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CT Scanner Shielding Guidelines

STATIONARY CT ROOMS

 

Computed tomography rooms typically have high workloads and high kilovoltage technique settings. As a result, at least 1/16-inch lead shielding or equivalent is required for the walls, doors, floors, ceilings, and operator's barrier. The concrete equivalence of 1/16-inch thick lead would be about 4 to 6 inches of standard-density concrete (147 pounds per cubic foot).  CT rooms with high workloads and with fully occupied uncontrolled space directly adjacent to the scanner may need shielding that is thicker than 1/16-inch lead or 4 to 6 inches of concrete to meet the recommended NCRP Report #147 shielding design goal of 0.02 mGy per week (1 mGy per year) for persons in uncontrolled areas. Facilities with CT x-ray equipment may also want to enlist the services of a qualified medical physicist for shielding advice.  In terms of ceiling and floor shielding, for example, the facility's physicist or consulting physicist may submit for our review facility-specific calculations for the actual thickness of concrete that will be needed, but for most facilities that thickness would typically range between 4 and 6 inches of standard concrete, depending on workload and distance factors.

 

Some CT rooms are designed to have a shielded door between the control room and the scanner room.  If a CT room has such a door and if the design is such that a CT machine operator is likely to leave the control room door open during patient scans to save time and effort in reaching the patient, this can potentially result in secondary radiation striking unshielded areas beyond the open door.  In such cases, consideration should be given to either of the following alternatives:

  • Shielding the area beyond the door to protect the area that could be exposed to secondary radiation if the control room door were to be left open, or
  • Redesigning the room.

 

CT SCANNERS IN VEHICLES

 

New vehicles

or

Existing vehicles with new or replacement CT scanners:

 

At least 1/16‑inch lead shielding or equivalent is required for the walls and doors of the CT enclosure, including the operator's barrier and lead-glass viewing window.  This shielding is required to extend from the floor to a height of at least 7 feet above the floor of the CT enclosure (or to the ceiling of the enclosure if the ceiling is less than 7 feet above the floor).  Shielding is required to be lapped along corners and any pull-out sections.  At least a 2-foot-wide perimeter of the floor of the CT enclosure is required to be shielded with 1/16‑inch lead, with the perimeter lead lapped with wall shielding.

 

A site plan is also needed for each location of use to ensure that the vehicle is not used in a location with nearby uncontrolled occupancy above a level protected by the shielding in the walls of the vehicle.  If the distance from a mobile scanner to an unshielded area above the height of the wall shielding is within 100 feet, the dose to the uncontrolled area should be evaluated.

 

Existing vehicles that have been serving Michigan facilities with registered older CT scanners:

 

Such vehicles might have initially been less adequately designed, perhaps with thinner lead shielding in some areas, exterior wall shielding extending to a lower height than specified above, or less perimeter shielding on the floor.  Shielding in such vehicles needs to be updated to meet the above requirements if the CT scanner is replaced.  Furthermore, operator's barrier and viewing window shielding need to meet the 1/16-inch lead equivalency requirements noted above for new vehicles regardless of the design date.

 

A site plan is also needed for each location of use to ensure that the vehicle is not used in a location with nearby uncontrolled occupancy above a level protected by the shielding in the walls of the vehicle.  If the distance from a mobile scanner to an unshielded area above the height of the wall shielding is within 100 feet, the dose to the uncontrolled area should be evaluated.

 

Existing vehicles and scanners that are new to Michigan:

 

Such vehicles need to be designed and shielded to meet the requirements above for new vehicles.

 

August 30, 2005



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