Now, engineers at Washington University in St. Louis have designed an experiment using magnetic resonance elastography (MRE) that directly assesses how skull motion direction impacts in vivo human brain deformation.
MRE is a process that has been used for years to learn about material properties, such as stiffness and damping, in soft tissue. It is based on measuring shear waves that pass through the tissue. For brain experiments, waves are induced by vibrating the head; previous studies have shown that some approaches to generating these waves have worked better than others.
“This probably applies to head impacts as well as harmonic vibrations, because you can approximate any input as the sum of harmonic components,” said lead researcher Phil Bayly, a mechanical engineering professor at Washington University in St. Louis. “We then needed a metric to compare the effects of different kinds of inputs. This metric needed to be non-dimensional and physically meaningful, which we determined was the ratio of strain energy (SE) to kinetic energy (KE).”
Bayly and his colleague, Ruth Okamoto, analyzed the strain energy and kinetic energy in the brain using MRE displacement data obtained by harmonic excitation of the head at five frequencies and in two different directions. One direction was created by occipital excitation applied to the back of the skull, which produces an anterior-posterior skull displacement motion—essentially, nodding motion. The other direction was due to lateral excitation applied to the right temple, producing left-right skull displacement plane that looks like shaking “no.”
When SE and KE are computed using the same harmonic displacement data, the SE/KE ratio serves as a nondimensional “transfer function” between excitation and deformation. “This is a potentially simple and powerful dimensionless metric that can be used to compare the brain’s response to different types of loading,” Bayly said.
The subject group consisted of 32 healthy people without self-reported neurological conditions ranging in age from 20 years old to 68 years old. Structural images of their brains were taken using an MRI scanner prior to application of skull vibrations.
Skull vibrations were then induced with a deformable actuator placed at either the lateral (right temple) or occipital (back of skull) position; vibrations were induced at 20, 30, 50, 70, and 90 Hz using acoustic pressure waves.
Phase-contrast and magnitude images were taken at each frequency using an echo-planar imaging process. Image data analysis also included rigid-body displacements, wave displacements and strains, and energy quantities.
Results showed that ratio of strain energy to kinetic energy (SE/KE) depended strongly on both direction and frequency. SE/KE was approximately four times larger for lateral excitation than for occipital excitation and was largest at the lowest excitation frequencies studied.
Okamoto and Bayly, along with five coauthors, reported their results in “Effect of Direct Motion and Frequency of Skull Motion on Mechanical Vulnerability of the Human Brain,” published in the November 2023 issue of ASME Journal of Biomechanical Engineering.
Response to loads
“SE/KE ratio is a simple but revealing index of the brain’s mechanical vulnerability to skull motion,” Bayly said. “The brain really does deform more readily when you rotate about neck axis in a sideways motion, compared to being in neck flexion-extension.”
Like many other solid structures—for example, aircraft frames or automobile bodies—the brain deforms more readily in response to loads in different directions and frequencies.
“This can be measured using advanced MR imaging methods that rely on exciting vibrations and waves in tissue, imaging the displacement fields with motion sensitive MRI, and analyzing the displacement fields to extract velocity strain, strain energy and kinetic energy,” Bayly said.
In future studies, Bayly and Okamoto will look more closely at what other factors determine vulnerability of the brain to skull motion. “What role do the size and shape of the brain play? Is it different in children or older adults? I think the SE/KE ratio can be used to understand the propensity of any compliant object to deform under dynamic loading,” he said.
Mark Crawford is a technology writer in Corrales, N.M.
