RF heating due to conductive wires during MRI depends on the phase distribution of the transmit field

Abstract

In many studies concerning wire heating during MR imaging, a "resonant wire length" that maximizes RF heating is determined. This may lead to the nonintuitive conclusion that adding more wire, so as to avoid this resonant length, will actually improve heating safety. Through a theoretical analysis using the method of moments, we show that this behavior depends on the phase distribution of the RF transmit field. If the RF transmit field has linear phase, with slope equal to the real part of the wavenumber in the tissue, long wires always heat more than short wires. In order to characterize the intrinsic safety of a device without reference to a specific body coil design, this maximum-tip heating phase distribution must be considered. Finally, adjusting the phase distribution of the electric field generated by an RF transmit coil may lead to an "implantfriendly" coil design.

MR imaging in assessing cardiovascular interventions and myocardial injury

“…Another limitation exists within the XMR suite; all ferro-magnetic cardiac monitoring equipment cannot be used in the MR suite. RF energy emitted by the scanner during image acquisition can be focused on long conductive structures (such as catheters and wires), producing localized heating causing skin burns and thrombi formation (119)(120)(121). The effect can be eliminated by breaking long conductive elements into multiple segments of shorter length (less than half the wavelength of the incident RF) (122).…”

Specific Absorption Rate Analysis of Heterogeneous Head Models with EEG Electrodes/Leads at 7T MRI

“…The interactions of the EEG leads and RF coils will induce changes in the EM field inside the head (i.e., "shielding effect" of the EEG leads) [Hamblin 2007] and local SAR enhancement at the interface between electrodes and skin [Armenean 2004, Yeung 2002 Table 1) determines an electric field enhancement at the interface between leads and head surface (i.e., epidermis). This observation is in line with theoretical models [Guy 1975] and physical evidence of reports of burns due to "antenna-effect" of leads [Dempsey 2001].…”

“…Capacitive coupling -represented by the conservative term of eq. (1) V  , occurs when the dimensions of the load (i.e., head with leads) are comparable with the incident wavelength, and the time-varying electric component is "picked up" by the load, acting as scattering antennas [Armenean 2004, Balanis 2005, Dempsey 2001, Yeung 2002. Hence, there is a difference of potential ( V  in eq.…”

Calculation of radiofrequency electromagnetic fields and their effects in MRI of human subjects

“…The use of numerical calculation methods to examine interactions of RF fields with implanted devices is increasing in recent years (30,81,82). Of particular interest is the high local heating expected at the tip of a wire in contact with or implanted into tissue (as for a pacemaker or deep brain stimulator) or conductive catheter (81,82). Efforts to produce MRI-compatible devices have led to increasingly sophisticated designs in these wires resulting in relatively complex geometries (83).…”