Z-axis over-beaming

• Due to finite size of focal spot, multi-slice beams are wider than the nominal value

• Usual beam width ~2-3 mm wider

  • e.g. 4 x 2.5 mm slice, 12.5 mm beam

• More significant for narrow beams

  • 10 mm beam ~ 25% wider
  • 5 mm beam ~50% wider than nominal

• Less significant as no of slices increases

  • Wider collimations routinely used