A contribution to MRI safety testing related to gradient-induced heating of medical devices

Purpose: To investigate theoretically the feasibility of a novel procedure for testing the MRI gradient-induced heating of medical devices and translating the results into clinical practice. Methods: The concept of index of stress was introduced, by decoupling the time waveform characteristics of the gradient field signals from the field spatial distribution within an MRI scanner. This index was also extended to consider the anisotropy of complex bulky metallic implants. This approach was applied on virtual models to put in evidence merits and drawbacks of the procedure, without introducing uncertainties due to the experiments. The results obtained using realistic orthopaedic implants placed within an ASTM phantom where compared with simulations performed with two anatomical body models (a man and a female) after applying a realistic surgery to implant the devices. Results: The analysis allowed to define regions of exclusion within the MRI bore, useful to determine if the imaging can be performed with the patient carrying a specific implant, previously characterized with standardized laboratory tests. It also proved that the power dissipated into the device by the eddy currents represents a good dosimetric quantity to estimate the maximum temperature increase in the tissues surrounding the implant. This result opens the route to adopt a laboratory characterization of the implants based on electrical measurements, rather than thermal measurements. Conclusion: Possible contributions to the future extension of the standards for testing the gradient-induced heating of medical devices have been proposed. The next step will be to implement this methodology into practical procedures. Final published version: http://doi.org/10.1002/mrm.2923