Sci-Fri AM: Quality, Safety, and Professional Issues 03: The Increasing Costs of Shielding Diagnostic CT Scanners in Ontario




In Ontario, shielding for all X-ray machines, including CT scanners, must be evaluated according to Safety Code 20A (Health Canada, 1983) which is based on NCRP-49 (NCRP, 1976).

NCRP-147 (NCRP, 2004) is the international standard for shielding calculations of CT scanners and is also referenced in Safety Code 35 (Health Canada, 2008) which, was published to supersede SC20A. The goal of this work is to demonstrate the cost effectiveness of NCRP-147 for CT scanner shielding.


CT scanner shielding calculations are performed using SC20A and NCRP-147:

  • 1.A room located on the third floor with the nearest building 75m away
  • 2.A room with high occupancy uncontrolled adjacent spaces
  • 3.Two side by side rooms on the main floor


1. SC20A: The exterior windows required 0.1mm of Pb to protect the public who may occupy the building at 75m.

1. NCRP-147: No additional shielding required.

2. SC20A: Two walls adjacent to high occupancy uncontrolled space required an additional 1.58mm Pb.

2. NCRP-147: No additional shielding required.

3. SC20A: The entire floor and ceiling slabs in both rooms required an additional 0.79mm Pb. In addition, 0.79mm Pb was added to the walls from the ceiling to overlap the existing Pb shielding in the walls.

3. NCRP-147: No additional shielding required.


The application of NCRP Report No. 147 affords the required protection to staff and the public, in the true spirit of the ALARA principle, taking into account relevant social and economic factors.