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.
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.
Zerdine® Hydrogel: Very good
ATS Urethane: Excellent
Notes; See “What About Stability?”
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.
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:
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.
Each CIRS ultrasound quality assurance phantom comes with a detailed user guide and a certificate of compliance. Final inspection data is available upon request.
No. The only calibration measurement required is with Zerdine® phantoms, which should be weighed at least once per year to check for water vapor loss through the membrane. We recommend that customers perform this measurement themselves, as all Zerdine® phantoms are labeled with their weight at manufacture. If signs of significant water loss are found (visible shrinking of the membrane or a weight loss of 2% or more), the phantom should be returned to CIRS for re-inspection and repair (if needed).
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 – 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 essentially is the same final ultrasound inspection each phantom originally 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 and is dependent on their particular situation.
No but contact CIRS customer service about trading your old phantom in for a discount on the purchase of a new CIRS phantom.
Still have questions?
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