NDT 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): • Sources Blurring in radiography: • Focal spot (all types
of
radiography) • Motion (all types of radiography) • Receptor blur -
depends on receptor • 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)

5. • Dual Receptors (Screens)
Noise and Image Quality
• mage 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
Radiography Image Receptor Blur

6. • 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

7. • he 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)

8. • 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

9. • atrix 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 bars+5 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)

10. • 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