Fundamentals of Digital Radiology

1. Fundamentals of Digital Radiology George David Medical College of Georgia

2. What we mean by Digital • Digital Radiographs • PACS • Picture Archival & Communication Systems • Reading from Monitors Filmless Department

3. 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 What is a digital image? • 2D array of #’s representing some image attribute such as • optical density • x-ray attenuation • Radiography • Fluoroscopy • CTDI • echo intensity • Magnetization • T1 • T2 • Proton Density

4. Number Array Forms Digital Image 194 73 22

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

6. What is this? 12 X 9 Matrix

7. Same object, smaller squares 24 X 18 Matrix

8. Same object, smaller squares 48 X 36 Matrix

9. Same object, smaller squares 96 X 72 Matrix

10. Same object, smaller squares 192 X 144 Matrix

11. Display of Digital Image • Each number of a digital image (pixel value) assigned a gray shade • Assignments can be changed • Window/level • Pixel values cannot

12. 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 Computer Storage • Computer image file is array of numbers • Same as any computer file 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

13. Digital Copies 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, 300 125, 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 = • If you’ve got the same numbers you have an IDENTICAL copy • Analog copies are never identical

14. All Digital Copies are Originals =

15. 125 25 311 111 199 192 85 69 111 87 77 103 118 139 118 155 145 301 256 223 Image Matrix & Image Size • Doubling the matrix dimension quadruples the # pixels 4 X 4 Matrix 16 pixels 2 X 2 Matrix 4 pixels

16. Image Matrix Doubling the matrix dimension quadruples # pixels • A 10242 matrix compared to a 5122 matrix quadruples • disk storage requirements • image transmission time • digital image manipulation Matrix # Pixels 512 X 512 => 262,144 1024 X1024 => 1,048,576 2048 X2048 => 4,194,304

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

18. Pixel Values & The Bit • Bit=>Fundamental unit of computer storage • Only 2 allowable values • 0 • 1 • Computers do all operations with 0’s & 1’sBUTComputers group bits together

19. 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

20. # of unique values which can be represented by 1 bit 2 unique values 1 2

21. # of unique values which can be represented by 2 bits 1 2 4 unique combinations / values 3 4

22. # of unique values which can be represented by 3 bits 5 1 6 2 7 3 8 4 8 unique combinations / values

23. Digital Image Bit Depth • the number of computer bits (1’s or 0’s) available to store each pixel value Values Bits # Values 1 2 3 . . . 8 0, 1 00, 01, 10, 11 000, 001, 010, 011, 100, 101, 110, 111 . . . 00000000, 00000001, ... 11111111 2 1 = 2 2 2 = 4 2 3 = 8 . . . 2 8 = 256

24. Bit Depth and Pixel Presentation on Image • 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

25. Bit Depth & Contrast Resolution The more bits per pixel the more possible gray shades and the better contrast resolution. 2 bit; 4 grade shades 8 bits; 256 grade shades

26. Computer Storage / Image Size • 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 262,144 pixels X 1 byte / pixel = 262,144 bytes 256 Kbytes 0.25 MBytes

27. Image Size • Depends on both matrix size & bit depth • Larger (finer) matrix requires more storage • doubling matrix size quadruples image size • Larger bit depth requires more storage • doubling bit depth (theoretically) doubles image size

28. 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 jpg 37, 37, 37, 37, 37, 37, 37, 37, 37, 37, 37, 37, 37, 37, 37, 37, 37, 37, 37, 37 gif (20) 37’s

29. 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

30. 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

31. 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

32. Computed Radiography (CR) • Re-usable metal imaging plates replace film & cassette • Uses conventional bucky & x-ray equipment

33. CR Exposure & Readout

34. CR Readout

35. Another View: CR Operation

36. - - - - - - - - - - - - - - - - - - - - - - - - - - - - Computer Radiography (CR) • photostimulable phosphor plate • radiation causes electrons to move to higher energy states • Excitation • Plate’s structure traps electrons in higher energy states • Form latent image H i g h e r E n e r g y E l e c t r o n S t a t e P h o t o n p u m p s e l e c t r o n t o h i g h e r e n e r g y s t a t e X - R a y P h o t o n L o w e r E n e r g y E l e c t r o n S t a t e

37. Reading Imaging Plate • laser scans plate with • laser releaseselectrons trapped inhigh energystates • electrons fall to lowenergy states giving upenergy as visible light • light intensity ismeasure of incidentradiation Lower Energy Electron State

38. Reading Imaging Plate • Reader scans plate with laser • Beam moved using rotating mirror • Plate pulled through scanner by rollers • Light emitted by plate measured byPM tube &recorded bycomputer

39. Laser & Emitted Light are Different Colors • Phosphor stimulated by laser light • Intensity of emitted light indicates amount of radiation incident on phosphor at each location • Only light emitted by phosphor measured by PMT • Filters remove laser light

40. CR Erasure • after read-out, plate erased using a bright light • plate can be erased and re-used • Erasure re-use cycle can be repeated without limit • Plate life defined not by erasure cycles but by physical wear

41. CR Resolution • Some vendor CR spatial resolution depends upon plate size. • Smaller pixels • More pixels / mm

42. CR Throughput • Generally slower than film processing • CR reader must finish reading one plate before starting the next • Film processors can run films back to back

43. CR Latitude • Much greater latitude than screen/film • Plate responds to many decades of input exposure • under / overexposures unlikely • Computer scale inputs exposure to viewable densities • Unlike film, receptor separatefrom viewer

44. Film Screen vs. CR Latitude • CR Latitude: .01 – 100 mR • Unlike film, small changes in incident radiation result in CR signal 100

45. Digital Radiography (DR) • Digital electronic bucky

46. DR Formats • Electronic bucky incorporated into x-ray equipment • Electronic wireless cassettes

47. Digital Radiography (DR) • Receptor provides direct digital output • No processor / reader required • Images available virtually immediately • Far fewer steps for radiographer

48. Types of DR Receptors TFT = THIN-FILM TRANSISTOR ARRAY

49. Digital Radiography (DR) • High latitude as for CR • DR portables now in available • Radiographer immediately sees image

50. Digital Raw Image • Unprocessed image as read from receptor • CR • Intensity data from PMT’s as a result of scanning plate with laser • DR • Raw Data read directly from TFT array • Not a readable diagnostic image • Requires computer post-processing • Specific software algorithms must be applied to image prior to presenting it as finished radiograph