MRI safety of implants: transfer function determination from MR images

Abstract

MRI is often considered to be a risk for patients with medical implants, like pacemakers and deep brain stimulators. The heating around the electrodes of these type of implants that are in contact with human tissue to deliver therapeutic doses is one of the most notorious challenges. Unintendedly, the implants can act as antennas and pick up energy from the radiofrequency field that is necessary to produce MR images and deposit it locally causing focused temperature hot spots. This process is described by the so-called transfer function of an implant. It is challenging to get an accurate estimate of the heating around the electrodes because it depends on many factors. The design of the implant, the anatomy of the patient, the type of RF coil, the relative positions of the implant, the patient and the RF coil all influence the heating. As shown in this thesis MRI can be used to measure the transfer function of an implant and hence get an accurate description of its interaction with the radio frequency field in more realistic scenarios. Nowadays transfer functions can only be measured in phantoms in straight trajectories in dedicated measurements setups. Translating these transfer functions to in-vivo situations introduce currently unknown uncertainties which can be probed using the MRI methods described in this thesis. This provides options for implant manufacturers to describe their products better and has the potential to improve care for an increasing patient population.