I asked my associate therapist/DC and here's his response:
Ultrasound works great for many conditions, I use it on everything from muscles strains, trigger points, or deeper structures, such as knees. Therapeutic ultrasound frequency (Waveform) used is, 0.7 to 3.3 MHz ( The lower the number the deeper penetration). Most units sold offer a 1MHz setting, which I like to use because it penetrates deeper. Ultrasound must always be moved constantly in a circular motion or it will burn the Periosteum (outer layer of the bone). Maximum energy absorption in soft tissue is about 3 inches, so you would not want to use the 1 MHz (deeper) setting over areas of decreased tissue, like the hand. Intensity decreases as the waves penetrate deeper. They are absorbed primarily by Connective tissue, Ligaments, Tendons and scar tissue. I believe most places won't sell a unit unless you have an active state license, (PT, DC, DO). When I bought mine I had to give my license number. Hope this helps.
-Dr. Brian Griffin
Austin Spine and Therapyhttp://www.austinspineandtherapy.com/