Ultrasound QA FAQs

 

Zerdine® Hydrogel or ATS Urethane?

Specifications Zerdine® Hydrogel ATS Urethane Notes

Speed of sound

1540 m/s

1450 m/s

See “Speed of Sound”

Elastography and multimodality

formulations

Excellent

Limited

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

Yes No

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

Very good

Excellent

See “What About Stability?”

Warranty

4 years

10 years

 

Effect of Temperature

May freeze with exposure to low temperatures

Durable, measurements sensitive to temperatures

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.

          
How does CIRS ensure the accuracy of my ultrasound QA phantom?

During Manufacture

Positional accuracy of targets is controlled by an internal solid CNC (computerized numerical controlled) machined frame.

To ensure the acoustic properties of the tissue mimicking materials meet CIRS specifications, a sample from each gel batch manufactured is obtained. The speed of sound and attenuation of each sample is measured in the CIRS acoustics lab using time of flight and pulse echo techniques with an accuracy of +/- 1.5m/s for SOS and +/-0.02dB/cm/MHz for attenuation. These measurements are traceable to NIST.

Phantom contrast is controlled by the concentrations of scattering agent per unit volume of tissue mimicking material. A precision in contrast of +/- 1 DB is achieved by controlling the weight of the scattering material.

Young’s modulus of the tissue mimicking material used to manufacture elasticity targets is verified using a compression/deformation method on standardized samples.

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

100% Final Inspection

A final inspection is performed on every ultrasound quality assurance phantom manufactured by CIRS. The inspection consists of two parts.

Part 1 - Visual Inspection of phantom housing, scanning surface and accessories.

Part 2 - Inspection with a diagnostic ultrasound machine for:

Homogeneity

Air voids in gel and around masses
Straightness of masses
Integrity and placement of wires
Measurement of vertical wire distance. 

The Diagnostic Ultrasound machine used for this test is calibrated to allow such measurements by a time-of-flight method.

What documentation is provided with my ultrasound QA phantom?

Each CIRS ultrasound quality assurance phantom comes with a detailed user guide and a certificate of compliance. Final inspection data is available upon request.

Can my ultrasound QA phantom be calibrated?

Calibration is not necessary. The phantom is sealed within an airtight enclosure and has no moving parts. There is no method by which CIRS ultrasound quality assurance phantoms can be calibrated after manufacture.

Can my ultrasound QA phantom be re-certified?

No, after original certificate of compliance at time of original manufacture, CIRS does not re-certify the acoustic or mechanical properties of the tissue mimicking materials or the location of the embedded targets.

Can my ultrasound QA phantom be re-inspected to ensure its suitability for continued use?

Yes, for a fee, CIRS can inspect your ultrasound QA phantom for signs of degradation that may impact the performance of the phantom. The inspection 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. Weight loss can be used to estimate changes to SOS and attenuation within the phantom from time of original certification. 

Step 3 – Thorough inspection with a diagnostic ultrasound system to identify any defects and confirm integrity of all targets. This essentially is the same final ultrasound inspection each phantom originally undergoes prior to being sold to the customer.

Step 4 – Generation of formal inspection report noting inspection results, special observations and manufacturers “opinion” regarding suitability for use. Final decision on usability resides with the customer and is dependent on their particular situation. Recommended non-warranty repairs will be quoted separately.

Can CIRS inspect phantoms supplied by other manufacturers?

No but contact CIRS customer service about trading your old phantom in for a discount on the purchase of a new CIRS phantom.