1. Influence of Pacing Lead Design on MRI-Induced Lead Heating Presenter: Stuart MacDonald VP Research and Development Heart Rhythm 2006 Boston, MA May 18, 2006

2. 2 Issue & Investigation Issue: Tissue proximal to implanted lead electrodes can significantly heat in MRI scanners. The heating depends on many factors including lead position within the MRI scanner bore and the lead’s electrical properties. Investigation: Two different bipolar pacing lead designs for reducing the MRI scanner’s RF induced heating were implemented.

3. 3 The Heating Mechanism The rotating B1 field of the MRI scanner sets up an electrical potential difference in the patient’s body. The electrical conductors of the implanted lead provide a short circuit path for induced currents. Induced current is concentrated around lead’s conductive electrodes. Current passes into and out of the resistive tissue and fluid surrounding the electrodes. Current through highly resistive tissue and fluid results in higher Ohmic heating than in the lead’s lower resistive conductors and electrodes.

4. 4 The Heating Mechanism Maxwell’s Equation: Rotating Field: Induced Electric Field: Lenz’s Law (Fixed Area 1 Loop):

5. 5 Thermal Measurements Thermal probe placement can be important. Heat sink effects can play a roll. Max heating can occur up to 0.5 mm away from surface of electrode.

6. 6 Two Possible Solutions Low pass filter requires an adequate electrical ground which is not available in, e.g. bipolar pacing lead in MRI scanner, because the induced voltage occurs on both conductors of bipolar lead. Could be implemented with significant changes to IPG. Solutions without reference to “ground”: 1.Adjust construction of lead to modify its impedance at 64MHz. (Coiling pitch, opposing coiling directions, polymer material.) 2.Block induced currents by resonant circuits. (Potentially at multiple MR operating frequencies.) Obvious approach and drawbacks:

7. 7 Materials and Methods Prototype bipolar active fixation pacing leads: 1) Different winding pitch of coiled conductors 2) Different resonant circuits in distal tip Standard active and passive fixation bipolar pacing leads Luxtron® model 3100 fluoroptic thermometry system with SFF 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 (GE), FSE-XL, Whole body avg. SAR: 1.79 W/kg

8. 8 Materials and Methods Samples positioned for maximum RF induced heating: Alignment Optical Temperature Probe IPG

9. 9 Changing Leads’ 64MHz Impedance

10. 10 With Resonant Circuits

11. 11 Additional Testing MR:comp, Gelsenkirchen, Germany Head/torso phantom, gelled (PAA) solution, OPTOCON optical temperature probes 1.5T GE scanner, Fast Spin Echo sequence, scanner reported WBA SAR = 2.0215 W/kg Scan time: ~18 minutes

12. 12 Conclusions Two different approaches to implantable lead design which reduce the MRI induced RF heating of surrounding tissue to an acceptable level: 1.By adjusting the construction of the bipolar lead such that the impedance of the pacing system at MR scanner operating frequency is large, the induced RF heating over approximately 3 minutes was < 0.5ºC. 1.By inserting small resonant circuits the induced RF heating over approximately 3 minutes was < 1.0ºC. Multiple circuits in the same lead can provide protection for different MR scanners (64MHz, 128MHz, etc.)

13. 13 Future Studies A.Gain a clear understanding of effect of variables: - Lead/IPG combination - Lead layout - Lateral position B.Get a clear demographic - Normal & abnormal lead layout in patient - Typical and atypical positioning in MR bore A x B => - Clear understanding of risk - Clear definition of requirements for removal of contraindication