Quantifying Copper Thickness Equivalence to Regulatory and Anatomical Phantoms In Fluoroscopy
Abstract
Purpose
To determine an equivalent copper (Cu) thickness that reproduces the Automatic Exposure Control (AEC) technique of fluoroscopy units relative to two reference phantoms: a standard size patient equivalent Lucite phantom (regulatory phantom) and an anatomical pelvis phantom (Manufacturer: RSD Phantoms, Model: RS-113).
Methods
Air kerma rates from fluoroscopy equipment, including mobile C-arms and interventional radiology (IR) units, were measured using both the regulatory and anatomical phantoms to establish baseline values. Thin copper sheets were stacked in 0.1-mm increments, from 3.2 mm to 4.0 mm total thickness, and exposures were made at each increment. For each phantom and copper thickness, the system-displayed air kerma rate (AKR; mGy/min), tube voltage (kVp), and tube current (mA) were recorded. Measurements were repeated three times and averaged. Deviation of copper-filtered measurements from phantom results was quantified using percent error to assess the suitability of copper as a phantom substitute.
Results
When comparing to the regulatory phantom, the best copper thickness equivalent is 3.5 mm Cu for the GE OEC 3D mobile C-arm fluoroscopy unit and 3.6 mm Cu for the Philips Allura Xper IR fluoroscopy unit. The best match for the anatomical phantom is 3.9 mm Cu for the GE OEC 3D mobile C-arm.
Conclusion
A copper thickness of 3.5 mm provides a suitable equivalent phantom for replicating the AEC technique relative to the regulatory phantom for both GE mobile C-arm fluoroscopy units and Philips IR fluoroscopy units. For the anatomical phantom, 3.9 mm Cu most closely reproduces AEC technique for GE mobile C-arms. Because prefabricated copper sheets are commonly available in 0.5-mm increments, 4.0 mm Cu represents a practical and cost-effective alternative, with only a modest increase in deviation (4.6% versus 1.5%).