Influence of electrical properties of lead insulation on radio frequency induced heating during MRI
Mikhail Kozlov1,2 and Gregor Schaefers1,3

1MR:comp GmbH, Gelsenkirchen, Germany, 2Max Planck Institute for Human Cognitive and Brain Sciences, Leipzig, Germany, 3Magnetic Resonance Institute for Safety, Technology and Research GmbH, Gelsenkirchen, Germany

Synopsis

We evaluated the dependence of RF-induced power deposited at a hot spot (p) on insulating electrical properties for insulated stainless steel wires of 1.5 mm in diameter with insulation thickness of 0.5 mm. Lead transfer functions (TF) were obtained by 3-D electromagnetic simulations. TF and p depended significantly on electrical properties of insulation. Increased insulator conductivity resulted in decreased p. For all insulated wires investigated non-uniform RF excitation resulted in higher power deposition than uniform RF excitation.

Introduction

It was experimentally discovered that addition of conductive material to lead insulation resulted in a reduction of radio frequency (RF) induced heating at the lead tip (hot spot) up to 60% [1]. In that experiment the leads were placed in a rectangular phantom so that the tangential electric field along the lead pathway (Etan(z)) varied significantly. ISO/TS 10974 Tier 3 [2] requires the assessment of power deposited at a hot spot (p). Tier 3 separates analysis of clinically relevant Etan(z) from calculations of the lead responses to these Etan(z) using a lead (implant) model expressed as: $$$p = A \times\ |\int_{0}^{L}S(z)\cdot E_{tan}(z)\cdot dz|^2 $$$, where A is the calibration factor, L is lead length, complex S(z) is the transfer function (TF). TF for an insulated lead was published in [3] but electrical properties of the insulator were not provided. If |Etan(z)| is constant the largest p (p_WC_uf) is generated, if phase distribution φ(Etan(z))=-φ(S(z)) [4]. However clinically relevant Etan(z) can be non-uniform. Thus p_WC_uf cannot be considered as the worst-case power deposition. Our goals in this study were: 1) to perform electromagnetic simulation of insulated leads, the insulation electrical properties of which varied, and to calculate p (EM_p); 2) to numerically obtain the lead models; 3) to calculate the largest p for uniform and non-uniform RF excitations using the lead models.

Method

The leads were insulated stainless steel wires of L={40:800:10} mm, ∅1.5 mm with insulation thickness of 0.5 mm. The wire was capped at one end and had a 10 mm long bare segment at the other end. A sensitivity study of insulator electrical properties was performed for a parameter matrix formed by εr={2.7,4}, and σ={0.007,0.000024} S/m. S(z) was calculated using the reciprocity approach [5]. The calibration factor was calculated from EM_p obtained for the leads excited from one side by an ideal 64 MHz uniform source. Hot spot integration volume (Fig. 1a) enveloped an area where the power deposition decayed more than 30 dB. For quantitative comparison all results were rescaled to Etan(z) equal to 1 V/m (peak-to-peak). Non-uniform linearly decayed Etan(z)=(-2/L)×z+2. p_WC_lr was calculated for φ(Etan(z))=-φ(S(z)).

Results and discussion

Two highest VLD areas were observed: one near the lead tip and one near the insulator end face (Fig. 1b). They are considered as one hot spot because one continuous area was visible from the temperature rise (Trise) profile (Fig. 1c). For L=400 mm there is significant discrepancy between the TF previously published [2] and TF calculated in our study (Fig. 2a). Taking into account TF for bare rod [3], it seems that the piece-wise excitation approach used in [2] is not robust at locations close to the lead tip. TF, EM_p (Fig. 3a), and p_WC_uf (Fig. 3b) depended significantly on electrical properties of insulation. The lead length, which resulted in the highest EM_p for plane wave excitation, was essentially shorter for εr=4. However for σ=0.000024 S/m, p_WC_ufr=4) was smaller than p_WC_ufr=2.7) only for L={360:600} mm and {760:800} mm. For a lead with L>200 mm located symmetrically about the Z axis in the ASTM phantom, p and Trise are essentially smaller than respective values for plane wave excitation due to: a) Etan(z) being essentially reduced at the lead ends in comparison with the value at the center, b) S(z) being significantly reduced along the lead pathway. For all L investigated non-uniform excitation resulted in higher power deposition than uniform excitation (Fig. 4a). The ratio p_WC_lr/p_WC_uf depended on lead length and the insulator conductivity (Fig. 4c). Thus even for electrically short implants, worst case analysis based on results obtained only for uniform Etan(z) can lead to significant underestimation. Increased insulator conductivity from σ=0.000024 S/m to σ=0.007 S/m resulted in significant reduction of EM_p for L<500 mm, p_WC_uf and p_WC_lr for all L.

Conclusion

Hot spot counting based on VLD or SAR data could result in an improper number of hot spots because thermal heating could merge several separated high-value VLD areas into one thermal hot spot. Electrical properties of lead insulator and clinically relevant Etan(z) are important initial implant data for RF-induced heating analysis. However, the specific results of this study, e.g., increased insulator conductivity resulted in decreased power deposition, cannot be simply extrapolated to other leads. A further extensive analysis should be conducted to cover different lead constructions.

Acknowledgements

This work was supported by the German Federal Ministry of Education and Research (BMBF) and within the European Joint Undertakings ENIAC JU, grant # 16ES0028, DeNeCoR.

References

[1] P. Nordbeck, et al. MRM 68:1963–1972 (2012).

[2] Technical specification ISO/TS 10974 1st edition 2012.

[3] E. Zastrow, et al. EMC 2014, Tokio.

[4] S-M. Park et al. JMRI, 26(5), 1278–1285. 2007.

[5] Shi Feng et al. MTT, Vol.63,No.1,305-313,2015.

Figures

Figure 1. a) Lead tip with hot spot integration volume in grey. b) Zoomed VLD profile in linear scale. c) Trise after 360s.

Figure 2. a) and b) TF amplitude for lead length of 400 mm; c) TF phase for lead length of 400 mm.

Figure 3. a) EM_p for plane wave excitation. b) and c) p_WC_uf for φ(Etan(z)) = - φ(S(z)).

Figure 4. Results for non-uniform Etan(z). a) and b) p_WC_lr for φ(Etan(z)) = - φ(S(z)). c) ratio p_WC_lr / p_WC_uf



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