Radiology

• Density


  Darkness or lightness of the radiographic image
• Factors affecting mAs & Density


  • Patient factors: size of pt., density / pathology of tissue
  • kVp
  • Collimation
  • Distance
  • Grids
  • Film/Screen Combinations
  • Processing
• Factors affecting mAs


  • Patient factors: size of pt., density of tissue, pt. compliance
  • kVp
  • Distance
  • Grids
  • Film/Screen Combinations
  • Processing
• Patient's Body Habitus (size)


  Influences technique & density on image
• Pathology like pneumothorax causes lung to collapse, making it easy to penetrate with x-ray photons
• Scatter


  Creates fog and lowers contrast (more grays)
• Scatter


  Increases as kV increases, field size increases, thickness of part increases
• Collimation


  Collimating to area of interest reduces scatter and radiation dose to the patient
• Grids


  A device with lead strips placed between the patient and the cassette to reduce the number of scattering photons from reaching the image
• Grids can leave lines on the image if placed backwards
• Detail


  The degree of sharpness in an object's borders and structural details, how "clear" the object looks on the radiograph
• Recorded Detail


  The degree of sharpness in an object's borders and structural details, also called sharpness of detail, definition, resolution, or degree of noise
• Spatial resolution


  • The ability to image small objects that have high subject contrast (e.g. bone-soft tissue interface, calcified lung nodules)
  • Determined by focal-spot size and other factors that contribute to blur
• Contrast resolution


  • The ability to image two separate objects and visually distinguish one from the other
• Factors affecting recorded detail


  • Geometric unsharpness
  • Motion unsharpness (blurring)
  • Intensifying Screens
  • Film Speed / Composition
  • Film – Screen contact
  • kVp & mAs (density / visibility)
• Main factors affecting recorded detail


  • kVp & mAs
  • Motion
  • Object Unsharpness
  • Focal Spot Size
  • SID (Source to Image Distance)
  • OID (Object to Image Distance)
  • Material Unsharpness
• Geometric qualities


  Detail, Distortion, Magnification
• Resolution test tools


  Line pairs/mm - depicts how well you can see the differences in structures, more lines = more detail
• Motion


  Can be voluntary or involuntary, best controlled by short exposure times, use of careful instructions to the patient, suspension of patient respiration, immobilization devices
• Decreasing motion unsharpness


  • Instruct patient not to move or breath
  • Use immobilization devices
  • Use short exposure times
  • Lock equipment in place
• SID (Source to Image Distance)


  The greater the distance between the source of the x-ray (tube) and the image receptor (cassette), the greater the image sharpness, standard distance = 40 in. most exams, exception = Chest radiography 72 in.
• Penumbra


  Fuzziness that obscures the true border (umbra) of an object, farther the x-ray source from the object the sharper the borders
• OID (Object to Image Distance)


  The closer the object to the film, the sharper the detail, structures located deep in the body require positioning to get the object closest to the film
• Focal spot size


  Determined by filament in cathode and surface area used at anode, smaller focal spot produces sharper image
• Distortion


  Misrepresentation of the true size or shape of an object, includes magnification (size distortion) and true distortion (shape distortion)
• Magnification


  Occurs when the tube is close to the part (small SID) and the part is far from the cassette (large OID)
• The best image is produced with a small OID and a large SID
• Size distortion

  Affected by SID and OID, as SID increases magnification increases, standardized SIDs allow radiologists to assume certain magnification factors
• Shape distortion

  Misrepresentation of the shape of an object, controlled by alignment of the beam, part, and image receptor, influenced by central ray angulation and body part rotation
• Central ray


  Radiation beam diverges from the tube in a pyramid shape, photons in the center travel along a straight line, photons along the periphery travel at an angle, when central ray is angled the image shape is distorted
• Central ray angulation


  Used to demonstrate certain details that can be hidden by superimposed body parts, body part rotation or obliquing can also help visualize superimposed anatomy
• Main factors affecting recorded detail


  • kVp & mAs
  • Motion
  • Object Unsharpness
  • Focal Spot Size
  • SID (Source to Image Distance)
  • OID (Object to Image Distance)
  • Material Unsharpness/ Film Screen Combo
• Factors affecting mAs


  • Patient factors: size of pt., density of tissue, pt. compliance
  • kVp
  • Distance
  • Grids
  • Film/Screen Combinations
  • Processing
• Focal spot size


  Smaller x-ray beam width will produce a sharper image, fine detail requires small focal spot, general radiography uses large focal spot
• Intensifying screens


  Equipment used that can contribute to image unsharpness, fast film/screen combinations decrease image sharpness, slower film/screen combinations increase image sharpness
• Cassettes with intensifying screens


  The cassette holds the film in a light tight container and consists of front and back intensifying screens
• Intensifying screens


  Lower patient dose (less photons needed), changes resolution of image, slow screens less light = better detail, faster screens less detail (more blurring on edges)
• Phosphors in intensifying screens


  Calcium Tungstate emits blue to purple light, Rare Earth emits green & ultraviolet light
• Tight contact between film and intensifying screens is needed to prevent gaps that can cause more light to reach the film and reduce image detail
• Film characteristics


  Film contains silver halide crystals in 2 layers - emulsion and base, emulsion thickness determines speed and resolution, larger/thicker crystals are faster but less detailed, finer/thinner crystals are slower but more detailed