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IEC TS 62736-2016: Ultrasonic Pulse-Echo Scanner Periodic Testing for Stability
Simple Methods for Verifying Stability of Medical Ultrasound Imaging Systems
1. Introduction and Background
IEC TS 62736-2016, prepared by IEC TC 87 (Ultrasonics), addresses an urgent clinical problem: studies cited in the standard show approximately 14% transducer failure rate and 10% system failure rate per year on first testing for medical ultrasound systems. With an estimated 100,000+ systems worldwide, this means tens of thousands of systems are performing suboptimal diagnostic exams at any given time. The standard provides simple, practical quality control (QC) methods that can be performed by clinical staff without specialized engineering support, organized into three levels of increasing complexity and decreasing frequency.
Tip: The key philosophy behind this standard is that many traditional ultrasound QC tests (such as spatial resolution measurement) are “low-yield” and better suited for performance evaluation rather than routine QC. The focus here is on detecting the most common failures: transducer element degradation, channel failure, and loss of penetration — which account for the majority of clinically significant performance issues.
2. Three-Level Quality Control Framework
| Level | Frequency | Key Tests | Who Performs | Time Required |
|---|---|---|---|---|
| Level 1 | Monthly | Visual inspection, display check, simple uniformity check | Clinical user | 5-10 minutes |
| Level 2 | Semi-annually | Image uniformity (element/channel integrity), mechanical inspection | QC professional | 30-45 minutes |
| Level 3 | Biennially (every 2 years) | All Level 2 + maximum depth of penetration, distance measurement accuracy, display system evaluation | QC professional | 2-3 hours |
Important: Level 2 and Level 3 tests require tissue-mimicking phantoms. The standard provides detailed guidance on phantom selection and verification. Annex A shows example phantom designs including the “three-in-one” phantom concept that combines distance measurement pins, uniformity sections, and penetration-depth targets in a single device for cost-effective QC.
3. Key Test Methods
3.1 Image Uniformity Test (Level 2)
This is the most important test in the standard for detecting transducer element and channel failures. The method involves imaging a uniform section of a tissue-mimicking phantom and analyzing the resulting image for brightness variations across the lateral dimension. The procedure:
- Position the transducer over a uniform region of the phantom with no visible targets
- Acquire a sequence of frames (e.g., 50 frames) and compute the median image to reduce speckle noise
- Plot the mean pixel value as a function of lateral position across the image
- Identify dips in the profile — a dip of more than 20% relative to the surrounding area indicates a failed transducer element or channel
3.2 Maximum Relative Depth of Penetration (Level 3)
This test measures the maximum depth at which echoes are detectable above the noise floor. The standard specifies:
- Use a tissue-mimicking phantom with a uniform region of known attenuation (typically 0.5-0.7 dB/cm/MHz)
- Acquire the image at maximum gain with the TGC set to a uniform profile
- Also acquire a “noise image” with the transducer decoupled from the phantom
- Plot mean pixel value vs. depth for both phantom and noise images
- The maximum depth of penetration is where the phantom signal curve crosses the noise floor + threshold
Engineering Insight: The image uniformity test can be automated using open-source software. Annex B describes one such tool that generates lateral profiles and control charts. Implementing a statistical process control (SPC) approach — plotting uniformity metrics on a control chart over time — enables early detection of gradual transducer degradation before it affects clinical image quality. A transducer showing a downward trend in uniformity should be scheduled for replacement before it reaches the failure threshold. This proactive approach can reduce unplanned downtime by 30-50%.
4. Equipment and Phantom Requirements
| Test Level | Phantom Required | Key Phantom Specifications |
|---|---|---|
| Level 1 | None (visual check) | N/A |
| Level 2 | Uniformity phantom | Homogeneous tissue-mimicking material, attenuation 0.5-0.7 dB/cm/MHz, speed of sound 1540 m/s |
| Level 3 | Multi-purpose phantom | Uniform region + anechoic targets + distance measurement pins, verified stability over time |
Critical: Phantom stability is a major concern. Tissue-mimicking materials can degrade over time due to dehydration, microbial growth, and temperature cycling. The standard requires that phantoms be verified periodically (typically annually) for continued stability of acoustic properties. Document the phantom serial number, purchase date, and verification status. Replace phantoms according to the manufacturer’s recommended lifespan, typically 2-5 years depending on construction and care.
5. Frequently Asked Questions
Q1: Why does the standard recommend monthly Level 1 tests instead of daily or weekly?
A: The standard is designed as a minimum set of tests. Many organizations choose to perform Level 1 checks (visual inspection and display check) more frequently — even daily — as part of their equipment startup procedure. The monthly recommendation represents the minimum acceptable frequency for maintaining QC accreditation.
Q2: What is the typical transducer failure rate in clinical practice?
A: The standard cites approximately 14% failure rate per year. Common failure modes include: broken or delaminated lens, failed piezoelectric elements (cracks or depolarization), cable/connector damage from repeated flexing, and electronic channel failure in the system front end. Array transducers (linear and curved arrays) are more susceptible to element failure than single-element or phased-array probes.
Q3: Can the test methods be used for Doppler and harmonic imaging modes?
A: The standard’s primary focus is on B-mode imaging. However, it notes that failures detected in B-mode will also affect other modes such as colour-flow, harmonic, elasticity, and compound imaging. Dedicated Doppler systems are excluded and require specialized test equipment. For harmonic imaging QC, the standard recommends using the fundamental frequency depth of penetration as a reference.
Q4: How does the standard address display system testing?
A: Display testing is included at Level 1 (visual inspection for dead pixels, brightness, contrast) and Level 3 (using test patterns from AAPM TG18). Modern LCD displays are more stable than CRTs but can still experience luminance drift, dead pixels, and colour uniformity issues. Annex C provides sample display test patterns and evaluation forms.
