Principles of Imaging

• Major controller of image resolution

  Focal spot size
• Controlled by line focus principle

  Focal spot size
• formula of focal spot blur
• Used for imaging microcalcifications

  Microfocus tube
• share area or area of complete shadows and receives no photons

  Umbra
• imperfect, unsharp shadow surrounding the umbra and receives more photons

  Penumbra
• Important factor in determining spatial resolution

  Penumbra
• Relationship of focal spot size and penumbra

  Directly proportional
• Angling the target

  focus line principle
• To improve resolution

  Minimize effective focal spot
• To absorb the heat

  Maximize actual focal spot
• Consequence of focus line principle

  anode heel effect
• Used for magnification radiography

  Small focal spot
• When does blooming occur

  with large mA
• magnified image

  short sid, long oid
• sharpest image detail

  long sid, short oid
• where is the reference point

  central ray
• penumbra is greater than effective focal spot

  near the source
• penumbra is less than the effective focal spot

  away from the source
• resolving power: raiographic film

  100lp/mm
• resolving power: very fast screen


  7lp/mm
• resolving power: unaided eye

  10lp/mm
• resolving power: fine detail screen

  15lp/mm
• resolving power: non-screen/direct contact


  50lp/mm