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Magnetic resonance imaging (MRI) scans may cause harmful tissue heating in patients with active and passive implants,¹ such as pacemakers, deep brain stimulators, stents and prosthetic implants, as a result of the applied RF magnetic field (B1) of 64 MHz at 1.5T and 128 MHz at 3T. Consequently, patients with implants are preventively excluded from receiving diagnostically valuable MRI scans.

Mechanism of RF Heating in MR

The two primary mechanisms for coupling the RF energy to the implant are 1) the B1 orthogonal to the implants forms a conductive loop and 2) the induced E-field components in the tissues are tangential to the implant path (without implant present) as a result of the B1 exposure of the patient. The collected RF power over the entire length of the implant may be deposited very locally, e.g. at the tip of implants. The heating depends on the following parameters:

• B1 rms
• body coil design
• position and posture of the patient in the MR scanner
• outer and inner anatomy of the patient
• implant trajectory
• implant length
• RF properties of the implant
• implant geometry at the location of maximum energy deposition
• 3-D distribution of the energy deposition
• thermal properties (heat capacity, conduction, perfusion, etc.) at the location of energy deposition

This list illustrates the complexity of analyzing worst-case heating even in a defined patient group independent of the scanner.

Objectives

Patients with active and passive implants will only be able to benefit from MRI diagnostics if the devices are inherently safe under worst-case conditions. This paper describes the possible methods and the available tools to demonstrate with known uncertainty that the induced heating under worst-case conditions is below the threshold of harmful thermal effects.