Subepithelial tumors (SETs) are covered with normal epithelium; they are often detected during gastrointestinal endoscopy and up to 20% of these may be neoplastic [1,2]. These comprise not only benign SETs such as leiomyomas, cysts, and lipomas but also potentially malignant tumors such as gastrointestinal stromal tumors (GISTs). GISTs are soft tissue sarcomas arising in mesenchymal tissues, and are the most common tumors of this origin in the gastrointestinal tract, accounting for approximately 3% of all gastrointestinal tumors and 5%–6% of all mesenchymal tumors [3,4]. However, it is difficult to distinguish between benign SET and GIST by only assessing SET size during upper gastrointestinal endoscopy.

Endoscopic ultrasound (EUS) has been reported to be useful in distinguishing GISTs from benign SETs [5,6]. EUS can differentially diagnose SETs by determining the wall layer of the origin and assessing its echogenicity. In EUS images, GISTs are usually hypoechoic masses that originate from the proper muscle layer. Some retrospective studies have shown that differences in shape, echogenicity, and homogeneity on EUS could differentiate GISTs from other benign SETs and assess the malignancy risk of GISTs [5,6]. However, it is reportedly difficult to differentiate small GISTs from leiomyomas because of a difference in the shape, echogenicity, and homogeneity of EUS [7].

It was reported that contrast-enhanced harmonic (CH) imaging of EUS was useful in distinguishing GIST from benign SET [8-12]; 10% to 30% of GISTs have a malignant clinical course. Therefore, it is necessary to estimate the malignancy potential of GIST by examining the pattern of microvasculature [10,13-15]. Contrast-enhanced ultrasound (US) has recently been found to be useful in the evaluation of treatment response by assessing blood flow in the tumors [16,17].

In this article, we review the principle of CH imaging, provide examples of CH imaging of GIST and benign SET, and describe the prediction of GIST malignancy potential and the evaluation of treatment response in GIST.

Three different second-generation US contrast agents are used in CH-EUS: SonoVue^® (Bracco Diagnostics, Milan, Italy), Sonazoid^® (Daiichi-Sankyo Co., Tokyo, Japan), and Definity^® (Bristol-Myers Squibb Co., New York, NY, USA). US contrast agents contain gas-filled microbubbles with a lipid monolayer, forming a lipophilic shell. On US insonation, the microbubbles resonate in response to pressure waves. The reflection signal from the resonated contrast microbubbles contains multiple insonated frequencies known as harmonic [18-20]. On the other hand, the harmonic component causing reflection signals from biological tissue is known as “tissue harmonics”. However, microbubbles, having larger nonlinearity, exceed the tissue harmonic component. The harmonic method depicts microbubbles selectively with high sensitivity by extracting only the harmonic component [21,22]. The value of mechanical index (MI) (a measure of acoustic power) that can resonate but break the microbubbles is 0.3–0.4 [21,22]. Therefore, US of the contrast agent can detect even a small blood vessel in the tissue (Fig. 1) [21-23].

After SET is detected by fundamental B-mode EUS, the MI is set to 0.2–0.4. The EUS screen is changed to dual image mode before the intravenous injection of contrast agent, with one image being the fundamental B-mode and the other being the contrast harmonic mode. The focal point is also set at the bottom. US beam penetration during CH is lesser than that in the fundamental B-mode; hence it is important to image the target lesion closely. A bolus infusion of contrast agent (Sonazoid^® 15 μL/kg bwt; SonoVue^®, 2.4–4.8 mL/bwt; Definity^®, 10 μL/kg bwt) is administered intravenously. Both continuous 0- to 15-second (vessel image) and 40- to 60-second (perfusion image) images were examined to allow for real time assessment of vascular structures.

There are three patterns used to quantify within-lesion blood flow on CH-EUS: hypo-enhancement, iso-enhancement, and hyper-enhancement. Quantification is done by comparing within-the target lesion blood flow with surrounding normal tissue blood flow, as described in a previous report [8]. The vessel image is reportedly used to detect irregular vessels in GIST. The enhancement patterns in the perfusion image are defined as either homogenous or heterogeneous enhancement.

It is difficult to distinguish GIST from benign submucosal lesions on fundamental B-mode EUS images. There have been some reports that CH-EUS is useful in distinguishing GIST from benign SET [8-12], as GISTs had hyper-enhancement patterns and benign SETs had hypo-enhancement patterns on CH-EUS images (Fig. 2). Literature on the use of CH-EUS for diagnostic differentiation of GIST from SET are summarized in Table 1. The sensitivity, specificity, and accuracy of the hyper-enhancement pattern on CH-EUS in the diagnosis of GIST ranged from 78 to 100%, 60 to 100%, and 82.2 to 100%, respectively [8-12].

Kamata et al. reported the homogeneity and heterogeneity of the contrast effect of GIST in CH-EUS images [9]. The homogeneity and heterogeneity of the contrast effect of SET were evaluated by the extent of blood flow in the perfusion image during the blood-pool phase (40–60 seconds after infusion of the contrast agent (Sonazoid) in the same study. This study found that 84.5% (49/58) of GISTs showed the hyper-enhancement pattern, whereas 26.7% (4/15) of benign SETs (lipoma, leiomyoma, schwannoma, glomus tumor, or ectopic pancreas) showed a hyper-enhancement pattern. Concurrently, 36.2% (21/58) of GISTs showed heterogeneous contrast enhancement, whereas only 13.3% (2/15) of benign SETs demonstrated heterogeneous contrast enhancement in the perfusion image. The sensitivity, specificity, and accuracy of the heterogeneous enhancement pattern on CH-EUS in the diagnosis of GISTs were 36.2%, 86.7%, and 46.6%, respectively. Almost all of the GISTs showed a hyper-enhancement pattern on CH-EUS. There was a tendency for smaller and larger GISTs to show homogeneous and heterogeneous enhancement patterns, respectively.

Lee et al. reported that the perfusion quantification analysis software was useful in differentiating GIST from SET by EUS [11]. They calculated and compared peak enhancement, wash-in rate (WiR), wash-in perfusion index (WiPl), and wash-in and wash-out areas time-intensity curve (WiWo-AUC) between the GISTs and benign SETs. The time-intensity curves were automatically analyzed to determine the CH-EUS parameters. The parameters for perfusion quantification include peak enhancement in arbitrary units (a.u), rise time in seconds, WiR in a.u, WiPl in a.u, and area under time-intensity curve in a.u. In GISTs, the WiR, WiPI, and WiWo-AUC were significantly higher than those in leiomyomas (p=0.024, 0.012, 0.006, respectively).

The contrast agents of US and EUS are useful in distinguishing GIST from benign SET and estimating the malignancy potential of GIST. In addition, recently, the contrast agent of US has been found to be useful in estimating the effectiveness of treatment of GIST with tyrosine kinase inhibitors (TKIs) such as imatinib [16]. TKIs are antiangiogenic agents that reduce the growth of new blood vessels that supply tumors, resulting in hemorrhage and/or necrosis. This can result in a clinical improvement, but tumor size does not necessarily reduce. Therefore, positron emission tomography (PET) with CT was used for estimating the treatment effectiveness of TKIs because PET has a high sensitivity in detecting effectiveness of TKI to metastatic GIST at an early stage. In contrast, PET-CT is prohibitively expensive for weekly monitoring to establish an early response [28,29]. In this regard, the European Society for Medical Oncology suggested that the use of contrast-enhanced US was useful for establishing an early response. Lassau et al. reported that 3 contrast-enhanced US variables were useful in predicting the effectiveness of TKIs in the treatment of GISTs: the area under the curve (AUC), the area under wash-in (AUWI), and the area under washout (AUWO) [16]. These measurements are all related to blood volume. In this regard, they were useful in evaluating the outcome of reduction in tumor vascularization by TKI treatment [16]. In the aforementioned study, 90% (18/20) of patients showed improvement and were treated for at least one year. There were no significant changes in the CH-US variables on day 1 and 7, when compared with those before treatment. However, there were significant reductions in the CH-US variables that relate to blood volume on day 15: AUC (p=0.004), AUWI (p=0.002), and AUWO (p=0.002). There were 63.6% (7/11) of patients with reduction in Standard Uptake Values (SUV). In addition, reduction of AUC, AUWI, and AUWO values on day 7 in the CH-US were highly predictive of the PET result. AUC, AUWI, and AUWO values on day 15 in the CH-US can be used as predictors of the response of GISTs to treatment with TKIs.

CH-EUS is useful in distinguishing GISTs from benign SETs and evaluating the malignancy potential of GIST. CH-EUS is an imaging modality that allows valuable assessment of the perfusion pattern of GISTs and SETs. Malignant GISTs have irregular vessels and heterogenous enhancement patterns in the tumor on CH-EUS. This method has a higher sensitivity than contrast CT and Doppler EUS for evaluating the malignancy potential of GIST. In addition, CH-EUS may be useful for the evaluation of treatment response by examining the blood flow in GIST. However, CH-EUS has some limitations in the diagnosis of gastrointestinal SETs. The limitations are that evaluation of SETs depends on performers’ skills and subjective analyses. Therefore, accumulation of more reports on CH-EUS is necessary to clarify its use.