5.3 Therapeutic Ultrasound and Phonophoresis
Key Takeaways
A piezoelectric applicator converts electrical input into acoustic waves. At 3 MHz attenuation is generally greater than at 1 MHz under comparable conditions, with no fixed depth cutoff.
Duty cycle is on time divided by total cycle time. Pulsing lowers temporal-average intensity when on-phase intensity stays fixed, and heating can still occur.
ERA is a measured effective emitting area. BNR relates spatial maximum to effective intensity and describes the beam in a test field.
Phonophoresis attempts to enhance topical drug delivery. Uptake and clinical effect depend on the medication, formulation, exposure and condition.
Use condition-specific evidence and the actual device labeling for treatment selection, coupling, safety screening and dosing.
Therapeutic Ultrasound and Phonophoresis
Therapeutic ultrasound transmits acoustic energy through a coupling medium into tissue. In a typical physiotherapy transducer, an alternating voltage deforms a piezoelectric element. Its vibration launches longitudinal compression and rarefaction waves in soft tissue. Distinguish the physical effect of a beam from the evidence that a course of treatment improves symptoms or function.
Frequency, attenuation and heating
Absorption converts acoustic energy into heat. Scattering and reflection can also reduce energy reaching the target. A boundary between tissues of different acoustic impedance reflects some energy. Refraction changes beam direction at an oblique boundary. The transmitted energy depends on the materials, frequency, angle and path. Keep the applicator coupled and watch for discomfort, especially near superficial bone.
| Frequency | Practical comparison in similar soft tissue |
|---|---|
| 1 MHz | Generally attenuated less per unit depth than 3 MHz and can deliver energy relatively deeper. With sound speed about 1,540 m/s, wavelength is about 1.54 mm. |
| 3 MHz | Generally attenuated more per unit depth and heats shallower tissue more readily under comparable conditions. Wavelength is about 0.51 mm at the same sound speed. |
Wavelength is speed divided by frequency. Penetration is gradual and varies with tissue and applicator. In one human study, 3 MHz also heated muscle at 2.5 cm. Measured heating rates depend on intensity, time, depth, tissue and equipment. They do not follow a fixed threefold rule. Measurements of muscle cooling after 1 and 3 MHz applications also differ by depth and protocol. If using heat before movement, plan the sequence with the measured response in mind. A fixed temperature rise or three-minute deadline cannot be inferred for every joint capsule.
Continuous and pulsed exposure
Continuous output has a 100% duty cycle. In pulsed output, divide on time by the whole cycle time. For 2 ms on and 8 ms off the fraction is 2/10, or 20%. If the spatial average during the on period is 1.0 W/cm² and the off-period output is zero, the spatial-average temporal-average intensity is 0.20 W/cm². If the display already reports a temporal average, use that quantity directly. Check the manufacturer's intensity convention.
Pulsing reduces average exposure at a fixed on-phase intensity. Heating can still occur. In a human gastrocnemius study, 3 MHz at 1.0 W/cm² with 50% duty cycle and continuous 3 MHz at 0.5 W/cm² produced similar temperature rises at 2 cm under the tested conditions. Mechanical effects include acoustic streaming, the movement of fluid driven by sound, and microstreaming around oscillating bubbles. Cavitation refers to bubble behavior, including oscillation or, under other conditions, collapse. These effects depend on the acoustic field and medium. A pulsed setting alone cannot establish cavitation or tissue repair in a patient.
Beam, applicator and coupling
Effective radiating area (ERA) describes the effective emitting area of an applicator measured by a defined test. Use the applicator's labeled ERA when relating power to intensity. The physical size of the faceplate is a separate measurement. The beam nonuniformity ratio (BNR) compares the spatial maximum intensity with the effective intensity using the same temporal basis. A BNR of 6 and an effective intensity of 1.5 W/cm² imply a spatial maximum of 9 W/cm² in the specified measurement field. This calculation describes beam output. It cannot predict the intensity at a particular point in living tissue.
Select an applicator and treatment area using the device instructions. For a 2.5 cm² ERA, 5 cm² is twice the ERA. That ratio describes coverage, while the exposure also depends on time, technique and output. With a conventional moving applicator, maintain full contact through approved gel and move the head smoothly over the area. Use a water or other indirect technique only if the specific device allows it. Air gaps impair transmission. Stop for sharp or unexpected discomfort and check the patient, contact and settings.
Phonophoresis
Phonophoresis uses ultrasound to try to increase delivery of a topical medication through the stratum corneum. Laboratory work supports a role for bubble activity and effects on the skin barrier. Delivery depends on the formulation, exposure and site. A study using occluded dexamethasone found greater serum appearance after 3 MHz pulsed ultrasound than after sham. A separate study using 10% hydrocortisone acetate found no difference in drug concentration in sampled connective tissue between active and sham application. Serum detection, target-tissue delivery and clinical benefit are different outcomes.
If considering phonophoresis, confirm medication authorization, contraindications and formulation compatibility as well as the ultrasound instructions. There is no universal drug mixture or dose. The 2022 APTA lateral elbow guideline recommends against specific hydrocortisone, prednisolone and diclofenac regimens and makes no recommendation for stand-alone ultrasound because results conflict.
Safety screening and clinical case
Follow the instructions for the actual applicator and indication. The adult Intelect Mobile 2 ultrasound instructions prohibit use over an eye, a pregnant uterus, a healing fracture or a cancerous lesion, and over or near active bone growth centers. They also exclude patients with specified implanted electronic devices. These are device-specific instructions. The manual identifies a post-laminectomy site and anesthetic areas as requiring extra precautions. Check local circulation, sensation, bleeding risk and ability to report symptoms. Defer local ultrasound if acute thrombosis is suspected and arrange appropriate assessment. For a patient with an implant, confirm implant type, site and restrictions rather than assuming that all materials behave the same way.
A tennis player has eight weeks of lateral elbow pain reproduced by gripping and resisted wrist extension. Knowing the extensor region is superficial helps explain which beam attenuates more quickly. It does not determine whether ultrasound will help the patient. The clinician starts graded wrist extensor loading and tracks pain with gripping and return to play. As a separate example, the 2025 rotator cuff guideline recommends against therapeutic ultrasound for both calcific and noncalcific rotator cuff tendinopathy in adults. These treatment decisions depend on the indication and outcome as well as device safety.
With two comparable applicators and similar soft tissue paths, which statement best describes a typical difference between 1 MHz and 3 MHz ultrasound?
One megahertz always heats more quickly near the skin
Three megahertz stops abruptly at a depth of 2 cm
Three megahertz generally attenuates more rapidly with depth, and the actual heating rate depends on exposure and tissue
The beam frequency has no relation to wavelength in a specified tissue
The display reports an on-phase spatial average of 1.0 W/cm². Each cycle has 2 ms on and 8 ms off with no off-phase output. What is the temporal average?
1.0 W/cm²
0.80 W/cm²
0.25 W/cm²
0.20 W/cm²
An applicator has a BNR of 6 and an effective intensity of 1.5 W/cm² on the same temporal basis. What follows from these specifications?
The spatial maximum is 9 W/cm² in the specified test field
The spatial maximum is 0.25 W/cm² in the test field
Every point in the patient’s tendon receives 9 W/cm²
The applicator’s ERA is 6 cm²
A clinician is considering ultrasound over the uterus of a pregnant adult. What do the Intelect Mobile 2 therapeutic ultrasound instructions require?
Use 20% duty cycle to allow the application
Do not apply the sound head over the pregnant uterus
Use 3 MHz and shorten the treatment time
Proceed with continuous output if the patient reports no warming
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