FAQs

Ultrasound FAQs

Speed of Sound
Zerdine® Hydrogel: 1540 m/s
ATS Urethane: 1450 m/s
Notes: See “Speed of Sound”

Elastography and multimodality formulations
Zerdine® Hydrogel: Excellent
ATS Urethane: Limited
Notes: Zerdine® is easily formulated to provide a range of elasticities. It can also be formulated for tissue-mimicking contrast in MRI and CT applications.

Desiccation
Zerdine® Hydrogel: Yes
ATS Urethane: No
Notes: Zerdine® phantoms come with durable vapor barrier membranes that protect from desiccation; airtight carry cases provide additional protection. Phantoms must still be handled with care.

Durability
Zerdine® Hydrogel: Very good
ATS Urethane: Excellent
Notes; See “What About Stability?”

Warranty
Zerdine® Hydrogel: 4 years
ATS Urethane: 10 years

Effect of Temperature
Zerdine® Hydrogel: May freeze with exposure to low temperatures
ATS Urethane: Durable, measurements sensitive to temperatures
Notes: See “Effect of Temperature”

Speed of Sound: Why is this Important?

All ultrasound machines use the speed of sound to convert time that an echo returns to a probe into a distance measurement. Most machines assume a sound velocity of 1540 m/s, the average speed of sound through human soft tissue, to make this conversion. The standard formulation of ATS Urethane Rubber has a sound velocity of 1450 m/s at room temperature (23 C), while a standard Zerdine® formulation has a sound speed of 1540 m/s at room temperature.

Because the speed of sound in ATS Urethane Rubber does not match the assumed speed of sound, the design of rubber phantoms must compensate for this difference. Adjusting the physical position of line targets and anechoic target structures allows users to make horizontal and vertical distance accuracy measurements. Unfortunately, the sound speed difference also results in a loss of image resolution, because most diagnostic imaging systems use the tissue-average sound speed to calculate where to send and focus ultrasound beams when forming images. Some newer ultrasound systems have a feature called aberration correction that provides better image quality in fatty tissue; this feature can be used with ATS Urethane Rubber to eliminate this loss of resolution. In Zerdine®-based phantoms, these corrections are not necessary.

What About Stability?

Zerdine® is the most stable hydrogel material on the market, and many Zerdine phantoms provide over a decade of useful service. The main failure mode with Zerdine® is water vapor loss through the membrane (or through a damaged housing). CIRS recommends periodically weighing your Zerdine phantom to check for signs of desiccation. Early signs of water loss (about 1%) can easily be corrected by adjusting storage conditions. Phantoms with water vapor loss exceeding 2% should be returned to CIRS for evaluation. In most cases, these phantoms can be repaired.

Urethane rubber has no such concerns with water vapor loss. The acoustic properties of ATS Urethane Rubber production batches have been continually tested since 1995, and in all cases these batch samples have remained within tolerance (± 1% for speed of sound, ±10% for attenuation). Mechanical damage to a phantom housing will not cause the ATS Urethane Rubber to change properties, and damaged phantoms can usually be repaired.

Effect of Temperature

The acoustic properties of all materials are affected by changes in temperature. Most diagnostic imaging systems and tissue-mimicking phantoms are calibrated at average room temperature (23 C). The speed of sound in ATS Urethane Rubber is more strongly influenced by temperature changes than Zerdine, and so all ATS Urethane Rubber phantoms include a thermometer strip affixed to the outside of the housing to indicate actual room temperature.

If a phantom is left in extreme temperatures for extended periods, wait approximately 24 hours for the phantom to reach room temperature to ensure the full phantom interior has reached room temperature.

Exposure to extreme temperatures will not damage urethane rubber phantoms. Zerdine® hydrogel phantoms may be damaged by the freeze/thaw cycle if allowed to freeze. High temperatures will not cause Zerdine® to melt (unlike other hydrogels), but they will cause the phantom to dry out faster than normal.

All quality assurance phantoms made by CIRS undergo a rigorous, multi-step inspection process to ensure consistency in the tissue mimicking properties of the gel batches we mix and the image quality of the phantom.

Tissue Mimicking Properties

To ensure the acoustic properties of the tissue mimicking materials meet CIRS specifications, a sample from each gel batch manufactured is tested using methodology recommended by the American Institute of Ultrasound in Medicine Technical Standards Committee.[1]  The speed of sound, attenuation and backscatter contrast of each sample is measured in a water test tank using a single-element pulse-echo ultrasound setup.

For elasticity targets, the Young’s modulus of the tissue mimicking material is verified using a compression/deformation method on standardized samples.  This measurement provides a consistent figure of merit, but may not suitable for reference measurements due to the lack of a suitable calibration standard.[2]

Volume accuracy is controlled by measuring the weight of the target and the density of the tissue mimicking material used in its manufacture.

Image Quality Assessment

Our manufacturing technicians have access to several diagnostic ultrasound machines to check for imaging artifacts and anomalies at each stage of phantom manufacture.  Work-in-process imaging inspections are supplemented by visual inspections and dimensional measurements of key subcomponents. 

A final imaging inspection is performed on every ultrasound quality assurance phantom manufactured by CIRS by one of our certified radiology technologists (or ultrasound engineers).  The diagnostic ultrasound machines used by CIRS are inspected quarterly using guidelines established by the American Association of Physicists in Medicine[3], the American College of Radiology[4] and the American Institute of Ultrasound in Medicine.[5],[6]

The final outgoing inspection of new phantoms and phantoms returned to CIRS for re-inspection consists of the following steps:

Step 1 – Artifact scan with a diagnostic ultrasound machine

Targets near the surface of the phantom are inspected with a small parts probe while deeper targets are inspected with a curved array as described below:

       Surface Inspection:  A detailed scan is performed with a small parts probe to a depth of 4 cm to check for artifacts and other anomalies that may affect the ability to perform QA measurements.  This inspection may be supplemented with a membrane inspection using a superficial imaging probe when necessary.  Regions designated for image uniformity measurement receive additional scanning with the ultrasound system settings optimized for dead element checks to ensure that the tissue mimicking material is uniform in these areas.

       Deep tissue artifact survey:  Additional scanning of targets deeper than 4 cm is performed with a curved array.  Due to the limited resolution of this inspection, the goal of this scan is to check for imaging artifacts and anomalies in target positioning, orientation and contrast.

Step 2 – Quantitative Assessments

Quantitative measurements to provide a reference measurement for assessing phantom stability over time.

       Wire Targets:  Distance measurements between key horizontal and vertical targets are performed with electronic calipers to ensure that the wire targets are positioned within tolerance and have not moved.  This measurement also provides a check on the stability of the speed of sound in the tissue mimicking material. 

Visual  Assessment

Additional visual inspections are performed prior to final release of the phantom:

       Water wells are filled with water and checked for leaks

       Outside labeling is checked for accuracy and aesthetics

       Housing, scanning surface and accessories are also checked for signs of damage

[1] In the standard ATS urethane formulation, the rate of change is -3 m/s/°C.  In the standard Zerdine formulation, it is +1.6 m/s/°C.

[1] AIUM Technical Standard Committee (2014).  Methods for Specifying Acoustic Properties of Tissue Mimicking Phantoms and Objects (2nd edition).  American Institute of Ultrasound in Medicine.

[1] Oudry J, Lynch T, Vappou J, Sandrin L, Miette V. Comparison of four different techniques to evaluate the elastic properties of phantom in elastography: is there a gold standard? Phys Med Biol. 2014 Oct 7;59(19):5775-93.

[4] Mitchell M Goodsitt, Paul L Carson, Scott Witt, David L. Hykes, James M. Kofler, Jr. “Real-time B-mode ultrasound quality control test procedures: Report of AAPM Ultrasound Task Group No. 1” Med Phys 25 (8) August 1998.

[5] ACR Ultrasound Accreditation Program Requirements, 5/18/18 revision: https://www.acraccreditation.org/-/media/ACRAccreditation/Documents/Ultrasound/Requirements.pdf?la=en

[6]  EJ Boote, F Forsberg, BS Garra, J Ophir, K Ophir and JA Zagzebski. “Routine Quality Assurance for Diagnostic Ultrasound Equipment,” American Institute of Ultrasound in Medicine. 2008.

[7] EL Madsen, BS Garra, JA Zagzebski, T Wilson, N Hangiandreou, D Tradup, M Ziskin, ZF Lu. “AIUM Quality Assurance Manual for Gray Scale Ultrasound Scanners.” American Institute of Ultrasound in Medicine. 2014.    


 

Each CIRS ultrasound quality assurance phantom comes with a detailed user guide and a certificate of compliance. 

No.  The acoustic properties of ATS Urethane and Zerdine® hydrogel are very stable over time, except in the case of accelerated water vapor loss through the membrane.  Premature desiccation can be checked by the customer by weighing the phantom annually.  The original weight of the phantom is provided on the label and on the certification sheet.  If minor weight loss if found (≈1% and a flat membrane), the QA measurements made with the phantom are still valid, but the storage conditions should be adjusted to prevent further weight loss.  Cooler, humid environments will slow the rate of water loss, and we recommend increasing the humidity of the storage container by adding a wet sponge to the case.  (Use of an anti-microbial additive is recommended to prevent mold growth.  Do not place the sponge directly on the membrane, as this may damage the membrane.)  If signs of significant water loss are found (over 2% or a visibly concave membrane), contact CIRS immediately for return instructions. In the case of phantoms less than 4 years old, the phantom will be repaired or replaced under warranty, while older phantoms can still be rejuvinated but a repair fee will be charged.

Phantoms made of ATS Urethane do not require periodic weighing, as urethane phantoms do not desiccate over time.

CIRS provides reinspection for customers that do not wish to perform this assessment of phantom stability themselves or are concerned that the phantom may have been damaged. This re-inspection process consists of four steps:

Step 1 – Careful visual inspection of phantom housing, scanning surface and accessories for signs of damage

Step 2 – Weigh the phantom to quantify any desiccation (if applicable)  

Step 3 – Quote for repair, if applicable.  Zerdine® phantoms can often be restored to their original condition by replacing the membrane and re-hydrating the gel inside.  If the phantom cannot be repaired, or the cost is significant, CIRS will either apply the cost of reinspection to a replacement phantom or proceed with re-inspection of the damaged phantom as below.  ATS Urethane phantoms, due to the stability of the material, rarely need repair. When they are damaged (usually due to tearing or rough handling), repair is generally not feasible.

Step 4 – Thorough inspection with a diagnostic ultrasound system to identify any defects and confirm integrity of all targets. This is the same as the final image quality inspection each phantom undergoes prior to being sold to the customer.

Step 5 – Generation of formal inspection report noting inspection results, special observations and manufacturer’s “opinion” regarding suitability for use. Final decision on usability resides with the customer, as each user sets different inspection criteria.

These phantoms can be used to perform all the quality assurance tests required by the ACR for ultrasound accreditation.  However, the ACR does not recommend any specific phantom designs or manufacturers for performing the tests that they require.  For more information on ACR Ultrasound Accreditation standards, see:  https://www.acraccreditation.org/modalities/ultrasound

 

 

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