Equipment changes and magnetic resonance imaging compatibility

Abstract

We wish to highlight the wider consequences caused by a simple equipment change made in our hospital. A patient developed severe back pain following the removal of an epidural catheter. We duly requested an urgent magnetic resonance imaging (MRI) scan to exclude an epidural haematoma. Of note, there was a large wound extending close to the pelvis that had been closed using metal skin staples. A new brand of skin staples (Autosuture Appose™ ULC 35W; Covidien (UK) Commercial Ltd, Gosport, Hampshire, UK) had been introduced to our hospital without communication with the anaesthetic or radiology departments. The equipment information leaflet contained no information on MRI compatibility. We contacted the manufacturer and were told that the staples were composed of steel, and categorically not MRI compatible. This uncertainty resulted in a delay to the imaging procedure and potential haematoma evacuation. The radiologist had to make a clinical decision, balancing the risk of missing an epidural haematoma against potential complications relating to the metal staples. The scan was performed without incident. Ferromagnetic implants are subject to a number of forces in the presence of strong magnetic fields. On the periphery of the field, they are attracted towards the centre and may be displaced in that direction. At the centre of the field, implants are subject to torque and may rotate, loosening them [1]. Implants may also be heated, in extreme cases causing burns [2]. These problems are magnified as the magnetic field strength increases. Current guidelines from the Medicines and Healthcare products Regulatory Agency (MHRA) recommend that medical implants be classed as MR safe, MR conditional or MR unsafe in accordance with international labelling and summarised in the AAGBI guidelines in 2010 [3, 4]. The MR conditional label means that an implant is safe within a specified magnetic field strength, amongst other factors. The MHRA recommends that if there is any doubt, assume that the implant is MR unsafe [3, 5]. We later discovered (via the Internet) a manufacturer’s document stating that the staples were MR conditional to a magnetic field strength of 3 Tesla [6]. We communicated this information to the company which finally established that the implants were, after all, MR conditional. Increasingly, it has been shown that skin staples may be safe within the MR scanner, though this may depend upon the field strength of the magnet, amongst other factors. As scanning equipment increases in strength, implants deemed MR safe under prior testing regimens may become less so [3].