Aiping Yao^{1,2}, Earl Zastrow^{1}, and Niels Kuster^{1,2}

Determining the local tissue temperature rise caused by the RF-induced deposition of an active implantable medical device (AIMD) requires a conversion between localized power deposition to temperature (p2∆T). We investigate the quasi-static limit by which both the distributions of power and temperature are assumed to depend only on the electrode geometry (when electrically small) and independent of the current distribution along the AIMD’s wire conductor. The results confirm that p2∆T conversion can be derived without the knowledge of incident conditions to the AIMD and complete geometry of the AIMD. The relationship between p2∆T and the electrode physical geometries is summarized.

Computational
electromagnetic (EM) and thermal (TH) simulations (S4L) are used to calculate
the spatial distributions of power deposition and temperature changes in the
vicinity of the electrode. The AIMD is embedded in an unbounded homogeneous
high-permitivity medium (HPM) defined in ISO/TS 10974 (σ= 0.47 S/m, εr= 78, K=
0.62 W/m/k, Cp= 4200 J/kg/K) No thermal perfusion and thermal conduction
through the AIMD conductor and insulation was assumed. The conversion factor
between power deposition and the maximum local temperature rise (p2∆T_{max}) is calculated from the ratio of the peak-spatial
∆T and the local deposited power in the vicinity of the electrode. The power deposition is calculated by
integrating the volume enclosing the –30 dB contour of the distribution
obtained from the EM simulations and peak-spatial ∆T is obtained
from the TH simulations at 1200 s.
The p2∆T_{max} conversion factors for AIMD samples of different electrode geometries and
lead physical geometries are derived. Figure 1 shows the AIMD parameters
considered in this work. We divided this work into two parts, as described
below:

Study 1: The
p2∆T_{max} is derived
for different electrode geometry, T = 2 – 10 mm. Two families of AIMD: (a, c) = (0.1 mm, 0.75
mm) and (0.5 mm, 0.75 mm) are considered. The quasi-static limit is
investigated by varying the conductor length of each AIMD family from L = 75 –
500 mm.

Study 2: The p2∆T_{max}
is derived for smallest electrode geometry, T = 2 mm. The dependent of p2∆T_{max} on both conductor and insulation
geometries are investigated from 15 families of AIMD with (a, c) = (0.5 mm, [0.75,
1.0, 1.5, 2.0, 2.5] mm); (0.6 mm, [0.9, 1.2, 1.8, 2.4,
3.0] mm); (0.8 mm, [1.2, 1.6, 2.4, 3.2, 4.0] mm) are considered. The
conductor length of each AIMD family is fixed to L = 500 mm.

1. Maurits K.K, Lambertus W.B, Henk F..M.S, Chris J.G.B. Heating around intravascular guidewires by resonating RF waves. JMRI, 2000;12:79–85.

2. ISO/TS 10974, Assessment of the safety of magnetic resonance imaging for patients with an active implantable medical device, (Draft), 2016.

3.Yeungr C.J, Susil R.C, Atalar E. RF Safety of Wires in Interventional MRI Using a Safety Index. MRM, 2002;47:187-193.

Figure 1: Generic AIMD in
ambient medium, parameters of AIMDs used in different study groups.

Figure 2. Results from Study
1: (a) p2∆T_{max} of each electrode
(T = 2, 4, 6, 8, 10 mm) as a function of AIMD length, L. (b) Deviation of p2∆T_{max}
from the values at the longest AIMD length, depicted as a function of L/T
(AIMD to electrode length ratio). Empirical limit of the quasi-static regime is
L/T >= 25.

Figure 3. Results from Study 2:
p2∆T_{max} as a
function of insulation-conductor thickness ratio, c/a, for 15 AIMDs with different
(a, c) combinations. All samples have L=500 and T=2 mm. The p2∆T_{max} approaches an asymptotic value when c/a approaches
∞.