1. Digital Radiography

2. Basic Concepts Image Quality Concepts Nature of the Digital Image
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 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)

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)
1000:1 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 < 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

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: 16:1 (between 0.5 and 2.5 OD) (exposure yielding 2.5 OD is 16x exposure yielding 0.5 OD)
CR: >10,000:1 (between minimum and maximum measurable scan levels)

29. CR and “Film Density”
1/8 x E
4 x E

30. Radiation Dose with DR How much is enough ? How much is too much?
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 Levels LgM=1.10 LgM=2.1

32. 0.1 Exposure Correct Exposure

33. FUTURE CR TECHNOLOGY New phosphors and scan head technology
Dual Energy

34. “Flat Panel” CR Receptor Devices

36. Dual Energy Imaging