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SU‐FF‐I‐61: Gypsum Barrier Shielding Estimates for Two Cone‐Beam CT Systems Dedicated to Head Imaging
Author(s) -
Larson S,
Goodsitt M,
Christodoulou E
Publication year - 2009
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.3181181
Subject(s) - electromagnetic shielding , gypsum , beam (structure) , imaging phantom , transmission (telecommunications) , materials science , cone beam computed tomography , optics , radiation protection , nuclear medicine , physics , computed tomography , medicine , composite material , computer science , radiology , telecommunications
Purpose: Cone‐beam CT is becoming increasingly popular for bone imaging of the head. Shielding practices for CT defined in NCRP Report No. 147 do not directly apply to a cone‐beam system, but standard methods for radiographic systems are also inadequate for the highly filtered x‐ray beams in cone‐beam CT systems. This study evaluates the use of published 120kVp CT transmission curves for gypsum [Simpkin 1990] to estimate required shielding for two cone‐beam CT systems at different energies in comparison to measured transmission data. Method and Materials: The head CTDI phantom was used with an E‐Woo Technologies Master3DS dental CT at 90kVp and a Xoran Technologies MiniCAT at 120kVp to produce scattered radiation. Transmission of scattered radiation through two 3‐foot by 3‐foot sheets of standard 5/8‐inch (16mm) gypsum wallboard was measured with an 1800 cc ionization chamber. Results: For the Master3DS, transmission through 2 layers (32mm) of gypsum wallboard was 0.30. The estimated shielding from the 120kVp transmission curves for a 1968 mAs/week workload and 2 mR/week permitted exposure at 2.09m distance was 32mm gypsum. The corresponding exposure determined with the measured transmission was 1.3 mR/week, indicating a calculated shielding over‐estimate of 22mm of gypsum. For the MiniCAT, transmission through 32mm of gypsum wallboard was 0.51. The estimated shielding from the 120kVp transmission curves for a 960 mAs/week workload and 2 mR/week permitted exposure at 1.46m distance was 32mm gypsum. The corresponding exposure determined with the measured transmission was 2.2 mR/week, indicating a calculated shielding under‐estimate of 4 mm gypsum. Conclusion: The transmission curve data for gypsum at 120kVp overestimates the shielding requirement for this 90kVp cone‐beam system, but slightly underestimates the shielding requirement for the 120kVp system. However, since lead is more likely to be necessary to shield the 120kVp system, a minor underestimate for gypsum may be insignificant.