Three-dimensional ultrasound molecular imaging of angiogenesis in colon cancer using a clinical matrix array ultrasound transducer

Motion correction of 3D dynamic contrast-enhanced ultrasound imaging without anatomical B-Mode images: Pilot evaluation in eight patients

BACKGROUND

Dynamic contrast-enhanced ultrasound (DCE-US) is highly susceptible to motion artifacts arising from patient movement, respiration, and operator handling and experience. Motion artifacts can be especially problematic in the context of perfusion quantification. In conventional 2D DCE-US, motion correction (MC) algorithms take advantage of accompanying side-by-side anatomical B-Mode images that contain time-stable features. However, current commercial models of 3D DCE-US do not provide side-by-side B-Mode images, which makes MC challenging.

PURPOSE

This work introduces a novel MC algorithm for 3D DCE-US and assesses its efficacy when handling clinical data sets.

METHODS

In brief, the algorithm uses a pyramidal approach whereby short temporal windows consisting of three consecutive frames are created to perform local registrations, which are then registered to a master reference derived from a weighted average of all frames. We applied the algorithm to imaging studies from eight patients with metastatic lesions in the liver and assessed improvements in original versus motion corrected 3D DCE-US cine using: (i) frame-to-frame volumetric overlap of segmented lesions, (ii) normalized correlation coefficient (NCC) between frames (similarity analysis), and (iii) sum of squared errors (SSE), root-mean-squared error (RMSE), and r-squared (R2) quality-of-fit from fitted time-intensity curves (TIC) extracted from a segmented lesion.

RESULTS

We noted improvements in frame-to-frame lesion overlap across all patients, from 68% ± 13% without correction to 83% ± 3% with MC (p = 0.023). Frame-to-frame similarity as assessed by NCC also improved on two different sets of time points from 0.694 ± 0.057 (original cine) to 0.862 ± 0.049 (corresponding MC cine) and 0.723 ± 0.066 to 0.886 ± 0.036 (p ≤ 0.001 for both). TIC analysis displayed a significant decrease in RMSE (p = 0.018) and a significant increase in R2 goodness-of-fit (p = 0.029) for the patient cohort.

CONCLUSIONS

Overall, results suggest decreases in 3D DCE-US motion after applying the proposed algorithm.

Biopathologic Characterization and Grade Assessment of Breast Cancer With 3-D Multiparametric Ultrasound Combining Shear Wave Elastography and Backscatter Tensor Imaging

OBJECTIVE

Despite recent improvements in medical imaging, the final diagnosis and biopathologic characterization of breast cancers currently still requires biopsies. Ultrasound is commonly used for clinical examination of breast masses. B-Mode and shear wave elastography (SWE) are already widely used to detect suspicious masses and differentiate benign lesions from cancers. But additional ultrasound modalities such as backscatter tensor imaging (BTI) could provide relevant biomarkers related to tissue organization. Here we describe a 3-D multiparametric ultrasound approach applied to breast carcinomas in the aims of (i) validating the ability of BTI to reveal the underlying organization of collagen fibers and (ii) assessing the complementarity of SWE and BTI to reveal biopathologic features of diagnostic interest.

METHODS

Three-dimensional SWE and BTI were performed ex vivo on 64 human breast carcinoma samples using a linear ultrasound probe moved by a set of motors. Here we describe a 3-D multiparametric representation of the breast masses and quantitative measurements combining B-mode, SWE and BTI.

RESULTS

Our results reveal for the first time that BTI can capture the orientation of the collagen fibers around tumors. BTI was found to be a relevant marker for assessing cancer stages, revealing a more tangent tissue orientation for in situ carcinomas than for invasive cancers. In invasive cases, the combination of BTI and SWE parameters allowed for classification of invasive tumors with respect to their grade with an accuracy of 95.7%.

CONCLUSION

Our results highlight the potential of 3-D multiparametric ultrasound imaging for biopathologic characterization of breast tumors.

Characterization of spatially mapped volumetric molecular ultrasound signals for predicting response to anti-vascular therapy

Quantitative three-dimensional molecular ultrasound is a promising technology for longitudinal imaging applications such as therapy monitoring; the risk profile is favorable compared to positron emission tomography and computed tomography. However, clinical translation of quantitative methods for this technology are limited in that they assume that tumor tissues are homogeneous, and often depend on contrast-destruction events that can produce unintended bioeffects. Here, we develop quantitative features (henceforth image features) that capture tumor spatial information, and that are extracted without contrast destruction. We compare these techniques with the contrast-destruction derived differential targeted enhancement parameter (dTE) in predicting response to therapy. We found thirty-three reproducible image features that predict response to antiangiogenic therapy, without the need for a contrast agent disruption pulse. Multiparametric analysis shows that several of these image features can differentiate treated versus control animals with comparable performance to post-destruction measurements, suggesting that these can potentially replace parameters such as the dTE. The highest performing pre-destruction image features showed strong linear correlations with conventional dTE parameters with less overall variance. Thus, our study suggests that image features obtained during the wash in of the molecular agent, pre-destruction, may replace conventional post-destruction image features or the dTE parameter.

Dynamic Contrast-Enhanced Ultrasound Modeling of an Analog to Pseudo-Diffusivity in Intravoxel Incoherent Motion Magnetic Resonance Imaging

Tumor perfusion and vascular properties are important determinants of cancer response to therapy and thus various approaches for imaging perfusion are being explored. In particular, Intravoxel Incoherent Motion (IVIM) MRI has been actively researched as an alternative to Dynamic-Contrast-Enhanced (DCE) CT and DCE-MRI as it offers non-ionizing, non-contrast-based perfusion imaging. However, for repetitive treatment assessment in a short time period, high cost, limited access, and inability to scan at the bedside remain disadvantages of IVIM MRI. We propose an analysis framework that may enable 3D DCE Ultrasound (DCE-US) - low cost, bedside imaging with excellent safety record - as an alternative modality to IVIM MRI for the generation of DCE-US based pseudo-diffusivity maps in acoustically accessible anatomy and tumors. Modelling intravascular contrast propagation as a convective-diffusive process, we reconstruct parametric maps of pseudo-diffusivity by solving a large-scale fully coupled inverse problem without any assumptions regarding local constancy of the reconstructed parameters. In a mouse tumor model, we demonstrate that the 3D DCE-US pseudo-diffusivity is repeatable, sensitive to treatment with an antiangiogenic agent, and moderately correlated to histological measures of perfusion and angiogenesis.

Early evaluation of treatment response to transarterial chemoembolization in patients with advanced hepatocellular carcinoma: The role of dynamic three-dimensional contrast-enhanced ultrasound

BACKGROUND

Dynamic three-dimensional contrast-enhanced ultrasound (3D-CEUS) with quantitative analysis is available in recent years. It can reduce the quantitative sampling error caused by the inconsistency of different sections in order to evaluate local treatment response of hepatocellular carcinoma (HCC) accurately.

OBJECTIVE

To investigate the value of dynamic 3D-CEUS in evaluating the early response to transarterial chemoembolization (TACE) treatment in patients with advanced HCC lesions.

METHODS

In this prospective study, both two-dimensional (2D) CEUS and dynamic 3D-CEUS were performed on 40 HCC patients who scheduled for TACE at baseline (T0) and 1-3 days (T1) after treatment. Tumor microvascular perfusion changes were assessed by CEUS time-intensity curve (TIC) and quantitative parameters. According to contrast-enhanced computed tomography (CT) and magnetic resonance (MR) imaging 1 month after treatment results, patients were divided into responders and non-responders groups. The changes of perfusion parameters of both 2D-CEUS and 3D-CEUS were compared between responders and non-responders groups before and after TACE treatment.

RESULTS

Before and after TACE treatment, no significant difference in maximum diameter of HCC lesions between the two groups could be found. There were more significant differences and ratios of perfusion parameters in 3D-CEUS quantitative analysis than in 2D-CEUS. The mutual significant differences and ratios of 2D-CEUS and 3D-CEUS included peak intensity (PI) difference, PI ratio, ratio of area under the curve (A), ratio of area under the wash-out part (AWO) and slope (S) difference. The former 4 corresponding parameters were better on 3D-CEUS than on 2D-CEUS.

CONCLUSION

Dynamic 3D-CEUS can be used as a potential imaging method to evaluate early treatment response to TACE in advanced HCC patients.

Molecular Ultrasound Imaging

In the last decade, molecular ultrasound imaging has been rapidly progressing. It has proven promising to diagnose angiogenesis, inflammation, and thrombosis, and many intravascular targets, such as VEGFR2, integrins, and selectins, have been successfully visualized in vivo. Furthermore, pre-clinical studies demonstrated that molecular ultrasound increased sensitivity and specificity in disease detection, classification, and therapy response monitoring compared to current clinically applied ultrasound technologies. Several techniques were developed to detect target-bound microbubbles comprising sensitive particle acoustic quantification (SPAQ), destruction-replenishment analysis, and dwelling time assessment. Moreover, some groups tried to assess microbubble binding by a change in their echogenicity after target binding. These techniques can be complemented by radiation force ultrasound improving target binding by pushing microbubbles to vessel walls. Two targeted microbubble formulations are already in clinical trials for tumor detection and liver lesion characterization, and further clinical scale targeted microbubbles are prepared for clinical translation. The recent enormous progress in the field of molecular ultrasound imaging is summarized in this review article by introducing the most relevant detection technologies, concepts for targeted nano- and micro-bubbles, as well as their applications to characterize various diseases. Finally, progress in clinical translation is highlighted, and roadblocks are discussed that currently slow the clinical translation.

Ultrasound Molecular Imaging of Renal Cell Carcinoma: VEGFR targeted therapy monitored with VEGFR1 and FSHR targeted microbubbles

Recent treatment developments for metastatic renal cell carcinoma offer combinations of immunotherapies or immunotherapy associated with tyrosine kinase inhibitors (TKI). There is currently no argument to choose one solution or another. Easy-to-use markers to assess longitudinal responses to TKI are necessary to determine when to switch to immunotherapies. These new markers will enable an earlier adaptation of therapeutic strategy in order to prevent tumor development, unnecessary toxicity and financial costs. This study evaluates the potential of ultrasound molecular imaging to track the response to sunitinib in a clear cell renal carcinoma model (ccRCC). We used a patient-derived xenograft model for this imaging study. Mice harboring human ccRCC were randomized for sunitinib treatment vs. control. The tumors were imaged at days 0, 7, 14 and 28 with ultrasound molecular imaging. Signal enhancement was quantified and compared between the two groups after injections of non-targeted microbubbles and microbubbles targeting VEGFR1 and FSHR. The tumor growth of the sunitinib group was significantly slower. There was a significantly lower expression of both VEGFR-1 and FSHR molecular ultrasound imaging signals in the sunitinib group at all times of treatment (Days 7, 14 and 28). These results confirm the study hypothesis. There was no significant difference between the 2 groups for the non-targeted microbubble ultrasound signal. This study demonstrated for the first time the potential of VEGFR1 and FSHR, by ultrasound-based molecular imaging, to follow-up the longitudinal response to sunitinib in ccRCC. These results should trigger developments for clinical applications.

Spatial Characterization of Tumor Perfusion Properties from 3D DCE-US Perfusion Maps are Early Predictors of Cancer Treatment Response

There is a need for noninvasive repeatable biomarkers to detect early cancer treatment response and spare non-responders unnecessary morbidities and costs. Here, we introduce three-dimensional (3D) dynamic contrast enhanced ultrasound (DCE-US) perfusion map characterization as inexpensive, bedside and longitudinal indicator of tumor perfusion for prediction of vascular changes and therapy response. More specifically, we developed computational tools to generate perfusion maps in 3D of tumor blood flow, and identified repeatable quantitative features to use in machine-learning models to capture subtle multi-parametric perfusion properties, including heterogeneity. Models were developed and trained in mice data and tested in a separate mouse cohort, as well as early validation clinical data consisting of patients receiving therapy for liver metastases. Models had excellent (ROC-AUC > 0.9) prediction of response in pre-clinical data, as well as proof-of-concept clinical data. Significant correlations with histological assessments of tumor vasculature were noted (Spearman R > 0.70) in pre-clinical data. Our approach can identify responders based on early perfusion changes, using perfusion properties correlated to gold-standard vascular properties.

Efficacy evaluation and mechanism study on inhibition of breast cancer cell growth by multimodal targeted nanobubbles carrying AMD070 and ICG

To construct targeted nanobubbles carrying both small-molecule CXCR4 antagonist AMD070 and light-absorbing material indocyanine green (ICG), and to study their in vitro multimodal imaging, as well as their mechanism and efficacy of inhibition of breast cancer cell growth. Nanobubbles carrying AMD070 and ICG (ICG-TNBs) were constructed by carbodiimide reaction and mechanical oscillation. The physical characteristics and in vitro multimodal imaging were determined. The binding potential of ICG-TNBs to human breast cancer cells were observed by laser confocal microscopy. CCK-8 and flow cytometry were used to analyze the role of ICG-TNBs + US in inhibiting proliferation and inducing apoptosis of tumor cells. Flow cytometry and Western blotting are used to analyse the ROS generation and molecular mechanisms. ICG-TNBs had a particle size of 497.0 ± 29.2 nm and a Zeta potential of -8.05 ± 0.73 mV. In vitro multimodal imaging showed that the image signal intensity of ICG-TNBs increased with concentration. Targeted binding assay confirmed that ICG-TNBs could specifically bind to MCF-7 cells (CXCR4 positive), but not to MDA-MB-468 cells (CXCR4 negative). CCK-8 assay and flow cytometry analysis showed that ICG-TNBs + US could significantly inhibit the growth of MCF-7 breast cancer cells and promote their apoptosis. Flow cytometry and Western blotting showed that ICG-TNBs + US could significantly raise generation of ROS, reduce the expression of CXCR4, inhibit phosphorylation of Akt, and increase the expression of Caspase3 and Cleaved-caspase3. This indicated that ICG-TNBs could effectively inhibit and block the SDF-1/CXCR4 pathway, thus leading to the apoptosis of MCF-7 cells. ICG-TNBs can specifically bind to CXCR4 positive breast cancer cells, furthermore inhibit growth and promote apoptosis of breast cancer cells combined with ultrasonic irradiation by blocking the SDF-1/CXCR4 pathway. This study introduces a novel concept, method and mechanism for integration of targeted diagnosis and treatment of breast cancer.

Imaging Methods for Ultrasound Contrast Agents

Microbubble contrast agents were introduced more than 25 years ago with the objective of enhancing blood echoes and enabling diagnostic ultrasound to image the microcirculation. Cardiology and oncology waited anxiously for the fulfillment of that objective with one clinical application each: myocardial perfusion, tumor perfusion and angiogenesis imaging. What was necessary though at first was the scientific understanding of microbubble behavior in vivo and the development of imaging technology to deliver the original objective. And indeed, for more than 25 years bubble science and imaging technology have evolved methodically to deliver contrast-enhanced ultrasound. Realization of the basic bubbles properties, non-linear response and ultrasound-induced destruction, has led to a plethora of methods; algorithms and techniques for contrast-enhanced ultrasound (CEUS) and imaging modes such as harmonic imaging, harmonic power Doppler, pulse inversion, amplitude modulation, maximum intensity projection and many others were invented, developed and validated. Today, CEUS is used everywhere in the world with clinical indications both in cardiology and in radiology, and it continues to mature and evolve and has become a basic clinical tool that transforms diagnostic ultrasound into a functional imaging modality. In this review article, we present and explain in detail bubble imaging methods and associated artifacts, perfusion quantification approaches, and implementation considerations and regulatory aspects.

Ultrasound Measurement of Vascular Density to Evaluate Response to Anti-Angiogenic Therapy in Renal Cell Carcinoma

BACKGROUND

Functional and molecular changes often precede gross anatomical changes, so early assessment of a tumor's functional and molecular response to therapy can help reduce a patient's exposure to the side effects of ineffective chemotherapeutics or other treatment strategies.

OBJECTIVE

Our intent was to test the hypothesis that an ultrasound microvascular imaging approach might provide indications of response to therapy prior to assessment of tumor size.

METHODS

Mice bearing clear-cell renal cell carcinoma xenograft tumors were treated with antiangiogenic and Notch inhibition therapies. An ultrasound measurement of microvascular density was used to serially track the tumor response to therapy.

RESULTS

Data indicated that ultrasound-derived microvascular density can indicate response to therapy a week prior to changes in tumor volume and is strongly correlated with physiological characteristics of the tumors as measured by histology ([Formula: see text]). Furthermore, data demonstrated that ultrasound measurements of vascular density can determine response to therapy and classify between-treatment groups with high sensitivity and specificity.

CONCLUSION/SIGNIFICANCE

Results suggests that future applications utilizing ultrasound imaging to monitor tumor response to therapy may be able to provide earlier insight into tumor behavior from metrics of microvascular density rather than anatomical tumor size measurements.

A new preclinical ultrasound platform for widefield 3D imaging of rodents

Noninvasive in vivo imaging technologies enable researchers and clinicians to detect the presence of disease and longitudinally study its progression. By revealing anatomical, functional, or molecular changes, imaging tools can provide a near real-time assessment of important biological events. At the preclinical research level, imaging plays an important role by allowing disease mechanisms and potential therapies to be evaluated noninvasively. Because functional and molecular changes often precede gross anatomical changes, there has been a significant amount of research exploring the ability of different imaging modalities to track these aspects of various diseases. Herein, we present a novel robotic preclinical contrast-enhanced ultrasound system and demonstrate its use in evaluating tumors in a rodent model. By leveraging recent advances in ultrasound, this system favorably compares with other modalities, as it can perform anatomical, functional, and molecular imaging and is cost-effective, portable, and high throughput, without using ionizing radiation. Furthermore, this system circumvents many of the limitations of conventional preclinical ultrasound systems, including a limited field-of-view, low throughput, and large user variability.

Chemistry and engineering of cyclodextrins for molecular imaging

Cyclodextrins (CDs) are naturally occurring cyclic oligosaccharides bearing a basket-shaped topology with an "inner-outer" amphiphilic character. The abundance of hydroxyl groups enables CDs to be functionalized with multiple targeting ligands and imaging elements. The imaging time, and the payload of different imaging elements, can be tuned by taking advantage of the commercial availability of CDs with different sizes of the cavity. This review aims to offer an outlook of the chemistry and engineering of CDs for the development of molecular probes. Complexation thermodynamics of CDs, and the corresponding implications for probe design, are also presented with examples demonstrating the structural and physiochemical roles played by CDs in the full ambit of molecular imaging. We hope that this review not only offers a synopsis of the current development of CD-based molecular probes, but can also facilitate translation of the incremental advancements from the laboratory to real biomedical applications by illuminating opportunities and challenges for future research.

Contrast-enhanced ultrasound of malignant liver lesions

Contrast-enhanced ultrasound (CEUS) is a safe, relatively inexpensive, and widely available imaging technique using dedicated imaging ultrasound sequences and FDA-approved contrast microbubbles that allow detection and characterization of malignant focal liver lesions with high diagnostic accuracy. CEUS provides dynamic real-time imaging with high spatial and temporal capability, allowing for unique contributions to the already established protocols for diagnosing focal liver lesions using CT and MR imaging. In patients with lesions indeterminate on CT and MRI, CEUS is a helpful problem-solving complementary tool that improves patient management. Furthermore, CEUS assists guidance of liver biopsies and local treatment. Variations of CEUS such as DCE-US and ultrasound molecular imaging are emerging for quantitative monitoring of treatment effects and possible earlier detection of cancer. In this review, basic principles of CEUS techniques and ultrasound contrast agents along with a description of the enhancement patterns of malignant liver lesions are summarized. Also, a discussion of the role of CEUS for treatment guidance and monitoring, intraoperative CEUS, and an outlook on emerging applications is provided.

Protective and recuperative effects of 3-bromopyruvate on immunological, hepatic and renal homeostasis in a murine host bearing ascitic lymphoma: Implication of niche dependent differential roles of macrophages

3-bromopyruvate (3-BP) possesses promising antineoplastic potential, however, its effects on immunological homeostasis vis a vis hepatic and renal functions in a tumor bearing host remain unclear. Therefore, the effect of 3-BP administration to a murine host bearing a progressively growing tumor of thymoma origin, designated as Dalton's lymphoma (DL), on immunological, renal and hepatic homeostasis was investigated. Administration of 3-BP (4 mg/kg) to the tumor bearing host reversed tumor growth associated thymic atrophy and splenomegaly, accompanied by altered cell survival and repertoire of splenic, bone marrow and tumor associated macrophages (TAM). TAM displayed augmented phagocytic, tumoricidal activities and production of IL-1 and TNF-α. 3-BP-induced activation of TAM was of indirect nature, mediated by IFN-γ. Blood count of T lymphocytes (CD4⁺ & CD8⁺) and NK cells showed a rise in 3-BP administered tumor bearing mice. Moreover, 3-BP administration triggered modulation of immunomodulatory cytokines in serum along with refurbished hepatic and renal functions. The study indicates the role of altered cytokines balance, site specific differential macrophage functions and myelopoiesis in restoration of lymphoid organ homeostasis in 3-BP administered tumor bearing host. These observations will have long lasting impact in understanding of alternate mechanisms underlying the antitumor action of 3-BP accompanying appraisal of safety issues for optimizing its antineoplastic actions.

Targeting of microbubbles: contrast agents for ultrasound molecular imaging

For contrast ultrasound imaging, the most efficient contrast agents comprise highly compressible gas-filled microbubbles. These micrometer-sized particles are typically filled with low-solubility perfluorocarbon gases, and coated with a thin shell, often a lipid monolayer. These particles circulate in the bloodstream for several minutes; they demonstrate good safety and are already in widespread clinical use as blood pool agents with very low dosage necessary (sub-mg per injection). As ultrasound is an ubiquitous medical imaging modality, with tens of millions of exams conducted annually, its use for molecular/targeted imaging of biomarkers of disease may enable wider implementation of personalised medicine applications, precision medicine, non-invasive quantification of biomarkers, targeted guidance of biopsy and therapy in real time. To achieve this capability, microbubbles are decorated with targeting ligands, possessing specific affinity towards vascular biomarkers of disease, such as tumour neovasculature or areas of inflammation, ischaemia-reperfusion injury or ischaemic memory. Once bound to the target, microbubbles can be selectively visualised to delineate disease location by ultrasound imaging. This review discusses the general design trends and approaches for such molecular ultrasound imaging agents, which are currently at the advanced stages of development, and are evolving towards widespread clinical trials.

Ultrasound Molecular Imaging of VEGFR-2 in Clear-Cell Renal Cell Carcinoma Tracks Disease Response to Antiangiogenic and Notch-Inhibition Therapy

Metastatic clear-cell renal cell carcinoma (ccRCC) affects thousands of patients worldwide each year. Antiangiogenic therapy has been shown to have beneficial effects initially, but resistance is eventually developed. Therefore, it is important to accurately track the response of cancer to different therapeutics in order to appropriately adjust the therapy to maximize efficacy. Change in tumor volume is the current gold standard for determining efficacy of treatment. However, functional variations can occur much earlier than measurable volume changes. Contrast-enhanced ultrasound (CEUS) is an important tool for assessing tumor progression and response to therapy, since it can monitor functional changes in the physiology. In this study, we demonstrate how ultrasound molecular imaging (USMI) can accurately track the evolution of the disease and molecular response to treatment. Methods A cohort of NSG (NOD/scid/gamma) mice was injected with ccRCC cells and treated with either the VEGF inhibitor SU (Sunitinib malate, Selleckchem, TX, USA) or the Notch pathway inhibitor GSI (Gamma secretase inhibitor, PF-03084014, Pfizer, New York, NY, USA), or started on SU and later switched to GSI (Switch group). The therapies used in the study focus on disrupting angiogenesis and proper vessel development. SU inhibits signaling of vascular endothelial growth factor (VEGF), which is responsible for the sprouting of new vasculature, and GSI inhibits the Notch pathway, which is a key factor in the correct maturation of newly formed vasculature. Microbubble contrast agents targeted to VEGFR-2 (VEGF Receptor) were delivered as a bolus, and the bound agents were imaged in 3D after the free-flowing contrast was cleared from the body. Additionally, the tumors were harvested at the end of the study and stained for CD31. Results The results show that MI can detect changes in VEGFR-2 expression in the group treated with SU within a week of the start of treatment, while differences in volume only become apparent after the mice have been treated for three weeks. Furthermore, USMI can detect response to therapy in 92% of cases after 1 week of treatment, while the detection rate is only 40% for volume measurements. The amount of targeting for the GSI and Control groups was high throughout the duration of the study, while that of the SU and Switch groups remained low. However, the amount of targeting in the Switch group increased to levels similar to those of the Control group after the treatment was switched to GSI. CD31 staining indicates significantly lower levels of patent vasculature for the SU group compared to the Control and GSI groups. Therefore, the results parallel the expected physiological changes in the tumor, since GSI promotes angiogenesis through the VEGF pathway, while SU inhibits it. Conclusion This study demonstrates that MI can track disease progression and assess functional changes in tumors before changes in volume are apparent, and thus, CEUS can be a valuable tool for assessing response to therapy in disease. Future work is required to determine whether levels of VEGFR-2 targeting correlate with eventual survival outcomes.

Construction of ultrasonic nanobubbles carrying CAIX polypeptides to target carcinoma cells derived from various organs

Background

Ultrasound molecular imaging is a novel diagnostic approach for tumors, whose key link is the construction of targeted ultrasound contrast agents. However, available targeted ultrasound contrast agents for molecular imaging of tumors are only achieving imaging in blood pool or one type tumor. No targeted ultrasound contrast agents have realized targeted ultrasound molecular imaging of tumor parenchymal cells in a variety of solid tumors so far. Carbonic anhydrase IX (CAIX) is highly expressed on cell membranes of various malignant solid tumors, so it’s a good target for ultrasound molecular imaging. Here, targeted nanobubbles carrying CAIX polypeptides for targeted binding to a variety of malignant tumors were constructed, and targeted binding ability and ultrasound imaging effect in different types of tumors were evaluated.

Results

The mean diameter of lipid targeted nanobubbles was (503.7 ± 78.47) nm, and the polypeptides evenly distributed on the surfaces of targeted nanobubbles, which possessed the advantages of homogenous particle size, high stability, and good safety. Targeted nanobubbles could gather around CAIX-positive cells (786-O and Hela cells), while they cannot gather around CAIX-negative cells (BxPC-3 cells) in vitro, and the affinity of targeted nanobubbles to CAIX-positive cells were significantly higher than that to CAIX-negative cells (P < 0.05). Peak intensity and duration time of targeted nanobubbles and blank nanobubbles were different in CAIX-positive transplanted tumor tissues in vivo (P < 0.05). Moreover, targeted nanobubbles in CAIX-positive transplanted tumor tissues produced higher peak intensity and longer duration time than those in CAIX-negative transplanted tumor tissues (P < 0.05). Finally, immunofluorescence not only confirmed targeted nanobubbles could pass through blood vessels to enter in tumor tissue spaces, but also clarified imaging differences of targeted nanobubbles in different types of transplanted tumor tissues.

Conclusions

Targeted nanobubbles carrying CAIX polypeptides can specifically enhance ultrasound imaging in CAIX-positive transplanted tumor tissues and could potentially be used in early diagnosis of a variety of solid tumors derived from various organs.

Electronic supplementary material

The online version of this article (doi:10.1186/s12951-017-0307-0) contains supplementary material, which is available to authorized users.

Quantitative Three-Dimensional Dynamic Contrast-Enhanced Ultrasound Imaging: First-In-Human Pilot Study in Patients with Liver Metastases

Purpose: To perform a clinical assessment of quantitative three-dimensional (3D) dynamic contrast-enhanced ultrasound (DCE-US) feasibility and repeatability in patients with liver metastasis, and to evaluate the extent of quantitative perfusion parameter sampling errors in 2D compared to 3D DCE-US imaging.

Materials and Methods: Twenty consecutive 3D DCE-US scans of liver metastases were performed in 11 patients (45% women; mean age, 54.5 years; range, 48-60 years; 55% men; mean age, 57.6 years; range, 47-68 years). Pairs of repeated disruption-replenishment and bolus DCE-US images were acquired to determine repeatability of parameters. Disruption-replenishment was carried out by infusing 0.9 mL of microbubbles (Definity; Latheus Medical Imaging) diluted in 35.1 mL of saline over 8 min. Bolus consisted of intravenous injection of 0.2 mL microbubbles. Volumes-of-interest (VOI) and regions-or-interest (ROI) were segmented by two different readers in images to extract 3D and 2D perfusion parameters, respectively. Disruption-replenishment parameters were: relative blood volume (rBV), relative blood flow (rBF). Bolus parameters included: time-to-peak (TP), peak enhancement (PE), area-under-the-curve (AUC), and mean-transit-time (MTT).

Results: Clinical feasibility and repeatability of 3D DCE-US using both the destruction-replenishment and bolus technique was demonstrated. The repeatability of 3D measurements between pairs of repeated acquisitions was assessed with the concordance correlation coefficient (CCC), and found to be excellent for all parameters (CCC > 0.80), except for the TP (0.74) and MTT (0.30) parameters. The CCC between readers was found to be excellent (CCC > 0.80) for all parameters except for TP (0.71) and MTT (0.52). There was a large Coefficient of Variation (COV) in intra-tumor measurements for 2D parameters (0.18-0.52). Same-tumor measurements made in 3D were significantly different (P = 0.001) than measurements made in 2D; a percent difference of up to 86% was observed between measurements made in 2D compared to 3D in the same tumor.

Conclusions: 3D DCE-US imaging of liver metastases with a matrix array transducer is feasible and repeatable in the clinic. Results support 3D instead of 2D DCE US imaging to minimize sampling errors due to tumor heterogeneity.

Early prediction of tumor response to bevacizumab treatment in murine colon cancer models using three-dimensional dynamic contrast-enhanced ultrasound imaging

Due to spatial tumor heterogeneity and consecutive sampling errors, it is critically important to assess treatment response following antiangiogenic therapy in three dimensions as two-dimensional assessment has been shown to substantially over- and underestimate treatment response. In this study, we evaluated whether three-dimensional (3D) dynamic contrast-enhanced ultrasound (DCE-US) imaging allows assessing early changes in tumor perfusion following antiangiogenic treatment (bevacizumab administered at a dose of 10 mg/kg b.w.), and whether these changes could predict treatment response in colon cancer tumors that either are responsive (LS174T tumors) or none responsive (CT26) to the proposed treatment. Our results showed that the perfusion parameters of 3D DCE-US including peak enhancement (PE) and area under curve (AUC) significantly decreased by up to 69 and 77%, respectively, in LS174T tumors within 1 day after antiangiogenic treatment (P = 0.005), but not in CT26 tumors (P > 0.05). Similarly, the percentage area of neovasculature significantly decreased in treated versus control LS174T tumors (P < 0.001), but not in treated versus control CT26 tumors (P = 0.796). Early decrease in both PE and AUC by 45-50% was predictive of treatment response in 100% (95% CI 69.2, 100%) of responding tumors, and in 100% (95% CI 88.4, 100%) and 86.7% (95% CI 69.3, 96.2%), respectively, of nonresponding tumors. In conclusion, 3D DCE-US provides clinically relevant information on the variability of tumor response to antiangiogenic therapy and may be further developed as biomarker for predicting treatment outcomes.

Recent advances in ultrasound-based diagnosis and therapy with micro- and nanometer-sized formulations

Ultrasound (US) is one of the most frequently used imaging methods in the clinic. The broad spectrum of its applications can be increased by the use of gas-filled microbubbles (MB) as ultrasound contrast agents (UCA). In recent years, also nanoscale UCA like nanobubbles (NB), echogenic liposomes (ELIP) and nanodroplets have been developed, which in contrast to MB, are able to extravasate from the vessels into the tissue. New disease-specific UCA have been designed for the assessment of tissue biomarkers and advanced US to a molecular imaging modality. For this purpose, specific binding moieties were coupled to the UCA surface. The vascular endothelial growth factor receptor-2 (VEGFR-2) and P-/E-selectin are prominent examples of molecular US targets to visualize tumor blood vessels and inflammatory diseases, respectively. Besides their application in contrast-enhanced imaging, MB can also be employed for drug delivery to tumors and across the blood-brain barrier (BBB). This review summarizes the development of micro- and nanoscaled UCA and highlights recent advances in diagnostic and therapeutic applications, which are ready for translation into the clinic.

Ultrasound Molecular Imaging With BR55 in Patients With Breast and Ovarian Lesions: First-in-Human Results

Purpose We performed a first-in-human clinical trial on ultrasound molecular imaging (USMI) in patients with breast and ovarian lesions using a clinical-grade contrast agent (kinase insert domain receptor [KDR] -targeted contrast microbubble [MB_KDR]) that is targeted at the KDR, one of the key regulators of neoangiogenesis in cancer. The aim of this study was to assess whether USMI using MB_KDR is safe and allows assessment of KDR expression using immunohistochemistry (IHC) as the gold standard. Methods Twenty-four women (age 48 to 79 years) with focal ovarian lesions and 21 women (age 34 to 66 years) with focal breast lesions were injected intravenously with MB_KDR (0.03 to 0.08 mL/kg of body weight), and USMI of the lesions was performed starting 5 minutes after injection up to 29 minutes. Blood pressure, ECG, oxygen levels, heart rate, CBC, and metabolic panel were obtained before and after MB_KDR administration. Persistent focal MB_KDR binding on USMI was assessed. Patients underwent surgical resection of the target lesions, and tissues were stained for CD31 and KDR by IHC. Results USMI with MB_KDR was well tolerated by all patients without safety concerns. Among the 40 patients included in the analysis, KDR expression on IHC matched well with imaging signal on USMI in 93% of breast and 85% of ovarian malignant lesions. Strong KDR-targeted USMI signal was present in 77% of malignant ovarian lesions, with no targeted signal seen in 78% of benign ovarian lesions. Similarly, strong targeted signal was seen in 93% of malignant breast lesions with no targeted signal present in 67% of benign breast lesions. Conclusion USMI with MB_KDR is clinically feasible and safe, and KDR-targeted USMI signal matches well with KDR expression on IHC. This study lays the foundation for a new field of clinical USMI in cancer.

In vitro characterization and in vivo ultrasound molecular imaging of nucleolin-targeted microbubbles

Nucleolin (NCL) plays an important role in tumor vascular development. An increased endothelial expression level of NCL has been related to cancer aggressiveness and prognosis and has been detected clinically in advanced tumors. Here, with a peptide targeted to NCL (F3 peptide), we created an NCL-targeted microbubble (MB) and compared the performance of F3-conjugated MBs with non-targeted (NT) MBs both in vitro and in vivo. In an in vitro study, F3-conjugated MBs bound 433 times more than NT MBs to an NCL-expressing cell line, while pretreating cells with 0.5 mM free F3 peptide reduced the binding of F3-conjugated MBs by 84%, n = 4, p < 0.001. We then set out to create a method to extract both the tumor wash-in and wash-out kinetics and tumor accumulation following a single injection of targeted MBs. In order to accomplish this, a series of ultrasound frames (a clip) was recorded at the time of injection and subsequent time points. Each pixel within this clip was analyzed for the minimum intensity projection (MinIP) and average intensity projection (AvgIP). We found that the MinIP robustly demonstrates enhanced accumulation of F3-conjugated MBs over the range of tumor diameters evaluated here (2-8 mm), and the difference between the AvgIP and the MinIP quantifies inflow and kinetics. The inflow and clearance were similar for unbound F3-conjugated MBs, control (non-targeted) and scrambled control agents. Targeted agent accumulation was confirmed by a high amplitude pulse and by a two-dimensional Fourier Transform technique. In summary, F3-conjugated MBs provide a new imaging agent for ultrasound molecular imaging of cancer vasculature, and we have validated metrics to assess performance using low mechanical index strategies that have potential for use in human molecular imaging studies.

Contrast-Enhanced Ultrasound with VEGFR2-Targeted Microbubbles for Monitoring Regorafenib Therapy Effects in Experimental Colorectal Adenocarcinomas in Rats with DCE-MRI and Immunohistochemical Validation

OBJECTIVES

To investigate contrast-enhanced ultrasound (CEUS) with VEGFR2-targeted microbubbles for monitoring therapy effects of regorafenib on experimental colon carcinomas in rats with correlation to dynamic contrast-enhanced MRI (DCE-MRI) and immunohistochemistry.

MATERIALS AND METHODS

Human colorectal adenocarcinoma xenografts (HT-29) were implanted subcutaneously in n = 21 (n = 11 therapy group; n = 10 control group) female athymic nude rats (Hsd: RH-Foxn1rnu). Animals were imaged at baseline and after a one-week daily treatment with regorafenib or a placebo (10 mg/kg bodyweight), using CEUS with VEGFR2-targeted microbubbles and DCE-MRI. In CEUS tumor perfusion was assessed during an early vascular phase (wash-in area under the curve = WiAUC) and VEGFR2-specific binding during a late molecular phase (signal intensity after 8 (SI8min) and 10 minutes (SI10min)), using a conventional 15L8 linear transducer (transmit frequency 7 MHz, dynamic range 80 dB, depth 25 mm). In DCE-MRI functional parameters plasma flow (PF) and plasma volume (PV) were quantified. For validation purposes, CEUS parameters were correlated with DCE-MRI parameters and immunohistochemical VEGFR2, CD31, Ki-67 and TUNEL stainings.

RESULTS

CEUS perfusion parameter WiAUC decreased significantly (116,989 ± 77,048 a.u. to 30,076 ± 27,095a.u.; p = 0.005) under therapy with no significant changes (133,932 ± 65,960 a.u. to 84,316 ± 74,144 a.u.; p = 0.093) in the control group. In the therapy group, the amount of bound microbubbles in the late phase was significantly lower in the therapy than in the control group on day 7 (SI8min: 283 ± 191 vs. 802 ± 460 a.u.; p = 0.006); SI10min: 226 ± 149 vs. 645 ± 461 a.u.; p = 0.009). PF and PV decreased significantly (PF: 147 ± 58 mL/100 mL/min to 71 ± 15 mL/100 mL/min; p = 0.003; PV: 13 ± 3% to 9 ± 4%; p = 0.040) in the therapy group. Immunohistochemistry revealed significantly fewer VEGFR2 (7.2 ± 1.8 vs. 17.8 ± 4.6; p < 0.001), CD31 (8.1 ± 3.0 vs. 20.8 ± 5.7; p < 0.001) and Ki-67 (318.7 ± 94.0 vs. 468.0 ± 133.8; p = 0.004) and significantly more TUNEL (672.7 ± 194.0 vs. 357.6 ± 192.0; p = 0.003) positive cells in the therapy group. CEUS parameters showed significant (p < 0.05) correlations to DCE-MRI parameters and immunohistochemistry.

CONCLUSIONS

CEUS with VEGFR2-targeted microbubbles allowed for monitoring regorafenib functional and molecular therapy effects on experimental colorectal adenocarcinomas with a significant decline of CEUS and DCE-MRI perfusion parameters as well as a significant reduction of specifically bound microbubbles under therapy, consistent with a reduced expression of VEGFR2.

Multimodal Genetic Approach for Molecular Imaging of Vasculature in a Mouse Model of Melanoma

PURPOSE

In this study, we evaluated a genetic approach for in vivo multimodal molecular imaging of vasculature in a mouse model of melanoma.

PROCEDURES

We used a novel transgenic mouse, Ts-Biotag, that genetically biotinylates vascular endothelial cells. After inoculating these mice with B16 melanoma cells, we selectively targeted endothelial cells with (strept)avidinated contrast agents to achieve multimodal contrast enhancement of Tie2-expressing blood vessels during tumor progression.

RESULTS

This genetic targeting system provided selective labeling of tumor vasculature and showed in vivo binding of avidinated probes with high specificity and sensitivity using microscopy, near infrared, ultrasound, and magnetic resonance imaging. We further demonstrated the feasibility of conducting longitudinal three-dimensional (3D) targeted imaging studies to dynamically assess changes in vascular Tie2 from early to advanced tumor stages.

CONCLUSIONS

Our results validated the Ts-Biotag mouse as a multimodal targeted imaging system with the potential to provide spatio-temporal information about dynamic changes in vasculature during tumor progression.

Intra-Animal Comparison between Three-dimensional Molecularly Targeted US and Three-dimensional Dynamic Contrast-enhanced US for Early Antiangiogenic Treatment Assessment in Colon Cancer

Purpose To perform an intra-animal comparison between (a) three-dimensional (3D) molecularly targeted ultrasonography (US) by using clinical-grade vascular endothelial growth factor receptor 2 (VEGFR2)-targeted microbubbles and (b) 3D dynamic contrast material-enhanced (DCE) US by using nontargeted microbubbles for assessment of antiangiogenic treatment effects in a murine model of human colon cancer. Materials and Methods Twenty-three mice with human colon cancer xenografts were randomized to receive either single-dose antiangiogenic treatment (bevacizumab, n = 14) or control treatment (saline, n = 9). At baseline and 24 hours after treatment, animals were imaged with a clinical US system equipped with a clinical matrix array transducer by using the following techniques: (a) molecularly targeted US with VEGFR2-targeted microbubbles, (b) bolus DCE US with nontargeted microbubbles, and (c) destruction-replenishment DCE US with nontargeted microbubbles. VEGFR2-targeted US signal, peak enhancement, area under the time-intensity curve, time to peak, relative blood volume (rBV), relative blood flow, and blood flow velocity were quantified. VEGFR2 expression and percentage area of blood vessels were assessed ex vivo with quantitative immunofluorescence and correlated with corresponding in vivo US parameters. Statistical analysis was performed with Wilcoxon signed rank tests and rank sum tests, as well as Pearson correlation analysis. Results Molecularly targeted US signal with VEGFR2-targeted microbubbles, peak enhancement, and rBV significantly decreased (P ≤ .03) after a single antiangiogenic treatment compared with those in the control group; similarly, ex vivo VEGFR2 expression (P = .03) and percentage area of blood vessels (P = .03) significantly decreased after antiangiogenic treatment. Three-dimensional molecularly targeted US signal correlated well with VEGFR2 expression (r = 0.86, P = .001), and rBV (r = 0.71, P = .01) and relative blood flow (r = 0.78, P = .005) correlated well with percentage area of blood vessels, while other US perfusion parameters did not. Conclusion Three-dimensional molecularly targeted US and destruction-replenishment 3D DCE US provide complementary molecular and functional in vivo imaging information on antiangiogenic treatment effects in human colon cancer xenografts compared with ex vivo reference standards. ^© RSNA, 2016 Online supplemental material is available for this article.

Ultrasound imaging of splenomegaly as a proxy to monitor colon tumor development in Apc(min716/+) mice

Animal models of colon cancer are widely used to understand the molecular mechanisms and pathogenesis of the disease. These animal models require a substantial investment of time and traditionally necessitate the killing of the animal to measure the tumor progression. Several in vivo imaging techniques are being used in both human clinics and preclinical studies, albeit at high cost and requiring particular expertise. Here, we report that the progression of splenomegaly coincides with and positively correlates to colon tumor development in Apc^min716/+ mice expressing a mutant gene encoding an adenomatous polyposis coli protein truncated at amino acid 716. Ultrasound image‐based spleen size measurement precisely mirrors splenomegaly development in vivo in the tumor‐laden Apc^min716/+ mice. Moreover, the spleen dimensions extracted from the ultrasound sonograms are positively correlated with normalized spleen weight and the number and area of colon tumors. Hence, we propose measuring the spleen size in vivo by ultrasound imaging as a novel approach to estimate splenomegaly development and to indirectly monitor colon tumor development in Apc^min716/+ mice. The widespread use of ultrasound machines in the laboratory setting, coupled with the fact that it is a noninvasive method, make it a straightforward and useful tool for monitoring the experimental progress of colon cancer in mice and determining end points without killing animals strictly for diagnostics purposes.

Ultrasound Molecular Imaging of the Breast Cancer Neovasculature using Engineered Fibronectin Scaffold Ligands: A Novel Class of Targeted Contrast Ultrasound Agent

Molecularly-targeted microbubbles (MBs) are increasingly being recognized as promising contrast agents for oncological molecular imaging with ultrasound. With the detection and validation of new molecular imaging targets, novel binding ligands are needed that bind to molecular imaging targets with high affinity and specificity. In this study we assessed a novel class of potentially clinically translatable MBs using an engineered 10^th type III domain of human-fibronectin (MB-FN3_VEGFR2) scaffold-ligand to image VEGFR2 on the neovasculature of cancer. The in vitro binding of MB-FN3_VEGFR2 to a soluble VEGFR2 was assessed by flow-cytometry (FACS) and binding to VEGFR2-expressing cells was assessed by flow-chamber cell attachment studies under flow shear stress conditions. In vivo binding of MB-FN3_VEGFR2 was tested in a transgenic mouse model (FVB/N Tg(MMTV/PyMT634Mul) of breast cancer and control litter mates with normal mammary glands. In vitro FACS and flow-chamber cell attachment studies showed significantly (P<0.01) higher binding to VEGFR2 using MB-FN3_VEGFR2 than control agents. In vivo ultrasound molecular imaging (USMI) studies using MB-FN3_VEGFR2 demonstrated specific binding to VEGFR2 and was significantly higher (P<0.01) in breast cancer compared to normal breast tissue. Ex vivo immunofluorescence-analysis showed significantly (P<0.01) increased VEGFR2-expression in breast cancer compared to normal mammary tissue. Our results suggest that MBs coupled to FN3-scaffolds can be designed and used for USMI of breast cancer neoangiogenesis. Due to their small size, stability, solubility, the lack of glycosylation and disulfide bonds, FN3-scaffolds can be recombinantly produced with the advantage of generating small, high affinity ligands in a cost efficient way for USMI.

VEGFR2-Targeted Three-Dimensional Ultrasound Imaging Can Predict Responses to Antiangiogenic Therapy in Preclinical Models of Colon Cancer

Three-dimensional (3D) imaging capabilities to assess responses to anticancer therapies are needed to minimize sampling errors common to two-dimensional approaches as a result of spatial heterogeneity in tumors. Recently, the feasibility and reproducibility of 3D ultrasound molecular imaging (3D USMI) using contrast agents, which target molecular markers, have greatly improved, due to the development of clinical 3D matrix array transducers. Here we report preclinical proof-of-concept studies showing that 3D USMI of VEGFR2/KDR expression accurately gauges longitudinal treatment responses to antiangiogenesis therapy in responding versus nonresponding mouse models of colon cancer. Tumors in these models exhibited differential patterns of VEGFR2-targeted 3D USMI signals during the course of antiangiogenic treatment with bevacizumab. In responding tumors, the VEGFR2 signal decreased as soon as 24 hours after therapy was started, whereas in nonresponding tumors there was no change in signal at any time point. The early decrease in VEGFR2 signal was highly predictive of treatment outcome at the end of therapy. Our results offer preclinical proof that 3D USMI can predict responses to antiangiogenic therapy, warranting further investigation of its clinical translatability to predicting treatment outcomes in patients. Cancer Res; 76(14); 4081-9. ©2016 AACR.

Quantitative correlational study of microbubble-enhanced ultrasound imaging and magnetic resonance imaging of glioma and early response to radiotherapy in a rat model

PURPOSE

Radiotherapy remains a major treatment method for malignant tumors. Magnetic resonance imaging (MRI) is the standard modality for assessing glioma treatment response in the clinic. Compared to MRI, ultrasound imaging is low-cost and portable and can be used during intraoperative procedures. The purpose of this study was to quantitatively compare contrast-enhanced ultrasound (CEUS) imaging and MRI of irradiated gliomas in rats and to determine which quantitative ultrasound imaging parameters can be used for the assessment of early response to radiation in glioma.

METHODS

Thirteen nude rats with U87 glioma were used. A small thinned skull window preparation was performed to facilitate ultrasound imaging and mimic intraoperative procedures. Both CEUS and MRI with structural, functional, and molecular imaging parameters were performed at preradiation and at 1 day and 4 days postradiation. Statistical analysis was performed to determine the correlations between MRI and CEUS parameters and the changes between pre- and postradiation imaging.

RESULTS

Area under the curve (AUC) in CEUS showed significant difference between preradiation and 4 days postradiation, along with four MRI parameters, T2, apparent diffusion coefficient, cerebral blood flow, and amide proton transfer-weighted (APTw) (all p < 0.05). The APTw signal was correlated with three CEUS parameters, rise time (r = - 0.527, p < 0.05), time to peak (r = - 0.501, p < 0.05), and perfusion index (r = 458, p < 0.05). Cerebral blood flow was correlated with rise time (r = - 0.589, p < 0.01) and time to peak (r = - 0.543, p < 0.05).

CONCLUSIONS

MRI can be used for the assessment of radiotherapy treatment response and CEUS with AUC as a new technique and can also be one of the assessment methods for early response to radiation in glioma.

Molecular Acoustic Angiography: A New Technique for High-resolution Superharmonic Ultrasound Molecular Imaging

Ultrasound molecular imaging utilizes targeted microbubbles to bind to vascular targets such as integrins, selectins and other extracellular binding domains. After binding, these microbubbles are typically imaged using low pressures and multi-pulse imaging sequences. In this article, we present an alternative approach for molecular imaging using ultrasound that relies on superharmonic signals produced by microbubble contrast agents. Bound bubbles were insonified near resonance using a low frequency (4 MHz) element and superharmonic echoes were received at high frequencies (25-30 MHz). Although this approach was observed to produce declining image intensity during repeated imaging in both in vitro and in vivo experiments because of bubble destruction, the feasibility of superharmonic molecular imaging was demonstrated for transmit pressures, which are sufficiently high to induce shell disruption in bound microbubbles. This approach was validated using microbubbles targeted to the αvβ3 integrin in a rat fibrosarcoma model (n = 5) and combined with superharmonic images of free microbubbles to produce high-contrast, high-resolution 3-D volumes of both microvascular anatomy and molecular targeting. Image intensity over repeated scans and the effect of microbubble diameter were also assessed in vivo, indicating that larger microbubbles yield increased persistence in image intensity. Using ultrasound-based acoustic angiography images rather than conventional B-mode ultrasound to provide the underlying anatomic information facilitates anatomic localization of molecular markers. Quantitative analysis of relationships between microvasculature and targeting information indicated that most targeting occurred within 50 μm of a resolvable vessel (>100 μm diameter). The combined information provided by these scans may present new opportunities for analyzing relationships between microvascular anatomy and vascular targets, subject only to limitations of the current mechanically scanned system and microbubble persistence to repeated imaging at moderate mechanical indices.

Assessment of Inflammation in an Acute on Chronic Model of Inflammatory Bowel Disease with Ultrasound Molecular Imaging

BACKGROUND

Ultrasound (US) molecular imaging has shown promise in assessing inflammation in preclinical, murine models of inflammatory bowel disease. These models, however, initiated acute inflammation on previously normal colons, in contrast to patients where acute exacerbations are often in chronically inflamed regions. In this study, we explored the potential of dual P- and E-selectin targeted US imaging for assessing acute inflammation on a murine quiescent chronic inflammatory background.

METHODS

Chronic colitis was induced using three cycles of 4% DSS in male FVB mice. Acute inflammation was initiated 2 weeks after the final DSS cycle through rectal administration of 1% TNBS. Mice at different stages of inflammation were imaged using a small animal ultrasound system following i.v. injection of microbubbles targeted to P- and E-selectin. In vivo imaging results were correlated with ex vivo immunofluorescence and histology.

RESULTS

Induction of acute inflammation resulted in an increase in the targeted US signal from 5.5 ± 5.1 arbitrary units (a.u.) at day 0 to 61.0 ± 45.2 a.u. (P < 0.0001) at day 1, 36.3 ± 33.1 a.u. at day 3, returning to levels similar to control at day 5. Immunofluorescence showed significant increase in the percentage of P- and E-selectin positive vessels at day 1 (P-selectin: 21.0 ± 7.1% of vessels; P < 0.05; E-selectin: 16.4 ±3.7%; P < 0.05) compared to day 0 (P-selectin: 10.3 ± 5.7%; E-selectin: 7.3 ± 7.0%).

CONCLUSIONS

Acute inflammation can be accurately measured in a clinically relevant murine model of chronic IBD using ultrasound molecular imaging with a dual P- and E- selectin-targeted contrast agent.

Ultrasound molecular imaging: Moving toward clinical translation

Ultrasound is a widely available, cost-effective, real-time, non-invasive and safe imaging modality widely used in the clinic for anatomical and functional imaging. With the introduction of novel molecularly-targeted ultrasound contrast agents, another dimension of ultrasound has become a reality: diagnosing and monitoring pathological processes at the molecular level. Most commonly used ultrasound molecular imaging contrast agents are micron sized, gas-containing microbubbles functionalized to recognize and attach to molecules expressed on inflamed or angiogenic vascular endothelial cells. There are several potential clinical applications currently being explored including earlier detection, molecular profiling, and monitoring of cancer, as well as visualization of ischemic memory in transient myocardial ischemia, monitoring of disease activity in inflammatory bowel disease, and assessment of arteriosclerosis. Recently, a first clinical grade ultrasound contrast agent (BR55), targeted at a molecule expressed in neoangiogenesis (vascular endothelial growth factor receptor type 2; VEGFR2) has been introduced and safety and feasibility of VEGFR2-targeted ultrasound imaging is being explored in first inhuman clinical trials in various cancer types. This review describes the design of ultrasound molecular imaging contrast agents, imaging techniques, and potential future clinical applications of ultrasound molecular imaging.