Recent advances in molecular, multimodal and theranostic ultrasound imaging

Abstract

Ultrasound (US) imaging is an exquisite tool for the non-invasive and real-time diagnosis of many different diseases. In this context, US contrast agents can improve lesion delineation, characterization and therapy response evaluation. US contrast agents are usually micrometer-sized gas bubbles, stabilized with soft or hard shells. By conjugating antibodies to the microbubble (MB) surface, and by incorporating diagnostic agents, drugs or nucleic acids into or onto the MB shell, molecular, multimodal and theranostic MBs can be generated. We here summarize recent advances in molecular, multimodal and theranostic US imaging, and introduce concepts how such advanced MB can be generated, applied and imaged. Examples are given for their use to image and treat oncological, cardiovascular and neurological diseases. Furthermore, we discuss for which therapeutic entities incorporation into (or conjugation to) MB is meaningful, and how US-mediated MB destruction can increase their extravasation, penetration, internalization and efficacy.

Introduction

Due to its non-invasive nature, low cost, broad diagnostic applicability and easy handling, ultrasound (US) imaging is the second-most used imaging modality in clinical practice after conventional x-ray radiography [1]. It is used by medical doctors from various different disciplines, including radiologists, gynecologists, cardiologists, gastroenterologists, surgeons and many more as an initial screening tool, as well as for fast-look follow-up examinations. Its ability to visualize blood flow, blood velocity and blood vessels by Power and Color Doppler further recommends US imaging for vascular diagnosis, e.g. for measuring the degree of stenosis in carotid arteries [2], and for looking at the perfusion of tumors [3] and organs after transplantation [4].

Besides these diagnostic applications, High-Intensity Focused US (HIFU) has been attracting ever more attention as a valuable therapeutic option to destroy ureteric stones [5], and to ablate benign uterus myomas and other benign and malignant tumors [6]. In this context, the acoustic energy focused to one defined spot is moved over the pathological tissue. Due to absorption of the acoustic energy and the resulting local temperature increase, the pathological tissue is destroyed. Recently, the first commercial HIFU-systems that can be used inside clinical MR scanners have been introduced which enable highly personalized and well-controlled tissue ablation by getting anatomical information about the pathology and the local temperature rise from MR imaging.

However, the diagnostic and therapeutic potential of US imaging has not yet been fully explored and translated to clinic. In this regard, US contrast agents, which are gas-filled microbubbles (MBs) stabilized by a shell made of lipids, proteins or polymers can enormously improve US imaging. In particular, the use of MB significantly expands the diagnostic potential of US for characterizing pathologies based on functional and molecular vascular characteristics. Furthermore, the use of MB-based contrast agents in US imaging offers possibilities for image-guided (theranostic) interventions. In the present manuscript, recent developments in this emerging and interdisciplinary field are summarized and discussed.

US contrast agents in combination with contrast agent-specific US imaging techniques are increasingly accepted in routine clinical practice for diagnostic imaging of several organs and pathologies. Particular interest is given to examinations of the liver, because of the significant improvement over conventional US in both, the detection and characterization of focal liver lesions. Recent studies even show that the diagnostic performance of contrast-enhanced ultrasound (CEUS) can reach that of contrast-enhanced computed tomography (CT) and magnetic resonance imaging (MRI) (Fig. 1) [7], [8], [9]. The high diagnostic accuracy of CEUS in liver imaging is based on two characteristics:

• the detection and early enhancement of a malignant liver lesion during the arterial phase

• the rapid wash-out of the contrast agent in malignant liver lesions.

A further benefit of US contrast agents in the clinical routine is their good safety profile, which enables the administration of contrast agents to patients who have contra-indications for contrast-enhanced CT or MRI (e.g. patients with severe renal dysfunction). As a consequence, focal liver diseases have evolved into the single most important application of CEUS. The recommendations for CEUS for liver imaging are summarized in the guidelines for good clinical practice of the EFSUMB [10].

A second major clinical application is contrast echocardiography, where MBs are used for left ventricular opacification and endocardial border delineation. The superior anatomical delineation of the cardiac boarders leads to specific clinical scenarios in which US contrast agents could/should be used, including the assessment of left ventricular systolic function, elevation of the left ventricular apex, mechanical complications of myocardial infarction and the characterization of intracardiac masses. The consensus statement on the use of ultrasound contrast agents was published in 2008 by the American Society of Echocardiography [11] and a summary of the clinical impact of the guidelines was provided two years afterwards [12].

In neurology and intensive care medicine, contrast-enhanced transcranial Doppler ultrasound has been established as a reliable tool to evaluate the cerebral circulation, e.g. to outline vessel stenosis and occlusion as well as ultimately, to diagnose brain death [13], [14].

Besides liver, cardiac and brain imaging, indications for CEUS have expanded to applications in the kidney [15], [16], in vesico-ureteric reflux [17], [18], in the pancreas [18], [19], [20], [21], in trauma patients [22] and in cerebral circulation, as well as in oncological studies [23], [24]. In this context, an emerging clinical field might represent the assessment of novel targeted drugs such as anti-angiogenic therapies. Here, contrast enhanced ultrasound enables the early identification of responders to an antiangiogenic treatment for gastrointestinal stromal tumors, renal cell carcinoma, and hepatocellular carcinoma [25], [26]. Despite this promising data derived in clinical trials, however, a broad application into the daily clinical routine has not yet been established.

In this context, the continuous revision of the existing “Guidelines for Good Clinical Practice” in consensus meetings of the US societies and the continuous medical training of “CEUS-examiners” in specialized CEUS courses are – besides advancements in US machines and contrast agents – the most important preconditions for maintaining and increasing the clinical success of CEUS.