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Digital Radiography

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Basic Concepts Image Quality Concepts Spatial Resolution (limiting resolution) Noise: Quantum Mottle Nature of the Digital Image Spatial Digitization Analog-to ... – PowerPoint PPT presentation

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Title: Digital Radiography


1
Digital Radiography
2
Basic Concepts
  • Image Quality Concepts
  • Spatial Resolution (limiting resolution)
  • Noise Quantum Mottle
  • Nature of the Digital Image
  • Spatial Digitization
  • Analog-to-Digital Conversion
  • Digital Radiography Factors
  • Spatial Digitization and Resolution
  • ADC and Noise
  • ADC and Dynamic Range

3
Basic Concepts Limiting Resolution
  • Limiting Resolution (simplest form)
    refers to the smallest, closely spaced objects
    for which separate images can be seen
  • Measurement and Units Bar pattern

4
Basic Concepts Limiting Resolution
  • Limiting Resolution (simplest form)
  • Measurement and Units Bar pattern Measured
    using bar pattern (lead strips separated by
    spaces) and expressed as smallest visible bar
    size or highest spatial frequency (line-pairs/mm)
  • Sources Blurring in radiography
  • Focal spot (all types of radiography)
  • Motion (all types of radiography)
  • Receptor blur - depends on receptor

5
Radiography Image Receptor Blur
6
Dual Receptors (Screens)
7
Noise and Image Quality
8
Image Noise Quantum mottle
  • Quantum mottle (QM) refers to the graininess of
    x-ray images
  • QM is caused by using a limited number of x-ray
    photons to make an image
  • QM interferes with ability to details
  • Using more photons (more mAs) reduces noise but
    increases radiation exposure

9
The Nature of the Digital Image
  • Basic Concepts Resolution and Noise
  • The Digitization Process
  • Spatial Digitization
  • Analog-to-Digital Conversion (ADC)
  • Radiation Dose, Noise and Resolution
  • Resolution versus Dose receptor thickness
  • Dose versus Image Noise (Quantum mottle)
  • Dynamic Range

10
The Digitization Process
  • Every image starts out in analog form
  • light image emitted by screen
  • light image from intensifier output phosphor
  • TV camera voltages
  • Stimulated light from computed radiography
  • Analog image must be converted (digitized) to
    matrix of pixels stored as binary numbers
  • Spatial digitization generation of pixels
  • Analog-to-Digital Conversion (ADC)

11
Spatial Digitization (pixels) Sampling
  • Must measure image along many rows (512, 1024,
    etc) and at many point along each row
  • Sampling done by
  • detector with discrete elements (eg, CCD
    camera, flat panel detector) or
  • Raster scan process

12
Matrix Size, Resolution and Bytes
  • Regular Film/Screen 5 line-pairs/mm
  • To Equal with Digital Image
  • 5 lp/mm 10 pixels/mm (to see 5 bars5 spaces)
  • 35 x 43 cm (14 x 17) image 350 x 430 mm
  • 350 x 430 mm at 10 pixels/mm 3500 x 4300 pixels
  • 3500 x 4300 x 2 bytes/pixel (16 bits/pixel) 30
    MB
  • Digital Radiography
  • Typically 2000 x 2500 pixels maximum (3 lp/mm)

13
Digital Spatial Resolution
14
Spatial Digizitation ADC
15
ADC and Noise How many bits?
16
Contrast vs Latitude (Dyamic Range)
17
ADC and Dynamic Range
  • Suppose we have
  • 10 bit ADC (1024 graylevels)
  • 10001 dynamic range (e.g. we can measure and
    record exposures from 1 mR to 1000 mR (1 R)
  • Need 1 mR difference for different graylevel
  • Differences between structures to see in image
    may be lt 1 mR in x-ray intensity reaching the
    receptor
  • Alternatives
  • throw out some dynamic range (limit range)
  • Increase number of bits (still uncommon)

18
Digital Detectors
  • Cassette-based Image Storage Phosphor (CR)
  • Image Intensifier
  • Scanned Projection
  • Direct Digitizing (Full Field)
  • CCD Camera
  • Selenium Flat Panel (Direct Digital
    Radiography)
  • Phosphor Flat Panel (Indirect Digital
    Radiography)
  • Future Technology

19
Digital Detectors
  • Cassette based Image Storage Phosphor (CR)
  • Image Intensifier
  • Scanned Projection
  • Direct Digitizing (Full Field)
  • CCD Camera
  • Selenium Flat Panel (Direct Digital
    Radiography)
  • Phosphor Flat Panel (Indirect Digital
    Radiography)
  • Future Technology

20
CR Clinical Use
21
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22
Conventional CR Scanning
23
  • Flying Spot CR Scan
  • In a conventional flying spot CR reader,
    stimulated output exposure (scan level) from the
    IP is proportional to the laser intensity I and
    dwell time Td

24
Absorption Efficiency
25
CR Blur
26
Dynamic Range (Latitude)
  • Dynamic Range, or latitude refers the range of
    exposures which provide useful diagnostic
    information. For film, is the the range of
    exposures that provide acceptable optical
    densities (ie, not too dark and not too light)

27
Dynamic Range
28
Dynamic Range (Latitude) CR vs Film
  • Dynamic Range (latitude) range of exposures
    providing useful diagnostic
    information
  • Regular F/S 161 (between 0.5 and 2.5 OD)
    (exposure yielding 2.5 OD is 16x exposure
    yielding 0.5 OD)
  • CR gt10,0001 (between minimum and maximum
    measurable scan levels)

29
CR and Film Density
  • 4 x E

1/8 x E
30
Radiation Dose with DR
  • How much is enough ?
  • Image Noise (Quantum mottle)
  • required image quality
  • How much is too much?
  • Patient radiation exposure concerns
  • possible saturation of parts of image (all black)

31
Noise and Proper Exposure LevelsLgM1.10
LgM2.1
32
0.1 Exposure Correct Exposure
33
FUTURE CR TECHNOLOGY
  • New phosphors and scan head technology
  • Dual Energy

34
Flat Panel CR Receptor Devices
35
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36
Dual Energy Imaging
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