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
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
2
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
3
A Well-known Electrode Characteristic:
the “Edge Effect”
Current density is non-uniform over the electrode surface and
is highest at the edges
I0
*
Jr 
2a 2 [1  (r a) 2 ]1/ 2
2
2
J / J0 (J0 = I0/a )
2.5
1.5
I0: Total current into the electrode
1
a: radius of circular disc electrode
0.5
0
-1
* Wiley and Webster, IEEE-BME 1982
-0.5
0
0.5
1
r/a
> 50% of
total current
-1
-0.5
0
0.5
1
4
-1
-0.5
0
0.5
1
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
??
5
??
?
?
??
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
6
4 mm
Current Density on a String of Electrodes
Connected to a Single Source (SS)
I
“Hot spots”
Single electrode source equivalent current(%) - SS
20
7
% of delivered current
% of delivered current
Single point source equivalent current(%) - SS
15
10
5
0
E1 E2 E3 E4 E5 E6 E7 E8
15
10
5
0
E1 E2 E3 E4 E5 E6 E7 E8
Edge Effect on a String of Electrodes
I
=
8
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
9
I
Effect on Neural Recruitment near Contact Array:
Extent of A-β Activation with Single-Source
Z (0.5 mm)
At Z = 3.5 mm
10
Current Density on a String of Electrodes Connected
to Multiple Independent Current Sources (MICS)
Single point source equivalent current(%) - MICC
20
11
% of delivered current
% of delivered current
Single point source equivalent current(%) - MICC
15
10
5
0
E1 E2 E3 E4 E5 E6 E7 E8
15
10
5
0
E1 E2 E3 E4 E5 E6 E7 E8
Effect on Neural Recruitment near Contact Array:
Extent of A-β Activation with MICS
Z (0.5 mm)
At Z = 3.5 mm
12
Comparison
Single Source (SS)
13
Multiple Independent
Current Source (MICS)
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
14