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 •

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 •

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 •

The Heating Mechanism

Maxwell’s Equation:

Rotating Field:

InducedElectric Field:

Lenz’s Law(Fixed Area 1 Loop):

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 •

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 •

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 •

Materials and Methods

Samples positioned for maximum RF induced heating:

Alignment

Alignment

Optical Temperature Probe

IPG

9 •

Changing Leads’ 64MHz Impedance

10 •

With Resonant Circuits

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 •

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 •

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