6.1 Therapeutic Ultrasound
Key Takeaways
- 1 MHz ultrasound penetrates approximately 5 cm and targets deep tissue; 3 MHz penetrates 1-2 cm and targets superficial tissue.
- Continuous-wave ultrasound at 100% duty cycle produces thermal effects; pulsed ultrasound at 20% or 50% duty cycle produces predominantly non-thermal (mechanical) effects.
- Treatment area should be no larger than two times the effective radiating area (ERA) of the sound head and the head must move continuously to avoid hot spots.
- Absolute contraindications include application over malignancy, a pacemaker, the pregnant uterus or low back, the eyes, open growth plates in children, active infection, deep vein thrombosis, and other vascular abnormalities.
- A Physical Therapist Assistant (PTA) applies ultrasound only within the plan of care (POC) established by the supervising Physical Therapist (PT).
Therapeutic Ultrasound
Therapeutic ultrasound uses high-frequency sound waves (typically 1 MHz or 3 MHz) delivered through a coupling medium to produce thermal or non-thermal (mechanical) effects in soft tissue. On the NPTE-PTA, ultrasound items most often test (1) frequency selection by tissue depth, (2) thermal vs non-thermal selection by duty cycle, (3) safe treatment-area sizing, and (4) recognition of absolute contraindications.
Frequency and Depth
Frequency is inversely related to depth of penetration. Lower frequency carries energy deeper because the longer wavelength is absorbed less rapidly.
| Frequency | Approximate depth | Typical target |
|---|---|---|
| 1 MHz | Up to ~5 cm | Deep muscle, hip rotators, gluteals, hamstrings |
| 3 MHz | ~1-2 cm | Lateral epicondyle, finger tendons, scar tissue, plantar fascia |
If the supervising PT writes "ultrasound to the right supraspinatus tendon," the PTA selects 3 MHz because the rotator cuff sits within the superficial-to-mid-depth window. If the order is "ultrasound to the piriformis," 1 MHz is the correct choice.
Duty Cycle: Thermal vs Non-Thermal
Duty cycle is the percent of time the ultrasound beam is actually on during a treatment second. It determines whether the dominant effect is heat or mechanical (cavitation/microstreaming).
| Duty cycle | Mode | Dominant effect | Typical use |
|---|---|---|---|
| 100% | Continuous wave | Thermal | Chronic contracture, deep heating before stretch, chronic tendinopathy |
| 50% | Pulsed | Mixed; mostly non-thermal | Subacute soft-tissue injury |
| 20% | Pulsed | Non-thermal | Acute injury, tissue healing, post-surgical edema |
Intensity is reported in W/cm² (spatial average temporal average, SATA). Typical thermal doses for the NPTE-PTA range from 1.0 to 2.0 W/cm² continuous; non-thermal doses use lower intensities at low duty cycles.
ERA and BNR
Two manufacturer-reported sound-head specifications are critical:
- Effective Radiating Area (ERA): the portion of the sound head that actually emits ultrasound. ERA is always smaller than the total head surface.
- Beam Nonuniformity Ratio (BNR): the ratio of the highest peak intensity in the beam to the spatial-average intensity. A BNR closer to 1:1 produces a more uniform beam; high BNR values (5:1 or 6:1) increase the risk of "hot spots."
Lower BNR is safer. Higher ERA covers more tissue per pass.
Treatment Area and Technique
A reliable exam rule: treatment area ≤ 2 × ERA. Treating a region larger than this dilutes the dose and the head cannot return to any given spot often enough to deliver useful energy.
Apply using a stroking (longitudinal or circular) technique. The head moves continuously at roughly 4 cm/second; never hold the head still over bone, hardware, or a fluid-filled cavity. Maintain firm coupling with gel or, for irregular surfaces such as the hand, an underwater technique.
Treatment duration is usually 5-10 minutes per area (longer for larger 2 × ERA fields). The PTA should monitor for periosteal pain, an early warning of bone heating, and reduce intensity or stop if it occurs.
Absolute Contraindications
Memorize these. A single contraindication makes ultrasound the wrong answer on the exam regardless of how well it would otherwise treat the impairment.
| Contraindication | Reason |
|---|---|
| Over a known or suspected malignancy | May accelerate tumor growth and metastasis |
| Over a pacemaker or implanted electronic device | Risk of device dysfunction |
| Over the pregnant uterus or low back during pregnancy | Risk to the fetus |
| Over the eyes | Cavitation in vitreous fluid |
| Over open growth plates (epiphyses) in children | Possible disturbance of bone growth |
| Over active infection in soft tissue | May spread infection |
| Over deep vein thrombosis or arterial insufficiency | Risk of embolus or thermal injury to ischemic tissue |
| Over the carotid sinus, anterior neck, or cervical ganglia | Autonomic and cardiovascular risks |
| Over areas of impaired sensation without strong rationale | Cannot detect overdose |
PTA Scope
The PTA selects the appropriate frequency, duty cycle, intensity, and technique within the parameters the supervising PT documented in the POC, monitors the patient's response, and reports any adverse response or lack of progress. The PTA does not initiate ultrasound for a new diagnosis, change the prescribed mode without supervision, or apply ultrasound when an absolute contraindication is present, even if the POC seems to allow it.
The supervising PT's plan of care directs the PTA to apply therapeutic ultrasound to a patient's left lateral epicondyle for chronic tendinopathy. Which parameter set is most appropriate?
Which patient presentation is an ABSOLUTE contraindication to therapeutic ultrasound, regardless of plan of care?