Principles of Exposure and Image Quality

• The prime factors of radiographic exposure:

  • Milliamperes (mA)
  • Exposure time (seconds)
  • Kilovolts (kVp)
  • Source-Image receptor distance (SID)
• Milliamperes (mA)

  • controls radiographic density
  • controls quantity of x-rays produced
  • controlled by adjusting the mA
  • quantity of exposure is directly proportional to mA
• Exposure Time (Seconds)

  • controls radiographic density
  • controls quantity of x-rays produced
  • controlled by adjusting the timer in x-ray circuit
  • controls duration of exposure
  • quantity of exposure is directly proportional to exposure time
• Kilovolts (kVp)

  • controls radiographic contrast
  • controls x-ray penetration
  • controls the quantity and quality of the x-ray beam
  • increased kVp results in increased quantity of photos and penetration of the body part
• Source-Image Receptory Distance (SID)

  • affects the density and intensity of the x-ray beam
  • quantity of exposure is inversely proportional to the square of the distance
• The distance between the tube target and the IR is the source-image receptor distance (SID)
• Density refers to the overall blackness or darkness of the radiographic image
• Density affects the visibility of detail
• An image that is too dark is said to be over-exposed
• An image that is said to be too light is under-exposed
• Density is primarily controlled by mAs, although kVp and SID do have an influence
• Contrast is the difference in radiographic density between adjacent portions of the image
• Contrast is what makes the anatomy more visible
• Contrast is primarily controlled by kVp
• Low kVp produces a high contrast radiograph
• High kVp produces a low contrast radiograph
• Subject contrast is the variation in beam intensity after it passes through the patient
• Radiographic contrast is the combination of IR and subject contrast
• Fog from scatter radiation or image processing can reduce contrast
• Optimal contrast may be high or low depending on the composition of the body part
• Low contrast is the lesser difference between densities
• High contrast is the greater difference between densities
• Size distortion is always in the form of magnification enlargment
• Shape distortion is the result of unequal magnification of the actual shape of the structure
• Object-image receptor distance (OID) is the distance between the subject/object and the IR
• Size distortion is affected by SID and OID
• The least shape distortion occurs when the plane of the subject is parallel to the plane of the IR and CR is perpendicular to both
• Angulation of the x-ray beam produces shape distortion
• Foreshortening projects the part so it appears shorter than it really is
• Elongation projects the object so it appears longer than it really is
• Spatial resolution refers to the sharpness of the image; resolution, sharpness, definition, detail
• Key factors that affect spatial resolution include patient movement, OID, SID, and the focal spot
• Umbra is the actual anatomic area, body part, or structure shown in the radiographic image
• Penumbra describes the "unsharp edges" of the umbra or body part
• The goal in radiographic imaging is to reduce the penumbra as much as possible
• Quantum mottle is used to describe the situation in which a grainy or mottled (spotty) image is created
• Quantum mottle occurs when the imaging system does not record the anatomic densities, usually because of the lack of photons
• Quantum mottle will occur when either the mAs or the kVp is set too low
• High contrast is best used for extremities
• High contrast is also called short-scale contrast because the range of densities is short