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.