Scatter and Grids

• Remnant x-rays are those that?


  exit the patient
• Which factors affect scatter radiation production?


  kVp, field size, patient thickness
• As kVp increases, scatter radiation will


  increase because of more compton interaction, contrast will decrease
• Approximately what % of x-rays are transmitted through the patient?


  1%
• At high kVp, most x-rays are 


  transmitted through the body WITH interaction
• Increasing kVp with compensating mAs what will be reduced?


  Patient dose
• Increase in field size will increase?


  scatter radiation
• Devices to reduce level of scatter radiation that reach image receptor:

  • beam restrictor
  • collimater
  • compression device
  • low kVp (not encouraged)
  • prone position
• What is primary radiation?

  x-rays that pass through patient without interacting
• Radiation that exits the grid and makes it to IR to form image?


  Image forming radiation
• What is primary effect on reducing patient dose?


  Proper collimation
• What is simplest form of all beam restricting devices?

  Aperture Diaphragm
• Cones and Cylinders: disadvantages

  Have fixed openings, no light field, patient positioning is critical due to small field size
• Variable Aperture AKA collimater is:


  Most commonly used light localizing beam and what has an exact center indicator
• Collimation will decrease IR exposure if not compensated for so to MAINTAIN you increase the mAs
• Poor collimation results in increased IR exp, patient exp, own exp, and makes image more difficult to interpret
• What is located at top of collimater and reduce off focus radiation?


  Entrance/ first stage shutters
• X-rays produced at areas on target outside the focal spot?

  off focus radiation- can reduce by placing PBL device close as possible to target or window
• How many lead leaf shutters are there and what is their purpose?


  4 individual with 2 sets of 2 and they are primary radiation barriers and there is longitudinal and transverse pairs
• To project light towards mirror, mirror is placed below x-ray tube port at 45 degree angle to light source
• PBL: positive beam limiting devices
• How much PBL must be accurate to the SID?

  2%
• When field size is reduced, in order to keep equal IR exposure the mAs must be increased
• Filtration removes low energy, long wavelengths from useful beam and main purpose is to reduce patient dose
• What is usually made of aluminum but uses copper to add more filtration?


  Aluminum filters
• 2 types of filtration:

  1. Inherent-located in glass envelope, .5MM and filters emerging useful xray beam
  2. Added- b/w housing and collimater at the port or window and attenuates higher percentage of low energy rays, 2MM but above 70kVp use 2.5MM
• Compensating filters: make remnant beam more uniform in intensity and even out IR exposure
• What is a grid made up of?

  strip :radiopaque material (LEAD)

  interspace: radiolucent material (Plastic fiber, aluminum or copper)
• Primary beam xrays incident on interspace and scatter will or will not be absorbed depending on the angle
• Grid ratio formula is height of grid/ width of interspace (h/D)
• What are effects of high grid ratio?

  Increased patient dose, cleanup, contrast, grid factor

  Decreased positioning latitude, IR exposure
• What is grid frequency?


  the number of lead strips or grid lines per cm or in (greater than 150 to be used with CR)
• What will be the most efficient at removing scatter?

  High grid ratio, high lead content and lower frequency
• The amount of scatter pickup (grid efficiency) will increase as ratio (affects efficiency the most) increases and frequency decreases.
• Aluminum in a grid increases absorption of primary photons at low kVp which results in higher patient dose (20%)
• What is used to measure grid performance?


  contrast improvement factor, bucky factor, and selectivity (lead content)
• Define: Contrast improvement factor (K)

  the ratio contrast of image taken with grid to contrast of image taken without a grid
• An increase in grid ratio , increase contrast improvement
• primary radiation is going to have more energy than scatter radiation
• What are the types of grids?

  linear parallel, crossed, linear focused, moving and stationary