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Thyroid Shields versus z-Axis Automatic Tube Current Modulation for Dose Reduction at Neck CT¹

¹ From the Department of Medical Imaging, Royal University Hospital and the University of Saskatchewan, 103 Hospital Dr, Saskatoon, SK, Canada S7N 0W8 (D.A.L., D.A.F.); and Saskatchewan Ministry of Advanced Education, Employment and Labour, Regina, Saskatchewan, Canada (M.M.H.) and Saskatoon, Saskatchewan, Canada (S.T.W.). Received August 10, 2007; revision requested October 16; revision received November 28; accepted January 28, 2008; final version accepted May 13. Supported in part by the Saskatoon Health Region and University of Saskatchewan Unified Department of Medical Imaging Continuing Research Fund. Address correspondence to D.A.L. (e-mail: david.leswick{at}saskatoonhealthregion.ca).

Purpose: To assess the effectiveness of in-plane bismuth thyroid shields and a z-axis automatic tube current modulation (ATCM) technique with respect to dose reduction and image noise in the thyroid and other regional tissues during neck computed tomography (CT).

Materials and Methods: Because this was a phantom study, neither institutional review board approval nor HIPAA compliance was required. A female phantom, thyroid shields, and an eight-section CT scanner were used. Radiation dose was measured by using thermoluminescent dosimetry (TLD) chips placed in the phantom's thyroid, cervical bone marrow, and soft tissues. Scans were performed by using fixed tube current and a z-axis ATCM technique with and without shields. Image noise was quantified as the standard deviation of the attenuation value (in Hounsfield units) on CT images.

Results: Thyroid dose was 76.9 mGy with an unshielded fixed tube current technique. Use of shields and ATCM reduced this dose to 44.7 mGy (42% reduction, P < .001) and 17.0 mGy (78% reduction, P < .001), respectively. The combination of shields and ATCM further reduced this dose to between 11.9 and 12.9 mGy (83%–85% reduction, P < .001), depending on the shield's presence in the scout scan. ATCM resulted in minimized tube current throughout the neck, which reduced dose-length product across the entire scan volume by 60% (926 vs 368 mGy · cm). Thyroid bed noise was lowest (3.9 HU) during the fixed tube current technique without shields, with noise slightly higher (6.9 HU) with the unshielded ATCM technique (P < .001). Thyroid image noise was markedly higher when using shields (P < .001), with noise measuring between 74 and 113 HU for fixed tube current and ATCM scans.

Conclusion: z-Axis ATCM is more effective than shields at reducing thyroid radiation dose during neck CT. Shields combined with ATCM slightly further reduces dose; however, this is associated with higher image noise.

© RSNA, 2008