A Spine-Specific Phased Array for Transvertebral Ultrasound Therapy: Design and Simulation

<italic>Objective:</italic> To design and simulate the performance of two spine-specific phased arrays in sonicating targets spanning the thoracic spine, with the objective of efficiently producing controlled foci in the spinal canal. <italic>Methods:</italic> Two arrays (256 elements each, 500 kHz) were designed using multi-layered ray acoustics simulation: a four-component array with dedicated components for sonicating via the paravertebral and transvertebral paths, and a two-component array with spine-specific adaptive focusing. Mean array efficiency (canal focus pressure/water focus pressure) was evaluated using forward simulation in neutral and flexed spines to investigate methods that reduce spine-induced insertion loss. Target-specific four-component array reconfiguration and lower frequency sonication (250 kHz) were tested to determine their effects on array efficiency and focal dimensions. <italic>Results:</italic> When neutral, two- and four-component efficiencies were <inline-formula><tex-math notation="LaTeX">$\text{32} \pm \text{11}$</tex-math></inline-formula>% and <inline-formula><tex-math notation="LaTeX">$\text{29} \pm \text{13}$</tex-math></inline-formula>%, respectively, spine flexion significantly increased four-component efficiency (<inline-formula><tex-math notation="LaTeX">$\text{36} \pm \text{18}$</tex-math></inline-formula>%), but not two-component efficiency (<inline-formula><tex-math notation="LaTeX">$\text{33} \pm \text{15}$</tex-math></inline-formula>%). Target-specific four-component re-configuration significantly improved efficiency (<inline-formula><tex-math notation="LaTeX">$\text{36} \pm \text{8}$</tex-math></inline-formula>%). Both arrays produced controlled foci centered within the canal with similar 50% pressure contour dimensions: 10.8–11.9 mm (axial), 4.2–5.6 mm (lateral), and 5.9–6.2 mm (vertical). Simulation at 250 kHz also improved two- and four-component efficiency (<inline-formula><tex-math notation="LaTeX">$\text{43} \pm \text{17}$</tex-math></inline-formula>% and <inline-formula><tex-math notation="LaTeX">$\text{36} \pm \text{13}$</tex-math></inline-formula>%, respectively), but doubled the lateral focal dimensions. <italic>Conclusion:</italic> Simulation shows that the spine-specific arrays are capable of producing controlled foci in the thoracic spinal canal. <italic>Significance:</italic> The complex geometry of the human spine presents geometrical and acoustical challenges for transspine ultrasound focusing, and the design of these spine-specific ultrasound arrays is crucial to the clinical translation of focused ultrasound for the treatment of spinal cord disease.