Convex Optimization of MRI Exposure for Mitigation of RF-Heating from Active Medical Implants

Juan Córcoles, Earl Zastrow, and Niels Kuster, Physics in Medicine and Biology, Volume 60, Issue 18, pp. 7293–7308, September 2015, online September 8, 2015

Magnetic resonance imaging (MRI) is a commonly used diagnostic imaging modality. The non-invasive nature of MRI and the resulting quality of soft tissue images make it an attractive diagnostic imaging method. Patients implanted with medical devices such as pacemakers, deep-brain stimulators, spinal cord stimulators, or cochlear implants are generally excluded from MRI examination or are subjected to strict scanning conditions and exclusions depending on the approved conditions of the implants because the strong radiofrequency (RF) fields to which the patient is subjected during MRI can cause significant heating of tissues near the implant, posing a significant health risk. The degree of patient risk depends on various parameters: in the MRI scanner, the RF magnetic field strength and frequency and the coil design; the patient's anatomy, posture, and imaging position; the location and RF coupling efficiency of the implant; and the bio-physiological responses associated with the induced local heating. We present three constrained convex optimization strategies that incorporate the implant's RF-heating characteristics for the reduction of local heating of medical implants during MRI. In the study, we emphasize the complementary performances of the different formulations. The analysis demonstrates that RF-induced heating of elongated metallic medical implants can be carefully controlled and balanced against MRI quality. A reduction of heating of up to 25 dB can be achieved at the cost of reduced uniformity of less than 5% in the magnitude of the B1+ field. The current formulations incorporate a priori knowledge of the clinically specific parameters assumed to be available. Before these techniques can be applied practically in the broader clinical context, further investigations are needed to determine whether reduced access to a priori knowledge regarding, e.g., the patient's anatomy, implant routing, RF-transmitter, and RF-implant coupling, can be accepted within reasonable levels of uncertainty.

The scientific and technical impact of the study can be summarized as:

  • The general applicability of heat mitigation strategies that can implemented to ensure patient safety against RF-induced heating caused by active medical implants during MRI examination is demonstrated 
  • The mitigation strategies proposed are applicable to any implant configuration that may have non negligible RF-coupling and can be adjusted with respect to the relevant patient-specific parameters 
  • The implementation of heat mitigation strategies in MRI has the potential to make the procedure more widely available to patients with implants 
  • The mitigation strategies proposed are readily applicable to higher field strengths
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