Probe Placement on the Right Upper Quadrant
A single bedside probe replaces the liver biopsy needle.
- ~5 min: Exam duration (bedside, no sedation)
- ≥10: Valid shots required (per exam)
- 25–65 mm: Probe depth range (skin to liver capsule)
- <30%: IQR/Median limit (reliability criterion)
Where the probe goes
Intercostal space, right lobe, patient lying supine.
Why placement matters
Rib shadow or ascites can invalidate the reading.
M and XL probes
XL probe suits obese patients with thicker chest wall.
Shear Wave Generation via Mechanical Impulse
A brief vibration launches a shear wave into the liver.
- 50 Hz: Push vibration freq (transient impulse)
- ~4000 Hz: Tracking pulse rate (ultrasound A-mode)
- ~4 cm: Wave travel distance (through parenchyma)
- VCTE: Method name (vibration-controlled transient elastography)
The mechanical tap
A tiny transducer-mounted piston taps the skin surface.
Shear vs compression waves
Shear waves move sideways, sensitive to tissue elasticity.
Ultrafast tracking
Pulse-echo ultrasound tracks the wavefront position over time.
Stiffness Measurement — Wave Speed to Kilopascals
Faster shear waves mean stiffer, more fibrotic liver tissue.
- E=3ρc²: Formula (Young modulus from speed)
- 2–6 kPa: Normal liver (F0 range)
- ≥12.5 kPa: Cirrhosis threshold (F4 range)
- 1.5–75 kPa: Reported range (device output)
Speed-to-stiffness conversion
Shear wave velocity c feeds directly into E=3ρc².
Fibrosis stiffens tissue
Collagen deposition raises tissue rigidity and wave speed.
Ten valid measurements
Median of ten shots gives the final kPa value.
CAP — Controlled Attenuation Parameter
Fat droplets scatter ultrasound, dimming the returning signal.
- dB/m: CAP unit (signal attenuation rate)
- <238 dB/m: Normal liver (S0 range)
- >290 dB/m: Severe steatosis (S3 range)
- Same probe: Measured with (simultaneous acquisition)
Attenuation physics
Fat droplets scatter sound, weakening the echo with depth.
Simultaneous with kPa
CAP is computed from the very same ultrasound shots.
Grading steatosis
CAP score bins liver fat into S0 through S3.
Fibrosis Staging — F0 to F4
The kPa value places the liver on the Metavir ladder.
- <7.0 kPa: F0–F1 (none to mild fibrosis)
- 7.0–9.4 kPa: F2 (significant fibrosis)
- 9.5–12.4 kPa: F3 (severe fibrosis)
- ≥12.5 kPa: F4 (cirrhosis)
Metavir mapping
Each kPa band corresponds to a Metavir fibrosis stage.
Biopsy avoidance
Staging replaces most diagnostic liver biopsies today.
Combined report
kPa and CAP together describe fibrosis plus steatosis.
Fibrosis (F) and steatosis (S) ladders