Room: Exhibit Hall
Purpose: Report No. 147 of the National Council on Radiation Protection and Measurements (NCRP-147) provides detailed and useful information for planning and calculating x-ray barrier shielding for imaging facilities. CTs, however, now have larger scan coverage per gantry rotation and dose optimization capability through automatic exposure control. Total scan times have been reduced, which also allows for increased patient throughput. A retrospective study was conducted in an outpatient clinic to determine how large coverage CTs with modern imaging protocols affect CT workload and scatter.
Methods: DICOM metadata was compiled from all CTs of an outpatient clinic, two GE Discovery CT750 HD scanners and one Siemens SOMATOM Force. CTDIvol and DLP were averaged per procedure. Calculated scanner specific scatter fractions (Îº) for the FDA phantoms as a function of collimation and tube voltage compared to NCRP-147 values. The CTs used to obtain values of Îº are a 750HD, Force and GE Revolution. Effects from dual energy CT (DECT), used for many patients, was considered, also.
Results: The weekly workload per outpatient clinic scanner was 36-mGy (59%) less than if were derived from NCRP DLP parameters. About less than 17% of weekly exams were acquired using DECT, for which the average DLP for body exams is approximately 286-mGy-cm. Dose to a given body region of equivalent length per exam is significantly lower than the values given in NCRP-147. The average Îº differed by 9% compared to NCRP-147 and the Îº-body from DECTs were 15% less on all scanners compared to NCRP-147.
Conclusion: The results indicate that the DLP values used in NCRP-147 for techniques are conservatively high per scanned length, as expected, due to use of current modulation and iterative reconstruction in modern CTs. The scatter fraction per centimeter given in NCRP-147 appears to be a good estimate for any scanner collimation.