Novel methods and circuits for field shaping in deep brain stimulation
2011
Deep Brain Stimulation (DBS) is a clinical tool used to treat various neurological
disorders, including tremor, Parkinson’s disease (PD) and dystonia. Today’s routine
use of this therapy is a result of the pioneering work of Benabid and colleagues, who
assessed the benefits of applying high-frequency stimulation to the ventral intermediate
nucleus and reported substantial long-term improvements in PD patients.
Clinical applications of DBS, however, have preceded research and left a number of
challenges to optimise this therapeutic technique in terms of quality, therapy costs
and understanding of its underlying mechanisms. DBS is based on monopolar or
bipolar stimulation techniques, which are characterised by a limited control over the
effects of stimulation and, in particular, over the shape and direction of the electric
field propagating around the electrode. This thesis proposes two approaches
to achieve dynamic electric field control during deep brain stimulation. The first
method is based on the use of current-steering multipolar electrode drive, adopted
to split the stimulation current between 2 or more contacts, in order to shift the
stimulation field to a desired location. The work included the design, development
and testing of an integrated circuit current-steering tripolar current source, developed
in AMS 0.35μm technology. The second method is based on the use of phased
arrays (PAs) in order to create an electromagnetic beam, which can be steered to
a desired location. Computational models have shown the ability to steer and focus
the electromagnetic fields in brain tissue by varying the phase and frequency of
stimulation. Modelling simulations have shown that the use of multipolar electrode
configurations is essential to achieve dynamic control over the shape and area of
tissue stimulated. Configurations with larger number of cathodes allow for several
stimulation patterns, making this stimulation approach beneficial in a clinical environment.
Tests on the performance of the integrated tripolar current source have shown its capability to generate stimulation currents up to 1.86mA, to linearly steer
the stimulation current to one of the anodes and to generate biphasic square and
exponential current pulses, with time constant up to 28ms. In vitro experiments,
carried out to map the electric potential generated by a dynamic tripolar current
source, validated the model results, by showing the ability to shape the potential distribution
around the electrode during stimulation. Finally, models of the behaviour
of PA fields in brain tissue have shown that PAs could be introduced to DBS to allow
for more accurate field steering and shaping in DBS. This thesis presents methods
and implementations to achieve dynamic field shaping in DBS, which can greatly
ameliorate the efficacy of clinical DBS.
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