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RAD 354 Chapt . 18 The Digital Image

RAD 354 Chapt . 18 The Digital Image. Spatial resolution Contrast resolution Contrast-detail curve Pt. dose considerations. Spatial Resolution.

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RAD 354 Chapt . 18 The Digital Image

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  1. RAD 354 Chapt. 18 The Digital Image • Spatial resolution • Contrast resolution • Contrast-detail curve • Pt. dose considerations

  2. Spatial Resolution • The ability of an imaging system top RESOLVE and RENDOR on the image a HIGH CONTRAST object of items in close proximity and similar atomic mass density (shades of gray differenced in close objects to one another)

  3. Spatial FREQUENCY • Refers to LINE PAIR! • Line pair per millimeter (lp/mm) • The HIGHER the spatial frequency, the HIGHER the SPATIAL RESOLUTION! • In digital imaging, spatial resolution is limited by PIXEL size.

  4. Line Pair Device

  5. Modulation Transfer Function (MTF) • The ability of an imaging system to render objects of DIFFERENT SIZES ONTO AN IMAGE (RATIO OF IMAGE TO OBJECT DUE TO SPATIAL RESOLUTION)

  6. Contrast Resolution • Shades of gray making various atomic mass densities visible • The HIGHEST possible contrast is TWO shades of gray = BLACK AND WHITE!

  7. Dynamic Range (OD) • Dynamic range is really the OD as shown on the H & D curve = ranges from base + fog (.2 or lower for film; and “base OD for digital” = 0) to the solarization point • The number of shades of GRAY that an imaging system can produce (remember, the human eye can only see about 30 different shades of gray)

  8. NOISE • Image noise limits contrast resolution (SNR) • Post processing of the image allows for manipulation and visualization of shades of gray (window and leveling – also called “smoothing”)

  9. Digital Imaging Pt. Dose • Digital imaging should reduce pt. dose (in reality, it has done the reverse!) – the “if in doubt, burn it out” syndrome and post image manipulation plus “dose creep” have significantly INCREASED patient dose in digital imaging

  10. Misc. Information • Focal spot size determines spatial resolution in film/screen systems • Post image manipulation = contrast resolution in digital REGARDLESS of pt. dose • Pt. dose in digital SHOULD be LOW because of DQE (probability of a photon interacting with an image receptor due to atomic number, etc.) • DQU = measure of x-ray absorption efficiency

  11. RAD 354 Chapt. 19 Viewing the Digital Image • Photometric Quantities • Hard/Soft Copy • Active Matrix Liquid Crystal • Display • Pre-Postprocessing digital image • PACS

  12. Photometric Quantities • Response of the human eye • Photopic • Scotopic

  13. Luminous Flux • Total intensity of light from a source • Measured in “luminous flux” in LUMENS

  14. Illuminance • Intensity of light incident on a surface • Goes back to the measure of “candle light” on a single square foot of surface

  15. Luminance Intensity • Amount of light emitted by a light source • A viewbox is a good example

  16. Luminance • Measure of the BRIGHTNESS of a source (like in a digital display) • Measured in CANDELA per meter squared = NIT

  17. Cosine Law • Relate to inverse square law and the amount of radiation at varied distance • Same for light at GREATER distances – like the CR vs PERIPHERY of the beam (Pathagora’s Theory • A2 + B2 = C2)

  18. Hard – Soft Copy • Hard = radiograph on film • Soft = Viewing image on a cathode ray tube (computer/TV monitor, etc)

  19. Active Matrix Liquid Crystal Display • Instead of using a “cathode ray tube” (TV type tube) viewing system, the active matrix, PIXEL by PIXEL, viewing monitor is used • Spatial resolution IMPROVES with larger matrix (PIXEL) displays

  20. Preprocessing the Image • YOU can program the computer with what YOU want the image to look like (convert an analogue image {radiograph} into a digital image) • i.e. Do we want a hand with a DYNAMIC range of 2? • Perhaps a CXR with a dynamic range of 60? • Shows what SCALE of contrast and shaded of gray to display • At the workstation, you tell the computer what anatomic area you radiographed and the computer shows it on the screen as it should look like (hand vs CXR)

  21. Postprocessing • Annotation (NOT L or R!!!) upright, decube, etc • Window and leveling (expand or shrink the shades of grays)

  22. PACS • Tim – Tell your thing!!!

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