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IEC TS 62791: Ultrasonics — Field Measurement for Physiotherapy
Technical specification for measuring ultrasonic fields produced by physiotherapy equipment to ensure safe and effective treatment
IEC TS 62791, published in 2015 as a Technical Specification, specifies methods for measuring and characterizing the ultrasonic fields produced by physiotherapy equipment operating in the frequency range of 0.5 MHz to 5 MHz. While therapeutic ultrasound has been used in physiotherapy for over 60 years, standardized field characterization methods have been essential for ensuring consistent treatment delivery and patient safety. The standard provides a framework for determining the Effective Radiating Area (ERA) and Beam Non-Uniformity Ratio (BNR), which are the two most critical parameters for clinical efficacy and safety of ultrasound physiotherapy treatments.
The standard was developed in response to growing evidence that many commercial physiotherapy ultrasound units delivered fields significantly different from their labeled specifications, leading to under-treatment or over-treatment of patients. Studies conducted before the standardization showed that up to 30% of physiotherapy ultrasound units in clinical use had output characteristics that deviated by more than 50% from their stated values, highlighting the critical need for standardized measurement and specification methods. The technical specification applies to both single-transducer and multi-transducer physiotherapy equipment used for thermal and non-thermal therapeutic effects.
Ultrasound physiotherapy operates through two primary mechanisms: thermal effects (tissue heating through absorption of acoustic energy) and non-thermal effects (cavitation, acoustic streaming, and micro-massage). The ERA and BNR parameters directly influence the spatial distribution and intensity of these effects within the target tissue volume.
Effective Radiating Area and Beam Non-Uniformity Ratio
Effective Radiating Area (ERA) is defined as the area of the transducer face that actually contributes to the radiated ultrasonic field. It is determined from a pressure field scan performed at a specified distance from the transducer face, typically 5 mm for most physiotherapy applicators. The ERA is calculated as the area within which the acoustic pressure squared integral (p2) exceeds 5% of the maximum value. This parameter is critical because it determines the effective treatment area and, together with the total output power, the average intensity delivered to the tissue.
Beam Non-Uniformity Ratio (BNR) quantifies how evenly the acoustic energy is distributed across the treatment area. It is defined as the ratio of the peak intensity to the average intensity across the ERA. A perfectly uniform beam would have a BNR of 1.0, while clinically acceptable applicators typically have BNR values below 6:1. Higher BNR values indicate “hot spots” in the beam that can cause patient discomfort, pain, or tissue damage during treatment. The standard specifies that BNR must be determined from the same spatial pressure scan used for ERA calculation, ensuring consistency between the two critical parameters.
| Parameter | Symbol | Definition | Typical Range |
|---|---|---|---|
| Effective Radiating Area | ERA | Area of transducer contributing to field | 1-20 cm2 |
| Beam Non-Uniformity Ratio | BNR | Peak intensity / average intensity | 2:1 to 8:1 |
| Operating frequency | f | Center frequency of transducer | 0.5-5 MHz |
| Total output power | Ptot | Total acoustic power output | 0.1-20 W |
| Spatial-average intensity | ISA | Average intensity across ERA | 0.1-3.0 W/cm2 |
| Spatial-peak intensity | ISP | Peak intensity within field | 0.2-12 W/cm2 |
The measurement procedure requires a computer-controlled scanning system capable of positioning a hydrophone with an accuracy of at least 0.1 mm in all three axes. The hydrophone must be calibrated with traceability to national standards, with a calibrated frequency response covering the operating frequency and its harmonics up to at least the third harmonic. The pressure field is scanned in a plane parallel to the transducer face at the specified measurement distance, with step sizes no larger than half the transducer radius or 2 mm, whichever is smaller. A sufficient number of measurement points must be acquired to resolve the field structure, with at least 100 points within the -6 dB beam area for accurate ERA determination.
An applicator with a high BNR (above 8:1) can create localized tissue heating exceeding 5 degrees C within seconds while the average tissue temperature rise remains below 1 degree C. This can cause pain, burns, or tissue damage even when the treatment appears to be within normal parameters based on average intensity readings alone.
Power Measurement and Clinical Implications
Total output power is measured using a radiation force balance method, where the acoustic beam impinges on a target (typically a rubber or metallic absorbing cone) suspended from a precision balance. The measured radiation force is directly proportional to the total acoustic power output. The standard requires power measurement accuracy of better than +/- 10% for the clinically relevant range of 0.5 to 15 W. Combined with the ERA measurement, the spatial-average intensity (ISA) is calculated as the total power divided by the ERA, providing the clinically relevant parameter for treatment planning.
The spatial-peak intensity (ISP) is derived from the pressure field scan and is used together with ISA to determine the BNR. Clinical studies have demonstrated that applicators with BNR below 4:1 provide more consistent treatment outcomes with fewer adverse effects compared to those with higher BNR values. The effective treatment time required to achieve a therapeutic temperature rise of 3-5 degrees C at a depth of 2-3 cm is inversely proportional to ISA for thermal treatments, while non-thermal treatments operating at lower intensities (0.1-0.5 W/cm2) rely primarily on ISP for initiating cavitation and acoustic streaming effects within the tissue. The standard recommends that manufacturers specify both ISA and ISP values on the equipment labeling to enable clinicians to select appropriate treatment parameters for specific clinical indications.
Frequency selection in physiotherapy involves a trade-off between penetration depth and absorption rate. At 1 MHz, approximately 50% of the acoustic energy reaches a depth of 3-5 cm, making 1 MHz suitable for treating deeper tissue structures such as muscles, tendons, and ligaments. At 3 MHz, the penetration depth reduces to 1-3 cm due to higher absorption in tissue, making it appropriate for superficial structures including skin, subcutaneous tissue, and superficial tendons. The standard requires that the effective field characteristics be specified for each operating frequency of a multi-frequency applicator, as the ERA and BNR often differ significantly between frequencies due to the transducer design and construction.
Modern physiotherapy ultrasound units with BNR below 4:1 and stable ERA specifications enable clinicians to deliver precise, reproducible treatments. Combined with calibrated output power, these parameters ensure that the prescribed acoustic energy dose is actually delivered to the target tissue, dramatically improving treatment outcomes.
Engineering Design Insights for Physiotherapy Ultrasound
From an engineering perspective, achieving a low BNR while maintaining adequate ERA requires careful transducer design. The vibration pattern of the piezoelectric element must be controlled through proper electrode geometry, backing layer design, and matching layer optimization. Thickness-mode resonators operating at their fundamental frequency produce the most uniform vibration patterns. Edge effects and mode coupling in the piezoelectric element can create localized vibration nodes that increase BNR, requiring careful mechanical isolation of the active element from the transducer housing.
The acoustic matching layer design is critical for efficient energy transfer from the transducer to the tissue. A quarter-wavelength matching layer with an acoustic impedance intermediate between the piezoelectric ceramic (30-35 MRayls) and tissue (1.5 MRayls) can improve transmission efficiency from 50% to over 90%. For physiotherapy applicators operating in the 1-3 MHz range, the matching layer thickness is typically 0.15-0.5 mm, requiring precise manufacturing tolerances. Multi-layer matching designs can further improve bandwidth and pulse characteristics but add manufacturing complexity and cost.
Field measurement itself requires careful attention to experimental conditions. The water tank used for measurements must be filled with degassed, deionized water to minimize bubble formation on the hydrophone and transducer surfaces, which would scatter the acoustic field and introduce measurement errors. The water temperature should be maintained at 22 +/- 3 degrees C, as the speed of sound in water varies with temperature by approximately 2 m/s per degree C, affecting spatial registration of the measured field. The standard also provides guidance on measurement uncertainty, with the combined uncertainty for ERA determination typically in the range of 10-15% (k=2) when the measurement protocol is carefully followed.
| Target Depth | Frequency | Intensity ISA | Treatment Mode | Min. ERA |
|---|---|---|---|---|
| Superficial (< 2 cm) | 3-5 MHz | 0.5-1.5 W/cm2 | Thermal | 2 cm2 |
| Medium (2-4 cm) | 1-3 MHz | 0.5-2.0 W/cm2 | Thermal / Non-thermal | 4 cm2 |
| Deep (4-6 cm) | 0.5-1 MHz | 0.2-1.0 W/cm2 | Non-thermal | 5 cm2 |
| Acupuncture points | 1-3 MHz | 0.1-0.5 W/cm2 | Non-thermal (pulsed) | 0.5 cm2 |
Q1: Why are ERA and BNR important for clinical physiotherapy?
A: ERA determines the effective treatment area and, combined with total power, the average tissue heating rate. BNR indicates beam uniformity; high BNR creates hot spots that can cause pain or burns even when average intensity appears safe.
A: ERA determines the effective treatment area and, combined with total power, the average tissue heating rate. BNR indicates beam uniformity; high BNR creates hot spots that can cause pain or burns even when average intensity appears safe.
Q2: How often should physiotherapy ultrasound equipment be calibrated?
A: The standard recommends annual calibration at minimum, with more frequent checks (every 6 months) for high-usage clinical equipment. Power output should be verified using a radiation force balance, and ERA/BNR should be re-measured whenever the transducer is replaced or repaired.
A: The standard recommends annual calibration at minimum, with more frequent checks (every 6 months) for high-usage clinical equipment. Power output should be verified using a radiation force balance, and ERA/BNR should be re-measured whenever the transducer is replaced or repaired.
Q3: What is the difference between thermal and non-thermal ultrasound therapy?
A: Thermal therapy uses continuous-wave ultrasound at 1-3 W/cm2 to heat tissue for pain relief and muscle relaxation. Non-thermal therapy uses pulsed ultrasound at lower intensities (0.1-0.5 W/cm2) to promote tissue healing through cavitation and acoustic streaming effects.
A: Thermal therapy uses continuous-wave ultrasound at 1-3 W/cm2 to heat tissue for pain relief and muscle relaxation. Non-thermal therapy uses pulsed ultrasound at lower intensities (0.1-0.5 W/cm2) to promote tissue healing through cavitation and acoustic streaming effects.
Q4: Can the same transducer be used for both 1 MHz and 3 MHz operation?
A: Some dual-frequency applicators use a single piezoelectric element designed to operate at both fundamental and third harmonic frequencies. However, the ERA and BNR typically differ between the two frequencies, and the standard requires separate characterization for each operating frequency.
A: Some dual-frequency applicators use a single piezoelectric element designed to operate at both fundamental and third harmonic frequencies. However, the ERA and BNR typically differ between the two frequencies, and the standard requires separate characterization for each operating frequency.
