I. Introduction
Hepatocellular carcinoma (HCC) accounts for 85%–90% of primary liver carcinoma [1]. Early detection of HCC is crucial for treatment. There are several common diagnostic methods such as clinical, medical imaging, biopsy, etc., among which biopsy is the gold standard procedure [1]. Although biopsy is the most accurate method, it is invasive and inappropriate for monitoring the progress of HCC [2]. Therefore, the common protocols are imaging techniques, e.g., ultrasound (US), magnetic resonance imaging (MRI), Computed tomography (CT), or contrast-enhanced US (CEUS). Among these protocols, US is usually used for screening or surveillance tests for its wide availability, low cost, and high patient acceptance [3]. For high-probability HCC visible on US, further diagnostic uses CEUS, CT, and MRI [4]. However, CT can produce harmful radiation to the human body. If the patient has metal implants in the body, MRI is prohibited. CT and MRI are expensive and inconvenient to employ. Moreover, CT and MRI depict vascular pseudolesion, such as an arterioportal shunt, which is a frequent cause of diagnostic confusion [4]. In contrast, CEUS has been developed to identify focal liver lesions (FLLs) because of its enhancement patterns to highlight the textural tissues. Compared to CT and MRI, CEUS is cheaper, safer, and more suitable for vascular pseudolesions. According to the definition in CEUS Liver Report and Data System (CEUS LI-RADS) [4], starting from contrast agent injection, a liver CEUS video is divided into three intervals: 10–40 s, arterial phase (AP); 40–120 s, portal venous phase (PVP); and after 120 s, late phase (LP). In the literature [5], [6], the CEUS has been proven to be more sensitive in testing the FLLs than US or CT.