Reduction of the E field at the tip of implanted wires generated by pTx coils using RF current measurements
Gerd Weidemann1, Frank Seifert1, and Bernd Ittermann1

1Physikalisch-Technische Bundesanstalt (PTB), Braunschweig and Berlin, Germany

### Synopsis

The possibility to reduce implant heating is an added value option of parallel transmission. An orthogonal-projection method (OPM) is presented to reduce the E fields at the tip of wire type implants by using voltage vectors orthogonal to the vector inducing the worst case RF current at the protruding end of the implant. Experiments confirm that the minimization of RF current at the protruding end leads to a distinct reduction of the electric field at the tip of the wire. Low-hazard steering conditions for n-element pTx coils can be determined in real time during an MR investigation from the measurement of only n complex valued RF currents at the protruding end of the implant.

### Purpose

The possibility to reduce implant heating is an added value option of parallel transmission (pTx). An approach to control the heating of wire-type implants by using pTx voltage vectors from a subspace orthogonal to the worst case steering vector was suggested and its virtue demonstrated for a 7T pTx coil using FDTD and thermal simulations [1]. Now, the value of this concept for minimizing the RF current in a wire using in-situ RF current measurements is tested. Instead of determining modes with low coupling to the implant [2] the worst case current is determined experimentally.

### Methods

An insulated copper wire (diameter 2 mm) was mounted in the head section of an ASTM like body phantom filled with tissue equivalent liquid based on TWEEN 20 (ε = 62.7 and σ = 0.81 S/m, measured at 128 MHz) with the tip (10 mm without insulation) pointing towards the central axis of the phantom (Fig 1). The head section of the phantom with the wire was placed inside an 8-channel 3T pTx coil (Rapid Biomedical). Calibrated RF current measurements were performed by a home-built Rogowski coil inside a toroidal shield with a slit which ideally is only penetrated by the magnetic field from the RF current in the wire [3]. A time domain electro-optic transducer (PH-0655, Seiko Giken) and a 10 m optical fiber to the controller of the OEFS sensor system outside the RF cabinet was used for readout. The signal was recorded with a high speed PCIe transient recorder card (M3i.4142, Spektrum) in a Linux host computer triggered by the MR sequence. The E$\space$field at the wire tip was measured using a 3-axis time domain sensor (OEFS–S1B, Seikoh Giken) together with the aforementioned system.

First, the complex valued current induced by each coil element for equal transmit voltage amplitude (UTx$\space$=$\space$20$\space$V) and equal phase was measured (Fig 2). Weighting the voltage amplitudes with the ratio of the induced currents and setting the phases for coherent superposition (Fig 3) yields the maximum current for a given input power, i.e. the worst case. Currents in the implant can effectively be suppressed by an orthogonal-projection method (OPM), i.e. by using only voltage vectors orthogonal to the worst case vector [1]. By projecting, e.g. the CP mode vector onto the seven-dimensional orthogonal subspace (Fig. 3) good image quality can be combined with low induced currents [1]. For both the worst case and the OPM-optimized vector an axial profile of the E field is measured in the vicinity of the wire tip in the central tube of the phantom (i.e. in a distance of 18 mm).

### Results ans Discussion

The worst case voltage vector generates the maximum current per transmitter forward power of 19.3$\space$mA/(1$\space$Wfwd)0.5, about twice as much current as the regular CP mode (8.9$\space$mA/(1$\space$Wfwd)0.5). The OPM-optimized voltage vector reduces the normalized current at the protruding end of the implant to 0.4$\space$mA/(1$\space$Wfwd)0.5, i.e. by a factor > 20 compared to the CP mode. This is achieved without much loss in image quality [1]. In spite of this reduction at the protruding end non-zero currents elsewhere along the wire are in principle possible. It was thus measured in how far the reduced current leads to a reduced E$\space$field at the tip of the wire (Fig 4). For the worst case condition the electric field is dominated by the component Ey parallel to the bent wire tip. This is no longer true for the suppressed current. The OPM optimization reduces the total electric field by a factor of 2 and the current generated Ey component by a factor of 3, thus reducing local heating by almost an order of magnitude.

### Conclusion

The minimization of the RF current at the protruding end of the wire by the described OPM method causes a distinct reduction of the electric field at the tip of the wire while maintaining good image quality. Consequently, the method can be used to determine low-hazard steering conditions for a pTx coil in real time during an MR investigation of patients with protruding wire type implants as neurostimulator leads, catheters or guide wires. For an n-element coil this requires only n complex valued RF current measurements at the protruding end of the implant but no simulations or model assumptions.

### Acknowledgements

This work was funded by European Metrology Research Program (EMRP) grant HLT06. The EMRP is jointly funded by the EMRP participating countries within EURAMET and the European Union.

### References

[1] Seifert F, Weidemann G, Ittermann B "Q matrix approach to control implant heating by transmit array coils" Proc. ISMRM 23(2015)3212.

[2] Etezadi-Amoli M, Stang P, Kerr A, Pauly J, Scott G "Controlling radiofrequency-induced currents in guidewires using parallel transmit" (2014) MRM doi:10.1002/mrm.25543.

[3] Weidemann G, Seifert F, Hoffmann W, Ittermann B "RF current measurements in implanted wires in phantoms by fiber optic current clamps" Proc. ISMRM 23(2015)1865.

### Figures

Figure$\space$1 Lateral (top) and front view (middle) of the experiment. Phantom, pTx coil and sensor on the protruding wire are placed together on the patient bed of the 3T MR scanner.

Bottom: Sketch of lateral and top view of experiment geometry (dimensions in mm). Arrows mark wire tips and senor.

Figure 2 Amplitude (bottom) and phase (top) of the RF current in the implant when the elements of the pTx coil are driven with equal phase and amplitude.

Steering voltage phases for a constructive superposition of currents are derived from the current phases.

Figure 3 Steering voltage amplitudes (left) for the worst case (red) as derived from the measured RF currents (Fig. 2). The OPM-optimized solution (green) was calculated by projecting the CP-mode voltage vector onto the orthogonal subspace of the worst case vector. Right: Phases for both voltage vectors.

Figure 4 E field components in a coordinate system with Ey parallel to the wire tip for the worst case steering condition (full squares) and the voltage vector orthogonal to worst case voltage vector with suppressed RF current (hollow diamonds). The lines are just meant to guide the eye.

Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)
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