Scattered radiation measurements and shielding verification

5.2.1. Scattered radiation measurements and shielding verification

5.2.1.1. Aim The aim of this test is to ensure that the radiation shielding specified

in the original design complies with the local regulatory CT shielding requirements.

5.2.1.2. Frequency Tests should be done at the time of acceptance, annually and at any time

when the technical factors (e.g., kVp and mAs) or the workload of the scanner may have changed.

5.2.1.3. Materials The materials required are professional reports [41–43], isodose maps

provided by the CT manufacturer, the design of the shielding, a 32 cm diameter CT dose phantom and a sensitive radiation detector (e.g., a survey meter and a large volume ionization chamber).

5.2.1.4. Procedure The amount of shielding material in the walls, floor and ceiling should be

determined from the original shielding design. The ideal approach is for the qualified medical physicist to monitor construction and to visually ensure that the appropriate lead shielding is applied to the specified walls, and that all seams and penetrations are appropriately shielded. Otherwise, it is necessary at acceptance testing to verify, by making physical measurements if possible, the presence of the appropriate thickness of shielding material [42, 43].

The CT dose phantom should be placed on the table and scanned with a high kVp–mAs technique. The integrity of the shielding over the surface of the walls, floor and ceiling should be determined while the scanner is producing radiation, using a sensitive radiation detector, with particular attention being paid to potential gaps in the shielding around shielding joints, electrical switches and junction boxes.

On the basis of the measured attenuation of the shielding material and other required parameters [42, 43], including the occupancy factors of adjacent areas, the effective dose to individuals should be determined under various On the basis of the measured attenuation of the shielding material and other required parameters [42, 43], including the occupancy factors of adjacent areas, the effective dose to individuals should be determined under various

The same procedure applies at acceptance testing, annually or when technical factors or workloads may have been changed. However, evaluation of the integrity of the shielding, i.e. looking for gaps in the shielding, is only necessary during acceptance testing, unless subsequent construction or physical changes have occurred in the radiation barriers.

The detailed procedures discussed in Refs [41–43] should be followed.

5.2.1.5. Analysis

A survey meter may be used to quickly scan for potential gaps in the radiation shielding. If any gaps are located then a large volume ionization chamber must be used to determine the actual amount of radiation leakage through this area.

The attenuation of the walls, floor and ceiling should be determined. These data are then used to determine the effective dose to radiation workers and members of the public in adjacent areas.

5.2.1.6. Suggested tolerances The effective doses determined on the basis of this procedure must not

exceed the maximum permissible doses to radiation workers or members of the public, as appropriate. Ideally, the effective doses should be limited to some fraction of the maximum permissible dose, for example, one half, based on local regulatory requirements.

5.2.1.7. Corrective action Any time that the effective doses, based upon measured attenuation of

the shielding and calculations taking into consideration the workload of the CT scanner, approach the maximum permissible doses for radiation workers or members of the public, consideration must be given to how to improve the radiation shielding of the CT scan room or of how to limit the dose to these individuals. Area monitoring using thermoluminescent dosimeters (TLDs) or other dosimeters can also be considered if there is concern amongst staff about the appropriate shielding levels.