MR Scanner Transmit Gain as a Potential Indicator For Lead Heating November, 2006

The Question Repeated MRI lead heating experiments using the same: Imaging parameters Lead samples Phantom Setup & position SAR level MRI scanner yielded different results. Why? 2 •

Materials Test Samples Used: An industry available standard active fixation bipolar pacing. Prototype bipolar active fixation pacing lead with resonant circuit in distal tip. Prototype bipolar active fixation pacing lead without resonant circuit in distal tip. 3 •

Methods Luxtron® model 3100 fluoroptic thermometry system with SSM model optical probes Gelled-saline solution: 5.8 g PAA, 0.8g NaCl per liter of de-ionized water in head/torso phantom GE 1.5-T MR system, FSE-XL, whole body average SAR = 2.0181 W/kg 4 •

Materials and Methods Three (3) samples positioned in identical configurations: 5 •

Transmit Gain Variable Observed 6 •

Further Study Three different leads tested at the same SAR level, different TG values. Linear scale Logarithmic scale 7 •

Clinical Scanner Transmit Gain History For a single clinical MR scanner using the same phantom and imaging sequence the reported transmit gain (TG) can vary. Date: 7/03/06 7/10/06 7/17/06 7/24/06 TG: 113 96 94 7/31/06 8/14/06 8/21/06 8/28/06 97 117 119 Min: 94 Max: 119 Ave: 104 * Data from University Medical Imaging, Rochester, NY 8 •

Discussion & Conclusions It is known that the scanner calculated and reported SAR level can be different between different MRI scanner types.* For the same experimental setup on the same MRI scanner using the same imaging sequence and reported SAR level, lead heating can significantly vary due to transmit gain. The MRI scanner’s transmit gain varies for the same reported SAR level using identical imaging parameters. *See: Baker et al., “Evaluation of Specific Absorption Rate as a Dosimeter of MRI-Related Implant Heating”, Journal of Magnetic Resonance Imaging, 20:315-320, 2004 9 •