital procedures and
  image contrast depend upon several factors.
• Two are
• X-ray beam spectrum
• Scattered radiation
• Similar to film.

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Digital Radiograph Contrast

• What is different is the ability to adjust and
  optimize contrast after the image is recorded.
• This is done through the digital processing of
  the image and then the adjustment of the window
  when the image is being viewed.

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Digital Radiographic Detail

• Visibility of detail is reduced and limited by
  the blurring that occurs at different stages of
  the imaging process.
• Some are common to both film and digital
  radiography.

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Common Sources of Blurring

• Common sources of blurring
• The focal spot (depends upon size and object
  location.)
• Motion if present
• The receptor (generally due to light spreading
  within the fluorescent or phosphor screen)

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Digital Sources of Blurring

• Additional blurring is the result of dividing the
  image into pixels.
• The size of a pixel (amount of blurring) is the
  ratio of the image size (image size relative to
  the anatomy) and the matrix size.

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Digital Sources of Blurring

• Pixel size is another factor that must be
  considered because it limits the detail of the
  image.
• 1024 x 1280 on a 18cm x 24 cm image would be
  sharper than on a 35 cm x 43 cm image.
• Generally, digital images lack the spatial detail
  of film images due to the digital processing.

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Noise in Digital Radiographs

• The most predominate source of noise in digital
  imaging is quantum noise associated with the
  random distribution of the x-ray photons received
  by the image receptor.

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Noise in Digital Radiographs

• The level of noise depends upon the amount of
  receptor exposure used to produce the image.
• With digital imaging, it can be adjusted over a
  wide range due to the wide dynamic range.

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Noise in Digital Radiographs

• Noise is controlled by using the appropriate
  exposure factors.

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Digital Image Noise

• Underexposed Proper exposure

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End of Lecture

• I must acknowledge Dr. Perry Sprawls of Emory
  University and the Sprawls Educational Foundation
  for the hard work in producing the slides used in
  this lecture.
• I also acknowledge the hard work done by the
  American College of Radiology in Drafting
  Practice Guidelines for Digital Radiography.