Characterizing speed of sound in the spine for focused ultrasound

Focused ultrasound is a relatively new technology for treating brain disorders such as Parkinson’s disease and essential tremor. The method uses large arrays of ultrasound transducers to focus sound waves through the skull to stimulate or treat a specific target.

Ultrasound distorts as it passes through bone. Simulations account for this effect for treatment in the skull but have not yet been developed for similar treatment in the spinal cord. Xu and O’Reilly characterized the speed of sound in vertebral laminae to develop simulation capabilities for focusing ultrasound to the spinal cord.

“The sound speed is very important because it affects how sound will be transmitted through the bone and the aberrations that will result,” said author Rui Xu.

The team conducted ex vivo experiments of vertebrae suspended in water, measuring the time delay between an ultrasound signal generated outside the vertebral canal and its reception at a hydrophone inside the canal. After building up their data set, the researchers simulated the same experiments using CT images of the vertebrae.

“From there we could change the simulated sound speed in the vertebrae to maximize the similarity between the experiment and simulation signal delays,” said Xu.

Using a simple linear model relating sound speed and bone density, the authors found a lower slope than previous works that measured the same parameters in the skull. This indicates the bone microstructure, and therefore sound speed, of vertebral laminae is different.

The scientists used a single spine for these results, but plan to include a more representative sample in the future and eventually conduct in-person trials.

Source: “Establishing density-dependent longitudinal sound speed in the vertebral lamina,” by Rui Xu and Meaghan A. O’Reilly, Journal of the Acoustical Society of America (2022). The article can be accessed at https://doi.org/10.1121/10.0009316 .