Fundamentals of Digital Radiology

2. So what is �Digital�?

3. What we mean by Digital Digital Radiographs PACS Picture Archival & Communication Systems Reading from Monitors

4. What we really mean by Digital

5. Expectations for Digital All images available on-line new images old / comparison images images from all modalities Convenience Eliminate file room headaches lost / stolen films Reduce report turn-around time

6. Important Considerations Resolution needed Changing role of radiologists with referring physicians everyone has instant access to all images Security Cost

7. Digital Image Formation

8. Digital Image Formation Place mesh over image

9. Digital Image Formation Assign each square (pixel) a number based on density Numbers form the digital image

10. Digital Image Formation The finer the mesh, the better the digital rendering

11. What is this?

12. Same object, smaller squares

13. Same object, smaller squares

14. Same object, smaller squares

15. Same object, smaller squares

16. Numbers / Gray Shades Each number of a digital image corresponds to a gray shade for one picture element or pixel

17. So what is a digital image? Image stored as 2D array of #�s representing some image attribute such as optical density x-ray attenuation echo intensity magnetization

18. Computer Storage 125, 25, 311, 111, 182, 222, 176, 199, 192, 85, 69, 133, 149, 112, 77, 103, 118, 139, 154, 125, 120, 145, 301, 256, 223, 287, 256, 225, 178, 322, 325, 299, 353, 333, 300

19. Digital Copies =

20. Digital Copies then you have an identical copy

21. Copies Digital copies are identical All digital images are originals Film copies never identical to original may be substantially degraded loss of original information`

22. The Bit Fundamental unit of computer storage Only 2 allowable values 0 1 Computers do all operations with 0�s & 1�sBUTComputers group bits together

23. Special Binary Digit Grouping Terms Nibble 4 binary bits (0101) Byte 8 binary bits (1000 1011) Word 16 binary bits (1100 0100 1100 0101) Double Word 32 binary bits(1110 0100 0000 1011 0101 0101 1110 0101)

24. Abbreviations Review Bit (binary digit) Smallest binary unit; has value 0 or 1 only Byte 8 bits Kilobyte 210 or 1024 bytes sometimes rounded to 1000 bytes Megabyte 213 or 1,048,576 bytes or 1024 kilobytes sometimes rounded to 1,000,000 bytes or 1,000 kilobytes

25. Computer Storage Storage = # Pixels X # Bytes/Pixel Example: 512 X 512 pixels; 1 Byte / Pixel 512 X 512 pixel array # pixels = 512 X 512 = 262,144 pixels Storage = 262,144 pixels X 1 byte / pixel = 262,144 bytes = 256 KBytes = .25 MBytes

26. Image Matrix Doubling the matrix dimension quadruples the # pixels

27. Image Matrix A 10242 matrix compared to a 5122 matrix quadruples disk storage requirements image transmission time digital image manipulation

28. Matrix Size & Resolution More pixels = better spatial resolution

29. # of unique values which can be represented by 1 bit

30. # of unique values which can be represented by 2 bits

31. # of unique values which can be represented by 3 bits

32. Digital Image Bit Depth the number of computer bits (1�s or 0�s) available to store each pixel value

33. Digital Image Bit Depth bit depth indicates # of possible brightness levels for a pixel presentation of brightness levels pixel values assigned brightness levels brightness levels can be manipulated without affecting image data window level

34. Bit Depth & Contrast Resolution The more bits per pixel the more possible gray shades and the better contrast resolution.

35. Image Size Related to both matrix size & bit depth higher (finer) matrix requires more storage doubling matrix size quadruples image size higher bit depth requires more storage doubling bit depth theoretically doubles image size Computer may require storage in multiples of 8 bits (bytes) 10 or 12 bits stored in 16 bit slot alters image size requirements

36. Image Compression reduction of digital image storage size by application of algorithm for example, repetitive data could be represented by data value and # repetitions rather than by repeating value

37. Image Compression Image Decompression calculating original digital image from previously compressed data Compression Ratio original image size--------------------------------compressed image size ratio depends upon data to be compressed algorithm

38. Compression Types Reversible Compression Image decompresses to original pixel values Low compression ratios only Non-reversable Compression Decompressed image�s pixel values not necessarily identical to original much higher compression ratios possible variation from original image may or may not be visible or clinically significant

39. Non-Reversable Compression variation from original image generally increases with increasing compression ratio but a higher compression ratio means less storage requirements variation less noticeable for dynamic (moving) images than for still images such as radiographs

40. Where do Digital Images Come From? Direct Digital MRI CT Computed Radiography (CR) direct digital TV output (CCD TV�s) Digitization of an Analog Image Film Digitizers Video �Frame Grabbers� (requires good low noise video signal) teleradiology TV looking at viewbox digitizing of fluoroscopic video

41. Film Digitizer films stacked in input tray film pulled through digitizer CCD measures transmission from collimated light source optical density

42. Film Digitizers Expensive faster models cost more Patient Demographics input manually DICOM assistance Digitizers can have artifacts

43. Computer Radiography (CR) Re-usable metal imaging plates replace film & cassette

44. Computer Radiography (CR) plate is photostimulable phosphor radiation traps electrons in high energy states higher statesform latent image

45. Reading Imaging Plate plate scanned with laser Releases electrons trapped inhigh energy states electrons fall to lowenergy states electrons give upenergy as visible light light intensity ismeasure of incident radiation

46. Reading Imaging Plate Reader scans plate with laser light using rotating mirror Plate pulled through scanner by rollers Light emitted by plate measured by PM tube &recorded by computer

47. CR Operation after read-out, plate erased using bright light plate can be re-used digital image can be printed on film Read on-line

48. CR Comments Throughput CR reader must finish reading one plate before reading next Film processors run films back-to- back Latitude Plate responds to many decades of input exposure Much greater latitude than screen/film Computer scales input exposure to viewable densities Unlike film, receptor separate from viewer

49. Digital Radiography (DR) Direct digital output No processor / reader Images available virtually immediately Greater throughput

50. Digital Radiography (DR) Potentially lower patient dose High latitude as for CR Built into imaging equipment Fragile requires special attention for use with portable x-ray equipment

51. Digital Radiography (DR) Direct imaging X-rays interact with semiconductor material Amorphous selenium X-rays converted directly into electrical charge Indirect imaging X-ray strike scintillator producing light Photodiode array converts light to electrons

52. Digital Radiography (DR) Indirect imaging uses light and photodiodes Light spreads / scatters Can degrade resolution Both direct & indirect read-out digitally Charge pattern stored in array Analog to digital conversion digitizes charge pattern

53. Digital Video Video Signal Digitized (�Frame Grabber�)

54. Teleradiology Frame grabbers used for teleradiology Quality depends upon TV camera viewbox can have artifacts from lighting matrix size affects transmission speeds display quality

55. Digital Spot Films Frame grabber or CCD TV digitizes image Radiographic Technique used required to control quantum noise High-quality camera required high signal to noise Operationally nice allows review of images in exam room allows image manipulation allows later selection of images for printing

56. Digital Fluoroscopy TV Image digitized real-time Digitized image can be manipulated / enhanced real-time

57. Last Image Hold computer displays last fluoro image after radiation shut off allows operator to review static processes without beam ideal for teaching environments ideal for orthopedic applications such as hip pinning Reduces dose for patient, staff

58. Frame Averaging �Normal� fluoro only current frame displayed Frame averaging computer calculates average of current & user-selectable number of previous frames reduces quantum noise lag results if too many frames averaged

59. Other Fluoro Features Edge Enhancement / Image Filtering real-time Can be turned on and off Option of using lower frame rates computer displays last image until next one reduces flicker dynamic studies may be jumpy

61. Digital Subtraction Immediate replay of run Free selection of mask before or after bolus >1 frame may be averaged for mask Note subtraction adds noise

62. Registration matching anatomy between two images to be subtracted compensates for motion registration adjustments often fine adjustment down to 1/10 pixel registration types translational (left, right, up, down) rotational warp

63. Digital Image Manipulations on-screen measurements distances angles volumes/areas stenosis image annotation peak opacification / roadmapping peak opacification displays vessels after a test injection allows visualization of live catheter on top to saved image of test injection

64. CR/DR Advantages post-processing & manipulation possible tremendous latitude virtually no technique repeats DR faster than film CR operationally slightly slower than film

65. Digital Disadvantages High up-front expenses CR: many readers may be required for spread out departments DR: new radiographic equipment required High resolution images very large Images require high speed transmission systems massive archival systems required Spatial resolution poorer than film May be offset by other advantages

66. Digital Possibilities Multi-modality imaging / Image fusion PET/CT

67. Digital and other Possibilities Tomosynthesis Multi-slice tomography from single pass Histogram Equalization Computer provides approximately equal density to various areas in image.

68. DR & Energy Subtraction 2 images taken milliseconds apart at 2 kVp�s Combine / subtract images

69. The End ?