IEC TS 62558 – Ultrasound Phantom with Cylindrical Cysts for 3D VDR Evaluation

Ultrasonics – Real-time pulse-echo scanners – Phantom with cylindrical, artificial cysts in tissue-mimicking material for VDR testing

Periodic quality assurance of diagnostic ultrasound systems is essential for maintaining diagnostic accuracy and patient safety. IEC TS 62558:2011 provides a standardized approach for evaluating the void-detectability ratio (VDR) of 3D ultrasound imaging systems using a specially designed phantom containing cylindrical artificial cysts embedded in tissue-mimicking material (TMM).

📋 Scope and Application

IEC TS 62558 specifies a measurement method and phantom design for evaluating and periodically testing the 3D distributions of void-detectability ratio in real-time pulse-echo ultrasound scanners. The standard is applicable to ultrasound systems operating in the frequency range from 1 MHz to 15 MHz, covering most diagnostic imaging applications including abdominal, obstetrics, gynecology, and musculoskeletal imaging.

The phantom-based approach allows healthcare facilities and manufacturers to:

Assess the ability of a 3D ultrasound system to detect anechoic voids in a tissue-equivalent background
Monitor system performance degradation over time through periodic measurements
Compare different ultrasound systems using a standardized test object
Verify system specifications during acceptance testing after installation or repair

The VDR metric represents the ratio of void volume detected by the system to the actual physical void volume in the phantom. A VDR of 1.0 indicates perfect detection capability, while values below 0.5 suggest significant image quality degradation requiring service intervention.

🔬 Phantom Design and TMM Specifications

Tissue-Mimicking Material (TMM)

The standard specifies rigorous requirements for the TMM used in constructing the phantom. The material must closely match the acoustic properties of soft tissue to ensure clinically relevant measurements.

Property	Specification	Tolerance	Measurement Method
Speed of sound	1540 m/s	±10 m/s	Time-of-flight method
Attenuation coefficient	0.5 dB/(cm·MHz)	±0.05 dB/(cm·MHz)	Insertion loss method
Backscatter coefficient	Mimics soft tissue	Per Annex specification	Reference phantom method
Density	1.04 g/cm³	±0.02 g/cm³	Gravimetric
Non-linearity parameter B/A	~6.9	±0.5	Finite amplitude method

Artificial Cyst Design

The phantom contains cylindrical anechoic cavities (voids) of various diameters arranged in a 3D matrix. Each void slice contains holes of different diameters, enabling evaluation across multiple ultrasound frequencies. The void diameters range from 1 mm to 8 mm, simulating cysts of various sizes in clinical practice.

The phantom scanning surface must be maintained at a consistent temperature (typically 22 ± 2 °C) during measurements. Temperature variations affect the speed of sound in TMM and can introduce measurement errors in both spatial positioning and VDR calculation.

🏗️ Engineering Design Insights

VDR Calculation Methodology

The VDR calculation involves voxel-level analysis of the acquired 3D ultrasound data compared to known void locations in the phantom. The standard defines a systematic process:

Step 1: Acquire 3D volumetric scan of the phantom under specified system settings
Step 2: Segment the void regions using automated or semi-automated algorithms
Step 3: Compare detected void volume with the known physical void volume
Step 4: Calculate VDR = (Detected void volume / Actual void volume) × 100%

The standard’s informative Annexes provide example construction procedures, test results, and system characterization data that help phantom manufacturers and test laboratories implement consistent evaluation protocols.

For optimal test sensitivity, perform VDR measurements using the ultrasound system’s default clinical presets for the intended application. Changing image processing parameters (e.g., dynamic range, edge enhancement, speckle reduction) can significantly alter VDR results and should be documented.

Phantom Stability and Longevity

The TMM formulation specified in the standard is designed for long-term stability. Properly constructed and stored phantoms typically maintain their acoustic properties for 2-3 years. The enclosure must be hermetically sealed to prevent dehydration and contamination. Routine stability checks involve periodic measurement of reference voids and comparison with baseline data established during phantom qualification.

❓ Frequently Asked Questions

Q1: How often should VDR testing be performed using the IEC TS 62558 phantom?
A: For clinical systems, quarterly testing is recommended as part of a routine quality assurance program. After major repairs or software upgrades, additional testing should be performed to establish new baseline performance data.

Q2: What is the typical lifespan of a TMM phantom?
A: With proper storage and handling, TMM phantoms typically last 2-3 years. The TMM should be stored in a controlled environment (18-25°C) away from direct sunlight. Signs of dehydration (surface cracking, altered echogenicity) indicate end of useful life.

Q3: Can the same phantom be used for 2D and 3D ultrasound quality assurance?
A: Yes, while designed specifically for 3D VDR evaluation, the phantom can also be used for conventional 2D quality assurance tests including B-mode uniformity, spatial resolution, and anechoic cyst detection.

Q4: How is the VDR affected by ultrasound frequency?
A: Higher frequencies generally provide better spatial resolution and thus higher VDR for small cysts, but suffer from reduced penetration depth. The standard’s phantom includes voids of various diameters to test system performance across the clinically relevant frequency range.