1. MR Scanner Transmit Gain as an Indicator For Lead Heating November, 2006

2. 2 MRI lead heating experiments using the same: Imaging parameters Lead samples Phantom Setup & position SAR level MRI scanner

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4. 4 Test Samples Used: An industry available standard active fixation bipolar pacing lead. Prototype bipolar active fixation pacing lead with resonant circuit in distal tip. (One of Biophan’s solutions to the MR lead heating problem.) Prototype bipolar active fixation pacing lead without resonant circuit in distal tip.

5. 5 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 (ASTM F21820-02a) GE 1.5 T Signa, Excite version 12x with M5 software patch

6. 6 Slice Thickness10 mm Spacing2.5 mm Freq.512 Phase512 Phase FOV1.0 NEX5 Bandwidth31.25 kHz Body Mass120 kg Field Strength1.5T CoilBody SequenceFSE-XL Imaging PlaneAxial TE60 ms TR4300ms Echo T.L.127 FOV48 cm Whole body average SAR = 2.0 W/kg

7. 7 Three (3) samples positioned in identical configurations:

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9. 9 Initially, it was believed that MR scanner reported SAR was the primary parameter to hold constant in our studies. It was determined that the Transmit Gain had significant impact on the amount of energy transferred to the lead even when MR scanner reported SAR (and all other parameters) were held constant.

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11. 11 Note: TG = (constant) x (Log of Power).

12. 12 Note: TG = (constant) x (Log of Power).

13. 13 For a single clinical MR scanner the reported transmit gain (TG) can vary. * Data from University Medical Imaging, Rochester, NY Transmit Gain: Min: 94; Max: 119; Avg: 104 % Δ: ~21% Date:7/03/067/10/067/17/067/24/06 TG:11396 94 Date:7/31/068/14/068/21/068/28/06 TG:9711711997

14. 14 *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 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 varying “transmit gain” values.

15. 15 Experimental Relevancy Concerns: Determination of SAR from calorimetry (heat generated in tissue) studies may not be the SAR level used during lead testing if TG is not fixed. How should one compare lead heating experiments with MR scanners that do not have a “transmit gain” parameter?

16. 16 Lead Construction for Patient Safety: The lead with Biophan’s resonant circuit solution to the lead heating problem performed well below the safety limits for all values of TG tested.