(Dec 2012 – Jan 2015)

A common occurrence in hospital MR units is that patients who would benefit from diagnostic information acquired from an MR procedure have one or more metallic implants within their body. However, the MR environment may be unsafe for some of these patients because of mechanical forces on ferromagnetic materials due interaction with magnetic fields or excessive heating due to interaction with radiofrequency (RF) fields; in the absence of further information, safety currently requires that the presence of the implant within a patient is a contradiction for the MR procedure.  Although WP6 will provide knowledge applicable to all metallic implants, it will focus on RF and specific absorption rate (SAR) aspects regarding hip prostheses and thin wires.

The use of MRI in the management of patients with orthopaedic implants is increasing and metal artefact reduction (MAR) sequences are available to facilitate diagnosis of soft tissue problems adjacent to these implants. However these are often associated with increased SAR and there is currently little information in the literature describing quantitative effects in terms of B1 field distortion and increase in local SAR due to the presence of a metallic prosthesis. Small wire like structures are important as they are known to be capable of producing significant heating in the presence of a RF field and they are frequently used components of a variety of implants (e.g. the leads from inactive pacemakers, guide-wires or deep brain stimulators).

Although SAR is a useful surrogate to assess RF safety, the actual hazard is the combination of elevated temperature and its duration. Excessive tissue heating is a risk when metallic implants are exposed to MR transmit RF fields and therefore prediction of the spatial and temporal variations in temperature are important factors when assessing the safety of MR procedures involving patients with a metallic implant.

The workpackage is organised into 4 tasks covering the following aspects. Mathematical models involving anatomically realistic body models with hip prosthesis or wire implants and realistic transmit coils (for 1.5 and 3 T systems) will be used to study internal and external E- and B-fields and SAR in each case. The dependencies of parameters such as RF frequency, implant shape and composition, relative position of the implant with respect to the transmit coil, and body shape and size will be investigated. To help validate the results of simulations, procedures for in-situ measurements of RF currents using phantoms with implanted wires will be developed.  The time-averaged spatial distributions of SAR will be used as inputs to a thermal model to predict the spatial and temporal variations in the resulting temperature distributions within the body.