Medical implants represent a 3 billion € market in the EU. Approximately 50 million EU citizens carry a medical implant and a majority of these will need a magnetic resonance imaging (MRI) scan during the lifetime of their device. However, the powerful magnetic field of MRI systems still represents a unique safety hazard for these patients. Therefore, it is vital for both patient safety and the success of a medical implant on the market, that implant manufactures can demonstrate safety compliance in an MRI environment. This project will improve the competitiveness of European implant manufacturers by providing innovative, metrologically sound and legally safe methods to demonstrate the compatibility of their products with MRI safety regulations.
With more than 30 million scans per year in EURAMET countries, MRI safety is an ongoing concern. Carriers of medical implants, making up almost 10 % of the EU population, are particularly at risk as fatal accidents have occurred due to the interference of the device with the electromagnetic fields (EMF) from the scanner. As the majority of these patients will need an MRI scan at one point in their lives, MRI compatibility of an implant is a key factor for the competitiveness of a manufacturer. This was exemplified in 2011 when Medtronic Inc. achieved the first ever MRI approval for a cardiac pacemaker and within a few years virtually all noncompatible devices disappeared from the market.
The currently applied procedures to demonstrate MRI compatibility are either outdated (ASTM F2182) or incomplete (ISO/IEC TS10974). The applications of those are costly and lengthy since a mistake can be fatal
for both patient and manufacturer. While large producers of high-end active implantable medical devices are facing technological challenges to demonstrate MRI compatibility, SMEs manufacturing passive medical
implants are overburdened by the necessity to demonstrate MRI safety for each new size and shape of a particular device, therefore limiting their innovation potential.
Numerical modelling of field distributions in human subjects is an established state of the art technique. However, even though this technique has been used to include the presence of metallic implants there were limitations as mostly generic or simplified implants with non-detailed features were modelled and there was limited experimental verification of the results.
Parallel-transmit (pTx) radiofrequency systems can be used in MRI scanners to steer, within certain limits, the electromagnetic field as well as temperature distributions in and around the implant. The use of pTx for risk mitigation has enormous potential to ensure safety for a wide range of different implants and boundary conditions, and these systems can be combined with sensor-equipped implants to provide real-time feedback. However, currently such pTx methodology is still in early development and further work is needed to prove its use.
In the EMRP project HLT06 MRI safety it was discovered that heating of metallic implants due to switched magnetic-field gradients is an underestimated hazard in MRI. Some normative documents ignore this effect
completely, whilst others mention the possibility of such effects only in the context of protecting the device rather than the patient. Recently, an experimental investigation of these effects hinted at the possibility that
the induced heating effect might not scale with the root-mean-square averaged field changes, which would be in contrast to the assumptions of existing standards on this subject. Therefore, more work is needed to
investigate the hazards associated with the interaction between bulk metallic implants and switched magnetic fields.
To enable manufacturers of medical implants to demonstrate that patients carrying their products can safely undergo an MRI scan, the project aims to achieve the following objectives:
- To develop anatomical models of human subjects with realistic medical implants and millimetre resolution. The models to be sufficiently detailed for use with in silico medicine concepts, with resolution to be determined according to image analysis needs.
- To develop validated computational tools for the numerical simulation of electromagnetic fields (EMF) and temperature distributions in a virtual human subject during MRI exposure. The computational tools should be able to process high-resolution anatomical models.
- To develop validated methods and sensor-equipped reference implants for quantifying real-time implant-induced hazards during MRI exposure. This should include an assessment of parallel transmit (pTx) radiofrequency (RF) systems in MRI with real-time feedback and the development of appropriate mitigation strategies.
- To investigate numerically and experimentally the hazards associated with the interaction between bulk metallic implants and switched magnetic fields in the kilohertz regime. In addition, to develop a reference set-up for testing metallic implant heating, using switched magnetic-field gradients of a few mT/m with a target gradient uncertainty below 5 %.
- To develop and apply a suitable statistical method to demonstrate MRI compliance for small (< 10 cm) orthopaedic implants without extensive testing or numerical modelling, by determining an upper limit for the hazard associated with the new implant by comparison with a similar surrogate implant, which has already been fully assessed, thus enabling small manufacturers of a large variety of similar small metallic implants to dramatically reduce their costs for compliance demonstration.
- To interact closely with manufacturers of implants, MRI and test equipment and with standards developing organisations (e.g. ISO/TS 10974, IEC TC/SC 62B and ASTM Subcommittee F04.15 on Material Test Methods) to align the project and facilitate the take up of the technology and measurement infrastructure developed in the project.
Progress beyond the state of the art
Safety testing of metallic implants for MRI is today largely based on unreliable phantom measurements. The project will improve this unsatisfactory state of the art by applying more meaningful numerical modelling of realistic virtual models. High precision numerical models will be developed and subsequently used as an input to electromagnetic field (EMF) simulations. While the utilisation of EMF modelling in this context is becoming more and more accepted as the adequate approach to the problem in the scientific community, it has still not fully arrived at the level of small manufacturers and test houses. Still unique to this project is the use of calibrated sensor measurements to validate those simulation results with a metrological rigour.
The project will go even further by exploring technical means to actively mitigate metallic-implant related hazards in MRI. This will be achieved by steering the RF electric field (E-field) away from the implant by using pTx technology, an approach with a high potential for ensuring safety for a wide range of implants and scan conditions. In contrast to some recently published work, where this E-field steering approach has been pursued using the regular RF transmit coil (“body coil”) driven as a two-channel coil, the present project aims to exploit the much larger parameter space of true parallel transmission with at least eight independent channels.
Compared to RF heating, the potential heating of an implant by the switched gradient fields of an MRI scanner is a largely unexplored field. While preliminary results convincingly demonstrated the existence of the problem, this project will be the first ever to systematically investigate this issue. Up to now, this young field is still driven by the work from the project consortium.
Finally, a procedure to demonstrate MRI safety compliance will be developed allowing the manufacturers of small implants, e.g. screws, clips, or fixation devices, to reduce their costs by simplified safety assessments. Instead of performing numerical simulations and heating experiments for each and every new item, as it is the present state of the art, simple scaling laws can be applied to infer a valid safety assessment for their new product from previous, extensive investigations of similar devices.