European Journal of Radiology 71 (2009) 527–535 Osteoid osteoma in atypical locations: The added value of dynamic gadolinium-enhanced MR imaging Virna Zampa ∗ , Irene Bargellini, Simona Ortori, Lorenzo Faggioni, Roberto Cioni, Carlo Bartolozzi Department of Diagnostic and Interventional Radiology, University of Pisa, Via Roma 56, 56126 Pisa, Italy Received 15 February 2008; accepted 21 May 2008 Abstract Purpose: To compare the results of dynamic gadolinium-enhanced magnetic resonance imaging (MRI), unenhanced MRI and computed tomog- raphy (CT), in terms of nidus conspicuity and diagnostic confidence of osteoid osteoma in atypical sites. Materials and methods: CT and MR (nonenhanced T1- and T2-weighted and dynamic MRI) images of 19 patients with histologically proven osteoid osteoma located in atypical sites were retrospectively reviewed. Time–enhancement curves of the nidus and the adjacent bone marrow were generated. Images from each technique were scored for nidus conspicuity by two independent radiologists. Another blinded radiologist was asked to assess final diagnosis of the bone lesion on MR and CT images, independently. Results: In all cases, nidus contrast uptake started in the arterial phase and was higher compared to the surrounding bone marrow. Dynamic MRI significantly increased nidus conspicuity compared to nonenhanced MRI (P < .0001) and CT (P = .04). In 6/19 (31.6%) cases nidus conspicuity was higher at dynamic MRI compared to CT. Confident diagnosis of osteoid osteoma was achieved in all patients with MRI and in 10/19 (52.6%) patients with CT. Conclusion: In patients with osteoid osteoma located in atypical sites, dynamic MRI increases nidus conspicuity, allowing confident diagnosis. © 2008 Elsevier Ireland Ltd. All rights reserved. Keywords: Magnetic resonance imaging; Contrast enhancement; Osteoid osteoma; Computed tomography; Comparative study 1. Introduction Osteoid osteoma is a small benign osteogenic tumor consist- ing of a central core of vascular osteoid tissue and a peripheral zone of sclerotic bone. It occurs mainly in children and young adults [1], and it is characterized by typical nocturnal pain, responding to nonsteroidal anti-inflammatory drugs. Because of its peculiar radiological appearance and typical clinical signs, the diagnosis of cortical osteoid osteoma does not usually represent a diagnostic problem. Computed tomography (CT) is considered the best imaging modality, because it is usually able to clearly identify the radi- olucent cortical nidus, which is sometimes calcified, surrounded by medullary sclerosis and periosteal reaction [2–9]. However, these typical features can vary according to the site. In particular, when the lesion is not cortical, medullary sclerosis and periosteal reaction can be absent [10]. ∗ Corresponding author. Tel.: +39 050 992509; fax: +39 050 551461. E-mail address: virnazampa@hotmail.com (V. Zampa). The role of magnetic resonance imaging (MRI) in the diag- nosis of osteoid osteoma is still controversial. In fact, MRI has been reported to be a misleading technique, because exten- sive edema and soft tissue reaction can hide the nidus and cause a falsely aggressive appearance of this benign lesion [3,11]. To our knowledge, only one study has been published on the use of dynamic gadolinium-enhanced MRI in the diagnosis of osteoid osteoma, reporting excellent results in terms of increased nidus conspicuity, thanks to the hypervascularity of the lesion [9]. Purpose of our retrospective study was to evaluate the results of dynamic MRI, compared with unenhanced MRI and CT, in terms of nidus conspicuity and diagnostic confidence, in a series of 19 patients with histologically proven osteoid osteoma, located in atypical sites. 2. Materials and methods We retrospectively reviewed images and records of 19 con- secutive patients with histologically proven osteoid osteoma, 0720-048X/$ – see front matter © 2008 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.ejrad.2008.05.010