High-Intensity Focused Ultrasound Scattering at Mammal Vertebrae

David Sanford MS ME 2019 California State University Northridge

P.I. Dr Christoph Schaal, California State University Northridge

High-intensity focused ultrasound (HIFU) is used to heat cells to produce medical therapies. Hifu may also destroy cells through heat or cavitation which challenges therapuetic treatments. The unobstructed focused waves produce maximum wave amplitudes at a predictable focal region in homogenous media but HIFU is not used near the vertebra because 1) wave absorption at the tissue interfaces causing heating of the bone surfaces and 2) wave scattering causes an unpredictable redistribution of the maximum amplitudes. Clinical trials of stroke treatments or neural drug delivery have effectively passed ultrasound waves through the skull. In contrast, the irregular geometry of the spine creates a targeting challenge. Scattered and absorbed waves may cause a damaging temperature rise. This limits therapeutic ultrasound treatments of lower back pain or those used to encourage nerve cell growth.  

This paper presents an experimental study involving imaging, pressure measurements and temperature measurements to determine the spatial distribution of pressure amplitudes and heat deposition from induced HIFU waves targeting vertebrae. A curved piezoelectric transducer focuses waves which propagate through deionized water toward the focal region. First, a bone-like rectangular composite plate, partially obstructing the induced waves, is shown to break the conical HIFU form into two regions. These are captured using pulsed laser shadowgraphy in which incident light is refracted by the differential pressures. Bright and dark bands related to the maxima and minima pressures are captured with a camera. Image processing separates background light and the resulting data is analyzed to determine the spatial pressure distribution. Hydrophone measurements over the same region are compared to the shadowgraphy intensity plots to support the procedure.

Next, individual, clean, ex-vivo feline vertebrae are mounted at the focal point with visibility through the central cavity to allow imaging of the inside and around the bone. Shadowgraphy is performed for each bone under the same ultrasound conditions as the plate and the resulting images are processed to determine the patterns of wave scattering and the redistribution of maximum amplitudes. Temperature is measured at select locations of interest to determine surface heating associated with the insonication. The results indicate that shadowgraphy can be used to determine general energy deposition patterns, and with hydrophone-based calibration, it can also be used to determine heating at a specific location. Overall, these laboratory experiments help determine the efficacy of warming specific nerve cells within mammal vertebrae without causing damage to adjacent tissue.