Changes in the specific absorption rate (SAR) of radiofrequency energy in patients with retained cardiac leads during MRI at 1.5T and 3T

Abstract

Purpose: To evaluate the local specific absorption rate (SAR) and heating around retained cardiac leads during MRI at 64 MHz (1.5T) and 127 MHz (3T) as a function of RF coil type and imaging landmark.

Methods: Numerical models of retained cardiac leads were built from CT and X-ray images of 6 patients with retained cardiac leads. Electromagnetic simulations and bio-heat modeling were performed with MRI RF body and head coils tuned to 64 MHz and 127 MHz and positioned at 9 different imaging landmarks covering an area from the head to the lower limbs.

Results: For all patients and at both 1.5T and 3T, local transmit head coils produced negligible temperature rise ( $\Delta T<0.1°C$ ) for $\Vert \Vert B_1^{+}\Vert \Vert \le 3\mu T$ . For body imaging with quadrature-driven coils at 1.5T, $\Delta T$ during a 10-min scan remained < 3°C at all imaging landmarks for $\Vert \Vert B_1^{+}\Vert \Vert \le 3\mu T$ and <6°C for $\Vert \Vert B_1^{+}\Vert \Vert \le 4\mu T$ . For body imaging at 3T, $\Delta T$ during a 10-min scan remained < 6°C at all imaging landmarks for $\Vert \Vert B_1^{+}\Vert \Vert \le 2\mu T$ . For shorter pulse sequences up to 2 min, $\Delta T$ remained < 6°C for $\Vert \Vert B_1^{+}\Vert \Vert \le 3\mu T$ .

Conclusion: For the models based on 6 patients studied, simulations suggest that MRI could be performed safely using a local head coil at both 1.5T and 3T, and with a body coil at 1.5T with pulses that produced $\Vert \Vert B_1^{+}\Vert \Vert \le 4\mu T$ . MRI at 3T could be performed safely in these patients using pulses with $\Vert \Vert B_1^{+}\Vert \Vert \le 2\mu T$ .

Keywords: RF heating; SAR; abandoned lead; anatomical models; cardiac implanted electronic device; computational modeling; defibrillator; finite element method; pacemaker; retained lead; safety.

Figure 1

3D rendering of CT images and frontal views of the chest X-rays of six representative patients with fractions of bare retained cardiac leads remaining in their torso. Details of leads’ structure and dimension such as diameter and variations in the pitch were extracted from the X-ray.

Figure 2

(A) Example of a reconstructed lead model for finite element simulations. (B) The mesh was visually inspected to ensure detailed features of the lead were well represented. (C) Lead model was then co-registered with the ANSYS human body model and inserted into the MRI RF coils model for simulations.

Figure 3

(A): Relative locations of retained leads in the body model. (B): A closer image of leads, showing their detailed structure and locations of the maximum 1gSAR as give in Tables 2 and 3. (C): Locations of different imaging landmarks. Bottom row middle: Electromagnetic simulations calculating B₁⁺ and 1gSAR. (D): Temperature maps showing the maximum temperature after 600 s of exposure.

Figure 4

Mean and standard deviation of Max1gSAR_Lead (averaged over all patients) at different imaging landmarks for both 1.5 T and 3 T simulations for the case of B₁⁺=3µT.

Figure 5

Max1gSARLead at (A) 1.5 T and (B) 3 T for the head coil and body coil at imaging landmarks L1–L9. The input power of the coils was adjusted to produce a mean B₁⁺=3µT on a transverse plane passing through coils’ iso-center.

Figure 6

Temperature profiles for the case of maximum temperature rise (patient 5, landmark L3) calculated for different values of $\Vert {\mathbf{B}}_{\mathbf{1}}^{+}\Vert$ at (A) 64 MHz RF exposure (1.5T) and (B) 127 MHz (3 T).