Do technical factors causing falsely low measurements apply to Shear Wave Elastography (SWE)?

Technical Factors Causing Falsely Low Measurements in Shear Wave Elastography

Yes, technical factors can cause falsely low measurements in SWE, though the primary concern with both transient elastography and SWE is falsely elevated readings rather than falsely low values. The most clinically relevant technical factor causing potentially lower measurements is the use of different probes or measurement depths, while most confounding factors actually cause overestimation of liver stiffness. 1

Technical Factors That Can Lower SWE Measurements

Probe and Depth-Related Factors

• Different probe types yield systematically lower values: In transient elastography, the XL probe produces measurements that are lower than the M probe by a median of 1.4 kPa, which is relevant since similar probe variations exist across elastography platforms. 1

• Measurement depth significantly affects readings: Liver stiffness measurements are greater at depths >5 cm (5.78 ± 1.66 kPa) compared with depths ≤5 cm (4.66 ± 0.77 kPa), suggesting that superficial measurements may underestimate true liver stiffness. 2

• Anatomical location within the liver matters: The lowest coefficient of variation (8%) for liver stiffness measurements occurs at segment V, and measurements vary significantly depending on which hepatic segment is interrogated. 2

Equipment and Technical Variability

• Intersystem variability exists even within the same manufacturer: Two 2D-SWE systems from the same manufacturer (LOGIQ E9 vs LOGIQ S8) showed significantly different mean measurements (6.57 ± 2.33 kPa vs 6.90 ± 6.64 kPa), demonstrating that equipment should not be used interchangeably even when from the same company. 3

• The narrow measurement range of point SWE (pSWE) limits discrimination: pSWE/ARFI has a narrow range (0.5-4.4 m/sec) compared to other modalities, which can limit the ability to distinguish between fibrosis stages and may result in measurements that appear falsely low relative to disease severity. 1

Factors That Cause Falsely ELEVATED Measurements (More Common)

It is crucial to understand that most confounding factors cause overestimation rather than underestimation of liver stiffness. 1

Physiologic and Pathologic Confounders

• Food intake falsely elevates measurements: Both SWE and transient elastography are influenced by food intake, with recommendations to fast for at least 2-4 hours before testing to avoid falsely increased measurements due to postprandial blood flow. 1, 4

• Inflammatory conditions cause overestimation: Intrahepatic inflammation, elevated ALT levels, necro-inflammatory activity, and acute hepatitis all lead to falsely elevated liver stiffness measurements independent of fibrosis. 1

• Cholestasis and congestion increase readings: Extra-hepatic cholestasis, congestive heart failure leading to liver congestion, and amyloidosis all cause overestimation of liver stiffness. 1, 4

• Alcohol consumption elevates measurements: Excessive alcohol intake is a recognized confounding factor that increases liver stiffness readings beyond what fibrosis alone would produce. 1

Quality Criteria to Ensure Reliable Measurements

For SWE Specifically

• IQR/median ratio thresholds must be met: For reliable SWE measurements, the IQR/median should be less than 30% when reported in kPa and less than 15% when reported in m/s. 1

• Standard deviation and ROI size matter: For optimal accuracy in diagnosing cirrhosis with 2D-SWE, aim for a standard deviation ≤1.75 kPa combined with a larger ROI size (optimally ≥18 mm). 5

• Low variability indicates high reliability: High-quality SWE elastograms ensure low variability (SD = 1.1 ± 1.5 kPa; SD/mean = 12 ± 9%) and high reliability regardless of clinical indication. 5

Clinical Pitfalls and How to Avoid Them

• Always interpret results in clinical context: The Korean Association for the Study of the Liver emphasizes that test results may be overestimated in cases of intrahepatic inflammation, cholestasis, right heart failure, amyloidosis, or food intake, requiring caution in interpretation. 1

• Ensure proper fasting status: Patients should fast for at least 2-4 hours before elastography to avoid falsely elevated measurements, with some guidelines recommending up to 4 hours for MR elastography. 1, 4

• Check for acute inflammatory processes: Always review aminotransferase levels and clinical history for acute hepatitis, as ALT flares can significantly overestimate fibrosis stage. 1, 4

• Consider technical success and reliability rates: SWE has lower failure rates (2.9% for pSWE, 2.2% for 2D-SWE) compared to transient elastography (6.4%), particularly in patients with ascites or obesity, but quality criteria must still be met. 1, 6