Contrast & Grayscale

• Contrast


  One of two photographic properties comprising detail visibility, the other being brightness/density
• Synonyms of contrast

  • Dynamic Range
  • Window Width
• Radiographic Contrast


  The difference between two or more brightness levels or densities on a radiograph, resulting from tissue's differential absorption of x-ray photons
• Lack of contrast makes an image appear homogenous
• Radiographic contrast


  • Affects visibility of structural lines making up recorded images
  • Results from multiple factors: anatomic structure, quality of radiation, image receptor type, computer processing and display
• Subject Contrast


  A result of the absorption characteristics of the anatomic tissue radiographed along with the quality of the x-ray beam
• Types of contrast

  • Low Contrast (Few shades of gray, Increased contrast, Low kVp, Short scale)
  • High Contrast (Many shades of gray, Decreased contrast, High kVp, Long scale)
• Long scale


  Decreased contrast, Higher kV used, Many shades of gray, Gradual changes in tones
• Short scale


  Increased contrast, Lower kV used, Fewer shades of gray, Black & white
• This chest x-ray appears to be a long scale: many shades of gray, gradual changes in tones, higher kV used
• Radiographic contrast


  Results from subject contrast and IR contrast
• Subject Contrast


  The ratio of the x-ray intensity transmitted through one segment of the part in a study to that transmitted through another part, affected by part thickness, atomic number, radiation quality, scatter radiation, use of contrast agents
• Contrast Resolution


  The ability of the system to distinguish between small objects that attenuate the x-ray beam similarly, specifically used in digital imaging
• The human eye has limited contrast resolution
• Window width


  A computer processing tool that adjusts radiographic contrast
• Grayscale


  The number of different shades of gray that can be stored and displayed by a computer system, determined by bit depth
• Wide window width


  Lower contrast
• Narrow window width


  Higher contrast
• Kilovoltage Peak (kVp)


  The penetrating power of the x-ray beam, alters the quantity and quality of the x-ray beam, changes radiation exposure to the IR, important for image quality in digital radiography
• Increase in kVp


  Increase in radiation exiting the patient, increase in scatter radiation, decrease in contrast
• Decrease in kVp


  Decrease in radiation exiting the patient, decrease in scatter radiation, increase in contrast
• In digital radiography, the computer will post process the image making any over- or under-usage of kVp mostly unnoticeable
• Too much radiation


  Image appears appropriately bright, but patient is overexposed
• Too little radiation

  Image appears appropriately bright, but quantum noise is increased which decreases detail
• When altering mAs is limited, the kVp must be changed to maintain exposure to the IR
• The 15% Rule - changing the kVp by 15% has the same effect as doubling or halving the mAs
• Increasing kVp


  Decreased contrast
• Decreasing kVp

  Increased contrast
• Radiographs not generally repeated due to contrast errors, if a repeat is necessary to change the contrast: adjust the kVp by 15%, and adjust mAs by an inverse factor of 2
• A higher kVp and a lower mAs should be used to reduce patient dose
• Secondary factors affecting radiographic contrast

  • OID
  • Collimation
  • Filtration
  • Grid
  • Anatomic part
  • Focal spot size
  • Anode heel effect
• OID (Object-to-Image Distance)


  The distance between the object being radiographed and the image receptor, increasing OID reduces scatter radiation reaching the IR, increasing contrast
• Collimation


  Decreasing the field of view, reduces x-ray photon quantity and scatter, increasing contrast
• Filtration


  Filters low-energy x-ray photons, leaving the x-ray beam to consist of mostly high-energy photons, high-energy = increased scatter = decreased contrast
• Filtration


  Inversely related to contrast, as filtration increases, contrast decreases
• Grids


  Absorb scatter radiation from the remnant beam, increase in grid ratio = decrease in scatter reaching IR = increase in contrast, but using a grid does increase patient dose
• Anatomic part size


  Inversely related to contrast, as part size increases, scatter produced increases, causing contrast to decrease
• Tissue density/atomic number


  Directly related to contrast, as the atomic number of tissue increases, more photoelectric absorption occurs, causing contrast to increase
• Focal spot size changes spatial resolution but does not affect contrast, and the Anode Heel Effect has little to no effect on contrast
• two photographic properties comprising detail visibility:

  contrast and brightness/density