When you mention ultrasound (US) most people think of pregnancy. That is diagnostic US. It is a great way to be able to see what is going on inside, like MRI, X-ray, etc… When you visit a chiropractor and they speak of US it is for an entirely different purpose.
Ultrasound therapy is done with a wand device. This device creates sound waves (outside of our range of hearing) and directs them into your body. These waves can go as deep as 3-4 inches into your body. At that depth the waves are absorbed and produce heat. Kind of like putting a hot pack on your deep muscles to relax them. (A hot pack on the outside only penetrates about an inch.) The sound waves also kind of stir things up. Imagine putting a scoop of sugar in water. It will dissolve on its own, but if you stir it, it dissolves faster. That’s a good way to imagine how US speeds healing.
This therapy has been shown to relax tissues, increase blood flow and even break down scar tissue. US is mainly absorbed by soft tissue like ligaments, muscles, fascia and scar tissue so can be beneficial in sprains, strains, acute and chronic issues. Another use for US is medicine delivery. Putting a topical medicine such as Cortisone on the skin allows the sound waves to drive it across the barrier of skin to the tissues underneath. This can be an alternative to injections or pills. Chiropractors do not prescribe pills or injections, but in many states utilize some topical applications in this way. US can also be used to diagnose fractures, as putting US over a broken bone can cause a sharp pain, you can think of two sides of the brake vibrating against each other.
Typically US treatment will be 3-5 min long depending on the size of the treatment area. Treatment to break down scar tissue can be longer. You should not feel anything during the treatment other than the probe touching you.
There are precautions with any treatment. With US the wand must be kept moving and be used with a gel to increase conductivity of the waves or it could theoretically burn you inside or break the device. You should not use US over a cancer, metal implants, infection, growth plates, or eyes.