Original Research

Dual-energy index variation when evaluating the potential ferromagnetism of ex vivo bullets

Francois A. van der Merwe, Eugene Loggenberg
South African Journal of Radiology | Vol 27, No 1 | a2701 | DOI: https://doi.org/10.4102/sajr.v27i1.2701 | © 2023 Francois A. van der Merwe, Eugene Loggenberg | This work is licensed under CC Attribution 4.0
Submitted: 21 April 2023 | Published: 27 July 2023

About the author(s)

Francois A. van der Merwe, Department of Clinical Imaging Sciences, Faculty of Health Sciences, University of the Free State, Bloemfontein, South Africa
Eugene Loggenberg, Department of Clinical Imaging Sciences, Faculty of Health Sciences, University of the Free State, Bloemfontein, South Africa


Background: An MRI is potentially hazardous for patients with retained ferromagnetic bullets. Recent studies have aimed to develop dual-energy computed tomography (DECT) as a screening tool for recognising highly ferromagnetic bullets. Inconsistent findings have been ascribed to inherent CT technology differences. Previous research demonstrated significant Hounsfield unit (HU) measurement variation among single-source CT machines.

Objectives: This study investigated the theoretical dual-energy index (DEI) variation between DECT machines when evaluating the potential ferromagnetic properties within the same sample of ex vivo bullets and metal phantoms.

Method: An experimental ex vivo study was conducted on eight metal phantoms and 10 unused bullets individually positioned in the same Perspex head phantom and scanned on two DECT machines. Two senior radiology registrars independently recorded the HU readings, and DEI values were calculated. Statistical analysis was performed using non-parametric methods for paired data, namely the Signed Rank Test. The DEI values based on mean HU readings between the DECT machines were compared.

Results: Inter- and intra-reader agreement was not statistically significant. The metal phantoms had poor interscanner agreement, with an overlap of the ferromagnetic and non-ferromagnetic ranges. The bullets had good interscanner agreement, with a similar ferromagnetic to non-ferromagnetic relationship.

Conclusion: The use of DEI values negates the previous assumption that significant interscanner variability exists among different DECT technologies while assessing highly attenuative ex vivo bullets.

Contribution: This investigation demonstrated that even though HU readings may be variable, the implementation of the DEI equation translates this into comparable values with good interscanner agreement.


dual-energy computed tomography; ferromagnetic bullets; DECT machines; inter- and intra-reader agreement; MRI safety


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