Ultrasound molecular imaging for differentiation of benign and malignant tumors in patients (2018)

The differentiation of benign and malignant tumors is of great significance for the preliminary evaluation of biological behaviors and prognosis of tumors prior to surgery or other interventions (1). Histopathological examination of the tumor biopsy samples serves as the gold standard for confirming the degree of malignancy of tumors. However, it is desirable and valuable to explore noninvasive imaging approaches to obtain diagnostic information at the earliest possible time points and without biopsy. When compared with other imaging modalities, ultrasound offers many advantages. It is safe, sensitive and portable. It offers a lower cost, yet also provides the features of real-time imaging and deep tissue penetration. Traditional ultrasound imaging technologies include B-mode-based grayscale ultrasonography, blood-flow-based color Doppler flow imaging (CDFI) and stiffness-based elastography. These techniques reflect local anatomy and can provide precursory information on the benign and malignant status of tumors (2,3). For example, the representative grayscale ultrasonography of malignant neoplasms is considered to be hypoechoic in grey scale, with fuzzy boundary often accompanied by microcalcification (4). CDFI is generally used to determine the benign vs. malignant tumor status from the blood vessel density and blood flow spectrum data (5,6). Contrast-enhanced ultrasound (CEUS) is capable of assessing the blood flow in the lesions through perfusion imaging of small capillary blood vessels (7,8). To some degree, all these methods provide diagnostic information on the benign or malignant status of the interrogated lesions. However, these ultrasound techniques mostly reflect the characteristics of tumor anatomy.

During the past 20 years, ultrasound elastography has developed as a supplement to traditional ultrasound technology for differentiation of benign and malignant tumors, especially in superficial organs (9). Based on the heterogeneity of stiffness between different tumor tissues, ultrasound elastography can distinguish between benign and malignant tumors by detecting the modulus of elasticity (10). Studies have confirmed that, compared with conventional ultrasonography, ultrasound elastography has better specificity and accuracy in differentiating benign from malignant breast tumors (11,12). Nevertheless, ultrasound elastography has limitations in precise tumor diagnosis. The results of ultrasound elastography were easily affected by the skills of the operators and the location of the lesion, which might cause false positive results and reduce the diagnosis accuracy (13). Therefore, it is very important to develop novel ultrasound imaging technology to acquire functional information, especially the information on the tumor status at the molecular level, which is currently only available from invasive techniques, such as biopsy and histology. Fortunately, ultrasound technology has emerged with an exciting molecular imaging potential in tumor diagnosis.

Ultrasound molecular imaging with targeted probes requires specific targeting to the receptors overexpressed on the angiogenic blood vessels. Such targeted contrast agents are administered intravenously, and accumulate in the vasculature in the areas of disease (14). Therefore, microbubbles, the most popular ultrasound contrast agents, need to be functionalized with appropriate ligands that have high affinity to the target vasculature biomarkers of disease, and allow specific binding and retention of these particles on the target vessel surface, despite the dislodging action of blood flow. Numerous targeting ligands, including antibodies, antibody fragments, peptides, and carbohydrates have been applied to construct targeted microbubble probes (15). By detecting the signals derived from the retained microbubbles in the target regions, ultrasound molecular imaging can determine the expression level of molecules and visualize biomarker status in situ. Multiple studies in vivo have demonstrated the diagnostic utility of ultrasound molecular imaging for the detection of inflammation, atherosclerosis and tumor angiogenesis in animal studies (16-19).