When subjected to the RF field of an MRI, a cardiac pacemaker or defibrillator may encounter heating of the lead electrode and/or voltage at the device header causing unintended stimulation of cardiac tissue. The RF field may be generated by a transmit body coil, head coil or extremity coil. Due to the size, power and positioning restrictions of head and extremity coils, the fields generated are much smaller than those generated by body coils. The purpose is to demonstrate that evaluation of transmit head and extremity coils is unnecessary for cardiac pacemakers or defibrillators that are shown to be safe with body transmit coils.

The tangential component of electrical field (Etans) were extracted along cardiac pacemaker and defibrillator lead pathways in five body models (FATS, Duke, Ella, Billie and Thelonious [2]) in 217 body coil simulations and 133 head and extremity simulations. For the body coils the simulations were performed at landmarks from the top of the head to the lower extremities in 10cm increments.

The body coil diameters ranged from 60cm to 73cm, lengths from 35cm to 73cm with high pass (HP), low pass (LP) and band pass (BP) configurations. LP and HP head coils with diameters from 27cm to 31cm and lengths from 27cm to 38cm were simulated. HP and LP upper and lower extremity coils with diameters from 14cm to 22cm and lengths 14cm to 22cm were also simulated. The simulations were validated through 60 E-field measurements taken in an ASTM phantom filled with 1.2S/m media centered in a 1.5T body coil.

The average value of Etan along the length of the implant was calculated and scaled to Normal Operating Mode, First Level Controlled Mode, and 30uT as defined in [3]. The 90th, 99th, 99.9th and 99.99th percentile average Etan was extracted from the distribution. This analysis was performed for the body coil simulations and then repeated for the combination of head and extremity coils simulations. The Ratio² was calculated from (Etan_body/Etan_HE)² since temperature rise and power deposition are proportional to Etan². The Ratio was calculated from (Etan_body/Etan_HE) since header voltage is proportional to Etan. The average Ratio² was calculated across the percentiles for Normal Operating Mode and repeated for First Level Controlled Mode. The average Ratio was calculated across the percentiles for 30uT values.

Validation of the Simulated E-fields is shown in Figure 1. The measurements and simulations were normalized. For the measured E-field values ≥ 10 V/m, the standard deviation of the errors between the normalized measured and normalized simulated values was 2.59%.

Etan comparison between body coils and head/extremity coils are shown in Table 1.

The Ratio² averaged across the percentiles for normal operating mode is 26. This means that, as a first order approximation, a temperature rise or power deposition calculated using body coils will be need to be divided by 26 to reach the levels generated by head and extremity coils alone. This increases to 37 for first level controlled mode. With an additional safety factor of three to account for second order effects there is still a 9x margin for normal operating mode and 12x for first level controlled mode.

The Ratio averaged across the percentiles for 30uT is 10. This means that, as a first order approximation, a header voltage calculated using body coils will need to be divided by 10 to reach the levels generated by head and extremity coils alone. With an additional safety factor of three to account for second order effects there is still a 3x margin for the header voltage.

If RF heating and voltage can be shown to be safe with unrestricted body coils for cardiac pacemaker and defibrillators pathways then no additional analysis is necessary for head and extremity coils since there is a 9x margin in RF heating and 3x margin in voltage.