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Precision and accuracy of low-dose CT protocols in the evaluation of skull landmarks

¹ Neuroradiology Department, King's College Hospital, London, UK; ² Medical Engineering and Physics Department, King's College Hospital, London, UK

^* Correspondence to: Dr SEJ Connor, Neuroradiology Department, Ruskin Wing, King's College Hospital, Denmark Hill, London SES 9RS, UK; Email: sejconnor{at}tiscali.co.uk

Received 28 April 2006; revised 30 June 2006; accepted 3 July 2006

Objectives: It was proposed to design a series of low-dose three-dimensional (3D) CT protocols with an effective dose similar to that of the conventional radiographic series (0.011–0.032 mSv) used to assess craniofacial asymmetry. It was then aimed to assess the precision and accuracy for skull landmarks recorded ex vivo using these CT protocols.

Methods: Four ultra low-dose CT protocols (5 mAs, pitch 1.375/1.75; 10 mAs, pitch 1.375/1.75) were constructed on the basis of published data, dose calculations and measurements. A high-dose CT protocol was used as a reference standard examination. The protocols were used to scan a skull immersed in water. For each protocol, two observers evaluated 17 skull landmarks on two separate occasions. 3D and two-dimensional (2D) accuracy relative to the reference standard examination and inter- and intraobserver precision was calculated.

Results: The 2D accuracy was superior for the 10 mAs protocols and the 10 mAs/1.375 pitch protocol allowed one observer to achieve an accuracy of less than 2 mm for all landmarks. The 2D interrater precision for the 10 mAs/1.375 pitch protocol was superior to the other low-dose protocols and also the high-dose protocol, achieving precision less than 2 mm for 15/17 points. The 10 mAs protocols enabled greater 2D intrarater precision than the 5 mAs protocols for both observers.

Conclusions: Using the 10 mAs/1.375 pitch CT protocol, it was possible to achieve 2D accuracy of less than 2 mm for almost all skull landmarks with an interrater precision similar to a high-dose CT protocol. Although the estimated effective dose of 0.035 mSv was slightly greater than that of the conventional radiographic series (0.011–0.032 mSv), there was the additional benefit of a 3D data set. The 10 mAs protocols allowed accuracy and precision superior to that of the 5 mAs protocols and hence they are more likely to be clinically applicable.

Keywords: cephalometry; radiation dosage; tomography scanning, X-ray computed

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