Ultrasmall superparamagnetic iron oxide particles (AMI 227) as a blood pool contrast agent for MR angiography: experimental study in rabbits

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.

Designing of Co0.5Ni0.5GaxFe2-xO4 (0.0 ≤ x ≤ 1.0) Microspheres via Hydrothermal Approach and Their Selective Inhibition on the Growth of Cancerous and Fungal Cells

The current study offers an efficient design of novel nanoparticle microspheres (MCs) using a hydrothermal approach. The Co_0.5Ni_0.5Ga_xFe_2−xO₄ (0.0 ≤ x ≤ 1.0) MCs were prepared by engineering the elements, such as cobalt (Co), nickel (Ni), iron (Fe), and gallium (Ga). There was a significant variation in MCs’ physical structure and surface morphology, which was evaluated using energy dispersive X-ray analysis (EDX), X-ray diffractometer (XRD), high-resolution transmission electron microscopy (HR-TEM), and scanning electron microscope (SEM). The anti-proliferative activity of MCs was examined by MTT assay and DAPI staining using human colorectal carcinoma cells (HCT-116), human cervical cancer cells (HeLa), and a non-cancerous cell line—human embryonic kidney cells (HEK-293). Post 72 h treatment, MCs caused a dose dependent inhibition of growth and proliferation of HCT-116 and HeLa cells. Conversely, no cytotoxic effect was observed on HEK-293 cells. The anti-fungal action was assessed by the colony forming units (CFU) technique and SEM, resulting in the survival rate of Candida albicans as 20%, with severe morphogenesis, on treatment with MCs x = 1.0. These findings suggest that newly engineered microspheres have the potential for pharmaceutical importance, in terms of infectious diseases and anti-cancer therapy.

The Effect of Inversion Time on the Relationship Between Iron Oxide Nanoparticles Concentration and Signal Intensity in T1-Weighted MR Images

Background:

Magnetic nanoparticles have been widely applied in recent years for biomedical applications. Signal intensity (SI) of magnetic resonance (MR) images depends on the concentration of nanoparticles. It is important to find the minimum concentration of iron oxide nanoparticles that produces maximum SI and determines the minimum injection dose for clinical studies.

Objectives:

This study was performed to determine the relationship between the iron oxide nanoparticle concentration and SI using inversion recovery (IR) sequence in T₁-weighted MR images.

Materials and Methods:

Different concentrations of carboxydextran-coated iron oxide nanoparticles 20 nm in size were prepared. In vitro MR imaging was performed with inversion times (TI) of 100-400 ms (interval of 20 ms) and IR Turbo-FLASH (Turbo fast low angle shot) pulse sequence using a 1.5 T MRI system. Then the SI produced by each concentration of nanoparticles was measured and the minimum nanoparticle concentration that led to the maximum SI was determined. Coil non-uniformity was also considered for measuring the accurate SI of each image.

Results:

The results indicate that SI depended on the concentration of nanoparticles and TI. In addition, SI increased by increasing the TIs ranging from 200 to 400 ms for all studied concentrations. The linear relationship between the nanoparticle concentrations and SI that gave a square correlation coefficient (R²) equal to 0.99 was seen up to 76.83 µmol Fe/L in 400 ms for long TI and 239.16 µmol Fe/L in 200 ms for short TI.

Conclusions:

TI is an important parameter to consider in the relationship between SI and nanoparticle concentrations. An increase in TI leads to a decrease in the range of linearity.

Steady-state equilibrium phase inversion recovery ON-resonant water suppression (IRON) MR angiography in conjunction with superparamagnetic nanoparticles. A robust technique for imaging within a wide range of contrast agent dosages

PURPOSE

To investigate the ability of inversion recovery ON-resonant water suppression (IRON) in conjunction with P904 (superparamagnetic nanoparticles which consisting of a maghemite core coated with a low-molecular-weight amino-alcohol derivative of glucose) to perform steady-state equilibrium phase MR angiography (MRA) over a wide dose range.

MATERIALS AND METHODS

Experiments were approved by the institutional animal care committee. Rabbits (n = 12) were imaged at baseline and serially after the administration of 10 incremental dosages of 0.57-5.7 mgFe/Kg P904. Conventional T1-weighted and IRON MRA were obtained on a clinical 1.5 Tesla (T) scanner to image the thoracic and abdominal aorta, and peripheral vessels. Contrast-to-noise ratios (CNR) and vessel sharpness were quantified.

RESULTS

Using IRON MRA, CNR and vessel sharpness progressively increased with incremental dosages of the contrast agent P904, exhibiting constantly higher contrast values than T1 -weighted MRA over a very wide range of contrast agent doses (CNR of 18.8 ± 5.6 for IRON versus 11.1 ± 2.8 for T1 -weighted MRA at 1.71 mgFe/kg, P = 0.02 and 19.8 ± 5.9 for IRON versus -0.8 ± 1.4 for T1-weighted MRA at 3.99 mgFe/kg, P = 0.0002). Similar results were obtained for vessel sharpness in peripheral vessels, (Vessel sharpness of 46.76 ± 6.48% for IRON versus 33.20 ± 3.53% for T1-weighted MRA at 1.71 mgFe/Kg, P = 0.002, and of 48.66 ± 5.50% for IRON versus 19.00 ± 7.41% for T1-weighted MRA at 3.99 mgFe/Kg, P = 0.003).

CONCLUSION

Our study suggests that quantitative CNR and vessel sharpness after the injection of P904 are consistently higher for IRON MRA when compared with conventional T1-weighted MRA. These findings apply for a wide range of contrast agent dosages.

Histological study of the biodynamics of iron oxide nanoparticles with different diameters

The biodynamics of ultrasmall and small superparamagnetic iron oxide (USPIO and SPIO, respectively) particles that were injected intraperitoneally into 36 C57BL/6 mice were investigated chronologically. Their distribution was studied histologically at six time points by measuring iron-positive areas (μm²) in organ sections stained with Prussian blue. The uptake of the differently sized particles was also compared by cultured murine macrophages (J774.1). Iron-positive areas in the liver were significantly larger in the mice injected with USPIO than those injected with SPIO at the first three time points (P < 0.05). The amount of USPIO in the lung parenchyma around the airway was larger than that of SPIO at four time points (P < 0.05); distribution to the lymph nodes was not significantly different. The amount of iron was significantly larger in SPIO- than USPIO-treated cultured cells (P < 0.05). In conclusion, it is suggested that intra peritoneally injected USPIO particles could be used more quickly than SPIO to make Kupffer images of the liver and that both agents could help get lymph node images of similar quality.

Nanomedicine strategies for molecular targets with MRI and optical imaging

The science of 'theranostics' plays a crucial role in personalized medicine, which represents the future of patient management. Over the last decade an increasing research effort has focused on the development of nanoparticle-based molecular-imaging and drug-delivery approaches, emerging as a multidisciplinary field that shows promise in understanding the components, processes, dynamics and therapies of a disease at a molecular level. The potential of nanometer-sized agents for early detection, diagnosis and personalized treatment of diseases is extraordinary. They have found applications in almost all clinically relevant biomedical imaging modality. In this review, a number of these approaches will be presented with a particular emphasis on MRI and optical imaging-based techniques. We have discussed both established molecular-imaging approaches and recently developed innovative strategies, highlighting the seminal studies and a number of successful examples of theranostic nanomedicine, especially in the areas of cardiovascular and cancer therapy.

Conquering the dark side: colloidal iron oxide nanoparticles

Nanomedicine approaches to atherosclerotic disease will have significant impact on the practice and outcomes of cardiovascular medicine. Iron oxide nanoparticles have been extensively used for nontargeted and targeted imaging applications based upon highly sensitive T2* imaging properties, which typically result in negative contrast effects that can only be imaged 24 or more hours after systemic administration due to persistent blood pool interference. Although recent advances involving MR pulse sequences have converted these dark contrast voxels into bright ones, the marked delays in imaging from persistent magnetic background interference and prominent dipole blooming effects of the magnetic susceptibility remain barriers to overcome. We report a T1-weighted (T1w) theranostic colloidal iron oxide nanoparticle platform, CION, which is achieved by entrapping oleate-coated magnetite particles within a cross-linked phospholipid nanoemulsion. Contrary to expectations, this formulation decreased T2 effects thus allowing positive T1w contrast detection down to low nanomolar concentrations. CION, a vascular constrained nanoplatform administered in vivo permitted T1w molecular imaging 1 h after treatment without blood pool interference, although some T2 shortening effects on blood, induced by the superparamagnetic particles, persisted. Moreover, CION was shown to encapsulate antiangiogenic drugs, like fumagillin, and retained them under prolonged dissolution, suggesting significant theranostic functionality. Overall, CION is a platform technology, developed with generally recognized as safe components, that overcomes the temporal and spatial imaging challenges associated with current iron oxide nanoparticle T2 imaging agents and which has theranostic potential in vascular diseases for detecting unstable ruptured plaque or treating atherosclerotic angiogenesis.

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.

Long-circulating liposomal contrast agents for magnetic resonance imaging

Contrast-enhanced magnetic resonance imaging (CE-MRI) is a dynamic technique for imaging vasculature. However, the currently used gadolinium (Gd) chelates, such as Gd-DTPA, restrict the time window for image acquisition due to their rapid elimination from blood and their rapid diffusion into the extravascular space, which prevents their use in steady-state imaging, particularly for MR angiography (MRA). The goal of this study was to prepare long-circulating polyethylene glycol-bearing ((PEG)ylated) liposomes encapsulating Gd chelate, and characterize and demonstrate their utility for MRA. The liposomes were prepared by hydrating a mixture of lipids with gadodiamide (Omniscan). The liposomes were sized down to around 100 nm by extruder and exhaustively dialysed to remove the unencapsulated gadodiamide. The Gd liposomes exhibited a significant sustained (>4 hr) contrast enhancement of the vasculature with improved spatial details in a rat model with little leakage relative to Gd-DTPA controls as shown by MRI. We suggest that such long-circulating liposomal formulations allow for high spatial resolution imaging without the confounding effects of clearance and extravascular diffusion of the agent complicating the data and image analysis.

Characteristics of ultrasmall superparamagnetic iron oxides in patients with brain tumors

OBJECTIVE

The aim of this study was to evaluate the characteristics of an ultrasmall superparamagnetic iron oxides (USPIO) agent in patients with brain tumors and to correlate changes on MRI with histopathologic data collected systematically in all patients.

SUBJECTS AND METHODS

Nine patients with brain tumors were imaged before and 24 hr after administration of a USPIO at a dose of 2.6 mg Fe/kg. Analysis of MR images included qualitative and quantitative comparison of the USPIO and gadolinium enhancement of brain tumors. Brain surgery was performed 25-112 hr after administration of the USPIO. The histopathologic workup included iron histochemistry with diaminobenzidine (DAB)-enhanced Perls stain.

RESULTS

In seven of nine patients, USPIO-related changes of signal intensity were observed in gadolinium-enhancing brain tumors on T1- and T2*-weighted sequences. The difference in signal intensity on T1-weighted USPIO series was 40.1% +/- 26.7% (mean +/- SD). On T2*-weighted USPIO series, the difference in signal intensity was -33.1% +/- 18.4% in solid tumor parts. Areas of suspected radiation necrosis did not enhance in three patients with prior radiation therapy. Iron histochemistry revealed the presence of iron deposits in macrophages in two patients.

CONCLUSION

USPIO agents will not replace gadolinium in the workup of patients with brain tumors. Our findings suggest that USPIO agents seem to offer complementary information and may help to differentiate between brain tumors and areas of radiation necrosis. Signal intensity changes on T2*-weighted images might be related to the blood pool properties of the agent, possibly reflecting steady-state susceptibility effects.

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

The purpose of this study was to study different doses for first-pass and equilibrium phase MRA of aortoiliac vessels with a superparamagnetic iron oxide (SPIO) intravascular MR contrast agent (SH U 555 C) after single i.v. bolus injection. Sixteen healthy volunteers were prospectively enrolled into this single-blind, placebo-controlled clinical trial. SHU 555 C was injected as an i.v. bolus at stepwise increased dose levels of 5, 10, 20 and 40 micromol Fe/kg bodyweight (b.w.) corresponding to injection volumes of 0.01, 0.02, 0.04 and 0.08 ml/kg b.w. Serial high-resolution three-dimensional MRA of the aortoiliac vessels was acquired during first-pass and equilibrium, at 6 min intervals up to 42 min after contrast application using a breath-hold three-dimensional FLASH sequence on a 1.5 T scanner. Intravascular enhancement was calculated within the abdominal aorta and the inferior vena cava and a statistical analysis for significant differences in vessel enhancement was performed during the bolus and equilibrium phases. The visibility of vessels was ranked and effects of potential artifacts on image quality were graded for each time point and dose group. SH U 555 C showed a dose-dependent intravascular enhancement during the observation period (42 min). The highest dose of 40 micromol Fe/kg b.w. revealed the highest image quality during first-pass and equilibrium phases. The intravascular enhancement in the aorta increased dose-dependently from 5 to 40 micromol kg b.w. during first-pass and equilibrium phases (p<0.05). Intravascular signal inhomogeneities were observed at lower doses and decreased with increasing doses. First-pass MRA was diagnostic at doses of 10, 20 and 40 micromol Fe/kg b.w. For equilibrium MRA, a dose of 40 micromol Fe/kg b.w. was considered to be diagnostic. SH U 555 C proved to be a contrast agent with a high T1-effect suitable for both first-pass MRA comparable to gadolinium-enhanced MRA and high resolution equilibrium MRA up to 42 min post-injection (p.i.).

The utility of superparamagnetic contrast agents in MRI: theoretical consideration and applications in the cardiovascular system

This review will discuss the in vivo physical chemical relaxation properties of superparamagnetic iron oxide particles. Various parameters such as size, magnetization, compartmentalization and water exchange effects and how these alter the behavior of the iron oxide particles in an in vitro vs an in vivo situation with special reference to the cardiovascular system will be exemplified. Furthermore, applications using iron oxide particles for vascular, perfusion and viability imaging as well as assessment of the inflammatory status of a given tissue will be discussed.

Magnetic resonance molecular imaging with nanoparticles

Molecular imaging agents are extending the potential of noninvasive medical diagnosis from basic gross anatomic descriptions to complicated phenotypic characterizations based on the recognition of unique cell surface biochemical signatures. Although originally the purview of nuclear medicine, molecular imaging is now a prominent feature of most clinically relevant imaging modalities, in particular magnetic resonance (MR) imaging. MR nanoparticulate agents afford the opportunity not only for targeted diagnostic studies but also for image-monitored site-specific therapeutic delivery, much like the "magic bullet" envisioned by Paul Erhlich 100 years ago. Combining high-resolution MR molecular imaging with drug delivery will facilitate verification and quantification of treatment (ie, rational targeted therapy) and will offer new clinical approaches to many diseases.

Superparamagnetic iron oxide nanoparticles: nodal metastases and beyond

Superparamagnetic iron oxide (SPIO) nanoparticles are unique MR contrast agents and are of great interest for their multiple potentials. SPIO nanoparticles have a higher diagnostic accuracy for detecting metastatic lymph nodes than conventional MR studies, particularly in head and neck. The impact of this unique MR contrast agent on treatment decision of patients with head and neck cancer needs to be investigated in comparison with contrast-enhanced CT. As MR technology advances, the accuracy of SPIO nanoparticles for detection of metastasis certainly improves; thus, 1 day we may be able to reliably detect metastases in stage N0 patients, so that treatment strategy is established for each individual patient. This article presents physiologic properties of SPIO, technical considerations and diagnostic accuracy for imaging with SPIO, and other potential applications of SPIO agents.

First-pass contrast-enhanced magnetic resonance angiography in humans using ferumoxytol, a novel ultrasmall superparamagnetic iron oxide (USPIO)-based blood pool agent

PURPOSE

To evaluate the feasibility of first-pass contrast-enhanced magnetic resonance angiography (MRA) using ferumoxytol in humans.

MATERIALS AND METHODS

First-pass and equilibrium phase MRA were performed using ferumoxytol in one healthy volunteer and 11 patients with a fast three-dimensional spoiled gradient recalled (SPGR) pulse sequence. The examined vessels included carotid arteries, thoracic aorta, abdominal aorta, and peripheral arteries. A dose of either 71.6 micromol Fe/kg (n = 9), or 35.8 micromol Fe/kg (n = 3) was used. Based on a phantom study, the agent with initial concentration of 537.2 micromol Fe/mL was diluted by either four-fold (134.3 micromol Fe/mL) or eight-fold (67.1 micromol Fe/mL) for first-pass MRA.

RESULTS

All subjects completed their studies without adverse events. First-pass MRA showed selective arterial enhancement, with both arterial and venous enhancement on delayed acquisitions. Selective venous enhancement could be obtained by subtraction of arterial phase images from equilibrium phase images. The findings in ferumoxytol MRA were consistent with the results of original vascular tests.

CONCLUSION

Our preliminary experience supports the feasibility of first-pass MRA with ferumoxytol. Satisfactory arterial enhancement during first-pass imaging is obtained with injection of diluted contrast agent. With ferumoxytol, arteries and veins can be selectively depicted in a single exam.

Coronary arteries: contrast-enhanced MR imaging with SH L 643A--experience in 12 volunteers

PURPOSE

To assess SH L 643A for three-dimensional breath-hold and respiratory-gated magnetic resonance (MR) imaging in the depiction of coronary arteries.

MATERIALS AND METHODS

Twelve healthy male volunteers underwent either three-dimensional breath-hold (n = 6) or respiratory-gated (n = 6) coronary MR angiography before and after intravenous injection of 0.1 mmol SH L 643A per kilogram of body weight. For nonenhanced and contrast material-enhanced examinations, signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) measurements were obtained. Image quality was assessed in consensus with a five-point scale. Statistical analysis of nonenhanced and contrast-enhanced images was based on a two-tailed paired Student t test. A P value at the.05 significance level was used.

RESULTS

Overall statistically significant improvement in CNR was observed after administration of SH L 643A compared with that on nonenhanced images (8.7 +/- 5.3 [SD] vs 23.6 +/- 7.2, P <.01). While SNR of contrast-enhanced images showed improvement over that of nonenhanced images, the difference was not statistically significant (25.4 +/- 0.8 vs 30.2 +/- 16.8, P >.2). Image quality improved from a mean of 3.1 +/- 0.8 for nonenhanced images to 4.0 +/- 0.8 (P <.01) for contrast-enhanced images.

CONCLUSION

SH L 643A causes significant improvement of the blood-myocardium contrast enhancement at coronary MR angiography compared with that with nonenhanced sequences.

Angiogenesis in glioma: molecular mechanisms and roadblocks to translation

Gliomas are characterized by very high levels of neo-vascularization holding out the hope that therapies aimed at angiogenesis will have a significant impact on this intractable family of tumors. Intense research into the molecular mechanisms that drive the formation of new blood vessels in response to tumor growth has revealed a great deal of complexity, at the heart of which are competing pro- and anti-angiogenic influences. The relevant signaling pathways, and how they might be manipulated to interfere in the promotion of vessel growth are discussed. Several types of anti-angiogenic lead compounds are already in clinical trials, but assessing their impact on brain tumors is not straightforward. We discuss in depth some of the practical aspects of using imaging to more meaningfully follow tumor progression and response to treatment, which is particularly relevant to the use of therapies that target blood flow directly, which is fundamental to modern imaging modalities.

Preoperative breast cancer staging: MR imaging of the axilla with ultrasmall superparamagnetic iron oxide enhancement

PURPOSE

To evaluate magnetic resonance (MR) imaging with ultrasmall superparamagnetic iron oxide (USPIO) enhancement for preoperative axillary lymph node staging in patients with breast cancer by using histopathologic findings as the standard of reference.

MATERIALS AND METHODS

MR imaging was performed with a 1.5-T system within 24-36 hours after the start of intravenous slow-drip infusion of USPIO in 20 patients with breast cancer who were scheduled for surgery, followed by gadolinium-enhanced MR imaging. Lymph nodes were staged prospectively by using newly established criteria, and results were correlated with histologic findings.

RESULTS

In two patients, preoperative findings led to a change in therapeutic approach, and neoadjuvant chemotherapy was given; both patients were excluded from statistical analysis. Results of axillary staging with USPIO-enhanced MR imaging were true-positive in nine, true-negative in seven, false-positive in zero, and false-negative in two of 18 patients (sensitivity, 82%; specificity, 100%; positive predictive value, 100%; second reader, kappa = 1.0). Four hundred five lymph nodes were detected with MR imaging. For first and second readers, respectively, lymph node-based sensitivity was 83% and 73% and specificity was 96% and 97% (kappa = 0.68). USPIO as the intravascular contrast agent could not replace gadolinium for assessment of the primary tumor; however, no clinically relevant interaction was seen. Thus, an integrated imaging approach was feasible in all patients.

CONCLUSION

USPIO-enhanced MR imaging has the potential to become an adjunct to conventional MR imaging of the breast for preoperative assessment of axillary lymph nodes in patients with breast cancer.

Standardized MR protocol for the evaluation of MRA sequences and/or contrast agents effects in high-degree arterial stenosis analysis

PURPOSE

To investigate the relative role of high resolution (spatial or temporal) magnetic resonance angiography (MRA) sequence and of contrast agent properties in the evaluation of high-degree arterial stenosis.

METHODS

We qualitatively and quantitatively studied both 50 and 95% (300 microm diameter) stenosis of a 6 mm arterial phantom with two contrast agents (CA), Gd-DOTA (r(1)=2.9 mM(-1) s(-1)) versus P760 (r(1)=25 mM(-1) s(-1)) at several CA concentrations, including arterial peak concentration after injection of either a single or double dose of CA, using either a high temporal (booster) or high spatial (HR) resolution 3D MRA sequences. Experimental data were then compared to theoretical data.

RESULTS

With the 3D HR sequence, both visual and quantitative analysis were significantly better compared to the 3D booster sequence, at each phantom diameter. Quantitative analysis was significantly improved by injection of a double versus a single dose of each CA (Gd-DOTA or P760), primarily in high degree stenosis.

CONCLUSION

Combined MRA spatial resolution and high CA efficiency are mandatory to correctly evaluate high degree stenosis.

Contrast-enhanced blood-pool MR angiography with optimized iron oxides: effect of size and dose on vascular contrast enhancement in rabbits

PURPOSE

To investigate intravascular enhancement of bolus-injectable small and ultrasmall superparamagnetic iron oxides (USPIOs) of different particle sizes and relaxivities for first-pass and blood-pool magnetic resonance (MR) angiography.

MATERIALS AND METHODS

Iron oxides with different particle sizes (hydrodynamic diameters, 21, 33, 46, and 65 nm) were bolus injected intravenously at three doses (10, 20, and 40 micromol per kilogram body weight). An extracellular contrast agent (gadopentetate dimeglumine) served as a control. MR angiography was performed multiple times after intravenous injection (5-120 minutes and 24 hours later). Signal enhancement was calculated from signal intensity measurements in the abdominal aorta and renal and iliac arteries.

RESULTS

Highest enhancement was seen during the first pass with all contrast agents. USPIO enhancement in the abdominal aorta increased significantly with decreasing particle size (65 nm vs 33 nm, 65 nm vs 21 nm; P <.01).

CONCLUSION

The smallest iron oxide provided signal enhancement comparable with that of gadopentetate dimeglumine at 40 micromol iron per kilogram for first-pass investigations, with prolonged signal enhancement up to 25 minutes, allowing multiple measurements after injection of a single bolus.

MRA of the lower extremities in patients with pulmonary embolism using a blood pool contrast agent: initial experience

PURPOSE

To evaluate the feasibility of blood pool contrast-enhanced magnetic resonance angiography (MRA) to visualize the arterial and venous vessel tree and to detect deep venous thrombosis (DVT) of the lower extremities.

MATERIALS AND METHODS

Nine consecutive patients with pulmonary embolism (mean age = 46 +/- 9) were randomized to various doses of NC100150 (between 0.75 and 6 mg of Fe/kg of body weight). A T1-weighted (T1W) 3D gradient recalled echo (GRE) sequence (TE = 2.0 msec, TR = 5.0 msec) was used. Two observers blinded to the dose of contrast agent assessed image quality, contrast attenuation, and appearance of thrombi.

RESULTS

Qualitative assessment of overall MRA image quality and semiquantitative vessel scoring revealed good to excellent delineation of venous and arterial vessel segments independent of the dose of NC100150. However, quantitative region of interest analysis revealed a significantly higher signal-to-noise ratio (SNR) in the high-dose group than in the mid- and low-dose groups of NC100150 (P < 0.01). Between dose groups, the SNR was independent of vessel type (artery or vein) and vessel segment localization (proximal or distal). All seven venous thrombi (mean length = 7.2 +/- 0.95 cm) were characterized by a very low signal intensity (SI), which was only 16.6 +/- 7% of the SI in adjacent venous segments (P < 0.0001).

CONCLUSION

High-quality MR angiograms of the lower extremities can be obtained using low concentrations of NC100150 in combination with a strong T1W 3D GRE sequence. The obvious delineation of venous thrombi suggests that this technique may be potentially used as a noninvasive "one-stop shopping" tool in the evaluation of thromboembolic disease.

Coronary magnetic resonance angiography: experimental evaluation of the new rapid clearance blood pool contrast medium P792

The signal-enhancing characteristics of a new monodisperse monogadolinated macromolecular MR contrast medium (P792) were evaluated for magnetic resonance angiography (MRA) of the coronary arteries. A total of 15 cardiac examinations were performed in pigs at 1.5 T using a 3D gradient-echo sequence. Images were acquired during breath-hold before and up to 35 min after IV injection of Gd-DTPA (0.3 mmol Gd/kg), Gd-BOPTA (0.2 mmol Gd/kg), and P792 (13 micromol Gd/kg). An increase in the signal-to-noise ratio (SNR) of 97% +/- 17%, 108% +/- 37%, and 109% +/- 31% in coronary arteries and of 82% +/- 19%, 82% +/- 24%, and 28% +/- 18% in myocardium, respectively, was measured during the first postcontrast acquisition. The blood-to-myocardium signal-difference-to-noise ratio (SDNR) was significantly higher for P792 than for the other Gd compounds (P <.05) for up to 15 min after injection. Qualitative assessment showed that visualization of the coronary arteries and their branches was significantly better for P792 compared to the low-molecular Gd compounds (P <.05). The blood pool contrast medium P792 is well suited for MRA of the coronary arteries.

MR imaging characterization of microvessels in experimental breast tumors by using a particulate contrast agent with histopathologic correlation

PURPOSE

To define the diagnostic potential of magnetic resonance (MR) imaging enhanced with ultrasmall superparamagnetic iron oxide (USPIO) particles for the quantitative characterization of tumor microvasculature.

MATERIALS AND METHODS

NC100150 injection, a USPIO in clinical trials, and albumin-(Gd-DTPA)(30) were compared at MR imaging on sequential days in the same 19 rats with mammary tumors. Kinetic analysis of dynamic T1-weighted three-dimensional spoiled gradient-recalled imaging data with a two-compartment bidirectional model yielded MR imaging estimates of microvascular permeability (K(PS)) and fractional plasma volume (fPV) for each contrast medium.

RESULTS

Strongly positive and significant correlations were observed between MR imaging-derived K(PS )estimates and histologic tumor grade with either the soluble albumin-(Gd-DTPA)(30) (r = 0.88; P <.001) or larger particulate USPIO (r = 0.82; P <.001). A significant correlation (P <.05) was observed with each contrast medium between K(PS) and the histologic microvascular density (MVD), an angiogenesis indicator. Despite the considerable difference in molecule and particle sizes, no significant difference was observed in the MR imaging-derived mean permeability values generated with the two contrast media.

CONCLUSION

USPIO, a macromolecular particulate MR imaging contrast agent, can be applied successfully to characterize tumor microvessels in animals. USPIO-derived K(PS) correlated strongly with histopathologic tumor grade, MVD, and K(PS) values derived by using albumin-(Gd-DTPA)(30) in the same tumors.

New generation of monomer-stabilized very small superparamagnetic iron oxide particles (VSOP) as contrast medium for MR angiography: preclinical results in rats and rabbits

The purpose of this study was to evaluate the signal enhancement characteristics of very small superparamagnetic iron oxide particles (VSOP)-C63, a new monomer-coated, iron oxide-based magnetic resonance (MR) blood pool contrast medium with a very small particle size and optimized physical properties. Equilibrium MR angiography (MRA) of rats (thoracic and abdominal vessels) was performed at 1.5 T with a three-dimensional gradient-recalled echo (3D GRE) technique (TR/TE 6.6/2.3 msec, flip angle 25 degrees ) before and after (every 3-5 minutes up to 50 minutes) i.v. injection of VSOP-C63 [dosages: 15, 30, 45, 60, 75, and 90 micromol Fe/kg; diameter: 8 nm; relaxivities at 0.47 T: R1 = 30 l/(mmol * s); R2 = 39 l/(mmol * s)]. First-pass MRA images (3D-GRE, TR/TE 4.5/1.7 msec, flip angle 25 degrees ) were obtained with 45 micromol Fe/kg VSOP-C63 in comparison with 0.2 mmol Gd/kg of gadolinium diethylene triamine pentaacetic acid (Gd DTPA; before and every 5 seconds p.i.). MRA (3D GRE, TR/TE 4.5/1.7 msec, flip angle 25 degrees) of coronary vessels in rabbits was performed after i.v. injection of 45 micromol Fe/kg of VSOP-C63. In rats maximal S/N ratio in thoracic and abdominal arteries directly after i.v. injection of VSOP-C63 was 25 +/- 1, 43 +/- 2, 49 +/- 4, 57 +/- 3, 64 +/- 3, and 63 +/- 3 for the different dosages. Blood half-life was dose dependent (15 +/- 2, 20 +/- 3, 29 +/- 6, 37 +/- 5, 61 +/- 16, and 86 +/- 21 minutes). At a dose of 30 micromol Fe/kg even small intrarenal arteries were sharply delineated. First-pass MRA showed no significant difference in the S/N ratio between Gd-DTPA (71.5 +/- 11.5) and VSOP-C63 (65.1 +/- 18. 3). The proximal segments of the coronary arteries in rabbits were clearly depicted at a dose of 45 micromol Fe/kg. The monomer-coated, iron oxide-based contrast medium VSOP-C63 exhibits favorable properties as a blood pool agent for both equilibrium and first-pass MRA. J. Magn. Reson. Imaging 2000;12:905-911.

Characterization of ultrasmall magnetite [correction of paramagnetic magnetite] particles as superparamagnetic contrast agents in MRI

RATIONALE AND OBJECTIVES

Very small dextran-coated magnetite particles were developed. These particles can be used either as immunospecific contrast agents for MRI by coupling to antibodies or as an interstitial contrast agent.

METHODS

The particles were synthesized from iron chloride/dextran solutions. Size was evaluated by electron microscopy and photon correlation spectroscopy. The iron concentration was determined by x-ray spectroscopy. T1 and T2 values as well as relaxivities RI and R2 were evaluated with a clinical MR scanner at 1.5 T. Biocompatibility assays were performed with the cell line U937 in methylcellulose cultures.

RESULTS

Superparamagnetic, dextran-coated magnetite particles with a hydrodynamic diameter of 10 nm were developed. The iron core size was 7 nm; R1,7 L/mmol x s; and R2, 19 L/mmol x s. These particles are smaller than those currently available commercially and therefore show a smaller R1 to R2 ratio. Biocompatibility tests have shown no toxic side effects so far.

CONCLUSIONS

Ultrasmall magnetite particles with a dextran coating were developed; the physical properties of these particles evaluated in vitro are described in this study.

Pilot MR evaluation of pharmacokinetics and relaxivity of specific blood pool agents for MR angiography

RATIONALE AND OBJECTIVES

To evaluate the use of two new blood pool contrast agents (P760, P775) compared with a low-molecular-weight gadolinium chelate in MR angiography.

METHODS

The r1 efficiency of P760 was evaluated in vitro at 1.5 T; 3D abdominal contrast-enhanced MR angiography with qualitative analysis was compared in four rabbits after injection of incremental doses of P760 and in one rabbit after Gd-DOTA. A dynamic MR study was performed using a 2D T1-weighted turbo-flash MR sequence after injection of P760, P775, and Gd-DOTA. Each compound was tested at equivalent doses in three rabbits to assess r1 efficiency. Quantitative analysis of signal intensity in the aorta, the inferior vena cava, the renal cortex, and the medulla was performed.

RESULTS

In vitro, the r1 efficiency of P760 was 23.3 mmol(-1) x L x sec(-1) at 1.5 T. Injection of a dose of P760 10 times less than Gd-DOTA allowed similar vessel visualization. The signal intensity peak and first-pass contrast kinetics in the aorta and the inferior vena cava were similar with the three products. Compared with P760 and Gd-DOTA, P775 allowed a greater renal cortex signal intensity at the first pass and a faster decrease on delayed images.

CONCLUSIONS

The superior r1 efficiency of P760 and P775 was confirmed in vitro and in vivo at 1.5 T compared with Gd-DOTA, and P775 proved to be a rapid-clearance blood pool agent.

Blood pool contrast agents for cardiovascular MR imaging

The distribution and elimination of contrast agents is mainly determined by their size. First-pass perfusion with the use of blood pool contrast agents (BPCAs) and/or rapid clearance blood-pool-like contrast agents may allow quantitative myocardial perfusion evaluation in patients. This requires contrast bolus injection with a very fast injection speed. A major profit from BPCAs is expected for magnetic resonance angiography (MRA). The persistent signal-enhancing effects of BPCAs allow for a longer acquisition time window, which may be used to increase both the signal-to-noise ratio and/or image resolution. This is of paramount importance for coronary imaging, in which high-resolution imaging is desired. Moreover, the improved acquisition time window can be used to make multiple scans after one contrast injection. The role of ultrasmall paramagnetic iron oxide particles (USPIOs) for MRA is not clear yet, as they are limited by T2* effects at higher doses. Several safety aspects have to be taken into account before BPCAs are applied in humans, for whom toxicity caused by the injection speed is a concern.

Equilibrium phase MR angiography of the aortic arch and abdominal vasculature with the blood pool contrast agent CMD-A2-Gd-DOTA in pigs

Meglumine-carboxymethyldextran-ethylenediamino-gadoterate (CMD-A2-Gd-DOTA) was evaluated as a blood pool contrast agent for magnetic resonance angiography (MRA). MRA of large body vasculature was performed in seven pigs using a gradient-echo sequence at 1.5 T before and after 0.05 mmol/kg CMD-A2-Gd-DOTA injection. The signal- and contrast-to-noise ratios (SNRs, CNRs) were measured, as well as the pharmacokinetic clearance pattern. CMD-A2-Gd-DOTA visualized the vasculature with a high SNR and CNR for over 110 minutes after injection, but for the renal arteries the CNR was only significant within 15 minutes. Image quality was maximum within 15 minutes, producing enhancement (mean +/- SD) as follows: aortic arch 738 +/- 272%, abdominal aorta 393 +/- 123%, left renal artery 202 +/- 95%, right renal artery 248 +/- 107%, inferior vena cava 371 +/- 129%, and portal vein 513 +/- 145%, all P values < or =0.001. The clearance pattern was triphasic. Due to the excellent enhancement of vasculature without background enhancement, CMD-A2-Gd-DOTA is potentially a useful MR blood pool contrast agent for equilibrium phase MRA.

Approaches and agents for imaging the vascular system

Several classes of vascular imaging agents are described: (1) liposome-based blood cell mimetics; (2) plasma protein mimetics; (3) small molecules that bind to plasma proteins in the circulation. The characteristic features of the different agents are described and critically compared, including the advantages and potential pitfalls of each individual type.