Liver Elastography & Liver Fat: Stiffness vs Fat

The one distinction to hold onto: fat is not fibrosis

A single elastography exam hands you two numbers that mean completely different things:

• Liver stiffness (kPa or m/s) — a physical surrogate for fibrosis (scarring). This is the number that speaks to risk.
• UDFF (ultrasound-derived fat fraction, %) — an estimate of liver fat (steatosis).

A high UDFF means a fatty liver; it says little about scarring. A high stiffness suggests fibrosis; it is not a measure of fat. Liver fat and liver fibrosis are measured differently because they are different things — the central rule of this whole section. → What Is MASLD?

What shear-wave elastography (ARFI) actually does

We use acoustic radiation force impulse (ARFI) technology — Siemens Virtual Touch Quantification — on a full diagnostic ultrasound system. A focused ultrasound "push" pulse creates a shear wave inside the liver, and the system measures how fast it travels. Stiffer, more scarred tissue carries the wave faster; the speed (m/s) is converted to stiffness (kPa).

Because it runs on a full ultrasound system, we see the liver while we measure: we place the sample under direct imaging, avoid vessels and bile ducts, check measurement quality, and look at the liver's texture, size and surface at the same time. The exam takes about 10–15 minutes, is painless, and is done after a few hours of fasting. We repeat it on the same machine to track change.

How liver stiffness (kPa) is interpreted

Cutoffs depend on the technique. For ARFI shear-wave elastography, following the Society of Radiologists in Ultrasound consensus and AASLD imaging guidance:

• <7 kPa (≈<1.5 m/s) — no or mild fibrosis (F0–F1); reassuring.
• 7–9 kPa (≈1.5–1.7 m/s) — mild-to-moderate fibrosis (F1–F2); treated in our practice, with liver-directed medication when indicated.
• 9–13 kPa (≈1.7–2.1 m/s) — possible advanced fibrosis (F3); referral to hepatology.
• ≥13 kPa — probable cirrhosis (F4); hepatology-led care. Above about 17 kPa suggests portal hypertension.

Treat these as a rule of thumb read alongside FIB-4, platelets and the ultrasound picture — not a fixed line. Stand-alone transient elastography (VCTE, "FibroScan") uses different cutoffs (roughly <8, 8–12 and >12 kPa), so outside results are interpreted against the device that produced them.

How UDFF (liver fat) is interpreted

UDFF estimates the percentage of fat in the liver from the same ultrasound exam. In prospective comparison it tracks closely with MRI-PDFF, the reference imaging measure of liver fat (correlation about 0.8). A value above about 5% indicates steatosis, and higher values mean more fat. Because it is quick and repeatable, it is useful for watching liver fat fall with weight loss or medication — often before stiffness changes. UDFF tells you about fat, not scarring.

MRE — the more accurate cousin

MRE (magnetic resonance elastography) maps stiffness across the whole liver by MRI and is the most accurate noninvasive measure of fibrosis, less affected by body habitus; MRI-PDFF is the reference for liver fat. Both cost more and are less available, so we use them when ultrasound results are unreliable or don't fit the clinical picture.

The limits — when elastography is unreliable

Elastography is powerful, but it can mislead:

• A recent meal raises stiffness — fast for a few hours beforehand.
• Liver inflammation (an ALT flare) can falsely raise stiffness.
• Congestion and cholestasis — heart failure or bile-duct obstruction raise stiffness.
• Body habitus and breath-hold — a thick abdominal wall or poor breath-hold reduces reliability; we require a full set of valid measurements with acceptable variability.
• Different machines — values from different technologies are not interchangeable.

Because of these, a single number is never the whole story. Discordant results are repeated or confirmed with MRE or a blood-based test such as ELF.

Where elastography sits: FIB-4 first, then stiffness

Elastography is second-line, after FIB-4:

FIB-4  (first-line, from routine bloodwork; age-adjusted ≥65)
   ↓  indeterminate or high
ELASTOGRAPHY  (in-office ARFI shear-wave kPa + UDFF — or MRE if unreliable/discordant)
   ↓
LOWER (<7 kPa)  ·  MILD–MODERATE (7–9 kPa)  ·  ADVANCED (≥9 kPa)
   ↓
MONITOR  ·  TREAT IN OUR PRACTICE  ·  REFER TO HEPATOLOGY

This sequence means most people with a fatty liver never need a biopsy. For the full MASLD/MASH care pathway, see our sister site MASHExperts.

What the evidence says: Elastography

• What we know: ARFI shear-wave elastography is a validated fibrosis surrogate that improves risk stratification after FIB-4, with technique-specific cutoffs (SRU consensus); UDFF correlates closely with MRI-PDFF for liver fat; MRE is the most accurate noninvasive stiffness measure.
• What we think: Elastography works best as a second-line test, repeated on the same system and interpreted with FIB-4 and the clinical picture.
• What we don't know: Exact cutoffs vary by vendor and population, and how well short-term changes in stiffness predict long-term outcomes is still being studied.

Questions patients ask

Written by Darius A. Schneider, MD, PhD · Board-Certified Endocrinologist (ECNU) · Last updated: 2026-09-30 · References: Barr RG, et al. SRU liver elastography consensus update, Radiology 2020; Sterling RK, et al. AASLD imaging-based noninvasive liver disease assessment guideline, Hepatology 2025; Dillman JR, et al. UDFF vs MRI-PDFF, AJR 2022; AASLD Practice Guidance on MASLD (2023); EASL–EASD–EASO clinical practice guidelines (2024). Cutoffs are technique-specific. Educational; not a diagnosis.