What is SW Elastography (Shear Wave Elastography)? What Does Liver Stiffness Measurement (kPa) Mean?
Liver diseases often progress insidiously; a person can develop liver fibrosis (scar tissue) for years without experiencing any symptoms. Therefore, methods that can provide an early and reliable answer to the question “how is my liver?” are important. This is where SW elastography (shear wave elastography) comes into play. SW elastography measures the hardness of liver tissue using an ultrasound-based method and helps to assess the risk of fibrosis/cirrhosis based on this stiffness.
While conventional ultrasound provides information about the shape, size and structure of the liver, ultrasound elastography reports a numerical value (usually kPa) of “how stiff” the tissue is. This measurement is frequently used to establish a follow-up plan, especially in conditions such as hepatitis B/C, fatty liver disease (NAFLD/NASH), alcohol-related liver disease.
In this article, we will address the most curious questions such as “what is elastography?”, how SW elastography works, how to measure liver stiffness, how to interpret the results and SW elastography FibroScan difference in a patient-friendly language. (Note: Content is for informational purposes; consult your physician for diagnosis and treatment)
What is Elastography? What Does Ultrasound Elastography Do?
**In its simplest form, elastography is an imaging approach that measures the stiffness of tissues. Just as the doctor gets an idea about the stiffness of the tissue by pressing with his hand during the examination, elastography does this in a more objective and quantitative way with the help of a device. In the case of the liver: When there is long-term damage to the liver (e.g. chronic hepatitis, adiposity, alcohol, some metabolic diseases), the body tries to repair this damage and over time, scar tissue called fibrosis accumulates. As scar tissue increases, the liver usually becomes more “hard”.
Ultrasound elastography measures liver stiffness with techniques that can be integrated into a standard ultrasound device or operate as a separate system. SW elastography is one of the important methods under this umbrella. The main advantage of this measurement in the clinic is the following:
- Non-invasive (no needle, no incision)
- Radiation free
- Short and repeatable
- A practical tool for follow-up (is the disease progressing, is there a response to treatment?)
Nevertheless, it is an important point: ** Liver stiffness alone is not a definitive diagnosis.** The results are evaluated in combination with blood tests, examination, other imaging findings and the patient’s clinical history. For a more general framework on liver health, you can also take a look at this content: Fatty Liver Disease (NAFLD) Symptoms and Treatment.
How SW Elastography (Shear Wave Elastography) Works and How Is It Performed?
Shear wave elastography, as the name suggests, works by generating a “shear wave” in the tissue. The device generates very small mechanical waves in the liver tissue through an ultrasound probe and measures the speed at which these waves propagate through the tissue. The general principle is as follows:
- The harder the tissue, the faster the wave spreads.
- The softer the tissue, the slower the wave propagates.
The results are often reported in kPa (kilopascal); in some systems they may also appear in m/s (meters per second). This is where the answer to the question “**what is kPa?” becomes important: kPa is the unit of pressure/tension; it is used in elastography as a numerical measurement corresponding to the elasticity of liver tissue.
Process (patient experience)
The measurement of liver stiffness usually proceeds as follows:
- The patient lies on his/her back and may be asked to raise the right arm (opening the inter-rib space).
- The probe is placed in the right upper abdomen.
- A short breath hold may be requested during the measurement (to reduce motion artifact).
- The instrument takes several measurements and the average/median value is reported.
For most people, the procedure is painless; sometimes there may be mild discomfort due to probe pressure. The procedure usually takes 5-15 minutes. There is no radiation.
Preparation: Fasted or fed for elastography?
Although it may vary from center to center, the common approach in practice is to fast for 3-6 hours before the procedure. This is because liver blood flow may change after a meal and this may affect liver stiffness kPa. The instructions given to you before the appointment take precedence.
Why is Liver Stiffness (kPa) Measured? Association with Liver Fibrosis
*the main purpose of measuring liver stiffness is to help estimate the degree of fibrosis in the liver (expressed in stages such as F0-F4). As fibrosis increases, the risk of cirrhosis increases and follow-up/treatment strategies may change. Therefore, SW elastography is a valuable tool both in the initial evaluation and in regular follow-up.
Who needs SW elastography?
In particular, physicians may recommend SW elastography when
- Suspicion or diagnosis of fatty liver disease (NAFLD/NASH)
- chronic viral hepatitis such as Hepatitis B and Hepatitis C
- Alcohol-related liver disease
- Autoimmune hepatitis and some cholestatic diseases
- Staging and follow-up in patients with suspected cirrhosis
- Monitoring treatment response (e.g., fibrosis monitoring after antiviral therapy)
If you want to better understand the concepts of cirrhosis and fibrosis, this article may be helpful: What is Liver Fibrosis? Stages and Follow-up.
How to interpret the results? “What should the liver stiffness be?”
This question is much sought after: “What should the liver stiffness be? “ However, it is not correct to give a single “one-size-fits-all” normal value. Because:
- Device/technique used (SW elastography, FibroScan etc.)
- Measurement region and quality criteria
- The patient’s body structure (such as obesity)
- Conditions such as concomitant inflammation/cholestasis may affect the outcome.
Still for general information: Lower kPa values are generally consistent with less fibrosis, while higher values may increase the likelihood of advanced fibrosis/cirrhosis. Although some sources give approximate thresholds for fibrosis stages, the answer to questions such as how many kPa for F2 fibrosis or how many kPa for cirrhosis may vary depending on the cause of the disease (NAFLD or hepatitis?), laboratory findings and device calibration. It is best to discuss your report with your attending physician.
Does “high output” always mean cirrhosis?
Answer. **The critical point of the question “What does the shear wave elastography result mean?” is this: Increased liver stiffness may not only be due to fibrosis. For example
- ALT elevation / active inflammation (hepatitis exacerbation)
- Cholestasis (impaired bile flow)
- Congestive heart failure (blood pooling in the liver)
- Measurement while full such conditions may “temporarily” elevate stiffness. Therefore, the first step for a person who says “liver elastography result was high” is not panic, but reassessment with the clinical picture.
SW Elastography vs FibroScan vs MR Elastography: Differences, Advantages and Limitations
There is more than one method to assess liver fibrosis. Let’s answer the frequently asked question “SW elastography or FibroScan?” with a practical framework.
SW elastography (ultrasound elastography)
- Pros:
- It can often be performed as an adjunct during a standard ultrasound examination
- Targeted measurement can be taken from a specific region of the liver
- No radiation, fast and reproducible
- Limitations:
- Measurement quality may decrease in obesity, narrow rib space, breathlessness
- Operator experience and device differences may affect the result
FibroScan (Transient Elastography - TE)
- Pros:
- It is a common and well-studied method for fibrosis assessment
- Practical use in clinical follow-up
- Limitations:
- Some patients (especially obesity) may have technical difficulties (XL probe may be required)
- Usually planned as a separate device/procedure (not all ultrasound devices)
MR elastography
- Pros:
- May offer the advantage of wider area assessment and higher accuracy in some situations
- Limitations:
- More expensive, takes longer, access may be limited
- Not applicable in patients who cannot undergo MRI
To summarize: SW elastography FibroScan difference often makes sense according to device infrastructure, accessibility and patient characteristics. The appropriate method is determined by the physician based on the suspected disease, previous results and the follow-up plan.
Frequently Asked Questions (FAQ): Curiosities About Liver Stiffness Measurement
Is SW elastography painful?
It is usually painless. There may be slight discomfort due to probe pressure but no needle/incision.
Is there radiation?
No. Shear wave elastography is ultrasound-based and radiation-free.
What should the elastography result be?
There is no single ideal number. “Normal” ranges are interpreted in combination with the device, the quality of the measurement, the cause of the disease and the clinical situation. It is therefore necessary to evaluate the report together with blood tests and ultrasound findings.
Does fatty liver disease affect the elastography result?
Fatty liver disease may indirectly affect the elastography measurement. Stiffness may be increased if there is inflammation (NASH) or fibrosis accompanying the fatty liver. However, it is not always easy to distinguish adiposity from fibrosis on its own, so physicians use elastography in combination with other tests.
Does elevated ALT affect the elastography result?
Yes, stiffness may be temporarily elevated during periods of active inflammation. The timing of the measurement is therefore important in some patients.
Is it a substitute for a biopsy?
Not always. Elastography is a powerful tool in the assessment of fibrosis, but in some complex cases (uncertain diagnosis, conflicting results, suspicion of other comorbidities) liver biopsy may still be necessary. For more details: When is a liver biopsy needed?.
When should I consult a doctor?
If your elastography result is high or if you have the following symptoms: jaundice, abdominal swelling (ascites), confusion, easy bruising/bleeding, rapid weight loss, severe weakness.
Short summary
SW elastography is a radiation-free and practical ultrasound elastography method that helps to assess the risk of liver fibrosis and cirrhosis by measuring the hardness (kPa) of the liver. The results should not be interpreted alone; factors such as fasting status, ALT elevation, cholestasis and clinical findings should be taken into account. The best approach is to discuss your report with the attending physician and create a personalized follow-up plan.
