POI

• Factors affecting detail and distortion

  • Motion
  • Focal spot size
  • Object-film distance
  • Focal-film distance
  • Intensifying screen speed
  • Non-screen holder
  • Film screen contact
  • Target-object-film alignment

  
• Focal spot size


  • Major controller of image resolution
  • Does not affect x-ray quantity or quality
  • Controlled by the line focus principle
  • Controls penumbra

  
• Umbra


  The distinctly sharp area or area of complete shadow that receives essentially no photons
• Penumbra


  The imperfect, unsharp shadow surrounding the umbra that receives more photons, an important factor for determining spatial resolution

  
• Line focus principle


  1. Angling the target to minimize effective focal spot and improve resolution

  2. Maximize actual focal spot to absorb the heat, resulting in anode heel effect

  
• Large focal spot size

  • Used for general imaging of thick or dense body parts
  • Provides for a shorter exposure time to minimize motion blur
  • More capacity to produce x-rays
  • Higher anode heat capacity

  
• Small focal spot size


  • Used for fine-detail imaging of thin or less dense body parts
  • Used for extremity radiography
  • Less capacity to produce x-rays
  • Always used for magnification radiography
  • Lower anode heat capacity

  
• Large focal spot blooms more at higher mA because the incident electron beam is not as easily focused by the focusing cup
• Magnification


  The ratio of image size to object size, directly proportional to OID and inversely proportional to SID

  
• Tube-object-film alignment


  If the arrowhead is near the source, the size of penumbra is greater than the effective focal spot size. If the arrowhead is away from the source, the size of penumbra is less than the effective focal spot size.

  
• Image receptor factors

  • Intensifying screen speed
  • Non-screen holder
  • Film screen contact
  • Target-object-film alignment

  
• Film-screen combination


  Mostly classified by speed, which is inversely proportional to image resolution
• Resolving power: radiographic film 100 lp/mm, very fast screen 7 lp/mm, fine-detail screen 15 lp/mm, non-screen/direct exposure film 50 lp/mm, unaided eye 10 lp/mm
• Intensifying screen speed


  • Very fast
  • Ultra high or hi-plus
  • High or fast
  • Medium, par or standard
  • Detail, slow, or high resolution
  • Ultra-detail
• Slow screen


  For examinations requiring higher resolution, such as extremity examinations, resulting in higher patient dose
• Fast screen


  Used to reduce patient dose for examinations that do not require higher resolution
• Intensifying screen factors


  Phosphor size, phosphor thickness, and phosphor concentration are inversely proportional to resolution
• Film-screen contact


  The intensifying screen and film are sandwiched together, and poor contact can result in darker areas of blurring, increased penumbra, and poor detail
• Wire-mesh test is used to check for screen-film contact, and poor contact can be addressed by repairing or discarding the cassette
• Types of motion blur

  • Voluntary motion under the direct control of the patient
  • Involuntary motion not under the direct control of the patient
  • Equipment motion that is difficult to detect

  
• Motion blur


  Blurring of an image that results from movement of the patient, x-ray tube, or equipment during exposure, leading to loss of radiographic quality and repeated radiographs

  
• Motion reduction


  1. Carefully instructing the patient

  2. Immobilization

  3. Short exposure time

  4. Large SID and small OID

  5. Low ripple power

  6. High-speed image receptor

  
• Moving grid mechanism is an auxiliary equipment that causes the table or restraining devices to move
• Factors that improve resolution

  • Reduce focal spot size
  • Reduce OID
  • Increase SID
  • Eliminate motion
  • Reduce intensifying screen phosphor size and concentration

  
• Size distortion


  Misrepresentation of the size of structures being examined, generally a matter of magnification
• Shape distortion


  Misrepresentation of the shape of structures being examined, more difficult to assess than size distortion
• Elongation


  Projects the object so it appears longer than it really is, caused by poor alignment of the tube or image receptor
• Foreshortening


  Projects the object so it appears shorter than it really is, caused by poor alignment of the anatomical part
• Factors affecting shape distortion

  1. Alignment of central ray, anatomical part, and image receptor

  2. Angulation of the tube

  3. Direction of tube angulation

  
• Numerous radiographic projections utilize angulation to avoid superimposition of parts
• Tube angulation changes the SID, which will produce changes in magnification unless compensated for
• Topics


  • Brightness digital display/density (film)
  • Exposure to image receptor
  • Calculations for receptor exposure-maintenance

  
• Calculations for receptor exposure-maintenance

  • Reciprocity law
  • 5 and 15 percent rule
  • Grid/bucky factor
  • Speed class
  • SID
• Tube angle


  The direction of the tube angle must also be reversed to maintain the relationship
• Tube angle


  • 25° cephalad for an AP projection is identical to 25° caudad for a PA projection
• Degree


  A method of describing the exact amount of angulation, usually stated as the angle between the central ray and the image receptor plane from the standard reference point of perpendicularity

  
• Standard reference point: 90° from the patient's head, radiographic angles must be added or subtracted from that point

  
• Degree


  • 5° cephalad is 5° from perpendicular, as is 5° caudad
• It is important to maintain the correct degree of angle specified for a given procedure
• Tube angulations


  Change SID, which will produce changes in magnification