Ultrasonic distance detection in spinal cord stimulation

Spinal cord stimulation (SCS) is a clinical method for primarily the management
of chronic, intractable pain. The method is based on electrical stimulation of
nerve fibers in the posterior part of the spinal cord. An electric field is applied by
an electrode array, implanted medially in the posterior epidural space, at some
distance from the spinal cord. An implantable pulse generator provides the electrical
stimulation pulses to the electrode array via a cable. When stimulating the
spinal cord with pulses of constant amplitude, the current density in the spinal
cord and thus the effects on nerve fibers will change with body position and/or
posture, due to the varying distance between the spinal cord and the stimulating
electrode array. As a result, the pain relieving effect may either vanish (when
the distance is too large), or be dominated by uncomfortable sensations. To
avoid these adverse conditions the current density in the spinal cord has to remain
within a small window. In present SCS systems the stimulus applied by the
electrode array is either set at a constant value, or the patient is allowed to select
one out of only two preset pulse amplitudes. Therefore, a major improvement
would be achieved if the distance between stimulation electrode array and
spinal cord could be measured and used as a feedback signal to control the
stimulus amplitude in a closed-loop system. Therefore, an ultrasonic distance
detection system has been proposed to measure the distance between stimulation
electrode array and the spinal cord. The ultrasonic system consists of a piezoelectric
transducer and an echo detection circuit. To enable the design and optimization
of the piezoelectric transducer an appropriate model description is
needed.