  Focused ultrasound spinal cord neuromodulation studies have demonstrated
spinal cord neuromodulation in small animals. The safe and efficacious
translation of these approaches to human scale requires an understanding of
ultrasound propagation and heat deposition within the human spine. To address
this, combined acoustic and thermal modelling was used to assess the pressure
and heat distributions produced by a 500 kHz source focused to the C5/C6 level
of the cervical spine via two approaches a) the posterior acoustic window
between vertebral posterior arches, or b) the lateral intervertebral foramen
from which the C6 spinal nerve exits. Pulse trains of 150 0.1 s pulses with a
pulse repetition frequency of 0.33 Hz and free-field spatial peak
pulse-averaged intensity of 10 W/cm^2 were simulated for the CT volumes of four
subjects and for $\pm$10 mm translational and $\pm$10{\deg} rotational source
positioning errors. Target pressures ranged between 20% and 70% of free-field
spatial peak pressures with the posterior approach, and 20% and 100% with the
lateral approach. When the source was optimally positioned with the posterior
approach, peak spine heating values were below 1{\deg}C, but source
mis-positioning resulted in bone heating up to 4{\deg}C. Heating with the
lateral approach did not exceed 2{\deg}C within the mispositioning range. There
were substantial inter-subject differences in target pressures and peak heating
values. Target pressure varied three to four-fold between subjects, depending
on approach, while peak heating varied approximately two-fold between subjects.
This results in a near ten-fold range in the target pressure achieved per
degree of peak heating between subjects. This highlights the importance of
developing trans-spine ultrasound simulation software for the assurance of
subject-specific safety and efficacy of focused ultrasound spinal cord
therapies.
