Stimulation Field Smoothing using Multiple Independent Current Sources: Mitigating the ‘Edge Effect’

1. Stimulation Field Smoothing using Multiple Independent Current Sources: Mitigating the ‘Edge Effect’ Changfang Zhu PhD, Dongchul Lee PhD, Kerry Bradley MS NANS 2010, Dec 03 2010

2. Disclaimers • All authors are employees of Boston Scientific Neuromodulation • Subcutaneous electrical stimulation is not approved by the Food and Drug Administration for Boston Scientific Implantable Neuromodulation Systems

3. Introduction To nerve fibers near the contact array: A single cathode creates a limited focal stimulation region Programming multiple contacts as a “string of cathodes” however, can create a wider stimulation region

4. A Well-known Electrode Characteristic: the “Edge Effect” > 50% of total current • Current density is non-uniform over the electrode surface and is highest at the edges * I0: Total current into the electrode a: radius of circular disc electrode * Wiley and Webster, IEEE-BME 1982

5. Current Density on a String of Electrodes? • When a string of electrodes are connected to a single current source – • What does the current density look like on each electrode? • What is the clinical effect of the current distribution? I ? ? ? ? ? ? ? ?

6. Computational Simulation • Using a computational model, we simulated the current density on each contact of a percutaneous lead with the following contact size and spacing: 3 mm 4 mm

7. Current Density on a String of Electrodes Connected to a Single Source (SS) % of delivered current % of delivered current “Hot spots” I

8. Edge Effect on a String of Electrodes I =

9. Effect on Neural Recruitment near Contact Array:Simulation Settings • Infinite, homogeneous and isotropic medium • Sheets of fibers running orthogonal to the electrode string, two fiber diameters: • A-β: 12 µm • A-δ: 7 µm • Distance of fiber sheets to electrodes: • 1 ~ 10 mm • Observe: • Location and threshold of first A-β & A-δ activation • Extent of A-β activation prior to the first A-δ activation I

10. Effect on Neural Recruitment near Contact Array: Extent of A-βActivation with Single-Source At Z = 3.5 mm Z (0.5 mm) X (5 mm) Y (7 mm)

11. Current Density on a String of Electrodes Connected to Multiple Independent Current Sources (MICS) % of delivered current % of delivered current

12. Effect on Neural Recruitment near Contact Array: Extent of A-βActivation with MICS At Z = 3.5 mm Z (0.5 mm) X (5 mm) Y (7 mm)

13. Comparison Multiple Independent Current Source (MICS) Single Source (SS)

14. Conclusions • Using a single source, the outer edges of the string have the highest current density, resulting in unbalanced fiber activation across the string • The use of MICS can effectively “smooth” out the stimulation field: • provides more consistent activation across the string of contacts • activates deeper regions near the middle of the contact array