Synopsis

Interventional MRI using long conductive guidewires, risks high Local SAR and reduced image quality. This work expands on previous work relating current ‘nulling’ techniques to reducing worst case SAR, by investigating the importance of wire placement and current sensing planning. Results showed that two very distinct wires displayed common features but differences in behaviour do not allow for a generic ‘nulling’ plan. Also, there is a trade-off between removing current-generating modes, and reducing B₁⁺ efficiency that subsequently increases RF power to achieve necessary image quality. Overall, a careful study of the system can help find the optimal ‘nulling’ plan.

Introduction

Interventional MRI using conductive guidewires (hereafter called ‘wires’) risks high local SAR and poor image quality due to B1 enhancement. These originate from uncontrolled wire current. It has been demonstrated that currents can be minimised by ‘decoupling’ the wire from the transmit system using parallel transmit architecture.^1,2 Such methods rely on independent current measurements, with physical ‘sensors’¹ to make measurements independently of the MRI system. Sensor positioning affects how well this can work, as demonstrated previously.²

In this work EM simulations are used to further investigate the importance of sensor position and number while looking for common features between two distinct exemplar wire configurations.

Methods

Current ‘nulling’ starts by performing a singular value decomposition (SVD) on matrices built from current measurements; the singular vectors identify modes of the transmit coil that couple to the wire, and the singular values reflect their strength. Meanwhile, the maximum eigenvalue of a Q-matrix^3,4 can be used to reflect worst case SAR (wcSAR) at its location. We used EM simulations (Sim4Life) to explore these two methods.

Two 8-channel Tx/Rx cardiac RF coils were simulated: one short; one long. Both on a model (Duke - ITIS ViP3.1), at 64MHz, with a simplistic or a realistic wire inside, resulting in four scenarios (Fig.1). The wires (core:∅ = 1mm; insulating layer: 0.1mm thickness) had their distal 5mm stripped of insulation. The Huygens Box method, implemented in Sim4Life, was applied, to reduce computation time. Analysis was performed in MATLAB.

Eigenvalue decomposition of a Q-matrix near the wiretip was used to determine wcSAR before ‘nulling’. J was sampled along the wire to simulate current sensor readings. The true coupling modes were identified using SVD of the entire current distribution. The SVD was repeated using one, two and three ‘sensor’ measurements of J, ranging from 100mm outside the body to the wiretip. For each sensing configuration, the Q-matrix was projected onto the sub-space defined by excluding the strongest detected coupling modes. Eigenvalue decomposition of this projected Q-matrix, near the wiretip, quantifies the remaining wcSAR after ‘nulling’. Coil efficiency must also be taken into account to determine wcSAR under useful imaging conditions. For that, a magnitude least squares B₁⁺ shimming algorithm⁵ was applied to the heart, and all wcSAR values normalised to the square of the average shimmed B₁ ⁺ in that region.

Results & Discussion

Fig.2 illustrates the currents associated with each mode: all scenarios have a minimum of 3 modes generating significant wire-currents. The vertical bar on each figure quantifies the wcSAR when modes are removed: the top value is wcSAR of the entire system, the next one is wcSAR without the highest current mode, etc, clearly showing that coupling modes contribute unequally. If true modes are identified, removing the degrees of freedom associated with the highest or, in some cases, two highest modes would reliably reduce wcSAR.

In reality, these modes are estimated from a few point-wise measurements, meaning the identified modes won’t be the true modes that lead to optimal ‘nulling’. Fig.3 and 4 show reduction in wcSAR as a function of number of ‘sensors’, their locations and the number of degrees of freedom remaining. Despite the dominance of a single mode seen in Fig.2, removal of one degree of freedom cannot reliably reduce wcSAR except when the ‘sensors’ are close to the wiretip. Removal of two degrees of freedom, reduces wcSAR while providing good B₁⁺ efficiency, except for the short coil+simplistic wire scenario that only yields significant results when at least one ‘sensor’ is close to the wiretip. Finally, removing three degrees of freedom results in a reliable wcSAR reduction but it hinders B₁⁺ efficiency, resulting in a comparable, sometimes worse outcome under imaging conditions. Again, the short coil+simplistic wire is an exception: in the regions where removing two degrees of freedom wasn’t enough, removing three works best.

Conclusion

Acknowledgements

References

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