The Digital Image Dr. John Ryan

1. The Digital Image Dr. John Ryan

3. What would this look like in grayscale?

4. grayscale

13. 48 50 43 124 134 100 250 234 187

14. Is this the lowest level we can go?

16. 00110010 00101011 00110000 10000110 01111011 01100100 11111001 11101101 10111010

17. Why process images? Source: Hornegger & Paulus, Erlangen University

18. Line Defect Interpolation Source: Hornegger & Paulus, Erlangen University

19. Contrast Enhancement Source: Hornegger & Paulus, Erlangen University

20. Noise Reduction Source: Hornegger & Paulus, Erlangen University

21. Edge Enhancement Source: Hornegger & Paulus, Erlangen University

22. The Difference? Original Result

23. What is a Digital Image? The digital image is sampled and mapped as a grid of dots or picture elements (pixels). Each pixel is assigned a tonal value (black, white, shades of gray or color), which is represented in binary code (zeros and ones).

24. Digital radiography • is a form of X-ray imaging, where digital X-ray sensors are used instead of traditional photographic film. Advantages include time efficiency through bypassing chemical processing and the ability to digitally transfer and enhance images. Also less radiation can be used to produce an image of similar contrast to conventional radiography

25. What is a Pixel? • A pixel is the smallest element within an image that has a single intensity value • The pixel value varies depending on intensity resolution (range, depth)

26. Pixels

27. What is a Voxel? • A voxel is like the 3D version of a pixel • It is the smallest unit within a volume

28. A Fly on the Ceiling Y X

29. Image Coordinate System • Top left corner is the origin. • Bottom right corner is the final pixel or (width, height). • Although this is the most common image coordinate system, it may vary. • There are many definitions for “pixel”, but in the context of medical imaging, a pixel is the smallest intensity or colour component of an image.

30. Bits and Bytes

31. 00110010 00101011 00110000 10000110 01111011 01100100 11111001 11101101 10111010

32. How Computers Store Images • Uncompressed images are stored as a sequence of pixel values • From left to right, then down to next row • For 8 bit, 1 byte per pixel • Deep down it’s stored as binary information: 101001010100101001110101101001010010101001010100101 010100101010001001010010010100101110100111010010110 100111010010101010111010100110011101010101001101100 100011010010100100101011001011010100101010011010101 100101001010100110110101101010011101000011010100101

33. Bits and Bytes A bit can have two values: 1 or 0, on or off 1 2 4 8 16 32 64 128 • 8 bits = 1 byte (Total is 256) • Computer data is stored in binary • Binary digits (bits) • Looks like 10010100101110101……

34. Getting a Value 128 64 32 16 8 4 2 1 Example 1: 10000001 = 128 + 1 = 129 Example 2: 00010010 = 16 + 2 = 18

35. Resolution • Intensity resolution • The range from totally white to totally black. • Intensity – relating to brightness value • Spatial resolution • The width and height of the image. • Temporal resolution • The rate of frames per second of animation or video. • Temporal – relating to time

36. contrast • Contrast is a measure of the • magnitude of the measured signal differences between physically different regions of the imaged object.

37. contrast Two versions of a wrist MR image. Image a plainly has higher contrast than image b – the bones are much brighter relative to the surrounding tissue background, even though in image b the average brightness of the bones is greater than in image a. Measures of contrast are based on the relative or absolute difference in average intensity of an object and its background

38. Artifacts • Any intensity or color fluctuations that make it difficult to see what you want to see that occur due to specific properties of the imaging method

39. Intensity Resolution • Bits per pixel • Or, number of gray levels • 8 bpp [256; (256 colours)], • 16 bpp [65536; (65,536 colours, known as Highcolour)], • 24 bpp [16777216; (16,777,216 colours, known as Truecolour)]. • 48 bpp [281474976710656; (281,474,976,710,656 colors, used in many flatbed scanners and for professional work)

40. Intensity Resolution 3 bit, 8 gray levels 8 bit, 256 gray levels

41. Lets Build a Chest X-RayFrom scratch, 0 bits – 0 levels

42. 1 bit, 2 levels

43. 2 bits, 4 levels

44. 3 bits, 8 levels

45. 4 bits, 16 levels

46. 5 bits, 32 levels

47. 6 bits, 64 levels

48. 7 bits, 128 levels

49. 8 bits, 256 levels

50. Intensity Resolution 3 bit, 8 gray levels 8 bit, 256 gray levels