Possibilities and Risks in Ultrasound Operation
Technion Researchers Develop a Model that Provides a Unique
Explanation for the Way in Which Ultrasound Influences Living Cells
Cell membranes absorb energy from the ultrasound, expanding and contracting
during its operation
Ultrasound is widely used in imaging devices. In the last decades, its use has increased
also for treatment and therapy because it is non-invasive and can be pinpointed. But in
most uses (at medium or low intensity) it is unclear how ultrasound heals and interacts
with living cells. Over the past year, Technion researchers have developed a unique
model that explains how ultrasound waves affect the living cell.
The researchers claim that the cell membranes absorb mechanical energy from the
ultrasound pressure wave, expanding and contracting during its operation. The model was
published in the American journal “Proceedings of the National Academy of Sciences”
(PNAS).
“The model we developed predicts that the lipid bilayer membranes of the cell are
capable of absorbing mechanical energy from the ultrasound field and translating it into
expansion and contraction of the membrane’s internal space,” explains Prof. Eitan
Kimmel of the Technion’s Faculty of Biomedical Engineering. “We have developed a
unique model that is able to explain the interaction mechanism between ultrasound and
biological tissue. The model integrates physics and bubble dynamics with the
biomechanics of the cell and enables estimating the dynamic behavior of the bilayer
membrane, which is made up of two layers of lipid molecules.”
Fifteen years ago, Prof. Kimmel exposed anesthetized goldfish to low intensities of
ultrasound together with Dr. Victor Frenkel (during his doctoral research) and then
showed under an electronic microscope the spread of cell membranes in the fish skin. But
only now he has succeeded in understanding the meaning, using a model he developed
together with Prof. Shy Shoham and Dr. Boris Krasovitski. In their collaborated studies
Kimmel and Shoham stimulate neuron cells using ultrasound, giving special attention to
the cell membrane.
“In the world of ultrasound, the common explanation is that during operation of the
ultrasound machine tiny gas bubbles are produced (cavitation) which explode against the
surface and are responsible for changes in the cells and tissue,” says Prof. Kimmel. “The
FDA [US Federal Drug Administration] has determined a pressure amplitude limit under
which bubbles are not anticipated and therefore, the assumption is that no damage is
caused. Up until now, it was not known where the bubbles seen during high intensity
ultrasound operation are produced in the body. We found the source of the bubbles.
Where there are cells that are not entirely enclosed by dense tissue, there will be bubbles.
If bubbles are seen in the blood vessels, we know that these are blood cells and their
membranes have reacted to the ultrasound. Cell membranes under ultrasound look like
layers of ‘bubble wrap.’ But this is a dynamic layer whose ‘bubble wrap’ expands and
contracts with ultrasound operation.”
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The Technion researchers have identified a wide range of medical uses such as drug
introduction through the blood-brain barrier, stimulating nerves, pain suppression and
facilitating blood vessel growth and shortening the healing times in wounds and fractured
bones. On the other hand, it could be that it will be necessary to reevaluate the criteria for
safe operation in order to reduce the risks of ultrasound. “When the membrane is opened
and stretched – forces operate on the cell inside of it, on the cell skeleton and on the
proteins found in the membrane,” states Prof. Kimmel. “Ultrasound interferes with their
operations, which are vital for the life of the cell. At certain intensity, this can cause
positive changes in the cell and at other intensities it could cause damage. Therefore, we
recommend continuing investigation of the subject using the model we developed.”
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