Safety of MRI in patients with retained cardiac leads

Abstract

Purpose: To evaluate the safety of MRI in patients with fragmented retained leads (FRLs) through numerical simulation and phantom experiments.

Methods: Electromagnetic and thermal simulations were performed to determine the worst-case RF heating of 10 patient-derived FRL models during MRI at 1.5 T and 3 T and at imaging landmarks corresponding to head, chest, and abdomen. RF heating measurements were performed in phantoms implanted with reconstructed FRL models that produced highest heating in numerical simulations. The potential for unintended tissue stimulation was assessed through a conservative estimation of the electric field induced in the tissue due to gradient-induced voltages developed along the length of FRLs.

Results: In simulations under conservative approach, RF exposure at B₁⁺ ≤ 2 µT generated cumulative equivalent minutes (CEM)₄₃ < 40 at all imaging landmarks at both 1.5 T and 3 T, indicating no thermal damage for acquisition times (TAs) < 10 min. In experiments, the maximum temperature rise when FRLs were positioned at the location of maximum electric field exposure was measured to be 2.4°C at 3 T and 2.1°C at 1.5 T. Electric fields induced in the tissue due to gradient-induced voltages remained below the threshold for cardiac tissue stimulation in all cases.

Conclusions: Simulation and experimental results indicate that patients with FRLs can be scanned safely at both 1.5 T and 3 T with most clinical pulse sequences.

Keywords: RF heating; SAR; cardiovascular implantable electronic devices; finite element methods; fragmented retained leads; safety.

FIGURE 1

Steps of image segmentation and fragmented retained lead (FRL) model construction. (A–C) Computed tomography images were used to extract the 3D trajectory of FRL, whereas X‐ray images were used to reconstruct the FRL’s structure (e.g., number and pitch of loops). White arrows show the FRL on each image. (D) A triangulated surface of patient’s ribcage (green) was created and aligned with the ANSYS multicompartment body model (blue) to position the patient‐derived FRL model inside the ANSYS human body model. (E) Body model positioned inside the MRI coil

FIGURE 2

Top row: Trajectories and detailed structures of FRL models extracted from 10 patients (namely FRL1‐FRL10). FRL 4 and 7 represent folded trajectories. FRL 9 was a case of a patient with two closely situated retained leads. Middle row: Front and side views of reconstructed FRLs and their relative locations in the human body model. Bottom row: Position of the body model inside the MRI coil at different imaging landmarks (i.e., abdomen, chest, and head). The high‐resolution mesh area around the lead in which 1g SAR was calculated is also shown

FIGURE 3

Comparison of maximum value of 1g‐averaged specific absorption rate (MaxSAR1g) of all leads for each combination of landmark position and frequency. For each simulation, the input power of the coil was adjusted such that the spatial mean of the complex magnitude of B₁ ⁺ (i.e., 1/2 ‖B_1x + jB_1y‖) was 2 μT on an axial plane passing through the iso‐center of the coil

FIGURE 4

Normalized specific absorption rate (SAR) histograms pooling N = 300 body models and FRL leads for each field strength and at each imaging landmark. An exponential probability density function (mean = μ) was fitted to the SAR distribution for each case. Red markers show the location of extreme cases associated with the body + FRL model that created the maximum SAR. The value of the maximum SAR and the probability of an individual SAR being less than this value are also given

FIGURE 5

Cumulative equivalent minute (CEM₄₃) values calculated at different B₁ ⁺ levels as a function of acquisition time. The CEM₄₃ values are calculated for the FRL model that generated the maximum temperature rise at each imaging landmark and RF frequency. Reported B₁ ⁺ values are spatial means of the complex magnitude of the B₁ ⁺ calculated on an axial plane passing through the coil’s iso‐center

FIGURE 6

(A) The experimental setup for FRL3 and the positions of temperature probes, as well as their temperature responses at 3 T at chest landmark in the P2 case. (B) Maximum temperature rise along length of FRL models at 1.5 T and 3 T, as well as the locations where the maximum temperatures were measured