Ultherapy uses ultrasound targeting the 4.5mm SMAS layer to stimulate collagen...

https://rylansultimatethoughts.talesignal.com/posts/is-standard-hifu-safe-if-done-by-a-non-doctor-operator

Ultherapy uses ultrasound targeting the 4.5mm SMAS layer to stimulate collagen deep under the skin, with results peaking around 3 to 6 months. RF heats tissues to 40-45°C for surface tightening but doesn’t reach as deep