Ultrasound is a therapeutic modality that has been used by physical therapists since the 1940s. Ultrasound is applied using a round-headed wand or probe that is put in direct contact with the patient’s skin. Ultrasound gel is used on all surfaces of the head in order to reduce friction and assist in the transmission of the ultrasonic waves. Therapeutic ultrasound is in the frequency range of about 0.8-3.0 MHz.
According to Wikipedia.org Therapeutic ultrasound refers generally to any type of ultrasonic procedure that uses ultrasound for therapeutic benefit. This includes HIFU, lithotripsy, targeted ultrasound drug delivery, trans-dermal ultrasound drug delivery, ultrasound hemostasis, cancer therapy, and ultrasound assisted thrombolysis  It may use focused ultrasound (FUS) or unfocused ultrasound.
Ultrasound is applied using a transducer or applicator that is in direct contact with the patient’s skin. Gel is used on all surfaces of the head to reduce friction and assist transmission of the ultrasonic waves. Therapeutic ultrasound in physical therapy is alternating compression and rarefaction of sound waves with a frequency of >20,000 cycles/second. Therapeutic ultrasound frequency used is 0.7 to 3.3 MHz. Maximum energy absorption in soft tissue occurs from 2 to 5 cm. Intensity decreases as the waves penetrate deeper. They are absorbed primarily by connective tissue: ligaments, tendons, and fascia (and also by scar tissue).
Conditions for which ultrasound may be used for treatment include the follow examples: Ligament Sprains, Muscle Strains, Tendonitis, Joint Inflammation, Plantar fasciitis, Metatarsalgia, Facet Irritation, Impingement syndrome, Bursitis, Rheumatoid arthritis, Osteoarthritis, and Scar Tissue Adhesion.
Some conditions treated with ultrasound in a chiropractic setting include tendonitis, non-acute joint swelling, muscle spasm, degenerative disc disease, non-acute sprains or strains, bursitis, sciatica, fibromyalgia, whiplash, cervicogenic headaches. Contraindications of ultrasound include local malignancy, metal implants below the area being treated, local acute infection, vascular abnormalities, and directly on the abdomen of pregnant women. It is also contraindicated to apply ultrasound directly over active epiphyseal regions (growth plates) in children, over the spinal cord in the area of a laminectomy, or over the eyes, skull.