RF safety of wires in interventional MRI: using a safety index

Abstract

With the rapid growth of interventional MRI, radiofrequency (RF) heating at the tips of guidewires, catheters, and other wireshaped devices has become an important safety issue. Previous studies have identified some of the variables that affect the relative magnitude of this heating but none could predict the absolute amount of heating to formulate safety margins. This study presents the first theoretical model of wire tip heating that can accurately predict its absolute value, assuming a straight wire, a homogeneous RF coil, and a wire that does not extend out of the tissue. The local specific absorption rate (SAR) amplification from induced currents on insulated and bare wires was calculated using the method of moments. This SAR gain was combined with a semianalytic solution to the bioheat transfer equation to generate a safety index. The safety index (°C/(W/kg)) is a measure of the in vivo temperature change that can occur with the wire in place, normalized to the SAR of the pulse sequence. This index can be used to set limits on the spatial peak SAR of pulse sequences that are used with the interventional wire. For the case of a straight resonant wire in a tissue with very low perfusion, only about 100 mW/kg/°C spatial peak SAR may be used at 1.5 T. But for ≤10-cm wires with an insulation thickness ≥30% of the wire radius that are placed in well-perfused tissues, normal operating conditions of 4 W/kg spatial peak SAR are possible at 1.5 T. Further model development to include the influence of inhomogeneous RF, curved wires, and wires that extend out of the sample are required to generate safety indices that are applicable to common clinical situations. We propose a simple way to ensure safety when using an interventional wire: set a limit on the SAR of allowable pulse sequences that is a factor of a safety index below the tolerable temperature increase. Magn Reson Med 47: 187-193, 2002.

Construction and modeling of a reconfigurable MRI coil for lowering SAR in patients with deep brain stimulation implants

“…This is in contrast with previous studies which used simple straight wires to model DBS leads and concluded that maximum power deposition always occurs at the tip of the wire [39-42]. Fig.9 shows the distribution of MaxSAR for a representative lead geometry at two different rotation angles (ID9: θ = 0°, θ = 90°).…”

Placement of deep brain stimulator electrodes using real‐time high‐field interventional magnetic resonance imaging

“…Thus, while SAR is an important measure that is readily available on MR scanners, SAR alone is likely insufficient for adequately assessing the risk of MR imaging of internalized conductive structures. Yeung et al (23) attempted to address permissible SAR levels by modeling clinical situations where internal conductors exist. They proposed that a “safety index” could be obtained for most situations whereby the permitted SAR level would be adjusted according to the potential for locally elevated SAR levels.…”

Interventional CMR: Clinical Applications and Future Directions

“…An alternative approach is to limit heating by limiting the energy delivered by the imaging pulse sequence [47]. The excitation flip angle relates to heating quadratically, and lengthening the repetition time reduces heating proportionally [48].…”