Quantitative and noninvasive assessment of chronic liver diseases using two-dimensional shear wave elastography

Table 3 Precautions and techniques of two-dimensional shear wave elastography

Key points	Precautions and techniques
Fasting and resting	▪ Patients should fast for a minimum of 2 h and rest for a minimum of 10 min before undergoing liver stiffness measurement with SWE
Position	▪ Measurement of liver stiffness by 2D-SWE should be performed in a supine position with the right arm maximally extended; this position ensures the best possible access for assessing the right liver lobe ▪ The transducer is placed in a right intercostal space to visualize the right liver lobe in B mode
Breathing train	▪ Instruct the patient not to breathe in or breathe out deeply in order to eliminate unreliable measurements induced by breathing movements ▪ It has been suggested that a breath hold for a few seconds during quiet breathing may lead to the best results
Clear 2D-US images	▪ Adequate B-mode liver image is a prerequisite for 2D-SWE measurements ▪ Must avoid the ribs, gas and other factors of routine ultrasound ▪ The appropriate pressure can be applied with the ultrasound probe to broaden the intercostal space and, thus, acquiring clear images. Contrary to the ordinary suggestion, this does not increase the liver’s stiffness, as the intervening tissues prevent distortion of the liver surface
Scale	▪ Generally, the Young’s modulus scale should not be less than 30 kPa and preferably not higher than 150 kPa
Depth	▪ Liver stiffness measured by 2D-SWE should be performed at least 10 mm under the liver capsule ▪ Measurements should not be performed too deep or too close, in order to avoid reverberation artifacts, insufficient penetration and acoustic shadow, as these factors will lead to incorrect results
Sampling frame	▪ The sampling frame should be placed in a well-visualized area of the right liver lobe, free of large vessels, the gallbladder, the liver capsule, and any other hollow organs ▪ In addition, the sampling frame should be placed in the center of the image
ROI	▪ For valid measurement quality of 2D-SWE, the ROI should be placed at a minimum of 1-2 cm and a maximum of 6 cm beneath the liver capsule ▪ The SWE acquisition is continued for 4-5 s once a stable SWE image is obtained ▪ The operator freezes the image, and the ROI should be placed in the most homogeneously colored area of the SWE ROI
Penetration mode	▪ When measuring patients with thick subcutaneous fat, fatty liver or advanced cirrhosis, SWE can be adjusted to “Pen” mode to improve the measurement success rate ▪ In 2D-SWE, if the signal is weak or unstable, the penetration mode can be activated

2D-SWE: Two-dimensional shear wave elastography; 2D-US: Two-dimensional ultrasound; Pen: Penetration; ROI: Region of interest; SWE: Shear wave elastography.