Targeted imaging using ultrasound contrast agents

The applications of ultrasound contrast agents have recently expanded from blood pool enhancement to include passive targeting of physiological systems (in particular, the lymphatic and reticuloendothelial systems) and molecular imaging of factors expressed in angiogenesis, atherosclerosis, and inflammation. This article summarizes the progress made in targeted imaging using ultrasound with an emphasis on the opportunities this research provides for both clinical and research applications. We begin with a summary of current ultrasound contrast technology and then review the latest research in the use of targeted ultrasound contrast agents.     

Tumor microvasculature observed using different contrast agents: a comparison between Gd-DTPA-Albumin and B-22956/1 in an experimental model of mammary carcinoma

OBJECTIVE: The aim of this study was to compare a pure macromolecular contrast agent (Gd-DTPA-albumin) with a new protein-binding blood pool contrast agent (B22956/1) in terms of their capacity to investigate the microvasculature in an experimental model of mammary carcinoma. MATERIALS AND METHODS: Tumors were induced by subcutaneous injection of 5 x 10(5) BB1 cells into the backs of 5-7 week-old female FVB/neuNT233 mice. The animals were observed using DCE-MRI when the longest diameter of the tumor was 10.2+/-2.0 mm. DCE-MRI experiments were carried out using B22956/1 and (24 h later) Gd-DTPA-albumin. RESULTS: DCE-MRI data showed that vasculature in the tumor rim was characterized by greater fractional plasma volume and transendothelial permeability than vasculature in the tumor core as measured by both contrast agents. Permeability to Gd-DTPA-albumin in the tumor core was hardly measurable while permeability to B22956/1 was substantial. Histologically the tumor core showed areas of well vascularized, viable tissue surrounded by necrotic regions. CONCLUSIONS: DCE-MRI experiments performed with B22956/1 are useful in the investigation of vasculature in those tumor regions that are characterized by low permeability to macromolecules.     

Quantification and localization of contrast agents using delta relaxation enhanced magnetic resonance at 1.5?T

Object

Delta relaxation enhanced magnetic resonance (dreMR) is a new imaging technique based on the idea of cycling the magnetic field B ₀ during an imaging sequence. The method determines the field dependency of the relaxation rate (relaxation dispersion dR ₁/dB). This quantity is of particular interest in contrast agent imaging because the parameter can be used to determine contrast agent concentrations and increases the ability to localize the contrast agent.

Materials and methods

In this paper dreMR imaging was implemented on a clinical 1.5?T MR scanner combining conventional MR imaging with fast field-cycling. Two improvements to dreMR theory are presented describing the quantification of contrast agent concentrations from dreMR data and a correction for field-cycling with finite ramp times.

Results

Experiments demonstrate the use of the extended theory and show the measurement of contrast agent concentrations with the dreMR method. A second experiment performs localization of a contrast agent with a significant improvement in comparison to conventional imaging.

Conclusion

dreMR imaging has been extended by a method to quantify contrast agent concentrations and improved for field-cycling with finite ramp times. Robust localization of contrast agents using dreMR imaging has been performed in a sample where conventional imaging delivers inconclusive results.     

Imaging E-selectin expression following traumatic brain injury in the rat using a targeted USPIO contrast agent

Introduction  The aim of this work was to map E-selectin expression in a traumatic brain injury model using a newly-designed MR contrast agent. Iron cores, responsible for susceptibility effects and therefore used as T2* contrast agents, need to be coated in order to be stabilized and need to be targeted to be useful. Methods  We have designed a molecule coating composed, at one end, of bisphosphonate to ensure anchorage of the coating on the iron core and, at the other end, of Fukuda’s defined heptapeptide known to target selectin binding sites. Conclusion  The synthesized nanoparticles were able to non-invasively target the traumatic brain lesion, inducing a specific T2* decrease of about 25% up to at least 70 min post-injection of the targeted contrast agent.     

Sonophoretic Delivery for Contrast and Depth Improvement in Skin Optical Coherence Tomography

Our previous studies demonstrated the feasibility of using a sonophoretic delivery method to enhance skin light transmittance with topical application of optical clearing agents using spectroscopy. In this study, we examined the effect of ultrasound [surgeon-performed (SP)] on optical coherence tomography (OCT) imaging depth and contrast of in vitro and in vivo skin. Sixty percent glycerol (G) and SP with a frequency of 1 MHz and a power of 0.75 W over a 3 cm probe was simultaneously applied for 15 min. We find that 60% G/SP results in a twofold increase in achievable OCT imaging depth for in vitro porcine skin and induces 11% shrinkage of the skin. For in vivo human skin, OCT imaging depth and contrast is significantly improved within 30 min of treatment. Imaging depth is increased from 1.4 to 2 mm, and dermal vasculature is clearly visualized in the deeper tissue. OCT imaging of the skin treated with 60% glycerol shows little enhancement in contrast or imaging depth over 60 min. We first demonstrate the superb ability of sonophoretic delivery for in vivo human skin optical clearing, particularly in accelerating the clearing rate. The greater clearing efficiency of glycerol implemented with ultrasound may be attributed to more effective dehydration.     

Assessment of myocardial perfusion using contrast-enhanced MR imaging: Current status and future developments

Excellent inherent tissue contrast is one of the great promises of clinical magnetic resonance (MR) imaging, but functional information is relatively limited. However, MR imaging complemented by the administration of contrast agents can provide such functional assessment. The perfusion status of the myocardium is one of the most important functional information in cardiovascular imaging. Because the clinical acceptance of a contrast agent is measured by its ability to improve patient outcome and to guide therapy, it is unlikely that detection of myocardial infarction, the final stage of ischemic heart disease, should be the target for contrast media development. It would obviously be better if occult regional myocardial perfusion deficits could be reliably detected. The current article was prepared to help the clinical radiologist to keep pace with new strategies for myocardial enhancement and their potential clinical applicability for detection of early perfusion deficits. Several techniques for noninvasive measurement of myocardial perfusion are currently evolving which have the potential to be introduced into routine MR imaging. Most investigators favor a first-pass analysis of the contrast agent bolus through the myocardium using ultrafast sequences. However, such a technique may require clinical introduction of a blood pool agent. There are good resons to favorT ₁-weighted sequences over susceptibility imaging in such first-pass studies. In the future, assessment of myocardial perfusion status using contrast-enhanced MR imaging may be done producing perfusion maps with high spatial resolution (e.g., 256×128), with sequences available on most scanners without special hardware requirements (e.g., IR-Turboflash, keyhole imaging), and requiring only a short period of time for examination (≈3 min).     

Anti-EpCAM scFv gadolinium chelate: a novel targeted MRI contrast agent for imaging of colorectal cancer

Objectives

The development of targeted contrast agents for magnetic resonance imaging (MRI) facilitates enhanced cancer imaging and more accurate diagnosis. In the present study, a novel contrast agent was developed by conjugating anti-EpCAM humanized scFv with gadolinium chelate to achieve target specificity.

Materials and methods

The material design strategy involved site-specific conjugation of the chelating agent to scFv. The scFv monomer was linked to maleimide-DTPA via unpaired cysteine at the scFv C-terminus, followed by chelation with gadolinium (Gd). Successful scFv-DTPA conjugation was achieved at 1:10 molar ratio of scFv to maleimide-DTPA at pH 6.5. The developed anti-EpCAM-Gd-DTPA MRI contrast agent was evaluated for cell targeting ability, in vitro serum stability, cell cytotoxicity, relaxivity, and MR contrast enhancement.

Results

A high level of targeting efficacy of anti-EpCAM-Gd-DTPA to an EpCAM-overexpressing HT29 colorectal cell was demonstrated by confocal microscopy. Good stability of the contrast agent was obtained and no cytotoxicity was observed in HT29 cells after 48 h incubation with 25–100 µM of Gd. Favorable imaging was obtained using anti-EpCAM-Gd-DTPA, including 1.8-fold enhanced relaxivity compared with Gd-DTPA, and MR contrast enhancement observed after binding to HT29.

Conclusion

The potential benefit of this contrast agent for in vivo MR imaging of colorectal cancer, as well as other EpCAM positive cancers, is suggested and warrants further investigation.

     

Detection of cancerous breasts by dynamic area telethermometry

This study demonstrated the following 3 new important concepts. 1) Dynamic imaging can yield meaningful clinical diagnostic information objectively, without need for human expertise. The same methodology can be applicable to any quantitative pathophysiological assessment. 2) Using classical FFT and elementary statistics (Student's t tests and z-value statistics, or alpha and beta statistics), the authors have shown how to reduce thousands of observations to a single quantitative clinical diagnostic parameter. This approach should be applicable to any multiparametric diagnostic technique. 3) After 3 decades of controversial reports on the use of thermal imaging in the diagnosis of breast cancer, the authors have shown that dynamic infrared imaging (in contrast to static thermal imaging), using a plausible pathophysiological model and up-to-date infrared equipment, can distinguish between noncancerous and cancerous breasts with a highly impressive sensitivity and specificity     

Real-time imaging and triggering of 3D contrast-enhanced MR angiograms using MR fluoroscopy

Although a variety of timing techniques and data acquisition strategies have been, used for three-dimensional contrast-enhanced MR angiography, many are still limited by inadequate overall reliability, limited spatial resolution, or complexity. A technique is presented in this work in which contrast arrival is detected in the targeted vasculature in real time using MR fluoroscopy. Upon detection the operator triggers a 3D MR angiographic acquisition which uses an elliptical centric view order. It is shown that the view order intrinsically provides a high degree of venous suppression which in turn allows acquisition times of 30 s or longer. permitting high spatial resolution. The reliability of fluoroscopic triggering in bolus detection is shown to be over 90%. The technique provides high quality contrast-enhanced MR angiograms for many vascular regions.     

Spectral polarization imaging of human rectum-membrane-prostate tissues

Human rectum-membrane-prostate tissue samples were studied using a near-infrared spectral polarization imaging technique to detect small objects and structural changes inside prostate tissues through the rectum. Four modeling samples were made with a small piece of absorber or prostate tissue dyed with a contrast agent (indocyanine green) embedded inside a large piece of prostate tissue in rectum-membrane-prostate structures. The depth of the foreign objects underneath the surface of the rectum-membrane-prostate structures was varied from a millimeter to a centimeter to obtain the critical imaging distance. Different spectral polarization imaging methods with and without contrast agents were performed and compared. The results show that small objects hidden inside the host prostate tissues in the rectum-membrane-prostate structures at depths of 2.5, 4.0, and 7.5 mm can be imaged and identified using the scattering light imaging, tissue emission wing imaging, and contrast agent fluorescence imaging methods, respectively. Our results indicate the potential of imaging and detecting structural changes and cancers inside prostate tissue though rectum-membrane-prostate tissues using this noninvasive spectral polarization imaging technique.     

Improvements in ultrasound contrast agents

This article highlights the current state of ultrasound contrast imaging, including recent improvements in ultrasound contrast agents that allow the agents to pass through pulmonary circulation and even to circulate throughout the human body. A brief history of the field is presented, and current contrast agents and their properties are discussed. Clinical applications in cardiology and potential applications in other areas are also detailed     

Contrast agents in acute myocardial infarction

The experimental design in examination of acute myocardial infarctions should be valid in terms of flow, perfusion and re-flow after intervention. The contrast agents concentration in experimental studies can be measured by microdialysis. We have assessed the usefulness of different extracellular and blood pool contrast agents for visualization of the area at risk in coronary artery occlusions. The double contrast technique, where Dy-DTPA-BMA was combined with Gd-DTPA-BMA yielded a superior infarct visualization. Blood pool agents for example NC100/150 injection is also promising in first path myocardial perfusion imaging.     

Ultrasound-mediated drug delivery

Although ultrasound-based drug delivery has only seen limited clinical use for transdermal drug delivery, there has been considerable momentum in research aimed at using ultrasound for a wide variety of medical applications. Ultrasound-mediated gene therapy using sonoporation and targeted delivery has progressed from in vitro proof-of-concept studies to produce biological effects in angiogenesis and diabetes studies. These techniques have also been used for cancer therapy, thrombolysis, and disruption of the blood-brain barrier in animal models. It is also worth noting that ultrasound can facilitate targeted drug delivery through thermal means, in addition to the mechanisms associated with cavitation. Ultrasound hyperthermia can be used to target thermally sensitive drug-carrying liposomes or to induce gene expression through localized heat shock response.     

Targeting and ultrasound imaging of microbubble-based contrast agents

Preparation and characterization of targeted microbubbles (ultrasound contrast agents) is described. Specific ligands were attached to the microbubble shell, and ligand-coated microbubbles were selectively attached to various targets, using either an avidin-biotin model system or an antigen-antibody system for targeting to live activated endothelial cells. Firm attachment of microbubbles to the target was achieved. Forces necessary to detach microbubbles from the target were estimated to exceed dozens of pN. Microbubbles were bound to the target even in the rapidly moving stream of the aqueous medium. Down to 20 ng of the ultrasound contrast material on the target surface could be detected by the ultrasound imaging with a commercial medical imaging system. At high bubble density on the target surface, strong ultrasound image attenuation was observed.     

Assessment of neovascular permeability in a pancreatic tumor model using dynamic contrast-enhanced (DCE) MRI with contrast agents of different molecular weights

Object  

We evaluated the relationship of dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI)-derived pharmacokinetic parameters and contrast agents with different molecular weights (MW) in a pancreatic tumor mouse model.     

Specific targeting of folate–dendrimer MRI contrast agents to the high affinity folate receptor expressed in ovarian tumor xenografts

The need to develop target-specific MRI contrast agents to aid in disease characterization remains highly essential. In this study, we present a generation four polyamidoamine (PAMAM) folate-dendrimer that specifically targets the high affinity folate receptor (hFR) overexpressed on more than 80% of ovarian tumors. In vitro, mouse erythroleukemia cells expressing the hFR bind the radiolabeled folate-dendrimer chelate resulting in over 2700% increase in binding compared with untreated cells. The binding was inhibited by free folic acid to levels observed on folate-receptor-negative cells. In vivo, ovarian tumor xenografts resulted in a 33% contrast enhancement, following the folate-dendrimer chelate administration, that was significantly different compared with results obtained with a non-specific, extracellular fluid space agent, Gd-HP-DO3A. In addition, this contrast enhancement was absent in saline-treated animals, folate-receptor-negative tumors, and was inhibited by free folic acid. Results suggest that a macromolecular, dendrimeric MRI agent with high molecular relaxivities (1646 mM⁻¹ s⁻¹) can be used in specifically targeting the hFR on tumor cells and ovarian tumors.     

Pre-clinical results with Clariscan™ (NC100150 Injection); experience from different disease models

A superparamagnetic nanoparticle (NC100150 Injection) was investigated in two different animal models; renal perfusion in pigs and tumour imaging in mice. In the pig model, qualitative first-pass perfusion maps following a bolus injection of NC100150 Injection enabled good visualisation of hypoperfused regions of the renal cortex following partial ligation of the renal artery. High temporal resolution was found to be essential to accurately capture the first passage of the contrast agent through the kidney due to the very rapid blood flow in normal renal cortex. In the tumour model (LS174T cells implanted in nude mice), NC100150 Injection was found to cause a gradual (over 60 min) signal increase on Tl-w images in part of the tumours which was attributed to contrast agent leakage from the vascular space to the extravascular space in areas of increased capillary permeability. This observation is consistent with previous reports on the molecular cut-off size for vascular extraction for this tumour cell line. The specific enhancement of tumour tissue suggest potential utility of NC100150 Injection as an angiogenesis marker.     

Magnetic resonance imaging in obstetrics and gynecology: progress and limitations

The introduction of phase-array coils, fast spin echo, and certain other pulse sequences together with use of contrast agents has refined the application of magnetic resonance imaging (MRI) in pelvic disease. It makes management decisions in a number of benign conditions including uterine anomalies, adenomyosis, and leiomyomas of the uterus and endometriosis, especially in the context of infertility; it facilitates identification and characterisation of adnexal masses. In uterine malignancy, the multiplanar capability and excellent soft tissue contrast permit accurate assessment of depth of tumor invasion, tumor volume, and extension to adjacent structures. Its precise role in the management of primary and recurrent ovarian cancer remains to be decided. In pelvic malignancy, contrast facilitates identification of viable tumor but does not improve tissue specificity. In obstetrics, MRI is an attractive alternative to X-ray pelvimetry and assists in the evaluation of associated uterine and pelvic pathology. The use of echo-planar imaging eliminates movement artifact and has the potential to complement ultrasound in the assessment of fetal abnormalities and provide a method of identifying growth retardation from volume measurements of body organs.     

Extinction coefficients of hemoglobin for near-infrared spectroscopy of tissue.

Extinction coefficients of hemoglobin have been studied for five decades by clinical chemists and biochemists, particularly for laboratory spectrophotometric measurements. In the last ten to 15 years, near infrared spectroscopy (NIRS) and imaging for tissue vascular oxygenation, breast tumor detection, and functional brain imaging have been intensively developed for in vivo measurements by groups of physicists, biomedical engineers, and mathematicians. In the approach of NIRS, NIR light in the wavelength range of 650-900 nm is utilized to illuminate tissue in vivo, and the transmitted or reflected light through tissue is recorded for the quantification of hemoglobin concentrations of the measured tissue vasculature. In order to achieve mathematical conversion from the detected light intensity at different wavelengths to hemoglobin concentration, extinction coefficients of hemoglobin, /spl epsiv/, must be used. While the engineers and physicists working in the NIR field have found the correct /spl epsiv/ values to use, there has been controversy on what /spl epsiv/ values should be used for in vivo NIRS in comparison with the conventional e/spl epsiv/ that most biochemists have used in the laboratories for in vitro measurements. The purpose of this article is to address this issue and help biomedical engineers and physicists gain a better understanding of e to be used for NIRS and NIR imaging.