Page 42 - Volume 8, Issue 4 - Winter 2012

ULTRASONIC BIOEFFECTS ON PERIPHERAL NERVES
Robert Muratore
Quantum Now LLC Huntington, New York 11743
and
Jeffrey J. Vaitekunas
Cybersonics, Inc. Erie, Pennsylvania 16510
Introduction
“...the nervous system
number of neurons involved with diges- tion is roughly equal to the order of magnitude of the number of neurons in the spinal cord. Because of this, the autonomic system of the gastrointestinal tract is often considered separately as the enteric system. The enteric neurons are diffusely distributed throughout the
Acentury of studies has demon-
strated that the nervous system
is sensitive to incident ultra-
sound. A comprehensive review of
these effects was published in 2011 by
Gavrilov.1 From his detailed listing of
acoustic carrier frequencies, pulse rep-
etition frequencies, intensities, and exposure time, one overarching lesson can be learned: the nervous system responds in some way to nearly any acoustic energy to which it is exposed.
The details of specific responses to specific stimuli vary with what the authors of each study were monitoring. In par- ticular, the responses depend on the portion of the nervous system being insonified. However it is possible to make some additional general statements about the response of periph- eral nerves to acoustic energy.
Although nerves are long, they can be influenced by insonifying just a small portion of their length. There is a spectrum of effects that varies with dose, from nothing at all at sub-threshold insonification levels through complete ther- mal ablation at high doses. These effects act differently on different neuronal fibers within the nerve. Finally, the mech- anisms of low-dose effects, e.g., reversible stimulus and inhi- bition, are not fully understood.
Of the four tissue types parsed by anatomists (epithelial, connective, muscular, and neural), all four make up or are intimate with the peripheral nerves. The implications for acoustics mean that the differential acoustic properties of these tissues can affect the delivered acoustic dose. In addi- tion, for thermal effects, the thermal properties will affect the temperature distribution. In particular, nerves are often co- located with blood vessels.
Anatomists further divide the nervous system into the central nervous system (CNS) and the peripheral nervous system (PNS). Acoustic manipulation of the CNS (brain and spinal cord) is beyond the scope of this paper, but is an important and growing field of study. The PNS has receptor, or sensory, pathways and effector, or motor, pathways. The motor pathways are somatic (under conscious control), typi- cally innervating skeletal muscle, or autonomic, typically innervating smooth and cardiac muscles, glands, and adipose tissue. The autonomic pathways are sympathetic or parasym- pathetic.
The gastrointestinal tract is innervated by sympathetic and parasympathetic fibers. The order of magnitude of the
38 Acoustics Today, October 2012
responds to nearly any
ultrasonic energy to which
it is exposed.”
abdomen, wrapping around the digestive organs and inner- vating the smooth muscles and other features.
The nervous system responds to insonification over a wide range of acoustic parameters
As an illustration of the wide range of ultrasound to which the nervous system responds, consider the fingertips. The sensory pathways of the PNS begin with the Pacinian corpuscles, Meissner corpuscles, Ruffini corpuscles, Merkel cells, and free nerve endings monitoring the receptive fields in the skin and other organs. In his review article, Gavrilov1 presented data for receptive fields in fingertips. Figure 1, adapted from his data, is a graph of the threshold intensities required for human sensations of touch, heat, and pain as a function of ultrasound frequency. The incident acoustic
2 intensities for just this modality range from 8 to 3200 W/cm .
It is not necessary to insonify the entire length of a neuron to evoke a response
Nerves and nerve fibers are highly asymmetrical targets, ranging from 0.1 micron (for a fine fiber) to 2 mm (for a large nerve) in cross sectional diameter with some lengths in excess of 1 m. Practical acoustic focal regions tend to be much smaller than the length of a neural fiber, but much larger than the width of the fiber; the extremely high fre- quencies needed to target an individual fiber would have lim- ited penetration into tissue. In addition, such precise target- ing would be difficult. Thus, multiple fibers and other tissues adjacent to the desired target will be insonified by the ultra- sound focal region, and only a very short segment of the tar- get will be insonified.
Fortunately, for some applications, it is not a problem to insonify multiple fibers within a nerve; for other applications, the differential response of nerve fibers can be exploited (see below). Furthermore, blocking conduction of a short fiber segment is often effective at blocking conduction along the entire fiber. This is particularly true with ablation.
One promising application of high intensity focused ultrasound ablation is renal sympathetic denervation2 to