Ghost Sign on Diffusion-Weighted Imaging Generated Apparent Diffusion Coefficient Map: Additional MRI Diagnostic Marker for Extremity Osteomyelitis

<b>Objective</b> The aim of this study was to determine the sensitivity and specificity and inter-reader reliability of previously known "ghost sign" and "penumbra sign" on T1-weighted (T1W) imaging and "ghost sign" on apparent diffusion coefficient (ADC) map in osteomyelitis (OM) of the extremities. <b>Materials and Methods</b> In this cross-sectional retrospective study, two fellowship-trained musculoskeletal readers blinded to final diagnosis of OM versus no OM were asked to report the penumbra sign and ghost sign on T1W images and ghost sign on ADC map, as well as diagnosis of OM. Cohen's kappa was used. Diagnostic performance measures including sensitivity, specificity, and accuracy were calculated. <b>Results</b> A sample of 178 magnetic resonance imaging (MRI) scans of pathology-proven cases were included in this study, with 41 being positive for OM and 137 being negative for OM. There was a fair inter-reader agreement for imaging signs, and moderate agreement of 0.60 for OM. The sensitivities of the penumbra sign on T1W imaging, ghost sign on T1W imaging, and ghost sign on ADC map for OM are 3.7, 9.8, and 19.5%, respectively, while their respective specificities are 98.9, 97.8, and 94.5%, respectively. All three imaging signs showed a similar (good) accuracy of 76 to 78%. <b>Conclusion</b> The ghost sign on ADC can be used as an additional marker for OM and is a similarly highly specific but a more sensitive sign for OM than the conventionally used penumbra sign and ghost sign on T1W imaging. <b>Key Points</b> The ghost sign on ADC can be used as a helpful indicator of osteomyelitis.Across two fellowship-trained musculoskeletal readers, there was a fair inter-reader agreement for imaging signs and moderate agreement for OM.The ghost sign on ADC is a similarly highly specific but a more sensitive sign for osteomyelitis than the conventionally used penumbra sign and ghost sign on T1W imaging. All three imaging signs showed a similar (good) accuracy of 76 to 78%.