First-pass and equilibrium-MRA of the aortoiliac region with a superparamagnetic iron oxide blood pool MR contrast agent (SH U 555 C): results of a human pilot study

Progress in ultrasmall ferrite nanoparticles enhanced T1 magnetic resonance angiography

Contrast-enhanced magnetic resonance angiography (CE-MRA) plays a critical role in diagnosing and monitoring various vascular diseases. Achieving high-sensitivity detection of vascular abnormalities in CE-MRA depends on the properties of contrast agents. In contrast to clinically used gadolinium-based contrast agents (GBCAs), the new generation of ultrasmall ferrite nanoparticles-based contrast agents have high relaxivity, long blood circulation time, easy surface functionalization, and high biocompatibility, hence showing promising prospects in CE-MRA. This review aims to comprehensively summarize the advancements in ultrasmall ferrite nanoparticles-enhanced MRA for detecting vascular diseases. Additionally, this review also discusses the future clinical translational potential of ultrasmall ferrite nanoparticles-based contrast agents for vascular imaging. By investigating the current status of research and clinical applications, this review attempts to outline the progress, challenges, and future directions of using ultrasmall ferrite nanoparticles to drive the field of CE-MRA into a new frontier of accuracy and diagnostic efficacy.

Enhanced fluorescence/magnetic resonance dual imaging and gene therapy of liver cancer using cationized amylose nanoprobe

Recently, various technologies for targeted gene release in cancer treatment have emerged. However, most of these strategies are facing the challenge of untraceable distribution and poor antitumour treatment effects. In this study, we constructed a gene delivery system that integrated a series of components to assemble multifunctional NPs, providing a promising theranostic nanoplatform for hepatocellular carcinoma (HCC) therapy. Cationized amylose (CA), superparamagnetic iron oxide (SPIO) nanoparticles (NPs), and tetraphenylethylene (TPE) were self-assembled to form nanospheres (CSP/TPE). The prepared NPs was modified with SP94 pepide through amidation reaction, and then survivin small interfering RNA (siRNA) were loaded into the NPs to form CSP/TPE@siRNA-SP94 NPs. Our results showed that the prepared NPs had good size distribution, high RNA condensation and transfection ability. CSP/TPE@siRNA-SP94 NPs exhibited excellent fluorescence and magnetic resonance (MR) imaging properties in vitro and in vivo. The prepared targeted NPs improved Huh-7 cellular uptake in vitro, and the biodistribution of CSP/TPE@siRNA-SP94 in vivo was observed through in/ex vivo fluorescence imaging system and MRI. As survivin siRNA effectively retained in tumour cells, CSP/TPE@siRNA-SP94 NPs considerably inhibited tumour growth in vivo. In addition, H&E staining results showed that all the prepared CSP-based NPs had good biocompatibilities, as few histological changes or tumour metastasis were observed in major organs of the mice in the treatment group. Therefore, we envisage that the prepared CSP/TPE@siRNA-SP94 NPs can represent a promising strategy for HCC diagnosis and treatment.

Ferucarbotran-loaded red blood cells as long circulating MRI contrast agents: first in vivo results in mice

AIM

The encapsulation of superparamagnetic iron oxide contrast agents in red blood cells (RBCs) could overcome their rapid removal by reticulo-endothelial system improving their stability in blood circulation.

MATERIALS & METHODS

Murine ferucarbotran-loaded RBCs were tested in vivo as new contrasting agents in MRI application.

RESULTS

A superior visualization of organs and cerebral vessels was evidenced in ferucarbotran-loaded RBCs-treated mice compared with the controls. The signal enhancement lasted for days, while the contrast from bulk ferucarbotran disappeared after few minutes.

CONCLUSION

Ferucarbotran-loaded RBCs showed to improve diagnostic imaging and their use may extend the time frame for MRI and magnetic resonance angiography since to date the time frame for data acquisition has been limited to the first pass.

Ultrasmall Ferrite Nanoparticles Synthesized via Dynamic Simultaneous Thermal Decomposition for High-Performance and Multifunctional T1 Magnetic Resonance Imaging Contrast Agent

Large-scale synthesis of monodisperse ultrasmall metal ferrite nanoparticles as well as understanding the correlations between chemical composition and MR signal enhancement is critical for developing next-generation, ultrasensitive T₁ magnetic resonance imaging (MRI) nanoprobes. Herein, taking ultrasmall MnFe₂O₄ nanoparticles (UMFNPs) as a model system, we report a general dynamic simultaneous thermal decomposition (DSTD) strategy for controllable synthesis of monodisperse ultrasmall metal ferrite nanoparticles with sizes smaller than 4 nm. The comparison study revealed that the DSTD using the iron-eruciate paired with a metal-oleate precursor enabled a nucleation-doping process, which is crucial for particle size and distribution control of ultrasmall metal ferrite nanoparticles. The principle of DSTD synthesis has been further confirmed by synthesizing NiFe₂O₄ and CoFe₂O₄ nanoparticles with well-controlled sizes of ∼3 nm. More significantly, the success in DSTD synthesis allows us to tune both MR and biochemical properties of magnetic iron oxide nanoprobes by adjusting their chemical composition. Beneficial from the Mn²⁺ dopant, the synthesized UMFNPs exhibited the highest r₁ relaxivity (up to 8.43 mM^-1 s^-1) among the ferrite nanoparticles with similar sizes reported so far and demonstrated a multifunctional T₁ MR nanoprobe for in vivo high-resolution blood pool and liver-specific MRI simultaneously. Our study provides a general strategy to synthesize ultrasmall multicomponent magnetic nanoparticles, which offers possibilities for the chemical design of a highly sensitive ultrasmall magnetic nanoparticle based T₁ MRI probe for various clinical diagnosis applications.

Molecular Imaging of Brain Tumors Using Liposomal Contrast Agents and Nanoparticles

The first generation of cross-sectional brain imaging using computed tomography (CT), ultrasonography, and eventually MR imaging focused on determining structural or anatomic changes associated with brain disorders. The current state-of-the-art imaging, functional imaging, uses techniques such as CT and MR perfusion that allow determination of physiologic parameters in vivo. In parallel, tissue-based genomic, transcriptomic, and proteomic profiling of brain tumors has created several novel and exciting possibilities for molecular targeting of brain tumors. The next generation of imaging translates these molecular in vitro techniques to in vivo, noninvasive, targeted reconstruction of tumors and their microenvironments.

Au Nanocage Functionalized with Ultra-small Fe3O4 Nanoparticles for Targeting T1-T2Dual MRI and CT Imaging of Tumor

Diagnostic approaches based on multimodal imaging of clinical noninvasive imaging (eg. MRI/CT scanner) are highly developed in recent years for accurate selection of the therapeutic regimens in critical diseases. Therefore, it is highly demanded in the development of appropriate all-in-one multimodal contrast agents (MCAs) for the MRI/CT multimodal imaging. Here a novel ideal MCAs (F-AuNC@Fe₃O₄) were engineered by assemble Au nanocages (Au NC) and ultra-small iron oxide nanoparticles (Fe₃O₄) for simultaneous T₁–T₂dual MRI and CT contrast imaging. In this system, the Au nanocages offer facile thiol modification and strong X-ray attenuation property for CT imaging. The ultra-small Fe₃O₄ nanoparticles, as excellent contrast agent, is able to provide great enhanced signal of T₁- and T₂-weighted MRI (r₁ = 6.263 mM⁻¹ s⁻¹, r₂ = 28.117 mM⁻¹ s⁻¹) due to their ultra-refined size. After functionalization, the present MCAs nanoparticles exhibited small average size, low aggregation and excellent biocompatible. In vitro and In vivo studies revealed that the MCAs show long-term circulation time, renal clearance properties and outstanding capability of selective accumulation in tumor tissues for simultaneous CT imaging and T₁- and T₂-weighted MRI. Taken together, these results show that as-prepared MCAs are excellent candidates as MRI/CT multimodal imaging contrast agents.

Luteinizing Hormone Releasing Hormone-Targeted Cisplatin-Loaded Magnetite Nanoclusters for Simultaneous MR Imaging and Chemotherapy of Ovarian Cancer

Given the superior soft tissue contrasts obtained by MRI and the long residence times of magnetic nanoparticles (MNPs) in soft tissues, MNP-based theranostic systems are being developed for simultaneous imaging and treatment. However, development of such theranostic nanoformulations presents significant challenges of balancing the therapeutic and diagnostic functionalities in order to achieve optimum effect from both. Here we developed a simple theranostic nanoformulation based on magnetic nanoclusters (MNCs) stabilized by a bisphosphonate-modified poly(glutamic acid)-b-(ethylene glycol) block copolymer and complexed with cisplatin. The MNCs were decorated with luteinizing hormone releasing hormone (LHRH) to target LHRH receptors (LHRHr) overexpressed in ovarian cancer cells. The targeted MNCs significantly improved the uptake of the drug in cancer cells and decreased its IC50 compared to the nontargeted formulations. Also, the enhanced LHRHr-mediated uptake of the targeted MNCs resulted in enhancement in the T2-weighted negative contrast in cellular phantom gels. Taken together, the LHRH-conjugated MNCs show good potential as ovarian cancer theranostics.

Ultra-large-scale production of ultrasmall superparamagnetic iron oxide nanoparticles for T1-weighted MRI

Ultra-large-scale production of ultrasmall superparamagnetic iron oxide nanoparticles for magnetic resonance angiography.

Clinical applications of iron oxide nanoparticles for magnetic resonance imaging of brain tumors

Current neuroimaging provides detailed anatomic and functional evaluation of brain tumors, allowing for improved diagnostic and prognostic capabilities. Some challenges persist even with today's advanced imaging techniques, including accurate delineation of tumor margins and distinguishing treatment effects from residual or recurrent tumor. Ultrasmall superparamagnetic iron oxide nanoparticles are an emerging tool that can add clinically useful information due to their distinct physiochemical features and biodistribution, while having a good safety profile. Nanoparticles can be used as a platform for theranostic drugs, which have shown great promise for the treatment of CNS malignancies. This review will provide an overview of clinical ultrasmall superparamagnetic iron oxides and how they can be applied to the diagnostic and therapeutic neuro-oncologic setting.

Surface and interfacial engineering of iron oxide nanoplates for highly efficient magnetic resonance angiography

Magnetic resonance angiography using gadolinium-based molecular contrast agents suffers from short diagnostic window, relatively low resolution and risk of toxicity. Taking into account the chemical exchange between metal centers and surrounding protons, magnetic nanoparticles with suitable surface and interfacial features may serve as alternative T1 contrast agents. Herein, we report the engineering on surface structure of iron oxide nanoplates to boost T1 contrast ability through synergistic effects between exposed metal-rich Fe3O4(100) facets and embedded Gd2O3 clusters. The nanoplates show prominent T1 contrast in a wide range of magnetic fields with an ultrahigh r1 value up to 61.5 mM(-1) s(-1). Moreover, engineering on nanobio interface through zwitterionic molecules adjusts the in vivo behaviors of nanoplates for highly efficient magnetic resonance angiography with steady-state acquisition window, superhigh resolution in vascular details, and low toxicity. This study provides a powerful tool for sophisticated design of MRI contrast agents for diverse use in bioimaging applications.

A multiple gadolinium complex decorated fullerene as a highly sensitive T1 contrast agent

A multiple gadolinium complex decorated fullerene (CGD_n) as an enhanced T₁ contrast agent was presented.

Comparison of USPIO-enhanced MRI and Gd-DTPA enhancement during the subacute stage of focal cerebral ischemia in rats

BACKGROUND

Being one class of magnetic resonance imaging (MRI) contrast agents, ultrasmall superparamagnetic particles of iron oxides (USPIO) bear the potential to study neuroinflammation following stroke, but there is still debate over whether the iron oxides particles may enter the brain tissue passively over a damaged blood-brain barrier (BBB).

PURPOSE

To compare the enhancement patterns of USPIO and gadopentate dimeglumine (Gd-DTPA) during the subacute stage of focal cerebral ischemia, to examine the relationship between USPIO enhancement and BBB disturbance, as well as with neuroinflammatory cell response.

MATERIAL AND METHODS

Multiple MR sequences were obtained on days 3 and 6 after transient middle cerebral artery occlusion induced in rats with and without the application of USPIO and Gd-DTPA. The enhancement patterns of the two contrast agents were compared and correlated to histology, including IgG for BBB permeability, Prussian Blue staining for iron particle detection, and CD68 immunohistochemistry staining to identify macrophage/microglia.

RESULTS

Gd-DTPA enhancement depicted BBB breakdown being in line with IgG leakage. The USPIO enhanced images demonstrated both diffuse and focal signal alteration in ischemic lesions. The diffuse enhanced pattern had a similar spatial and temporal profile as with Gd-DTPA enhancement. In addition, focal enhanced signal loss was visible on T1-, T2-, and T2*-weighted images, with a peak tendency of MR signal loss, macrophage/microglia concentration and iron particle accumulation at a later phase of the study.

CONCLUSION

After focal cerebral ischemia, Gd-DTPA-enhanced MRI showed a higher sensitivity in detecting BBB alterations than did USPIO enhancement. USPIO provided complementary information regarding inflammatory cell activity in neuroinflammatory to cerebral ischemia that had not been visualized using conventional Gd-DTPA. The assessment using multiple MR parameters may identify intracellular and extracellular USPIO in vivo.

One-step synthesis of water-dispersible ultra-small Fe3O4 nanoparticles as contrast agents for T1 and T2 magnetic resonance imaging

Uniform, highly water-dispersible and ultra-small Fe3O4 nanoparticles were synthesized via a modified one-step coprecipitation approach. The prepared Fe3O4 nanoparticles not only show good magnetic properties, long-term stability in a biological environment, but also exhibit good biocompatibility in cell viability and hemolysis assay. Due to the ultra-small sized and highly water-dispersibility, they exhibit excellent relaxivity properties, the 1.7 nm sized Fe3O4 nanoparticles reveal a low r2/r1 ratio of 2.03 (r1 = 8.20 mM(-1) s(-1), r2 = 16.67 mM(-1) s(-1)); and the 2.2 nm sized Fe3O4 nanoparticles also appear to have a low r2/r1 ratio of 4.65 (r1 = 6.15 mM(-1) s(-1), r2 = 28.62 mM(-1) s(-1)). This demonstrates that the proposed ultra-small Fe3O4 nanoparticles have great potential as a new type of T1 magnetic resonance imaging contrast agents. Especially, the 2.2 nm sized Fe3O4 nanoparticles, have a competitive r1 value and r2 value compared to commercial contrasting agents such as Gd-DTPA (r1 = 4.8 mM(-1) s (-1)), and SHU-555C (r2 = 69 mM(-1) s(-1)). In vitro and in vivo imaging experiments, show that the 2.2 nm sized Fe3O4 nanoparticles exhibit great contrast enhancement, long-term circulation, and low toxicity, which enable these ultra-small sized Fe3O4 nanoparticles to be promising as T1 and T2 dual contrast agents in clinical settings.

Prolonged in vivo circulation time by zwitterionic modification of magnetite nanoparticles for blood pool contrast agents

Long circulation time is critical for blood pool contrast agents used in high-resolution magnetic resonance angiography. For iron oxide particle contrast agents, size and surface properties significantly influence their in vivo performance. We developed a novel long-circulating blood pool contrast agent by introducing zwitterionic structure onto the particle surface. Zwitterionic structure was fabricated by 3-(diethylamino)propylamine (DEAPA) grafted onto the surface of ployacrylic acid coated magnetite nanoparticles via EDC/NHS [N-(3-dimethylaminopropyl)-N'-ethylcarbo-diimide hydrochloride/N-hydroxysuccinimide] coupling chemistry. Zwitterionic particles demonstrated five times lower macrophage cell uptake than the original particles and low cell toxicity. Magnetic resonance angiography indicated that zwitterionic nanoparticles had much longer in vivo circulation time than the original particles and were an ideal candidate for blood pool contrast agent. We suppose that zwitterionic modification by DEAPA and EDC/NHS can be used generally for coating nanoparticles with carboxyl surface and to prolong their circulating time.

Feasibility of semiquantitative liver perfusion assessment by ferucarbotran bolus injection in double-contrast hepatic MRI

PURPOSE

To evaluate the feasibility of semiquantitative measurement of liver perfusion from analysis of ferucarbotran induced signal-dynamics in double-contrast liver MR-imaging (DC-MRI).

MATERIALS AND METHODS

In total 31 patients (21 men; 58 ± 10 years) including 18 patients with biopsy proven liver cirrhosis prospectively underwent clinically indicated DC-MRI at 1.5 Tesla (T) with dynamic T2-weighted gradient-echo imaging after ferucarbotran bolus injection. Breathing artefacts in tissue and input time curves were reduced by Savitzky-Golay-filtering and semiquantitative perfusion maps were calculated using a model free approach. Hepatic blood flow index (HBFI) and splenic blood flow index (SBFI) were determined by normalization of arbitrary perfusion values to the perfusion of the erector spinae muscle resulting in a semiquantitative perfusion measure.

RESULTS

In 30 of 31 patients the evaluated protocol could successfully be applied. Mean HBF was 7.7 ± 2.46 (range, 4.6-12.8) and mean SBF was 13.20 ± 2.57 (range, 8.5-17.8). A significantly lower total HBF was seen in patients with cirrhotic livers as compared to patients with noncirrhotic livers (P < 0.05). In contrast, similar SBF was observed in cirrhotic and noncirrhotic patients (P = 0.11).

CONCLUSION

Capturing the signal dynamics during bolus injection of ferucarbotran in DC-MRI of the liver allows for semiquantitative assessment of hepatic perfusion that may be helpful for a more precise characterisation of liver cirrhosis and focal liver lesions.

Contrast-enhanced magnetic resonance angiography in rabbits: evaluation of the gadolinium-based agent p846 and the iron-based blood pool agent p904 in comparison with gadoterate meglumine

OBJECTIVES

: To evaluate the performance of a gadolinium-based contrast compound (P846) as well as an ultra-small particle of iron oxide agent (P904) in contrast-enhanced magnetic resonance angiography (MRA) in rabbits and to compare those agents with gadoterate meglumine (Gd-DOTA) for first pass and steady state imaging.

MATERIALS AND METHODS

: A total of 6 rabbits underwent contrast-enhanced MRA of the aorta and its branches at 3 different time points. All examinations were performed on a 1.5T MR (Siemens HealthCare, Magnetom Espree), and the contrast agents were applied in random order. Image data were acquired using a time-resolved MRA sequence (time-resolved angiography with stochastic trajectories) during the first pass to assess the bolus phase and a high-resolution MRA sequence followed by repetitive measurements over the next 10 minutes for all 3 agents to evaluate the postbolus phase. Two radiologists reviewed the images in consensus blinded to the contrast agent used. Signal-to-noise ratio and contrast-to-noise ratio for three-dimensional high-resolution MRA were calculated for each time point and agent. Image quality was consensually evaluated on a 4-point Likert scale. A Wilcoxon-Mann-Whitney U test was used for comparison with P < 0.05 as level of statistical significance.

RESULTS

: All agents led to diagnostic MR angiograms in all 6 rabbits. The time-resolved angiography with stochastic trajectories datasets provided detailed information about the bolus phase for all the 3 agents. During the first pass, P904 and P846 proved to be superior to Gd-DOTA with the highest peak enhancement for P846. In the postbolus phase up to 10 minutes postcontrast injection, P904 proved to be superior to the other agents. All the agents led to excellent image quality, with no statistical difference to a maximum of 3 minutes postinjection, whereas thereafter images with Gd-DOTA and P846 were assessed as nondiagnostic.

CONCLUSIONS

: P846 and P904 proved to be superior to Gd-DOTA for time-resolved MRA. The ultra-small particle of iron oxide compound P904 showed continuous high signal over 10 minutes and seems to be best suited for first pass and steady-state imaging.

Monitoring of bevacizumab-induced antiangiogenic treatment effects by "steady state" ultrasmall superparamagnetic iron oxide particles magnetic resonance imaging using robust multiecho ΔR2* relaxometry

RATIONALE AND OBJECTIVES

The purpose of this study was to evaluate whether "steady state" magnetic resonance imaging (MRI) using a robust multiecho ΔR2* MR relaxometry technique is suitable for the early assessment of a clinically approved antiangiogenic treatment regimen using bevacizumab (Avastin).

METHODS

A673 rhabdomyosarcoma-bearing mice were treated with bevacizumab (n = 6) or saline as control, respectively (n = 6). MRI using a multigradient echo sequence was performed before and after 2 doses of 100 μg bevacizumab at baseline and day 7. Ultrasmall superparamagnetic iron oxide particles (SH U 555 C) induced changes of the transverse relaxation rate R2* (ΔR2*) were measured in regions of interest. From these results, the vascular volume fraction was estimated, providing a surrogate marker for the microvessel density (MVD). The actual MVD was determined by immunohistochemistry and correlated with the MRI results.

RESULTS

Bevacizumab treatment resulted in a significant reduction of the ΔR2* values compared with the control group (bevacizumab: 10.47 ± 0.78 seconds(-1) vs. control: 17.91 ± 2.63 seconds(-1); P = 0.01), reflecting the significant decrease of the vascular volume fraction by 33% (bevacizumab: 2.21% ± 0.15% vs. control: 3.31% ± 0.22%; P = 0.001). Immunohistochemistry confirmed the MR results showing an approximately 25% reduction of the MVD after treatment (bevacizumab: 7.11 ± 0.3 vs. control: 9.45 ± 0.38; P = 0.001).

CONCLUSION

Multiecho ΔR2* MR relaxometry allows an early and quantitative assessment of tumor vascularization changes in response to an antiangiogenic treatment with a clinically approved vascular endothelial growth factor inhibitor. With the availability of long circulating ultrasmall superparamagnetic iron oxide particles s for clinical use, this imaging technique could be instantly translated to antiangiogenic treatment monitoring in clinical studies.

Characterization of carotid artery plaques with USPIO-enhanced MRI: assessment of inflammation and vascularity as in vivo imaging biomarkers for plaque vulnerability

To evaluate ultra small superparamagnetic iron oxide particles (USPIO) enhanced magnetic resonance (MR) imaging for characterization of atherosclerotic carotid plaques by assessing vascularity and plaque inflammation, besides contrast-enhanced MR angiography (CE-MRA) of the carotid artery stenosis. Twelve patients with severe carotid artery stenosis, scheduled for endarterectomy, underwent MRI of the carotid artery bifurcation using SHU 555 C at a dose of 40 μmol Fe/kg BW. The MR imaging protocol comprised pre- and post-contrast T2*-w, a first-pass CE-MRA and dynamic T1-w sequences. For quantitative data analysis, the signal intensities (SI) were measured and SNR-data (SNR = SI(blood/plaque/bone marrow)/standard deviation(noise)) as well as ΔSI-data (SNR(post)-SNR(pre)) were calculated. In addition, two radiologists rated the diagnostic performance of first-pass MRA according to a four level decision scale. Staining of anti-dextran (SHU 555 C) and anti-CD68 (macrophages) was performed for immunohistological confirmation. Plaque sections with a T2*-w signal decline (intracellular USPIO accumulation in macrophages) showed significantly changes (mean -14%, 95% CI, -5 to -20%; P < 0.01) and corresponding plaque regions had significantly higher (15.15 ± 1.76 vs. 5.22 ± 1.50; P < 0.01) T1-w enhancement data (global estimation of vascularity). The first-pass MRA of the supra-aortal vessels provided images of diagnostic quality. Representative immunohistology sections revealed colocalization of dextran- and CD68-immunoreactive cells. USPIO-enhanced MRI is feasible for in vivo assessment of vascularity and macrophage content in atherosclerotic carotid plaques, determining an association of these potential imaging biomarkers of plaque vulnerability. Diagnostic MRA of the supra-aortal vessels can be imaged additionally with a single administration of SHU 555 C.

On the dual contrast enhancement mechanism in frequency-selective inversion-recovery magnetic resonance angiography (IRON-MRA)

The susceptibility of blood changes after administration of a paramagnetic contrast agent that shortens T(1). Concomitantly, the resonance frequency of the blood vessels shifts in a geometry-dependent way. This frequency change may be exploited for incremental contrast generation by applying a frequency-selective saturation prepulse prior to the imaging sequence. The dual origin of vascular enhancement depending first on off-resonance and second on T(1) lowering was investigated in vitro, together with the geometry dependence of the signal at 3T. First results obtained in an in vivo rabbit model are presented.

High resolution ultra high field magnetic resonance imaging of glioma microvascularity and hypoxia using ultra-small particles of iron oxide

OBJECTIVES

This study assessed whether ultra-small particles of iron oxide (USPIO) intravascular contrast agent could enhance visualization of tumor microvascularity in F98 glioma bearing rats by means of ultra high field (UHF) high-resolution gradient echo (GRE) magnetic resonance imaging (MRI). In an effort to explain differences in visualization of microvascularity before and after USPIO administration, hypoxia and vessel diameters were assessed on corresponding histopathologic sections.

MATERIALS AND METHODS

F98 glioma cells were implanted stereotactically into the brains of syngeneic Fischer rats. Based on clinical criteria, rats were imaged 1 to 2 days before their death with and without USPIO contrast on an 8 Tesla MRI. To identify hypoxic regions of the brain tumor by immunohistochemical staining, a subset of animals also received a nitroimidazole-based hypoxia marker, EF5, before euthanasia. These sections then were compared with noncontrast enhanced MR images. The relative caliber of tumor microvasculature, compared with that of normal brain, was analyzed in a third group of animals.

RESULTS

After USPIO administration, UHF high-resolution GRE MRI consistently predicted increased microvascular density relative to normal gray matter when correlated with histopathology. The in-plane visibility of glioma microvascularity in 22 rats increased by an average of 115% and signal intensity within the tumor decreased by 13% relative to normal brain. Tumor microvascularity identified on noncontrast MR images matched hypoxic regions identified by immunohistochemical staining with a sensitivity of 83% and specificity of 89%. UHF GRE MRI was able to resolve microvessels less than 20 micro in diameter, although differences in tumor vessel size did not consistently account for differences in visualization of microvascularity.

CONCLUSIONS

USPIO administration significantly enhanced visualization of tumor microvascularity on gradient echo 8 T MRI and significantly improved visualization of tumor microvascularity. Microvascularity identified on precontrast images is suspected to be partly associated with hypoxia.

Ultrasmall superparamagnetic iron oxides (USPIOs): a future alternative magnetic resonance (MR) contrast agent for patients at risk for nephrogenic systemic fibrosis (NSF)?

Gadolinium (Gd) based contrast agents (GBCAs) in magnetic resonance imaging (MRI) are used in daily clinical practice and appear safe in most patients; however, nephrogenic systemic fibrosis (NSF) is a recently recognized severe complication associated with GBCAs. It affects primarily patients with renal disease, such as stage 4 or 5 chronic kidney disease (CKD; glomerular filtration rate <30 ml/min per 1.73 m(2)), acute kidney injury, or kidney and liver transplant recipients with kidney dysfunction. Contrast-enhanced MRI and computed tomography (CT) scans provide important clinical information and influence patient management. An alternative contrast agent is needed to obtain adequate imaging results while avoiding the risk of NSF in this vulnerable patient group. One potential alternative is ultrasmall superparamagnetic iron oxide (USPIO) nanoparticles, which provide enhancement characteristics similar to GBCAs. We review our experience in approximately 150 patients on the potential benefits of the USPIOs ferumoxtran-10 and ferumoxytol. We focus on central nervous system (CNS) MRI but also review imaging of other vascular beds. Safety studies, including USPIO administration (ferumoxytol) as iron supplement therapy in CKD patients on and not on dialysis, suggest that decreased kidney function does not alter the safety profile. We conclude that for both CNS MR imaging and MR angiography, USPIO agents like ferumoxytol are a viable option for patients at risk for NSF.

Prediction of antiangiogenic treatment efficacy by iron oxide enhanced parametric magnetic resonance imaging

RATIONALE AND OBJECTIVES

Tools for monitoring modern target-specific antiangiogenic and antivascular therapies are highly desirable because treatment strategies are time consuming, expensive, and yet sometimes ineffective. Therefore, the aim of this experimental study was to evaluate the predictive value of steady-state ultrasmall particles of iron oxide (USPIO; SH U 555 C)-enhanced magnetic resonance imaging (MRI) for early assessment of antivascular tumor-treatment effectiveness.

METHODS

Mice were inoculated with an HT-1080 fibrosarcoma xenograft and subjected to target-specific antivascular therapy using a selective thrombogenic vascular-targeting agent (truncated tissue factor fused to RGD peptide) or saline as control. Four to 8 hours after treatment, the USPIO-induced change in the transverse relaxation rate DeltaR2* was measured by MRI, and the vascular volume fraction (VVF) was calculated by calibrating DeltaR2* of the tumor by DeltaR2* of muscle tissue. Treatment response was defined by histologic grading of vascular thrombosis and tumor necrosis.

RESULTS

After thrombogenic treatment, half of the HT-1080 xenograft-bearing animals showed only minor (=nonresponder) whereas the other half showed extensive tumor thrombosis (=responders). For responders, a significant decrease of DeltaR2* and VVF was observed compared with the control group (DeltaR2*: controls: 16 +/- 1 s-1 vs. responder: 4 +/- 2 s-1; P < 0.001) whereas DeltaR2* and VVF remained nearly unchanged for nonresponders (DeltaR2*: nonresponder 14 +/- 2 s-1). VVF and DeltaR2* values correlated inversely with the histologic grading of vascular thrombosis and tumor necrosis (VVF: r = -0.8; DeltaR2*: r = -0.71; P < 0.01).

CONCLUSION

USPIO-enhanced MRI allows a noninvasive, early assessment of treatment efficacy of thrombogenic vascular-targeting agents.

Differential effects of cannabinoid receptor agonists on regional brain activity using pharmacological MRI

BACKGROUND AND PURPOSE

Activation of cannabinoid CB1 and/or CB2 receptors mediates analgesic effects across a broad spectrum of preclinical pain models. Selective activation of CB2 receptors may produce analgesia without the undesirable psychotropic side effects associated with modulation of CB1 receptors. To address selectivity in vivo, we describe non-invasive, non-ionizing, functional data that distinguish CB1 from CB2 receptor neural activity using pharmacological MRI (phMRI) in awake rats.

EXPERIMENTAL APPROACH

Using a high field (7 T) MRI scanner, we examined and quantified the effects of non-selective CB1/CB2 (A-834735) and selective CB2 (AM1241) agonists on neural activity in awake rats. Pharmacological specificity was determined using selective CB1 (rimonabant) or CB2 (AM630) antagonists. Behavioural studies, plasma and brain exposures were used as benchmarks for activity in vivo.

KEY RESULTS

The non-selective CB1/CB2 agonist produced a dose-related, region-specific activation of brain structures that agrees well with published autoradiographic CB1 receptor density binding maps. Pretreatment with a CB1 antagonist but not with a CB2 antagonist, abolished these activation patterns, suggesting an effect mediated by CB1 receptors alone. In contrast, no significant changes in brain activity were found with relevant doses of the CB2 selective agonist.

CONCLUSION AND IMPLICATIONS

These results provide the first clear evidence for quantifying in vivo functional selectivity between CB1 and CB2 receptors using phMRI. Further, as the presence of CB2 receptors in the brain remains controversial, our data suggest that if CB2 receptors are expressed, they are not functional under normal physiological conditions.

Nephrogene systemische Fibrose (NSF) – Implikationen für die Radiologie

Nephrogenic systemic fibrosis (NSF) is a systemic disease with a 5% mortality which was first described in 1997 and which only occurs in patients with severely impaired renal function (GFR <30 ml/min per 1.73 m(2)) and for which an association with previous administration of several Gd-chelates has been observed. According to retrospective case control studies the odds ratio for a patient with severely impaired renal function to develop NSF was increased by a factor of 22-32 when gadodiamide was administered. At this time there are approximately 250 confirmed cases of NSF of which 177 are associated with the administration of gadodiamide and 78 are associated with gadopentetate dimeglumine. This review article elucidates the postulated pathogenesis of NSF and provides an overview of the published statements and recommendations from international regulatory authorities and from international advisory boards. Even though the pathogenesis is not completely understood at this time, the European Pharmacovigilance Working Party has decided that gadodiamide and gadopentetate dimeglumine must not be used in high-risk patients. Other Gd-containing contrast agents should only be administered after thorough assessment of the indication and with minimized Gd dose. In the USA, the FDA has issued a black box warning for Gd-containing contrast agents.

Antiangiogenic Tumor Treatment: Early Noninvasive Monitoring with USPIO-enhanced MR Imaging in Mice

PURPOSE

To prospectively investigate steady-state blood volume measurements for early quantitative monitoring of antiangiogenic treatment with ultrasmall superparamagnetic iron oxide (USPIO)-enhanced magnetic resonance (MR) imaging.

MATERIALS AND METHODS

The institutional animal care committee approved all experiments. HT-1080 fibrosarcoma-bearing nude mice were injected with a thrombogenic vascular targeting agent (VTA) (11 nude mice, 20 tumors) or saline (12 nude mice, 20 tumors). USPIO-enhanced (SH U 555C) MR imaging was performed after the VTA was administered. USPIO-induced changes in tissue R2* (DeltaR2*) were measured with a T2-weighted dual-echo echo-planar imaging sequence, and the vascular volume fraction (VVF) was calculated. Parametric DeltaR2* maps were analyzed with respect to tumor perfusion patterns. Correlative histologic analysis was performed for grading of tissue thrombosis, and tissue perfusion was quantified with fluorescent microbeads. Unpaired Student t test and Spearman nonparametric correlation coefficient were used for statistical analysis.

RESULTS

The DeltaR2* values were significantly (P < .001) reduced shortly after treatment initiation (mean DeltaR2*, 0.017 msec(-1) +/- 0.0014 [standard error] in control animals vs 0.005 msec(-1) +/- 0.0007 in animals that received VTA), which was also reflected by a decrease in the VVF (2.47% +/- 0.18 vs 0.41% +/- 0.48, P < .001). Histologic analysis revealed various degrees of tumor thrombosis after VTA treatment that correlated inversely with the DeltaR2* values (r = -0.83). Moreover, tumor perfusion measurements corroborated the MR results, indicating a significant reduction in tissue perfusion after VTA treatment (mean tissue fluorescence, 570.4 arbitrary units [au] per gram +/- 27 vs 161.7 au/g +/- 17; P < .05).

CONCLUSION

USPIO-enhanced MR imaging enables early monitoring of antiangiogenic treatment of tumors.

MR angiography with blood pool contrast agents

Contrast-enhanced magnetic resonance angiography (CE-MRA) with standard extracellular contrast material is well established for vascular imaging. Recently, the first blood pool contrast agent (BPA) has become clinically available. This paper reviews characteristics and classification of BPA as well as first clinical experience in various vascular territories. BPAs comprise gadolinium-based compounds, synthetic compounds, and ultrasmall superparamagnetic iron-oxide (USPIO) particles. Such BPAs are retained in blood with a prolonged time-window of enhancement as compared to extracellular gadolinium chelates. Promising results from USPIO at first-pass and steady-state angiography have been published, but no USPIO is approved yet. Gadofosveset is the first clinically approved BPA. After bolus injection, gadofosveset binds noncovalently to serum-albumine, thus enhancing relaxivity. First published results from carotid, coronary, renal, and peripheral angiography are encouraging; particularly helpful is prolonged enhancement during steady state. More BPAs have been clinically evaluated, but no approval has been granted. Bolus-injectable BPAs allow for first-pass CE-MRA similar to standard extracellular contrast media, but with higher relaxivity, allowing lower doses and reduced injection rates. An additional feature of BPA is the steady-state phase with a broad time window enabling high-resolution angiography or double-gated angiography of coronary arteries to compensate for the complex motion pattern.

Contrast-enhanced magnetic resonance imaging of central nervous system tumors: agents, mechanisms, and applications

Brain tumors are one of the most common neoplasms in young adults and are associated with a high mortality and disability rate. Magnetic resonance imaging (MRI) is widely accepted to be the most sensitive imaging modality in the assessment of cerebral neoplasms. Because the detection, characterization, and exact delineation of brain tumors require a high lesion contrast that depends on the signal of the lesion in relation to the surrounding tissue, contrast media is given routinely. Anatomical and functional, contrast agent-based MRI techniques allow for a better differential diagnosis, grading, and especially therapy decision, planing, and follow-up. In this article, the basics of contrast enhancement of brain tumors will be reviewed. The underlying pathology of a disrupted blood-brain barrier and drug influences will be discussed. An overview of the currently available contrast media and the influences of dosage, field strength, and application on the tumor tissue contrast will be given. Challenging, contrast-enhanced, functional imaging techniques, such as perfusion MRI and dynamic contrast-enhanced MRI, are presented both from the technical side and the clinical experience in the assessment of brain tumors. The advantages over conventional, anatomical MRI techniques will be discussed as well as possible pitfalls and drawbacks.

3D contrast-enhanced MR angiography

Safe, fast, accurate contrast arteriography can be obtained utilizing gadolinium (Gd) and 3D MR data acquisition for diagnosing vascular diseases. Optimizing contrast enhanced MRA (CE MRA), however, requires understanding the complex interplay between Gd injection timing, the Fourier mapping of 3D MR data acquisition and a multitude of parameters determining resolution, anatomic coverage, and sensitivity to motion artifacts. It is critical to time the bolus peak to coincide with central k-space data acquisition, which dominates image contrast. Oversampling the center of k-space allows reconstruction of multiple 3D acquisitions in rapid succession to time-resolve the passage of the contrast bolus. Parallel imaging increases resolution, shortens scan time and compresses the center of k-space into a shorter period of time, thereby minimizing motion and timing artifacts. Absence of ionizing radiation allows MRA to be repeated and combined with additional sequences to more fully characterize anatomy, flow, and physiology. Utilizing stepping table technology and thigh compression, whole body MRA is possible with a single contrast injection. As MR technology continues to advance, CE MRA becomes better and simpler to perform, increasing its efficacy in the diagnosis and management of vascular diseases.

Assessment of Bone Marrow Angiogenesis in Patients with Acute Myeloid Leukemia by Using Contrast-enhanced MR Imaging with Clinically Approved Iron Oxides: Initial Experience

PURPOSE

To prospectively assess bone marrow (BM) angiogenesis in patients with acute myeloid leukemia (AML) by using iron oxide-enhanced magnetic resonance (MR) imaging.

MATERIALS AND METHODS

The study was institutional ethics committee approved. Informed signed consent was obtained from each study participant. The requirement for informed consent for use of data from a reference database was waived. Eleven patients (seven women, four men; mean age, 53 years+/-4.40 [standard deviation]) with an initial diagnosis of AML were enrolled in the study and underwent T2*-weighted two-echo echo-planar MR imaging of the pelvis before and after intravenous injection of a clinically approved iron oxide blood-pool contrast agent. Six healthy control subjects (one woman, five men; mean age, 35 years+/-2.31) were examined with the same MR protocol. The iron oxide-induced change in R2* relaxation rate (DeltaR2*) was calculated, and the vascular volume fraction (VVF) of the BM was derived by dividing the DeltaR2* of the BM by the DeltaR2* of the muscle. Parametric DeltaR2* maps were calculated to visualize vessel distribution. Patients underwent BM biopsy for correlative determination of microvessel density (MVD) and vascular endothelial growth factor (VEGF). Differences in DeltaR2*, VVF, VEGF, and MVD were compared by using the Wilcoxon rank sum test.

RESULTS

DeltaR2* maps showed prominent areas of highly vascularized BM in the patients with AML, whereas the control subjects had moderately vascularized BM with homogeneous vessel distribution. Quantitative analysis revealed VVF values to be significantly higher in patients with AML than in control subjects: The mean VVF in the pelvis was 9.18%+/-1.54 for patients versus 3.91%+/-0.61 for control subjects (P=.010). In accordance with MR results, MVD (P=.009) and VEGF expression (P=.017) were significantly elevated in the AML group compared with values in the control group.

CONCLUSION

Iron oxide-enhanced MR imaging enables assessment of BM angiogenesis in patients with AML.

Pharmacological MRI in awake rats reveals neural activity in area postrema and nucleus tractus solitarius: relevance as a potential biomarker for detecting drug-induced emesis

Drug-induced vomiting (emesis) is a major concern in patient care and a significant hurdle in the development of novel therapeutics. With respect to the latter, rodents, such as the rat and mouse, are typically used in efficacy and safety studies; however, drug-induced emesis cannot be readily observed in these species due to the lack of an emetic reflex. It is known that emesis can be triggered by neural activity in brain regions including area postrema (AP) and nucleus tractus solitarius (NTS). In this study, using pharmacological magnetic resonance imaging (phMRI) and a blood-pool contrast agent, we imaged the hemodynamic consequences of brain activity in awake rats initiated by the administration of compounds (apomorphine 0.1, 0.3 micromol/kg i.v. and ABT-594 0.03, 0.1, 0.3 micromol/kg i.v.) that elicit emesis in other species. Regional drug-induced relative cerebral blood volume (rCBV) changes and percent activated area within the AP and NTS were calculated, in which a dose-dependent relationship was evident for both apomorphine and ABT-594. Additionally, to correlate with behavioral readouts, it was found that the activation of AP and NTS was observed at plasma concentrations consistent with those that induced emesis in ferrets for both drugs. Our data thus suggest that phMRI in awake rats may be a useful tool for predicting emetic liability of CNS-acting drugs and may provide insights into depicting the underlying emetic neural pathways in vivo.

Ultrasmall supraparamagnetic iron oxide-enhanced magnetic resonance imaging of antigen-induced arthritis: a comparative study between SHU 555 C, ferumoxtran-10, and ferumoxytol

OBJECTIVES

We sought to compare the ability of 3 ultrasmall superparamagnetic iron oxides (USPIOs) to detect and characterize antigen-induced arthritis with MR imaging.

MATERIALS AND METHODS

A monoarthritis was induced in the right knee of 18 rats. The left knee served as a normal control. Knees underwent magnetic resonance (MR) imaging before, up to 2 hours, and 24 hours after injection (p.i.) of 200 mumol Fe/kg SHU 555 C (n= 6), ferumoxtran-10 (n = 6), or ferumoxytol (n = 6), using T2-2D-SE 100/20,40,60,80/90 (TR/TE/flipangle), T2*-3D-spoiled gradient recalled (SPGR) 100/15/38, and T1-3D-SPGR 50/1,7/60 sequences.

RESULTS

Quantitative signal to noise ratio and DeltaSI data of arthritic knees on T1- and T2*-weighted MR images showed no significant differences between the 3 USPIOs (P > 0.05). At 2 hours p.i., SNR and DeltaSI data were significantly increased from baseline on T1-weighted images and significantly decreased on T2*-weighted images (P < 0.001). At 24 hours p.i., the T1-enhancement returned to baseline, whereas the T2*-enhancement remained significantly elevated (P < 0.001). Immunostains demonstrated an USPIO compartmentalization in macrophages in the arthritic synovium.

CONCLUSIONS

Based on the relatively small number of animals in our study group, inflammation in antigen-induced arthritis can be equally detected and characterized with any of the three USPIOs evaluated.

Effect of Field Strengths on Magnetic Resonance Angiography

OBJECTIVES

We sought to compare the intravascular enhancement of an ultrasmall superparamagnetic iron oxide (USPIO) blood-pool contrast agent to gadopentetate dimeglumine for contrast-enhanced magnetic resonance angiography (CE-MRA) at field strengths of 1.5 and 3.0 T in rabbits.

MATERIALS AND METHODS

CE-MRA at 1.5 and 3.0 T was performed at several time points (50 seconds and 5, 10, 20, and 30 minutes) after the manual intravenous injection of 40 micromol Fe/kg body weight of an USPIO (SH U 555 C; Schering AG, Berlin, Germany) and 100 micromol/kg body weight gadopentetate dimeglumine (Magnevist; Schering AG, Berlin, Germany). MRA was performed with comparable acquisition parameters at both field strengths (Turbo-gradient sequence; 1.5 T: TR/TE/alpha: 5.5/1.7 milliseconds/40 degrees ; 3.0 T: TR/TE/alpha: 5.1/1.8 milliseconds/40 degrees ) on clinical imaging systems (both: Gyroscan Intera, Philips Medical Systems, Best, The Netherlands). At either field strength, 6 rabbits were studied with both contrast agents (n = 24 in total). Signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) were calculated from signal intensity measurements in the abdominal aorta.

RESULTS

Compared with 1.5 T, the SNR and CNR of gadopentetate dimeglumine significantly increased at 3.0 T by a factor of 2.2 and 2.3, respectively (P <or= 0.01), measured 50 seconds after intravenous injection. SNR and CNR of SH U 555 C, measured 50 seconds after intravenous injection, did not change significantly with increasing field strength (P >or= 0.05). At both field strength and either time point, CNR and SNR of SH U 555 C were significantly higher compared with gadopentetate dimeglumine at 3.0 T (P <or= 0.01).

CONCLUSIONS

SNR and CNR of gadopentetate dimeglumine significantly increased with increasing field strength. No SNR or CNR gain was observed for SH U 555 C. However, blood-pool MRA with SH U 555 C is feasible at 3.0 T. Compared with gadopentetate dimeglumine, SNR and CNR of SH U 555 C were significantly higher from 5 to 30 minutes at both field strengths (P <or= 0.01).

Ultrasmall superparamagnetic iron-oxide-enhanced MR imaging of normal bone marrow in rodents: original research original research

RATIONALE AND OBJECTIVES

The objective is to compare three different ultrasmall superparamagnetic iron oxides (USPIOs) for magnetic resonance (MR) imaging of normal bone marrow in rodents.

MATERIALS AND METHODS

Femoral bone marrow in 18 Sprague-Dawley rats was examined by using MR imaging before and up to 2 and 24 hours postinjection (PI) of 200 mumol of Fe/kg of SHU555C (n = 6), ferumoxtran-10 (n = 6), or ferumoxytol (n = 6), using T1-weighted (50 ms/1.7 ms/60 degrees = repetition time [TR]/echo time [TE]/flip angle) and T2*-weighted (100 ms/15 ms/38 degrees = TR/TE/flip angle) three-dimensional spoiled gradient recalled echo sequences. USPIO-induced bone marrow was evaluated qualitatively and quantified as signal-to-noise ratio (SNR) and change in signal intensity (DeltaSI) values. A mixed-effect model was fitted to the SNR and DeltaSI values, and differences among USPIOs were tested for significance by using F tests.

RESULTS

At 2 hours PI, all three USPIOs showed marked positive signal enhancement on T1-weighted images and a corresponding marked signal loss on T2*-weighted images. At 24 hours PI, the T1 effect of all three USPIOs disappeared, whereas T2*-weighted images showed persistent signal loss on SHU555C and ferumoxytol-enhanced MR images, but not ferumoxtran-10-enhanced MR images. Corresponding SNR and DeltaSI values on T2*-weighted MR images at 24 hours PI were significantly different from baseline for SHU555C and ferumoxytol, but not ferumoxtran-10.

CONCLUSION

All three USPIO contrast agents, ferumoxtran-10, ferumoxytol, and SHU555C, can be applied for MR imaging of bone marrow. Ferumoxtran-10 apparently reveals a different kinetic behavior in bone marrow than ferumoxytol and SHU555C.

Future Horizons in MR Imaging

In this article, we defined the major areas of active research in clinical MR imaging. Further increases in the number of parallel coils within an imaging array and in advances in parallel imaging pulse sequences and postprocessing will lead to further reductions in imaging time analogous to the impact of multidetector CT on helical CT. The synergism between parallel and high-field imaging will aid the development of high-field imaging. The combined dynamic and hepatic parenchymal enhancement of new contrast agents that have or may soon receive FDA approval will enable improved detection and characterization of liver lesions. The lymphotropic SPIO agents will remain an active area of clinical research to further assess their role in oncologic staging. Molecular imaging contrast research using magnetic particles and MR microscopy will continue to flourish. Screening examinations by MR imaging will re-main an area of research for the short- and intermediate term, with the final outcome dependent more on socioeconomic costs than the underlying capability of achieving high-quality screening studies.

Future directions in body magnetic resonance imaging

Technologic innovations in instrumentation and contrast agents naturally lead to new clinical and research applications in body MRI. Although long-range predictions of innovation are an uncertain process, short-term trends in development are more readily discernable. This review will provide examples of recent developments in magnetic resonance spectroscopic imaging, contrast agent development and molecular imaging, instrumentation, post-processing, and screening in an attempt to describe areas of active research.