6.1 Therapeutic Ultrasound

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 National Physical Therapy Examination for Physical Therapist Assistants (NPTE-PTA), ultrasound items most often test (1) frequency selection by tissue depth, (2) thermal versus non-thermal selection by duty cycle, (3) safe treatment-area sizing, and (4) recognition of absolute contraindications. The mechanical energy travels poorly through air, so an acoustic coupling agent (gel, gel pad, or water) is mandatory.

Frequency and Depth

Frequency is inversely related to depth of penetration. Lower frequency carries energy deeper because the longer wavelength is absorbed more slowly by superficial tissue.

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. A common trap pairs a deep target with 3 MHz: the energy is absorbed before it reaches the tissue, so no clinically useful heating occurs.

Duty Cycle: Thermal vs Non-Thermal

Duty cycle is the percent of time the ultrasound beam is actually on during a treatment cycle. It determines whether the dominant effect is heat or mechanical (cavitation and acoustic microstreaming).

Intensity is reported in W/cm-squared (spatial average temporal average, SATA). Typical thermal doses for the NPTE-PTA range from 1.0 to 2.0 W/cm-squared continuous; non-thermal doses use lower intensities (0.3-0.5 W/cm-squared) at low duty cycles. A vigorous thermal dose raises tissue temperature 4 degrees Celsius to increase collagen extensibility before stretch; a mild dose of 1 degree Celsius increases metabolism without a true stretching window.

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, so a 5 cm-squared head may have an ERA of only 3-4 cm-squared.
• 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. The Food and Drug Administration limits BNR to a maximum of 8:1 for commercial units.

Treatment Area and Technique

A reliable exam rule: treatment area is less than or equal to 2 times 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 with the head held about 1 cm from the skin.

Treatment duration is usually 5-10 minutes per area (longer for larger 2-times-ERA fields). The PTA should monitor for periosteal pain, an early warning of bone heating from standing waves, and reduce intensity or stop if it occurs.

Absolute Contraindications

Memorize these. A single contraindication makes ultrasound the wrong answer regardless of how well it would otherwise treat the impairment.

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.