Comparison of Analog and Digital X-ray Imaging Functions

1. CHAPTER 15 Creating the Digital Image

2. WHAT DO WE MEAN BY “DIGITAL”? • Watch = analog • No further refinement in telling time beyond the hour, minute, and second • Series of numbers = digital • Discreet measurement of time; carried out infinitesimally

3. 4 BASIC FUNCTIONS FOR X-RAY IMAGE PRODUCTION • Acquisition • Processing • Display • Storage

4. COMPARISON OF 4 FUNCTIONS ANALOG DIGITAL LATENT IMAGE 1. ACQUISITION # X-RAYS IN PIXELS DARKROOM & PROCESSOR2. PROCESSING IN ROOM/COMPUTER VIEWBOX3. DISPLAYCOMPUTER MONITOR FILM LIBRARY4. STORAGEPACS

5. OVERVIEW OF ANALOG AND DIGITAL MAMMOGRAM EXAM

6. DIGITAL MAMMOGRAPHY MACHINE • Note the similar appearance to an analog unit.

7. DIGITAL ARRAY AND ITS PIXELS • The digital array is composed of millions of pixels.

8. RESOLUTION • The smaller the pixel, the higher the resolution.

9. IMAGING MODALITIES AND MATRIX SIZE

10. A HUMAN HAIR IN COMPARISON TO A ROW OF PIXELS

11. DISPLAY OF SHADES OF GRAY

12. DISPLAY OF SHADES OF GRAY • Varying optical densities of film versus how the digital system interprets optical densities

13. DIGITAL ARRAY A. Before the exposure, all the pixels are empty. B. After exposure, pixels record number of x-rays that interacted with each pixel. C. How the detector “sees” the number of photon interactions.

14. DIGITAL MACHINE & ITS FUNCTIONS • The x-ray gantry and technologist acquisition workstation

15. THE TECHNOLOGIST STAYS IN THE ROOM FOR THE ENTIRE EXAM SINCE IT IS NO LONGER NECESSARY TO LEAVE TO DEVELOP FILMS IN A DARKROOM

16. PHOTOTIMING • Digital machine phototimers rarely produce a suboptimal image. • Be vigilant for dose creep.

17. THE DIGITAL READING ROOM • A radiologist at his review station

18. RADIOLOGIST’S KEYPAD • Rather than point, click, and drag using a mouse, some vendors supply the radiologist with a more compact keypad to streamline the interpretation process. Each button controls an image enhancement tool

19. IMAGE ENHANCEMENT TOOLS • Hanging protocols • Electronic wax pencil • Annotations • Window/level • Zoom • Inversion • Electronic ruler

20. IMAGE ENHANCEMENT TOOLS • Primarily used by the radiologist during interpretation, but many of these same features are also available to the technologist. • Keep in mind that the technologist’s display monitor is not a high-resolution monitor like those available to the radiologist.

21. HANGING PROTOCOLS • Radiologists program the computer to display images in the order and location on the monitors that they prefer.

22. HANGING PROTOCOLS

23. WAX PENCIL & ANNOTATIONS • Radiologists now use an electronic wax pencil. • They use an electronic annotation instead of a Post-it note stuck on a film.

24. WINDOW WIDTH & WINDOW CENTER • The dynamic range of a digital image allows electronic changes to brightness and contrast levels without having to re-expose the patient. • Better known as window/level

25. FULL-RESOLUTION DISPLAY • The area inside the mag-box displays at the highest resolution the digital system offers. • Note: This is not a magnified view.

26. FULL-RESOLUTION DISPLAY A. Decreased resolution required to display entire breast on monitor B. Breast displayed at full resolution; aka “zoom”

27. INVERSION A. What was white is now black; what was black is now white B. It is possible to use tool combinations: inversion plus zoom

28. ELECTRONIC RULER

29. CAD • CAD is an available option when purchasing a digital system.

30. CONNECTIVITY • Digital images move effortlessly throughout “the network.”

31. 3 METHODS TO PRODUCE DIGITAL IMAGES • Direct-to-digital • 1. Indirect conversion (CsI) • 2. Direct conversion (a-Se) • Nondirect digital + indirect conversion • 3. CR

32. CR MAMMOGRAPHY • Uses a special CR cassette • Uses analog mammography machine

33. CR MAMMOGRAPHY • Image plate processor

34. CR ADVANTAGES • Less expensive • Use analog mammography machine • Mammography software upgrade for general radiology’s CR processor • Two sizes of IRSD

35. CR DISADVANTAGES • Not able to adapt to future imaging technologies that require direct-to-digital platforms • No improved productivity

36. INDIRECT CONVERSION • Indirect conversion detectors work by converting x-ray photons into light.

37. DIRECT CONVERSION • X-ray photons are directly converted into an electrical charge.

38. EVALUATING THE DIGITAL IMAGE • SNR • CNR • MTF • DQE

39. SNR • Signal-to-noise ratio • Measures the quality of information in the image • Compares level of desired signal to level of background noise • Determined by the number of x-ray photons absorbed by the digital detector

40. CNR • Contrast-to-noise ratio • Ability to differentiate the x-ray attenuation co-efficiencies for all breast structures and types of tissue • Essential so small differences in x-ray attenuation are visible and allow detection of cancer among the clutter of normal tissue

41. MTF • Modulation transfer function • Evaluates overall system resolution • How well does the system transfer shapes/structures from the incident to the output x-ray pattern • Measured by the manufacturer under perfect laboratory conditions

42. DQE • Detective quantum efficiency • Best method to measure detector performance of contrast and noise • Measures percentage of x-rays that strike the detector and are absorbed • Compares dose