CT Room Shielding Requirements: A SimpleGuide

CT scanners use X-rays to create detailed pictures of the body. They may perform many scans each day. Because of this, a CT room needs a shielding design that is based on the scanner and the way the room will be used. There is no single lead thickness that works for every CT room.

How is CT shielding different?

A CT scanner sends a narrow X-ray beam through the patient toward a detector. The detector and scanner gantry intercept the main beam. Because of this, NCRP Report No. 147 says the radiation that reaches the room barriers is secondary radiation rather than the direct primary beam.

Most of this radiation is scatter from the patient and scanner area. That is different from a normal X-ray room where the primary beam may be directed toward a wall or floor.

Does that mean CT rooms need less shielding?

Not necessarily. CT scanners can have a high workload. NCRP 147 notes that CT operating potentials and workloads can be much higher than those for general radiography or fluoroscopy. The report describes typical CT operating potentials in the range of about 80 to 140 kVp. A busy CT scanner can produce a large amount of scattered radiation over a week. That is why CT walls, floors, and ceilings need to be evaluated carefully.

What affects CT shielding?

The amount of shielding needed depends on several things:

• CT scanner manufacturer and model
• Number of patients
• Types of CT exams
• Scanner workload
• Scanner location in the room
• Gantry orientation
• Distance to nearby areas
• How nearby rooms are used
• Existing walls and concrete floors

The same scanner can need different shielding in two different buildings.

Why does scanner location matter?

Radiation gets lower as you move farther away from its source. This means the distance between the CT scanner and a nearby office or hallway matters. Moving the scanner within the room can sometimes change the shielding requirement. That is why the physicist needs the planned scanner location, not just the size of the CT room.

Why does gantry direction matter?

CT scatter is not exactly the same in every direction. NCRP 147 notes that radiation levels can be lower in the direction of the gantry than along the direction of the patient table. For its shielding method, however, the report uses a conservative model that assumes the scattered radiation spreads more evenly.

This is important when an old scanner is replaced. If the new CT scanner faces a different direction, the shielding should be checked again.

Do CT walls need lead?

Sometimes. The answer comes from the shielding calculation. Possible shielding materials include lead, gypsum wallboard, concrete, lead glass, lead acrylic, and other suitable building materials. An existing concrete wall or floor may already provide useful shielding. The physicist should evaluate the actual construction before specifying additional lead.

What about the CT control room?

The CT technologist normally works behind a protected control barrier. The control wall and viewing window should provide enough shielding for the technologist's work area. The technologist should also be able to see and communicate with the patient. The required lead equivalence of the wall and window should be shown clearly on the shielding plans.

Does the CT door need lead?

It may. If the shielding calculation shows that protection is needed at the doorway, the design may call for a lead-lined door or another suitable barrier. The door, frame, and surrounding wall should work together as one shielding system. The shielding should not stop at the wall and leave an unprotected opening.

What about the floor and ceiling?

These are very important in CT shielding. NCRP 147 specifically calls attention to CT floors, walls, and ceilings because CT can produce a large amount of secondary radiation. This matters when the CT room has occupied space above or below it. For example, there may be offices, exam rooms, waiting areas, patient rooms, or other clinical spaces.

Concrete floor slabs often provide a lot of radiation shielding, but the thickness and type of concrete should be known.

What shielding goals are used?

NCRP 147 recommends different design goals for different areas. For controlled radiation work areas, the report uses 0.1 mGy per week. For uncontrolled areas used by patients, visitors, the public, or workers who do not routinely work with radiation, it uses 0.02 mGy per week. These values help the physicist calculate how much protection each barrier needs.

Should an old CT room be checked before installing a new scanner?

Yes. Do not assume that a room is safe for a new scanner just because a CT scanner was already there. The new scanner may have a different location, a different orientation, a different workload, different operating characteristics, or more patients. NCRP 147 specifically notes that scanner orientation is an important issue when equipment is replaced.

Checking the shielding before installation can prevent costly changes later.

When should CT shielding be designed?

The best time is before final construction drawings are complete. The physicist should have the floor plan, CT model, scanner position, nearby room uses, and expected workload. Then the shielding requirements can be added to the plans before walls, doors, and windows are ordered.

There is no standard CT lead thickness

A CT room does not automatically need 1/16-inch lead, 1/8-inch lead, or any other standard thickness. The right amount depends on the actual project. A project-specific shielding calculation helps protect workers and the public while avoiding unnecessary shielding costs.

Need a CT shielding design? Heartland Physics provides CT shielding calculations for new installations, renovations, and scanner replacements. Send us the floor plan, CT manufacturer and model, expected workload, and project location to get started.