Shear-Wave Elastography
Ultrasound elastography measures tissue stiffness, which is often altered by disease (fibrosis, inflammation, tumour).
In shear wave elastography (SWE), an acoustic radiation force pulse (a focused push pulse) generates shear waves that travel perpendicular to the beam within tissue. Ultrafast imaging tracks their speed. Speed is converted to tissue stiffness through the formula E = 3 x rho x c squared (E is Young's modulus in kilopascals, rho is density, and c is shear wave speed in metres each second).
Stiffness is displayed in kilopascals (kPa) or shear wave speed in metres each second. Display is either a colour map overlaid on B-mode (two dimensional SWE) or a single point measurement (point SWE). Alternatively, vibration controlled transient elastography (VCTE) uses an external mechanical push and a one dimensional probe.
Strain (compression) elastography measures relative stiffness without absolute values and is not covered here. The examination takes 5 to 15 minutes, with no radiation or contrast.
Several measurements are taken in a region of interest placed in a homogeneous area (usually 5 to 10 valid measurements, with the median and the interquartile range to median ratio reported). The transducer is perpendicular to the surface. Measurement is during a held breath at the end of normal expiration, in the supine position with the right arm raised (for liver).
Liver stiffness is the most established application and a non-invasive marker of fibrosis in chronic liver disease (hepatitis B, hepatitis C, metabolic dysfunction associated steatotic liver disease, alcohol related liver disease, autoimmune and cholestatic disease).
Ultrasound elastography has an area under the curve of 0.85 to 0.95 for diagnosis of significant fibrosis (F2 or more), 0.90 to 0.97 for advanced fibrosis (F3 or more) and 0.90 to 0.98 for cirrhosis (F4). Sensitivity and specificity are 85 to 90% when proper cut offs are used. A liver stiffness under 10 kPa (VCTE) in the absence of other signs rules out advanced chronic liver disease.
SWE is also used for breast lesions (distinguishing benign from malignant masses and reducing unnecessary biopsy), thyroid nodules, prostate, lymph nodes, musculoskeletal tissue (tendon, muscle), kidney transplant, spleen (portal hypertension), and pancreas.
Indications.
- Staging of liver fibrosis in chronic liver disease (viral hepatitis, metabolic dysfunction associated steatotic liver disease, alcohol related, autoimmune).
- Screening for advanced fibrosis in at risk groups (type 2 diabetes, obesity, harmful alcohol use, persistently abnormal liver enzymes, incidental steatosis).
- Monitoring of fibrosis progression or regression with treatment (antiviral therapy, weight loss).
- Identification of clinically significant portal hypertension (liver stiffness over 20 to 25 kPa, with platelet count under 150, and spleen stiffness) to avoid unnecessary endoscopy.
- Prognosis in cirrhosis.
Assessment of breast masses (complement to B-mode ultrasound and mammography; adjustment of the assessment category). Thyroid nodules (suspicious stiffness) and cervical lymph nodes. Prostate (targeted biopsy). Musculoskeletal conditions: tendinopathy, muscle spasticity, and plantar fasciitis. Kidney transplant fibrosis.
Fatty liver and steatosis quantification when combined with attenuation measurement (controlled attenuation parameter, ultrasound attenuation imaging). Not as a stand alone tool for diagnosis of cancer or inflammatory disease.
Contraindications and precautions. Liver SWE requires fasting for at least 2 to 3 hours (liver stiffness increases by 20 to 30% after a meal in portal hypertension).
Avoid in patients with ascites (VCTE cannot propagate shear waves in fluid; 2D SWE is possible with care), pacemakers or implanted active devices (relative for VCTE, as the manufacturer states; ultrasound based SWE is generally acceptable), and very narrow intercostal spaces.
Acute hepatitis with markedly elevated alanine aminotransferase (over 5 times the upper limit), extrahepatic cholestasis (biliary obstruction), acute heart failure with hepatic congestion, and infiltrative disease (amyloidosis) falsely elevate liver stiffness. Obesity needs a larger probe (XL probe for body mass index over 30) or alternative measurement.
The operator should be trained, with recorded measurement quality criteria.
Preparation and technique.
For VCTE:
- Fasting 3 hours, supine with right arm in maximum abduction.
- Place the probe on the skin in the intercostal space over the right lobe (at the level of the tip of the xiphoid process). Choose the position with the time motion image.
- 10 valid measurements are required, with a success rate over 60% and interquartile range to median ratio under 30%.
For 2D SWE:
- B-mode first to choose an acoustic window, 4 to 5 cm deep to the capsule, avoiding vessels, ribs and the capsule.
- Place the region of interest in a homogeneous area on the colour map, and take the median of several measurements (5 or more).
- Reliability criteria are a stable colour map, adequate quality maps, and ratio of interquartile range to median under 15 to 30%.
For breast: gentle application of the probe without compression, region of interest over the stiffest part of the lesion and surrounding tissue, with mean, maximum and ratio of lesion to fat measured. For thyroid: no compression and neck extended; avoid carotid pulsation artefacts.
