IEC 61220 — Ultrasonics — Fields — Guidance for Measurement and Characterization of Medical Ultrasonic Fields (0.5 to 15 MHz)

Standard: IEC 61220 | Scope: Measurement and characterization of medical ultrasonic fields in the frequency range 0.5 MHz to 15 MHz

💡 Key Insight: IEC 61220 provides systematic guidance for the measurement and characterization of medical ultrasonic fields in the 0.5 MHz to 15 MHz frequency range. It defines key acoustic parameters, specifies hydrophone measurement methods, scanning techniques, and data processing procedures. This standard is the technical foundation for performance assessment and safety evaluation of diagnostic ultrasound equipment.

1. Standard Overview and Scope

IEC 61220, developed by IEC Technical Committee 87 (Ultrasonics), is the fundamental guidance standard for medical ultrasonic field measurement. It covers the frequency range from 0.5 MHz to 15 MHz, which encompasses the operating frequencies of the vast majority of medical diagnostic ultrasound equipment — abdominal (3.5-5 MHz), cardiac (2-3.5 MHz), small parts (7-14 MHz), and ophthalmic (10-20 MHz) applications.

The standard’s purpose is to establish a unified, reproducible measurement methodology framework that ensures comparability of measurement data across different laboratories and manufacturers. It defines key acoustic field parameters, specifies hydrophone selection and usage conditions, provides recommended practices for field scanning and data processing, and offers guidance for measurement uncertainty evaluation.

⚠️ Important Note: The acoustic output parameters of medical ultrasound devices directly relate to clinical safety. IEC 61220 measurement methods are directly referenced by IEC 60601-2-37 (Particular requirements for the safety of ultrasonic medical diagnostic and monitoring equipment) as the standard method for acoustic output measurement. Understanding this standard is essential for ultrasound equipment manufacturers and testing laboratories.

2. Ultrasonic Field Parameters and Measurement Methods

2.1 Key Acoustic Parameters

The standard defines the following core field parameters: acoustic pressure (instantaneous pressure p(t), peak-positive pressure p+, peak-negative pressure p-); acoustic intensity (spatial-peak temporal-average intensity I_SPTA, spatial-peak pulse-average intensity I_SPPA, spatial-average temporal-average intensity I_SATA); acoustic power (total output power W); beam width and beam non-uniformity ratio (BNR). Among these, I_SPTA and I_SPPA are the critical indicators for ultrasound safety assessment.

2.2 Hydrophone Measurement Techniques

The hydrophone is the primary measurement tool for ultrasonic field characterization. IEC 61220 specifies detailed selection criteria: membrane hydrophones offer broadband measurement (up to ~50 MHz) with flat frequency response but lower sensitivity; needle hydrophones provide higher sensitivity but narrower bandwidth and poorer directional response. The standard requires that the hydrophone’s effective aperture be less than half the wavelength of the measured acoustic field to avoid spatial averaging errors.

Field Parameter	Symbol	Definition	Typical Diagnostic Values
Peak-negative pressure	p-	Maximum absolute rarefactional pressure	1.0 – 4.5 MPa
Peak-positive pressure	p+	Maximum compressional pressure	2.0 – 8.0 MPa
Spatial-peak temporal-average intensity	I_SPTA	Temporal-average intensity at spatial peak	10 – 720 mW/cm²
Spatial-peak pulse-average intensity	I_SPPA	Pulse-average intensity at spatial peak	10 – 300 W/cm²
Total acoustic power	W	Total ultrasonic power from transducer	1 – 300 mW
Beam width (-6 dB)	BW	Width at 50% of peak pressure	0.5 – 5 mm
Beam non-uniformity ratio	BNR	I_SPTA / I_SATA	2 – 20

3. Measurement Systems and Engineering Practice

3.1 Measurement System Configuration

A complete ultrasonic field measurement system comprises: precision positioning system (3-axis or 5-axis scanning stage, step resolution better than 0.01 mm); hydrophone (membrane or needle type with preamplifier); digitizing oscilloscope or data acquisition system (sampling rate at least 100 MS/s, bandwidth at least 50 MHz); water tank (degassed water, temperature controlled at 22±3 °C); and acoustic absorber material to prevent reflection artifacts. The entire system requires careful calibration before testing.

3.2 Scanning Strategy

The standard recommends automated scanning for field measurements. A typical scanning protocol includes: coarse scanning to locate the acoustic beam axis and point of maximum pressure; fine axial scanning (along the beam axis) and lateral scanning (perpendicular to the beam axis); and finally, two-dimensional planar scanning to obtain the complete acoustic field distribution. For phased array probes, multi-focus scanning is required to evaluate field characteristics at different focal depths.

🌟 Practice Recommendation: Hydrophone positioning accuracy directly affects measurement results. Before each measurement session, calibrate the hydrophone’s effective sensitive element position using the “needle pulse” or “reciprocity” method. High-intensity fields may damage hydrophones through cavitation effects; always verify the hydrophone’s pressure withstand limit before measuring high-output probes.

3.3 Measurement Uncertainty Evaluation

IEC 61220 requires uncertainty analysis for all measurement results. Major sources of uncertainty in ultrasonic field measurement include: hydrophone calibration uncertainty (typically ±10%-15%); positioning system errors (±0.05 mm); temperature effects on sound speed (±0.2%); hydrophone spatial averaging effects (±5%-20% at lower frequencies); electrical noise and digitization errors; and water attenuation correction uncertainty. The standard recommends evaluation following the GUM (Guide to the Expression of Uncertainty in Measurement) methodology.

Probe Type	Typical Frequency	Typical p- (MPa)	Typical I_SPTA (mW/cm²)	Primary Application
Convex abdominal	3.5-5 MHz	1.5-3.0	50-300	Abdominal organ exam
Phased array cardiac	2-3.5 MHz	1.8-4.5	100-720	Echocardiography
Linear small parts	7-14 MHz	2.0-4.0	10-100	Thyroid, breast
Endocavity	5-9 MHz	1.0-2.5	30-150	Gynecology, urology
Color Doppler mode	2-5 MHz	2.0-4.5	200-720	Blood flow imaging

4. Frequently Asked Questions (FAQ)

❓ What is the relationship between IEC 61220, IEC 61157, and IEC 60601-2-37?

These three standards form the regulatory framework for medical ultrasound safety assessment. IEC 61220 provides the fundamental measurement methodology; IEC 61157 specifies the requirements for declaring acoustic output parameters of diagnostic ultrasound equipment (mandatory parameters, standard measurement conditions, and declaration format); IEC 60601-2-37 is the particular safety standard for ultrasound diagnostic equipment, referencing the first two for acoustic output measurement methods.

❓ Why is degassed water used as the propagation medium for ultrasonic field measurements?

Degassed water contains minimal dissolved gas, greatly reducing bubble scattering and attenuation along the ultrasound propagation path. Water gas content should be below 4 mg/L (achieved by boiling then cooling, or vacuum degassing). Additionally, particulate matter and microbial content should be minimized to reduce acoustic scattering and absorption losses. Water temperature must be tightly controlled because sound speed varies by approximately 0.2%/°C with temperature.

❓ What are the comparative advantages of membrane versus needle hydrophones?

Membrane hydrophones offer: extremely wide bandwidth (up to 50 MHz+), flat frequency response, small effective aperture (0.2-0.5 mm), and excellent directional response. Disadvantages include: lower sensitivity (typically requiring 40-60 dB preamplification), sensitivity to pressure shock, and careful handling requirements. Needle hydrophones offer: high sensitivity, robust construction, and ease of use. Disadvantages include: narrower bandwidth (typically < 20 MHz), larger effective aperture (0.5-1 mm), non-uniform low-frequency response, and poor directionality. For diagnostic ultrasound frequencies, membrane hydrophones are the preferred choice.

❓ How is ultrasound device safety assessed using IEC 61220 methods?

The safety assessment procedure includes: (1) measure acoustic output parameters (p-, I_SPTA, I_SPPA, etc.) under standard measurement conditions per IEC 61220; (2) compare measured values against the limits specified in IEC 60601-2-37 (e.g., I_SPTA should not exceed 720 mW/cm², I_SPPA should not exceed 190 W/cm²); (3) calculate Thermal Index (TI) and Mechanical Index (MI) and verify that clinical operating modes stay within recommended limits; (4) declare output parameters and safety indices in the equipment instructions for use as required by applicable regulations.