Article information

Clin Ultrasound. 2025;10(2):53-58

Publication date (electronic) : 2025 November 30

doi : https://doi.org/10.18525/cu.2025.10.2.53

Jae Seung Lee

Department of Internal Medicine, Yonsei University College of Medicine, Seoul, Korea

이재승

연세대학교 의과대학 내과학교실

Address for Correspondence: Jae Seung Lee, M.D., Ph.D. Department of Internal Medicine, Yonsei University College of Medicine, 50-1 Yonsei-ro, Seodaemun-gu, Seoul 03722, Korea Tel: +82-2-2228-2286 E-mail: sikarue@yuhs.ac

Received 2025 September 13; Accepted 2025 October 16.

Trans Abstract

INTRODUCTION

Hepatic fibrosis refers to scar-like changes in the liver that result from persistent long-term inflammation in patients with chronic hepatitis [1]. Clinical evidence indicates that the degree of fibrosis predicts both liver-related and extrahepatic clinical outcomes in patients with chronic hepatitis due to viral infection, chronic alcohol use, or steatohepatitis [1-4]. Therefore, accurate staging of fibrosis is essential for guiding treatment decisions, assessing prognosis, and planning surveillance strategies [1].

While liver biopsy has historically been the reference standard, its invasiveness, sampling variability, and potential complications have driven the adoption of non-invasive diagnostic tools [1]. Recent studies have increasingly highlighted the clinical usefulness of laboratory-based tests and liver stiffness measurements for routine surveillance [5-7]. Shear-wave elastography (SWE) is a pivotal imaging modality that provides quantitative measurements of liver stiffness, enabling non-invasive prediction of histological fibrosis [1]. SWE offers advantages in clinical practice by allowing real-time ultrasound (US) B-mode guidance for accurate placement of the region of interest (ROI) and by demonstrating high reproducibility under standardized protocols. Accordingly, this review will cover the conceptual framework of SWE and outline standardized approaches for its clinical application.

PHYSICAL PRINCIPLES OF SWE

SWE operates by inducing localized tissue displacement through an acoustic radiation force impulse (ARFI), which generates shear waves propagating transversely to the US beam [8]. In soft tissues, shear wave velocities typically range from 1 to 10 m/s, creating substantial differences in shear modulus among tissues to provide suitable contrast for elastography measurements [8]. The US transducer detects the speed of these shear waves (m/s) and calculates stiffness metrics (kPa) [9]. The shear modulus (G) is calculated as the product of tissue density and the square of shear wave velocity [8]. Young’s modulus (E), which refers to tissue stiffness, is approximately three times the shear modulus under the assumption that the tissue is isotropic and incompressible [8,10]. Therefore, faster wave propagation indicates stiffer tissue. For clinical use, liver stiffness can be expressed either as wave velocity (m/s) or as calculated elasticity (kPa).

SWE MODALITIES: POINT SWE (p-SWE) AND TWO-DIMENSIONAL SWE (2D-SWE)

p-SWE uses a single ARFI push to generate waves within a small, fixed ROI and can be performed under B-mode visualization. In contrast, 2D-SWE applies multiple ARFI pushes to create a real-time elastogram over a wide field of view (FOV), enabling ROI placement within the stiffness map and generally requiring fewer a cquisitions (Fig. 1). A lthough e arlier US s ystems typically supported only one of these two modalities, technological advances have enabled devices capable of incorporating both.

Figure 1.

Comparison between P-SWE and 2D-SWE. P-SWE, point shear-wave elastography; SW, shear-wave; ROI, region of interest; 2D-SWE, two-dimensional SWE.

Both p-SWE and 2D-SWE have demonstrated high diagnostic performance in assessing histological liver fibrosis [1]. Ac-cording to the 2024 guidelines of the Korean Association for the Study of the Liver, significant fibrosis (≥F2) corresponds to 1.23–1.59 m/s and 6.9–8.2 kPa, advanced fibrosis (≥F3) corresponds to 1.54–1.81 m/s and 7.4–9.2 kPa, and cirrhosis (F4) corresponds to 1.75–1.98 m/s and 8.0–21.4 kPa for p-SWE and 2D-SWE, respectively. These cutoffs are associated with high area under the receiver operating characteristic curve (AUC) values (0.72–0.98), indicating excellent discriminatory ability (Table 1) [1]. However, since the cutoff values reported in the literature vary to some extent, the recent 2024 World Federation for Ultrasound in Medicine and Biology (WFUMB) guidelines proposed the “Rule of Four,” which uses a 4 kPa increment as a basis to aid in the interpretation of SWE for predicting the degree of fibrosis, chronicity of liver disease, and portal hypertension (Table 2) [11].

Table 1.

Cutoff values and diagnostic performance of SWE in liver fibrosis staging according to the 2024 KASL guidelines

Table 2.

Interpretation of liver stiffness measurement obtained using ARFI-SWE techniques (rule of four)

Nevertheless, among the two modalities, 2D-SWE may be considered superior to p-SWE. A recent study from Korea by Lee et al. [12], which directly compared the diagnostic performance of 2D-SWE and p-SWE in 87 patients who underwent liver biopsy, demonstrated that 2D-SWE was associated with significantly fewer unreliable measurements (1.1% vs. 9.2%, p < 0.001) and provided superior diagnostic accuracy for significant fibrosis (AUC 0.965 vs. 0.872, p = 0.022) and cirrhosis (AUC 0.994 vs. 0.886, p = 0.042). These advantages may be attributable to the larger ROI box with color coding in 2D-SWE, which allows more appropriate ROI placement away from artifact-prone areas, as well as the fewer acquisitions required by 2D-SWE than p-SWE [12].

PRE-PROCEDURE PREPARATION

Proper patient preparation is essential for obtaining accurate and reproducible SWE measurements. Fasting is recommended for at least four hours according to WFUMB guidelines or a minimum of two hours according to European Federation of Societies for Ultrasound in Medicine and Biology (EFSUMB), as food intake can transiently increase liver stiffness and lead to overestimation of fibrosis [11,13]. Patients should rest for at least 10 minutes before the examination to minimize physiological variability, and alcohol consumption and heavy physical activity should be avoided [11,13].

DURING THE PROCEDURE: PATIENT POSITIONING AND BREATH CONTROL

The examination is typically performed with the patient in a supine or left lateral decubitus position with a tilt less than 30° [11], although a full left lateral decubitus position may also be used when appropriate. The right arm should be abducted overhead, with care to avoid shoulder strain or injury, to widen the intercostal spaces and improve acoustic access to the right lobe of the liver [11,13]. Measurements are ideally obtained during a neutral or end-expiration breath-hold [11,13]. Deep inspiration immediately before the hold should be avoided, as this can alter diaphragmatic position and introduce variability [11,13].

DURING THE PROCEDURE: PROBE PLACEMENT AND ROI SELECTION

Measurements are usually obtained using an intercostal approach at the site with the best acoustic window [11,13]. Adequate B-mode liver imaging is a prerequisite, without shadowing from the lung or ribs [11,13]. The transducer should be positioned perpendicular to the liver capsule to ensure accurate wave propagation, and the transducer, liver capsule, and top of the ROI box should be aligned in parallel (Fig. 2) [11,13]. The ROI is typically placed 1.5–2.0 cm below the capsule to minimize reverberation artifacts from the liver surface (Fig. 2) [11,13]. Measurements are most often performed in segment 8 of the right lobe, avoiding large vessels, bile ducts, and rib shadows. For 2D-SWE, the ROI size should be at least 10 mm; for p-SWE, the ROI size is fixed according to vendor specifications [11,13].

Figure 2.

Positioning of the liver capsule and ROI box in shear-wave elastography. The transducer, liver capsule, and top of the ROI box should be parallel. Measurement should be performed 1.5–2.0 cm below the liver capsule to avoid reverberation artifacts. ROI, region of interest.

DURING THE PROCEDURE: MEASUREMENT PROTOCOL

Obtaining a sufficient number of valid measurements is essential for accuracy. However, the appropriate number and method of acquisition differ slightly depending on the guideline. For p-SWE, the WFUMB recommends 5–10 valid measurements from the same site, whereas the EFSUMB recommends at least 10 (Fig. 3A) [11,13]. For 2D-SWE, the WFUMB recommends 3–5 independent acquisitions from a separated FOV [11], whereas the EFSUMB allows a minimum of 3 measurements regardless of the separation of the FOV (Fig. 3B) [13]. For this issue, the WFUMB notes that, although most vendors with 2D-SWE allow placement of multiple ROIs within the same elastogram FOV, this practice is discouraged because any error in that image would be reproduced across all derived measurements (Fig. 3C) [11]. Therefore, in practice, one possible approach when performing 2D-SWE is to obtain approximately 10 valid measurements by acquiring 2–3 measurements from each of 3–4 FOVs. Furthermore, some of the more recently developed systems provide the option to store multiple elastogram frames in advance, enabling operators to obtain conveniently a sufficient number of measurements.

Figure 3.

Examples of SWE measurements according to protocol. (A) Seven valid measurements obtained with point-SWE, showing poor quality control (IQR/M 20%). (B) Ten valid measurements in an unseparated but adequate FOV with good quality control (IQR/M 6.5%) using 2D-SWE. (C) Examples of concordance (left) and discordance (right) between biopsy-proven fibrosis stage and SWE stiffness under acceptable quality control, depending on FOV reliability. SWE, shear-wave elastography; IQR/M, interquartile range-to-median ratio; FOV, field of view; 2D-SWE, two-dimensional SWE.

Quality control should include assessment of the interquartile range-to-median ratio (IQR/median), which is considered acceptable at ≤15% for p-SWE when expressed in m/s and ≤30% for 2D-SWE when expressed in kPa (Fig. 3A, B) [11,13]. Reli-ability should also be verified using vendor-specific quality indices such as propagation maps or reliability measurement index, and acquisitions with poor wave propagation or evident artifacts should be excluded [14].

PITFALLS AND CONSIDERATIONS

Several artifacts can affect the accuracy of SWE, including reverberation from the liver capsule or ribs, attenuation in obese patients, motion related to respiration or cardiac pulsation, and inadvertent probe compression, all of which can falsely increase the measured stiffness values. In addition, certain physiologic and pathologic conditions, such as acute hepatitis, hepatic congestion due to heart failure, cholestasis, and steatohepatitis, may elevate the measured stiffness independently of actual fibrosis and should be considered when interpreting results.

CONCLUSIONS

SWE is a reliable and reproducible non-invasive tool for pre-dicting liver fibrosis, providing real-time quantitative assessment during standard US examinations. Strict adherence to technical guidelines, including patient preparation, probe positioning, breath-hold technique, and quality control criteria, is essential to ensure measurement reliability. Interpretation must also take into account physiological and pathological confounders such as inflammation, congestion, and steatosis and always be integrated with the clinical context. With ongoing technological advances and the adoption of standardized protocols supported by adequate training, SWE is poised to become an increasingly integral component of hepatologic imaging, extending its role across diverse clinical applications.

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