Most therapeutic ultrasound machines come with a choice of 1 or 3 megahertz frequencies and the clinician is left determining which of those frequencies is needed. This is a definition of megahertz and which frequency is indicated in ultrasound therapy.
A megahertz is defined as, abbreviated MHz, a unit of alternating current (AC) or electromagnetic (EM) wave frequency equal to one million hertz (1,000,000 Hz). This frequency is inaudible to humans since for humans the audible frequency is lost at 20,000 Hz.
The frequency of therapeutic ultrasound is important, depending on the area being treated for pain relief, or for accelerating healing due to injury. There is an inverse relationship between higher frequency and depth of penetration. As the frequency increases from 1 to 3 MHz, the depth of penetration decreases, ie. higher the frequency the less penetration.
The depth of penetration is important if the ultrasound treatment is on an ankle, knuckle, elbow, TMJ (tempro mandibular joint), knee or other area that is close to the surface that is being treated. In order to create heat for an ankle the clinician will set the frequency to 3 MHz because the area being treated is close to the surface and higher frequency has less penetration so the treatable tissues are being targeted better.
If a patient is suffering from back pain, sciatica, shoulder pain, hip pain or other areas where the tissues are deeper near joints with muscle tissue, or where muscle tissue is the targeted tissue then 1 MHz is used since defect penetration is needed to treat the affected area.
Many therapeutic ultrasound units come with a choice of frequencies, however for home use it is necessary to have multiple frequency choices, but more important to have the frequency necessary for the patient's condition. In acute situations it is most beneficial for a patient to rent the ultrasound unit for home use rather than go back and forth at greater cost to clinics for short ultrasound treatments.