Ultrasound Elastography QC: Physics and QA

Ultrasound elastography QC is the process of confirming that an elastography system reports tissue-stiffness values that are accurate against a known phantom, repeatable within a defined tolerance, and accompanied by the reliability criteria and system-specific context that make them clinically defensible. Elastography converts a physical shear wave measurement into a number that can change how a patient with chronic liver disease is staged and followed, so the measurement chain deserves the same QC discipline as any other quantitative imaging biomarker.¹³⁴

Unlike a grayscale image, where a radiologist interprets a picture, an elastography result is a number: a shear wave speed in meters per second or a stiffness in kilopascals. That number is only useful if it is traceable to a known standard, reproducible across days and operators, and reported with the context needed to compare it over time.⁶⁷

Introduction

Ultrasound elastography has moved from a research tool to a routine part of abdominal, breast, thyroid, and musculoskeletal imaging. In chronic liver disease especially, shear wave elastography is now used to estimate fibrosis and to assess the risk of compensated advanced chronic liver disease, often reducing the need for biopsy.³⁴ With that clinical weight comes a QC obligation: if a facility is going to report liver stiffness as a decision-driving value, it must be able to demonstrate that the value is accurate and repeatable.

This guide explains what elastography actually measures, how the underlying physics turns a wave speed into a stiffness value, and what a defensible QC program looks like. It covers phantom-based accuracy and repeatability testing, cross-system variability, the reliability criteria that flag unusable measurements, and the acceptance and constancy testing a medical physicist performs. DRPS supports elastography quality control as part of its ultrasound physics testing and accreditation support services across Florida, Maryland, Virginia, Washington DC, California, Nevada, and beyond.

Topic Explanation

What is ultrasound elastography?

Ultrasound elastography is a family of techniques that image or quantify the mechanical stiffness of tissue rather than its acoustic echogenicity. Conventional B-mode reveals differences in how tissues reflect sound; elastography reveals differences in how tissues deform or how fast a shear wave travels through them. Because many tissues share similar echogenicity but differ in stiffness, elastography adds contrast that B-mode cannot provide, and in quantitative modes it produces an actual number.¹²

All elastography methods share a common idea: apply a mechanical stimulus, then watch how the tissue responds. What differs is the stimulus and what is measured. The main categories a QC program must distinguish are:

• Strain elastography (SE) — quasi-static, using manual or physiologic compression, producing a qualitative or semi-quantitative strain image or strain ratio.
• Point shear wave elastography (pSWE), also called acoustic radiation force impulse (ARFI) quantification — a focused acoustic push generates a shear wave whose speed is measured in a small region of interest.
• Two-dimensional shear wave elastography (2D-SWE) — multiple pushes build a real-time color stiffness map with a selectable region of interest.
• Vibration-controlled transient elastography (VCTE) — an external mechanical piston generates a shear wave and reports liver stiffness, most familiar from a dedicated non-imaging device.

The QC concern is different for each. A strain image cannot be checked against a known kilopascal value because it is not a stiffness measurement; a shear wave method can and should be.¹²⁴ For the broader picture of routine ultrasound performance testing, see our guide to diagnostic ultrasound QC.

Why elastography needs its own QC

A shear wave speed measurement passes through a long chain: the push pulse, the tracking beams, the shear wave detection, the region-of-interest selection, and the vendor's calculation of a representative value. An error or drift anywhere in that chain changes the reported number without changing the grayscale image the operator sees. QC exists to confirm that the chain still produces the right answer on a known input, and that it does so repeatably.⁶⁷

Key Technical Principles

From shear wave speed to stiffness

The physics that underpins quantitative elastography is the relationship between shear wave speed and the mechanical properties of tissue. In an elastic medium, the shear wave speed $c_s$ depends on the shear modulus $\mu$ and the tissue density $\rho$:¹²

Rearranging gives the shear modulus directly from the measured speed:

Soft tissue is nearly incompressible, so Young's modulus $E$ — the "stiffness" most systems report in kilopascals — is approximately three times the shear modulus:

This single relationship explains several QC-relevant facts. First, the directly measured quantity is a speed; the kilopascal value is derived using assumed density and incompressibility. Second, because $E$ scales with the square of speed, a small speed error is amplified in the kilopascal value.

Worked example: converting a measurement

Consider a measured shear wave speed of $c_s=2.0$ m/s in tissue with density $\rho =1000$ kg/m³:

Now suppose the system's speed is biased high by 5%, reading 2.1 m/s instead of 2.0 m/s. The derived Young's modulus becomes $E\approx 3\times 1000\times (2.1{)}^2=13230$ Pa ≈ 13.2 kPa — roughly a 10% shift in the reported stiffness from a 5% speed error. This quadratic sensitivity is why a QC program should verify the measured value against a phantom of known modulus rather than assuming the displayed number is correct.⁶⁷

Repeatability and bias

Two different questions must be answered by QC. Bias asks whether the average measured value equals the true value of a known phantom. Repeatability asks how much the value scatters when the measurement is repeated under the same conditions. A common way to express scatter is the coefficient of variation:

where $\overline{x}$ is the mean of repeated measurements and SD is their standard deviation. A within-subject or within-phantom standard deviation can also be turned into a repeatability coefficient — approximately $2.77\times \mathrm{SD}$ — the value below which the difference between two measurements is expected to fall about 95% of the time. In phantom studies across five commercial systems, measured elastography values showed excellent intra- and interobserver agreement and coefficients of variation in the low single digits to about 0.2, with larger differences at higher stiffness values — a useful reminder that repeatability itself can depend on the stiffness range.⁵

Comparison of elastography methods

The table is a starting point for planning QC, not a substitute for the manufacturer's specifications. Accuracy and repeatability tolerances should be based on the specific system, probe, and mode in use.⁴⁶

Clinical Impact

Because elastography numbers cross clinical thresholds, small measurement errors have real consequences. In liver fibrosis assessment, the difference between adjacent stages can be only a few kilopascals, so a systematic bias or poor repeatability can move a patient across a decision boundary.⁴⁶ The Society of Radiologists in Ultrasound consensus responded to this by promoting a simplified, vendor-neutral interpretation framework and by emphasizing that harmonization efforts have reduced — but not eliminated — inter-system variability.³

Three practical points follow for any facility reporting stiffness:

• Trend on the same system. Because vendors can report modestly different values for the same tissue, longitudinal follow-up (for example, before and after antiviral therapy) is most reliable when performed on the same system and probe, tracking the change rather than comparing absolute values across machines.³⁴
• Report the context. A stiffness value without the system, mode, probe, depth, and reliability indicators is difficult for the next reader to trust. The report should make the measurement reproducible in principle.
• Respect confounders. Non-fasting state, hepatic inflammation or transaminase flares, hepatic venous congestion, cholestasis, and amyloidosis can all raise liver stiffness independent of fibrosis. A technically perfect measurement in the wrong physiologic state is still misleading.³⁴

For the physics of the acoustic output that drives shear waves and the safety indices displayed during scanning, see our companion post on the ultrasound thermal and mechanical index.

Practical Optimization Tips

A defensible elastography QC program has both an equipment side and a technique side.

1. Establish phantom-based accuracy at acceptance

At installation, and after major service or software updates, measure a tissue-mimicking elastography phantom with regions of known Young's modulus. Confirm that the measured stiffness agrees with the nominal value within the manufacturer's stated tolerance, and record the result as the baseline. Elastography phantoms with multiple embedded stiffness targets allow accuracy to be checked across the clinically relevant range rather than at a single value.⁵⁶

2. Characterize repeatability, not just accuracy

Repeat the phantom measurement multiple times, ideally with more than one operator, and compute the coefficient of variation and limits of agreement. This establishes the expected day-to-day scatter so that later drift can be distinguished from normal variation. Repeatability at high stiffness may be worse than at low stiffness, so characterize it where it matters clinically.⁵

3. Check depth and uniformity dependence

Shear wave methods can lose accuracy at greater depth as the push loses energy. Measuring a uniform phantom region at several depths documents how far into tissue the system remains reliable and informs a maximum measurement depth for clinical use.²⁶

4. Enforce reliability criteria in clinical practice

Operational quality depends on technique. Build reliability criteria into the workflow:

• Acquire the guideline-recommended number of valid measurements and report the median.
• Use the system's propagation-quality map (for 2D-SWE) to place the region of interest in a region of uniform, parallel wavefronts, avoiding vessels and artifacts.
• For transient elastography, apply the interquartile-range-to-median ratio as a validity check and record the applicability (success) rate.
• Standardize patient preparation, including fasting for liver studies, and a consistent breath-hold.³⁴

5. Common pitfalls to avoid

• Treating a strain ratio as a stiffness value. Strain elastography is qualitative or semi-quantitative and cannot be checked against a kilopascal standard.
• Comparing absolute values across vendors. Report and trend on a consistent system where possible.³
• Ignoring the physiologic state. Inflammation, congestion, and non-fasting status inflate liver stiffness.⁴
• Skipping phantom testing because the image "looks fine." The grayscale image does not reveal a stiffness-calculation drift.⁶⁷
• Reporting a single acquisition. Guidelines call for a median of multiple valid measurements with a stated reliability indicator.³⁴

Regulatory Considerations

Ultrasound is non-ionizing, so elastography systems fall outside the radioactive-material and x-ray-machine regulatory frameworks that govern most of medical imaging physics. They are not licensed under NRC rules (10 CFR Parts 20 and 35 apply to byproduct material), and they are not regulated under state x-ray-machine programs. In Florida, for example, the radiation-machine provisions of Chapter 64E-5, Florida Administrative Code, apply to x-ray-producing equipment, not to ultrasound.

That does not mean elastography quality is unregulated in practice. The binding expectations come from other places:

• Accreditation programs. The ACR Ultrasound Accreditation Program sets image-quality, protocol, personnel, and QC expectations for accredited practices, and accreditation is frequently tied to reimbursement and to hospital or payer requirements.
• Professional-society guidance. The WFUMB and EFSUMB elastography guidelines and the Society of Radiologists in Ultrasound consensus define acquisition technique, reliability criteria, and interpretation, and the Quantitative Imaging Biomarkers Alliance profile defines the technical performance a system should meet to serve as a quantitative biomarker.¹²³⁴⁷⁸
• The Joint Commission and facility policy. Where the Joint Commission or another accreditor applies, imaging-quality and equipment-maintenance standards create additional documentation expectations.

The practical takeaway: because no single agency mandates elastography QC the way MQSA mandates mammography QC, the responsibility falls on the facility and its medical physicist to build a program that satisfies the applicable accreditation body and stands up to review. For how this fits into a broader accreditation posture, see ACR accreditation physics requirements. DRPS provides this support through medical physics consulting and accreditation support.

Frequently Asked Questions (FAQs)

What is ultrasound elastography QC?

Ultrasound elastography QC is the quality-control process that confirms an elastography system reports accurate and repeatable tissue-stiffness values. It combines phantom testing against a known Young's modulus, repeatability and bias checks, reliability-criteria review, and attention to system- and operator-dependent confounders so that stiffness numbers are clinically defensible and comparable over time.

Why do elastography stiffness values differ between ultrasound systems?

Different vendors use different acoustic radiation force push sequences, tracking methods, region-of-interest handling, and proprietary calculations. As a result, the same tissue can yield modestly different shear wave speed or kilopascal values on different systems. Quantitative Imaging Biomarkers Alliance harmonization has narrowed this spread, but follow-up comparisons should still use the same system and probe when possible.

Should elastography stiffness be reported in meters per second or kilopascals?

Both are used. Shear wave speed in meters per second is the directly measured quantity; kilopascals are derived by assuming tissue density near 1000 kilograms per cubic meter and near-incompressibility, giving Young's modulus of about three times the shear modulus. Because the kilopascal value depends on those assumptions, reports should state the units, the system, and the measurement mode.

How often should elastography QC be performed?

Acceptance testing should be performed at installation and after major service or software changes, with periodic constancy testing on a schedule set by the physicist and the accreditation program. Daily operational checks focus on system self-tests and image quality, while phantom-based accuracy and repeatability checks are typically performed at longer intervals or when results drift.

What makes an individual elastography measurement unreliable?

Common causes include patient motion or breathing during acquisition, a region of interest placed in a vessel or artifact, inadequate depth or rib shadowing, a non-fasting state for liver studies, hepatic congestion or inflammation, and too few valid acquisitions. Reliability indicators such as the interquartile-range-to-median ratio and propagation-quality maps help flag these measurements.

Is ultrasound elastography regulated like x-ray equipment?

No. Ultrasound is non-ionizing, so elastography systems are not regulated under NRC rules or state x-ray-machine programs such as Florida Chapter 64E-5. Quality expectations come primarily from accreditation programs such as the ACR Ultrasound Accreditation Program, from manufacturer specifications, and from professional-society guidance, along with any Joint Commission requirements that apply to the facility.

Can a medical physicist help build an elastography QC program?

Yes. A qualified medical physicist can perform acceptance and constancy testing with an appropriate elastography phantom, establish baseline stiffness accuracy and repeatability, define reliability criteria and reporting conventions, train staff on measurement technique, and align the program with accreditation requirements so that stiffness values are accurate, repeatable, and defensible.

Key Takeaways

• Elastography reports a number, not just a picture. QC exists to confirm that the number is accurate against a known phantom and repeatable within tolerance.⁶⁷
• The measured quantity is a speed; stiffness is derived. Because Young's modulus scales with the square of shear wave speed, small speed errors produce larger stiffness errors.¹²
• Vendors differ. Harmonization has narrowed inter-system variability, but longitudinal follow-up is most reliable on the same system and probe.³
• Reliability criteria matter as much as the equipment. Median of multiple valid acquisitions, propagation-quality maps, and IQR/median checks separate trustworthy values from noise.³⁴
• Physiologic confounders can invalidate a technically perfect measurement. Fasting state, inflammation, and congestion all affect liver stiffness.⁴
• Ultrasound is non-ionizing. QC expectations come from accreditation and professional guidance, not from NRC or state x-ray rules, so the facility and its physicist own the program.

Conclusion

Ultrasound elastography is a quantitative imaging biomarker, and it should be treated like one. The value of a stiffness measurement depends entirely on whether it is accurate, repeatable, and reported with enough context to be compared over time. A QC program that verifies accuracy against a known phantom, characterizes repeatability across the clinical range, enforces reliability criteria at the point of care, and accounts for cross-system variability and physiologic confounders turns elastography from an impressive display into a defensible measurement. As elastography takes on more clinical weight, that discipline is what lets a facility stand behind the numbers it reports.³⁴⁶

How DRPS Can Help

Diagnostic Radiation Physics Services helps imaging facilities build elastography quality-control programs that satisfy accreditation and stand up to review. This includes acceptance and constancy testing with appropriate elastography phantoms, baseline accuracy and repeatability characterization, reliability-criteria and reporting-convention development, staff technique training, and integration with the facility's overall ultrasound QC and accreditation posture through our ultrasound physics testing, accreditation support, and medical physics consulting services.

DRPS supports facilities across our service locations, including Florida, Maryland, Virginia, Washington DC, California, Nevada, New York, Pennsylvania, New Jersey, and Delaware.

A strong elastography program is not just about passing accreditation. It is about making sure that every stiffness value a facility reports can be trusted by the clinician who acts on it.

Related Resources

• Diagnostic ultrasound QC
• Doppler ultrasound quality control
• Ultrasound thermal and mechanical index safety
• ACR accreditation physics requirements
• Ultrasound physics testing
• Accreditation support
• Medical physicist consulting

References

1. Shiina T, Nightingale KR, Palmeri ML, et al. WFUMB guidelines and recommendations for clinical use of ultrasound elastography: Part 1: basic principles and terminology. Ultrasound in Medicine & Biology. 2015;41(5):1126-1147. doi:10.1016/j.ultrasmedbio.2015.03.009. PubMed
2. Bamber J, Cosgrove D, Dietrich CF, et al. EFSUMB guidelines and recommendations on the clinical use of ultrasound elastography. Part 1: Basic principles and technology. Ultraschall in der Medizin. 2013;34(2):169-184. doi:10.1055/s-0033-1335205. PubMed
3. Barr RG, Wilson SR, Rubens D, Garcia-Tsao G, Ferraioli G. Update to the Society of Radiologists in Ultrasound Liver Elastography Consensus Statement. Radiology. 2020;296(2):263-274. doi:10.1148/radiol.2020192437. PubMed
4. Ferraioli G, Barr RG, Berzigotti A, et al. WFUMB Guideline/Guidance on Liver Multiparametric Ultrasound: Part 1. Update to 2018 Guidelines on Liver Ultrasound Elastography. Ultrasound in Medicine & Biology. 2024;50(8):1071-1087. doi:10.1016/j.ultrasmedbio.2024.03.013. PubMed
5. Mulabecirovic A, Mjelle AB, Gilja OH, Vesterhus M, Havre RF. Repeatability of shear wave elastography in liver fibrosis phantoms — Evaluation of five different systems. PLoS One. 2018;13(1):e0189671. doi:10.1371/journal.pone.0189671. PubMed
6. Piscaglia F, Salvatore V, Mulazzani L, Cantisani V, Schiavone C. Ultrasound Shear Wave Elastography for Liver Disease. A Critical Appraisal of the Many Actors on the Stage. Ultraschall in der Medizin. 2016;37(1):1-5. doi:10.1055/s-0035-1567037. PubMed
7. Radiological Society of North America, Quantitative Imaging Biomarkers Alliance. QIBA Profile: Ultrasound Shear Wave Speed (SWS). 2021. qibawiki.rsna.org
8. Radiological Society of North America, Quantitative Imaging Biomarkers Alliance. Ultrasound SWS Biomarker Committee. qibawiki.rsna.org