Potential health risk from local tissue heating caused by implant-RF interactions during Magnetic Resonance Imaging (MRI) is a well-recognized subject in MRI safety assessments studies. The ASTM-2182¹ and ISO/TS 10974² are the current guiding documents for the assessment of RF-heating risk; both limit the exposure conditions for safety assessment to a nominal circular polarization (CP) RF fields from 1.5T or 3T cylindrical bore MR systems. Our recent study³ has shown that RF-heating from medical implants may be mitigated in a controllable manner through careful adjustment of RF exposure, if some clinically specific parameters are known a priori and feasibility was demonstrated for a limited set of parameters at 64 MHz. We have extended our evaluation to 128 MHz and the coverage of possible clinical scenarios by increasing the permutations of RF-coil, patient’s anatomy, and imaging positions. The results of the study may be used to determine whether a reduced access to a priori knowledge can be tolerated and to what extent the sacrifice in heating-mitigation performance shall be.

Three generic 2-channel birdcage coils, operating at 128 MHz (Fig 1, top-right); two virtual patients; and four imaging positions marked (a) – (d) (Fig 1, left) are considered. Inclusion of a 400-mm generic leaded-cardiac implant, similar to that described in previous work³, is assumed in both patients. Ten unique implant routings with proximal termination in the left side of the chest and the electrode terminated inside the heart, are considered (Fig 1, bottom-right).

The convex optimization strategies described in previous work³, based on a common formulation for RF shimming^4-6 with a constraint based on the piece-wise excitation model of implant-induced RF-heating⁷, are used. The piece-wise excitation model of the considered implant at 128 MHz is illustrated in Fig 2.

The coefficient of variation of |B₁⁺| (B₁⁺_,cov) over a region of interest (RoI) and the estimated local power deposited in tissue by the implant (P_ind) are evaluated and used as figures of merit for comparison. The RoI is a rectangular cuboid centered at the isocenter of the RF coil with dimensions of 300 mm (coronal) x 300 mm (sagittal) x L/2 mm (axial), where L is the length of the RF coil.

B₁⁺_,cov and P_ind are evaluated under the RF-exposure conditions, defined by the driving vectors of the two-port RF coils obtained from the following methods:

(1) Unconstrained optimization: based on common method used in RF-shimming^4-6. The solution provides an exposure with the best homogeneity of |B₁⁺|.

(2) |B₁⁺| optimization with implant-induced heating null constraint: a null-constraint on the induced electric field at the electrode is introduced. A straightforward matrix solution provides an exposure with the lowest implant-induced heating.

(3) |B₁⁺| optimization with implant-induced heating constraint: the constraint is adjusted to compromise between implant-induced deposited power and |B₁⁺| homogeneity. The problem is solved with routines from CVXOPT package⁸.

(4) CP exposure: the implant-induced heating and |B₁⁺| homogeneity under nominal exposure condition considered by safety standards.

(5) Maximum implant-heating exposure: a generalized eigenvalue problem. The solution provides an exposure with the largest implant-induced deposited power.

All results are normalized to the first-level controlled operating mode of the MRI⁹. Fig 3 shows the averaged P_ind over 10 unique routings (<P_ind>) and B₁⁺_,cov, obtained from the methods summarized in (1) – (5). The results for both patients are illustrated together for each imaging position. The polarizations of the driving vectors obtained from (1) – (5) are illustrated top-right of each figure. As expected, (1) provides the best B₁⁺_,cov whereas (2) provides the lowest <P_ind> but with the trade-off on reduced |B₁⁺| homogeneity, for all imaging positions; and the upper bound for <P_ind> is established by the maximum implant-heating exposure. For all imaging positions, (3) successfully established solutions that provide lower B₁⁺_,cov and <P_ind> than CP, and <P_ind> can be reduced by 2 – 8 dB while maintaining similar B₁⁺_,cov as CP-exposure, depending on the imaging scenarios.

It is evident from Fig 4 that different RF coil lengths do not cause large variation of <P_ind> and the variation introduced by the different anatomy is much more dramatic in comparison. It is also worth noting that CP-exposure gives a fairly good approximation of maximum implant heating (within 1dB of difference).

The results show that a reduction of a priori information is possible and some common parameters describing the exposure conditions (e.g. RF coil length) may be assumed. The permutation of patient’s body type must be increased to determine the extent of the variation introduced by the different anatomy and other implant locations must be considered.