You'll note that it's measured in dB/(MHz-cm), and that soft tissue is 0.5, while bone is 6.9. Just like light, anytime ultrasound hits an interface, there are reflections and refractions. Furthermore, since bone attenuates ultrasound so much, you have to increase your amplitude greatly (and even more so with higher frequencies) in order to get enough thermal energy to be deposited at your target. If you don't do it right, you can create dangerous hotspots at the bone-tissue interface, and you can also deposit energy where you didn't intend to. Given that ultrasound therapy intends to be more precise than other techniques, you're kind of defeating the purpose!
Edit: in response to your edit, yes that's exactly what they do. They create an array of transmitters to be able to precisely target various locations within the skull. It wasn't my area of research, but I'm pretty sure they used them to create interference patterns as well, so many emitters were activated to work in concert to target one area.
You'll note that it's measured in dB/(MHz-cm), and that soft tissue is 0.5, while bone is 6.9. Just like light, anytime ultrasound hits an interface, there are reflections and refractions. Furthermore, since bone attenuates ultrasound so much, you have to increase your amplitude greatly (and even more so with higher frequencies) in order to get enough thermal energy to be deposited at your target. If you don't do it right, you can create dangerous hotspots at the bone-tissue interface, and you can also deposit energy where you didn't intend to. Given that ultrasound therapy intends to be more precise than other techniques, you're kind of defeating the purpose!
Edit: in response to your edit, yes that's exactly what they do. They create an array of transmitters to be able to precisely target various locations within the skull. It wasn't my area of research, but I'm pretty sure they used them to create interference patterns as well, so many emitters were activated to work in concert to target one area.
See this image: https://sunnybrook.ca/uploads/1/_research/about/fus/3-ultras...