CT Envirnomental Surveys: Practical Considerations
G Barnes, R Al-Senan, Department of Radiology, Univ Alabama Birmingham, Birmingham, ALPO-BPC-Exhibit Hall-13 Saturday Room: Exhibit Hall
Purpose: In many states environmental surveys of new x-ray installations are required. The most critical in terms of radiation levels to occupationally exposed personnel and the general public are CT installations. Presented are a methodology for doing a thorough and time efficient CT environmental survey, and the results of a survey.
Methods: The CTDI body phantom is suspended on the CT gantry shroud with its bottom slightly above the table so that the phantom remains stationary as the table moves during an image acquisition. The phantom is suspended by two Lucite bars with adjustable brackets attached to the bottom of the phantom. On a given wall or barrier a fast response hand held scintillation meter is utilized to scan the barrier and determine the area where the highest radiation level occurs. The location is marked with tape. The long CT scan time permitted by suspending the phantom allows one to survey a large barrier area during one scan. Subsequently an exposure rate reading is made at the location with a sensitive ionization survey meter. The readings are normalized to estimate weekly radiation levels. Shielding design Workload was 6,400 mA-min/wk @ 140 kVp. Survey technique was 140 kVp, 100 mA and 20 seconds.
Results: The readings one foot from the walls, doors and control window ranged from 0.5 to 5.5 mR/hr. The associated normalized readings for uncontrolled areas ranged from 0.07 to 1.2 mR/wk. The controlled area normalized readings ranged from 1.5 to 4.5 mR/wk.
Conclusions: The CT environmental radiation survey was done in a relatively short time (less than two hours) and was done for a worst case scenario (i.e., body CTDI phantom, and a busy workload at 140 kVp); and for this worst case scenario the normalized readings are well below regulatory limits for both controlled and uncontrolled areas.