Written and maintained by CASRAI Editorial Board
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General information about equipment and terminology, not medical advice. It does not describe how to treat any person or condition.
Short answer
Therapeutic ultrasound is the use of ultrasonic sound energy to act on body tissue, rather than to image it. In physical therapy it usually means an unfocused 1-3 MHz beam from a hand-held applicator that warms tissue. In the United States, FDA regulates the heating version as Class II ultrasonic diathermy under 21 CFR 890.5300, and ultrasonic diathermy for other uses as Class III.
Definition and scope
A 2012 review in the Journal of Ultrasound in Medicine (AIUM), “Overview of Therapeutic Ultrasound Applications and Safety Considerations”, separates therapeutic uses from diagnostic ones. Diagnostic ultrasound is typically in the 2-12 MHz range and uses short pulses of one to five cycles to build images. Therapeutic ultrasound delivers energy to produce an effect in tissue. The review groups therapeutic uses into heating applications (physical therapy, hyperthermia, high-intensity focused ultrasound), nonthermal applications (such as lithotripsy and low-power kHz devices) and applications whose mechanism is uncertain (low-intensity pulsed ultrasound for fracture healing). The same review notes that therapeutic ultrasound was established for physiotherapy by the 1970s.
When people search for “therapeutic ultrasound” in a clinic or equipment context, they usually mean the physical-therapy device, which is the one the rest of this page focuses on.
How does therapeutic ultrasound work?
The physical-therapy setup described in the review has a base unit that generates an electrical signal and a hand-held transducer. The transducer is applied to the skin through a coupling gel and is operated by trained physical therapy staff. Its purpose is to warm tissue such as tendon and muscle.
Mechanically, absorbed acoustic energy is converted to heat. The Miller review also describes nonthermal routes, including cavitation and mechanical stress.
Types and related concepts
Table 1 of the Miller review lists FDA-approved therapy modes. The table below summarizes the main types.
| Type | Description |
|---|---|
| Physical-therapy ultrasound | Unfocused beam, tissue warming, portable hand-held applicator, 1-3 MHz, continuous or repeated bursts |
| Hyperthermia and high-intensity focused ultrasound | Listed as heating applications, separate from physical-therapy use |
| Lithotripsy and low-power kHz devices | Listed as nonthermal applications |
| Low-intensity pulsed ultrasound (LIPUS) for fracture healing | Pulsed ultrasound at 1.5 MHz, with an intensity in the range of diagnostic ultrasound; the review states the biophysical mechanisms are uncertain |
| Diagnostic ultrasound | Imaging, typically 2-12 MHz, short 1-5-cycle pulses |
How FDA classifies these devices
In the United States, 21 CFR 890.5300 (ultrasonic diathermy) splits the device into two classes by intended use. The threshold is energy at a frequency beyond 20 kilohertz.
| Regulation paragraph | Intended use as written | Class |
|---|---|---|
| 890.5300(a) | Energy beyond 20 kHz intended to generate deep heat within body tissues for selected medical conditions, but not for the treatment of malignancies | Class II (performance standards) |
| 890.5300(b) | Ultrasonic diathermy for all other uses except treatment of malignancies, meaning treatment by means other than the generation of deep heat | Class III (premarket approval) |
The openFDA device classification database (last updated 2026-10-19) lists three product codes under this regulation:
| Product code | Name in the database | Class |
|---|---|---|
| IMI | Ultrasonic Diathermy For Use In Applying Therapeutic Deep Heat | 2 |
| PFW | Stationary Ultrasonic Diathermy Device For Use In Applying Therapeutic Deep Heat (stationary applicators enabling hands-free operation) | 2 |
| LXF | Diathermy, Ultrasonic, For Use Other Than Applying Therapeutic Deep Heat | 3 |
Where it matters for labs, clinics and supply decisions
For clinic staff and procurement teams, the practical points that the sources support are narrow. The device class depends on the intended use stated for the device, so a deep-heat physical-therapy unit and a non-heating ultrasonic diathermy device sit in different regulatory classes. The FDA database records each of these categories under its own product code (IMI, PFW or LXF above). And because the transducer is applied with coupling gel, gel is a recurring consumable in this kind of setting. CASRAI’s physical therapy clinic supply checklist and chiropractic office supply checklist list ultrasound gel among consumables, and the pain management and sports medicine checklists cover ultrasound gel for point-of-care diagnostic ultrasound. Medical-device quality systems for manufacturers are covered in ISO 13485 certification.
Common misunderstandings
It is not the same as diagnostic ultrasound
Imaging ultrasound and therapeutic ultrasound both use sound, but the Miller review describes different purposes, and the 1-3 MHz physical-therapy range overlaps with, but is not the same as, the 2-12 MHz diagnostic range. Frequency alone does not tell you which one a device is; intended use does.
“Therapeutic ultrasound” is not one technology
The term covers physiotherapy warming, focused heating, nonthermal mechanical devices and LIPUS. Their mechanisms and regulatory status differ.
The evidence for physical-therapy ultrasound is described cautiously
The Miller review states that the level of clinical benefit from physical-therapy ultrasound “remains uncertain” and that the risk of harm, such as burns, appears to be low when the modality is properly applied. Its overall summary is a modest level of efficacy and patient benefit with a low level of risk.
FAQ
What is therapeutic ultrasound?
It is the use of ultrasonic energy to produce an effect in tissue, most commonly warming through an unfocused 1-3 MHz hand-held applicator in physical therapy.
How does therapeutic ultrasound work?
A base unit sends an electrical signal to a hand-held transducer applied through coupling gel. Absorbed acoustic energy is converted to heat, and the Miller review also describes nonthermal effects such as cavitation and mechanical stress.
Is a therapeutic ultrasound machine an FDA-regulated medical device?
Yes. Ultrasonic diathermy is classified under 21 CFR 890.5300: Class II for deep-heat use and Class III for other uses (excluding treatment of malignancies).
Is this medical advice?
No. This page is general reference information. Decisions about any person’s care belong with a qualified clinician.
Sources
Checked 2026-10-30.
- 21 CFR 890.5300, Ultrasonic diathermy: https://www.ecfr.gov/current/title-21/section-890.5300
- openFDA device classification, regulation number 890.5300: https://api.fda.gov/device/classification.json?search=regulation_number:890.5300
- Miller DL, Smith NB, Bailey MR, Czarnota GJ, et al. Overview of Therapeutic Ultrasound Applications and Safety Considerations. J Ultrasound Med, 2012: https://pmc.ncbi.nlm.nih.gov/articles/PMC3810427/
