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Fundamentals of Digital Radiology

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

  2. So what is “Digital”?

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

  4. What we really mean by Digital No more File Room!!!

  5. Digital Image Formation • Place mesh over image

  6. Digital Image Formation • Assign each square (pixel) a number based on density • Numbers form the digital image 194 73 22

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

  8. What is this? 12 X 9 Matrix

  9. Same object, smaller squares 24 X 18 Matrix

  10. Same object, smaller squares 48 X 36 Matrix

  11. Same object, smaller squares 96 X 72 Matrix

  12. Same object, smaller squares 192 X 144 Matrix

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

  14. 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 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

  15. 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 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

  16. 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 ...

  17. Digital Copies then you have an identical copy =

  18. Digital Copies • Digital copies are identical • All digital images are originals =

  19. 125 25 311 111 199 192 85 69 111 87 77 103 118 139 118 155 145 301 256 223 Image Matrix • Doubling the matrix dimension quadruples the # pixels 2 X 2 Matrix 4 pixels 4 X 4 Matrix 16 pixels

  20. 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

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

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

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

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

  28. 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

  29. 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

  30. 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

  31. 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

  32. 1 2 3 4 5 6 7 8 9 1 0 1 1 1 2 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

  33. 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

  34. 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

  35. 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

  36. 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

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

  38. CR Exposure & Readout

  39. CR Readout

  40. Another View: CR Operation

  41. - - - - - - - - - - - - - - - - - - - - - - - - - - - - Computer Radiography (CR) • plate is photostimulable phosphor • radiation traps electrons in high energy states • higher statesform 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

  42. Reading Imaging Plate • reader scans plate with laser • laser releaseselectrons trapped inhigh energystates • electrons fall to lowenergy states • electrons give upenergy as visible light • light intensity ismeasure of incident radiation Lower Energy Electron State

  43. Reading Imaging Plate • Reader scans plate with laser light using rotating mirror • Film pulled through scanner by rollers • Light given off by plate measured byPM tube &recorded by computer

  44. 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 color of light emitted by phosphor measured by PMT

  45. CR Operation • after read-out, plate erased using a bright light • plate can be erased virtually without limit • Plate life defined not by erasure cycles but by physical wear

  46. CR Resolution • Small cassettes have better spatial resolution • Smaller pixels • More pixels / mm

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

  48. 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 separate from viewer

  49. Film Screen vs. CR Latitude CR Latitude: .01 – 100 mR 100

  50. Digital Radiography (DR) • Digital bucky • Incorporated into x-ray equipment