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Experimental investigations of high-intensity focused ultrasound interactions with vertebrae
High intensity focused ultrasound (HIFU) is used clinically to heat cells therapeutically or to destroy cells through heat or cavitation. In homogenous media, the highest wave amplitudes occur at a predictable focal region allowing precise targeting. Bone obstructions in the conical wave form reflect and absorb part of the wave energy and conduct the remaining energy at a different speed. This disrupts the waveform and poses a targeting challenge. Also, wave absorption at interfaces causes heating of the bone surfaces. Some clinical trials use HIFU through the skull to treat strokes or deliver drugs within the brain through mild heating effects. However, other bone structures are generally avoided. For example, the complex geometry of the spine currently prohibits therapeutic ultrasound treatments to treat lower back pain. This work presents a comprehensive experimental study involving imaging, pressure and temperature measurements to determine the spatial distribution of pressure amplitudes from induced HIFU waves on a vertebra. A semi-spherically faced piezoelectric transducer is stimulated at its resonant frequency to produce a wave packet which propagates through deionized water towards the focal region. A bone-like composite plate and cylinder obstruct the induced waves and redistribute the conical HIFU form. Images of the waves are captured using pulsed light shadowgraphy in which a very short duration light pulse is refracted by pressure gradients which produces bright and dark banding in a digital photograph. Image processing separates background light and the remaining data is analyzed to determine the spatial pressure distribution. Hydrophone measurements over the same region are compared to the shadowgraphy intensity plots to validate the imaging procedure. Feline vertebrae (dessicated ex vivo) are then mounted at the acoustic focal point with visibility through the central cavity to allow light to pass through and around the bone. Shadowgraphy is performed for each bone and the resulting images are processed to determine the patterns of wave scattering and the redistribution of maximum pressure amplitudes. Numerical simulations of pressure waves at the plate, cylinder, a vertebra and the unobstructed case are compared to the experimental data. Focal point temperatures are also measured during insonication for all cases and surface temperature changes are compared to fluid temperature changes. Results indicate that the numerical simulations adequately predict pressure intensity and patterns of scattering.Shadowgraphy can indicate the location of the highest amplitudes but it is not as effective outside of a very small field of view or as acoustic energy attenuates. Overall, vertebrae pose experimental challenges to verify pressure and to use shadowgraphy, but numerical simulation and thermocouple measurements are straightforward for these complex organic geometries. During the experiments, the use of a very simple shadowgraph is shown to be a conditionally effective visualization tool. Numerical simulations are shown to have good parity with pressure measurements from a hydrophone and temperature measurements are shown to correlate with pressures. Thus, these laboratory experiments demonstrate some approaches to determine pressure and heat distribution around a vertebra and discover the relationship between induced pressure and generated heat for a complex organic geometry. This provides empirical data to support the development of therapeutic ultrasound treatments for the spine.