Superparamagnetic iron oxide: pharmacokinetics and toxicity

Using superparamagnetic iron oxide-enhanced MRI to differentiate metastatic hepatic tumors and nonsolid benign lesions

OBJECTIVE

We evaluated the ability of superparamagnetic iron oxide (SPIO)-enhanced MRI to differentiate solid metastatic tumors and nonsolid benign lesions by clarifying the characteristic signal-intensity pattern of each lesion on SPIO-enhanced T2-weighted and heavily T1-weighted gradient-echo images.

MATERIALS AND METHODS

SPIO-enhanced MRI was performed using a 1.5-T system in 33 consecutive patients without cirrhosis who had 81 focal hepatic lesions (42 cysts, 13 hemangiomas, 26 metastatic tumors). The relative signal intensity of lesions on SPIO-enhanced heavily T1- and T2-weighted gradient-echo images was classified into one of the following three categories: high intensity, isointensity, or low intensity relative to the surrounding liver parenchyma. The diagnostic accuracy for differentiating solid metastatic tumors from nonsolid benign lesions (cysts or hemangiomas) was determined.

RESULTS

A combination of the relative signal intensity of the lesion on T2- and heavily T1-weighted gradient-echo images could be classified into the following five categories: high intensity and high intensity (category 1), high intensity and isointensity (category 2), high intensity and low intensity (category 3), isointensity and isointensity (category 4), and isointensity and low intensity (category 5). According to these categories, category 1 contained two hemangiomas, category 2 had 11 hemangiomas, category 3 had 25 metastatic tumors and two cysts, category 4 had three cysts, and category 5 had 37 cysts and one metastatic tumor. When a tumor with a relative signal intensity of categories 1 or 2 was considered to be a hemangioma (category 3 metastatic tumors and categories 4 and 5 cysts), diagnostic accuracy for characterizing such hepatic lesions was 96% (78/81).

CONCLUSION

When evaluating metastatic liver tumors on SPIO-enhanced MRI, we recommend that heavily T1- and T2-weighted gradient-echo images be obtained with our parameters to exclude hemangiomas or cysts.

Intracytoplasmic tagging of cells with ferumoxides and transfection agent for cellular magnetic resonance imaging after cell transplantation: methods and techniques

BACKGROUND

Superparamagnetic iron oxides (SPIO) are being used to label cells for in vivo monitoring by magnetic resonance imaging (MRI). The purpose of this study is to present protocols using SPIO and a polycationic transfection agent for magnetic labeling of cells as a basis for cellular MRI.

METHODS

Various concentrations of ferumoxides (FE)-poly-l-lysine (PLL) complexes were used to magnetically label cells. Iron incorporation into cells along with cell viability and short- and long-term toxicity were evaluated.

RESULTS

Rapidly growing cell suspension and adherent cells were effectively labeled by means of endocytosis into endosomes at low concentrations of FE (25 microg/mL media) and PLL (0.75 microg/mL media). Hematopoietic stem cells and lymphocytes required higher concentrations of PLL (1.5 microg/mL) in serum-free media during initial FE-PLL complex formation before labeling the cells in culture. Total iron concentration in cells depended on the cell type, concentration of FE-PLL complexes in media, cellular density, and incubation time. Iron concentrations after overnight incubation with given FE at 25 microg/mL media resulted in, for example, T cells being labeled with 1 to 3 pg/cell of intracytoplasmic endosomal iron and 15 to 20 pg/cell of intracytoplasmic iron in mesenchymal stem cells compared with 0.01 to 0.1 pg/cell for unlabeled cells. Protocols developed for this study demonstrated no adverse effect on the cell viability, functional capacity, or toxicity.

CONCLUSION

This technique can be used to label cells for in vivo MRI tracking of stem cells and lymphocytes. FE at a concentration of 25 to 50 microg/mL with a ratio of SPIO to PLL of 1:0.03 to 1:0.06 would be sufficient to label cells for cellular MRI.

Focal hepatic lesions: detection and characterization with combination gadolinium- and superparamagnetic iron oxide-enhanced MR imaging

PURPOSE

To compare gadolinium- and superparamagnetic iron oxide (SPIO)-enhanced magnetic resonance (MR) imaging for detection and characterization of focal hepatic lesions when different contrast agent administration sequences are used.

MATERIALS AND METHODS

Unenhanced, dynamic gadolinium-enhanced, and SPIO-enhanced hepatic MR images were obtained in 134 patients. SPIO-enhanced MR imaging was performed immediately after gadolinium-enhanced dynamic MR imaging in 50 patients, 1 day after gadolinium-enhanced dynamic MR imaging in 40 patients, and before gadolinium-enhanced dynamic MR imaging in 44 patients. Two radiologists independently reviewed the gadolinium image set (unenhanced and gadolinium-enhanced dynamic MR images) and the SPIO image set (unenhanced and SPIO-enhanced MR images) in random order. Lesion detection sensitivity and lesion characterization accuracy were compared by analyzing the area under the receiver operating characteristic curve (Az).

RESULTS

Overall lesion detection accuracy for pooled data was significantly higher with the SPIO set (Az = 0.903) than with the gadolinium set (Az = 0.857) (P <.05). When hypovascular lesions were excluded, the detection rate was similar with the two sets. When hepatocellular carcinomas were excluded, the detection rate was significantly higher with the SPIO set (P <.01). Readers were more accurate in differentiating benign from malignant lesions with the gadolinium set (Az = 0.915) than with the SPIO set (Az = 0.847) (P <.01). Detection accuracy tended to be better with the images obtained after the second contrast agent was used.

CONCLUSION

Hypovascular lesion detection was better with SPIO-enhanced MR images than with gadolinium-enhanced MR images. Detection and characterization of hypervascular lesions were improved with gadolinium-enhanced MR images.

Targeting of hematopoietic progenitor cells with MR contrast agents

PURPOSE

To label human hematopoietic progenitor cells with various magnetic resonance (MR) imaging contrast agents and to obtain 1.5-T MR images of them.

MATERIALS AND METHODS

Hematopoietic progenitor cells, labeled with ferumoxides, ferumoxtran, magnetic polysaccharide nanoparticles-transferrin, P7228 liposomes, and gadopentetate dimeglumine liposomes underwent MR imaging with T1- and T2-weighted spin-echo and fast field-echo sequences. Data were analyzed by measuring MR signal intensities and R1 and R2* relaxation rates of labeled cells and nonlabeled control cells. Mean quantitative data for the various contrast agent groups were assessed for significant differences compared with control cells by means of the Scheffe test. As a standard of reference, MR imaging data were compared with electron microscopic and spectrometric data.

RESULTS

For all contrast agents, intracellular cytoplasm uptake was demonstrated with electron microscopy and was quantified with spectrometry. When compared with nonlabeled control cells, progenitor cells labeled with iron oxides showed significantly (P <.05) increased R2*. Cells labeled with gadopentetate dimeglumine liposomes showed significantly increased R1. Detection thresholds were 5 x 10(5) cells for gadopentetate dimeglumine liposomes and ferumoxtran, 2.5 x 10(5) cells for ferumoxides and P7228 liposomes, and 1 x 10(5) cells for magnetic polysaccharide nanoparticles-transferrin.

CONCLUSION

Hematopoietic progenitor cells can be labeled with MR contrast agents and can be depicted with a standard 1.5-T MR imager.

Enhancing and targeting nucleic acid delivery by magnetic force

Insufficient contact of inherently highly active nucleic acid delivery systems with target cells is a primary reason for their often observed limited efficacy. Physical methods of targeting can overcome this limitation and reduce the risk of undesired side effects due to non-target site delivery. The authors and others have developed a novel means of physical targeting, exploiting magnetic force acting on nucleic acid vectors associated with magnetic particles in order to mediate the rapid contact of vectors with target cells. Here, the principles of magnetic drug and nucleic acid delivery are reviewed, and the facts and potentials of the technique for research and therapeutic applications are discussed. Magnetically enhanced nucleic acid delivery - magnetofection - is universally applicable to viral and non-viral vectors, is extraordinarily rapid, simple and yields saturation level transfection at low dose in vitro. The method is useful for site-specific vector targeting in vivo. Exploiting the full potential of the technique requires an interdisciplinary research effort in magnetic field physics, magnetic particle chemistry, pharmaceutical formulation and medical application.

Stem cells and cardiovascular disease

Several recent discoveries have shifted the paradigm that there is no potential for myocardial regeneration and have fueled enthusiasm for a new frontier in the treatment of cardiovascular disease-stem cells. Fundamental to this emerging field is the cumulative evidence that adult bone marrow stem cells can differentiate into a wide variety of cell types, including cardiac myocytes and endothelial cells. This phenomenon has been termed stem cell plasticity and is the basis for the explosive recent interest in stem cell-based therapies. Directed to cardiovascular disease, stem cell therapy holds the promise of replacing lost heart muscle and enhancing cardiovascular revascularization. Early evidence of the feasibility of stem cell therapy for cardiovascular disease came from a series of animal experiments demonstrating that adult stem cells could become cardiac muscle cells (myogenesis) and participate in the formation of new blood vessels (angiogenesis and vasculogenesis) in the heart after myocardial infarction. These findings have been rapidly translated to ongoing human trials, but many questions remain. This review focuses on the use of adult bone marrow-derived stem cells for the treatment of ischemic cardiovascular disease and will contrast how far we have come in a short time with how far we still need to go before stem cell therapy becomes routine in cardiovascular medicine.

Macrophage infiltration into the rat knee detected by MRI in a model of antigen-induced arthritis

Three-dimensional (3D) MR images were obtained from the knees of rats in a model of antigen-induced arthritis, elicited by the intraarticular administration of methylated bovine serum albumin (mBSA) to previously immunized rats. Superparamagnetic particles of iron oxide (SPIO) were administered i.v. 24 hr before each imaging session. Starting 4 days postantigen injection, images from arthritic knees exhibited distinctive signal attenuation in the synovium. This signal attenuation was significantly smaller in knees from animals treated with dexamethasone, a glucocorticosteroid, and completely absent in contralateral knees that had been challenged with vehicle. A significant negative correlation was found between the MRI signal intensity in the synovium and the histologically determined iron content in macrophages located in the same region. These results suggest the feasibility of detecting macrophage infiltration into the knee synovium in this model of antigen-induced arthritis by labeling the cells with SPIO. This readout could provide an early marker of disease progression, before more aggressive changes like cartilage and bone erosion take place. Monitoring early changes associated with arthritis can have an impact in preclinical studies by shortening the duration of the experimental period and by facilitating the investigation of novel immunomodulatory therapies acting on macrophages. Also, the approach can be potentially adapted to clinical studies.

The magnetofection method: using magnetic force to enhance gene delivery

In order to enhance and target gene delivery we have previously established a novel method, termed magnetofection, which uses magnetic force acting on gene vectors that are associated with magnetic particles. Here we review the benefits, the mechanism and the potential of the method with regard to overcoming physical limitations to gene delivery. Magnetic particle chemistry and physics are discussed, followed by a detailed presentation of vector formulation and optimization work. While magnetofection does not necessarily improve the overall performance of any given standard gene transfer method in vitro, its major potential lies in the extraordinarily rapid and efficient transfection at low vector doses and the possibility of remotely controlled vector targeting in vivo.

Atypical focal nodular hyperplasia of the liver: imaging features of nonspecific and liver-specific MR contrast agents

OBJECTIVE

The objective of our study was to describe the functional and differential uptake features of atypical focal nodular hyperplasia using different MR contrast agents and to evaluate their potential role in the diagnosis and characterization of focal nodular hyperplasia.

MATERIALS AND METHODS

Contrast-enhanced MR images of 45 patients with 85 focal nodular hyperplasia lesions were retrospectively reviewed. In these patients, sonographic findings were nonspecific (n = 37), or CT features were inconclusive (n = 8). Non-liver specific gadolinium chelates were used in 18 patients (48 lesions) suspected of having either focal nodular hyperplasia or hemangioma. The following liver-specific agents were used in patients with suspected focal nodular hyperplasia or metastases: mangafodipir trisodium, 30 patients (55 lesions); ferumoxides, six patients (16 lesions); and SHU 555 A, six patients (six lesions). Individual lesions were quantified by signal intensity and assessed qualitatively by homogeneity, contrast enhancement, and presence of a central scar.

RESULTS

At unenhanced MR imaging, the triad of homogeneity, isointensity, and central scar was found in 22% of the focal nodular hyperplasia lesions. On mangafodipir trisodium-enhanced T1-weighted images, all focal nodular hyperplasia lesions showed contrast uptake: in 64% of the lesions, uptake was equal to parenchyma; 25%, greater than the parenchyma; and 11%, less than the parenchyma. On iron oxide-enhanced T2-weighted images, all focal nodular hyperplasia lesions showed uptake of the contrast agent, but contrast uptake in the lesions was less than in the surrounding parenchyma. Dynamic gadolinium chelate-enhanced MR imaging showed early and vigorous enhancement of focal nodular hyperplasia lesions with rapid washout in 88%. Atypical imaging features of the lesions included hyperintensity on T1-weighted images, necrosis and hemorrhage, and inhomogeneous or only minimal contrast uptake.

CONCLUSION

For patients in whom the diagnosis of focal nodular hyperplasia cannot be established on unenhanced or gadolinium-enhanced MR imaging, homogeneous uptake of liver-specific contrast agent with better delineation of central scar may help to make a confident diagnosis of focal nodular hyperplasia.

Interstitial MR lymphography with MS-325: characterization of normal and tumor-invaded lymph nodes in a rabbit model

OBJECTIVE

The aim of our study was to evaluate the performance of a new blood-pool contrast agent, MS-325, in depicting regional lymph nodes when injected interstitially and in allowing the subsequent classification of the lymph nodes as normal or tumor-bearing (VX2 tumor).

MATERIALS AND METHODS

Six New Zealand white rabbits underwent adapted fast three-dimensional (3D) MR imaging before implantation of VX2 tumor cells in the flank and again 3 weeks after the implantation. For each imaging session, 0.5 mL of undiluted MS-325 was injected subcutaneously into both dorsal foot pads. For more than 120 min, the rabbits underwent repeated 3D MR imaging. The size of the individual lymph nodes and the amount of contrast agent uptake in the nodes were measured 5, 10, 15, 30, 60, and 120 min after the injection. After the rabbits had been sacrificed, their lymph nodes were removed and histopathologically analyzed. RESULTS. In normal as well as tumor-bearing hindlegs, the subcutaneous administration of MS-325 resulted in rapid delineation of popliteal, inguinal, iliac, and paraaortal lymph nodes. Tumor invasion into lymph nodes presented as circumscribed signal voids in the areas infiltrated by tumor, whereas the surrounding residual lymphatic tissue showed enhancement identical to that of normal nodes.

CONCLUSION

In addition to providing a safe means of displaying the normal lymphatic system, MS-325-enhanced 3D MR lymphography depicts direct tumor invasion in lymph nodes.

MRI of transgene expression: correlation to therapeutic gene expression

Magnetic resonance imaging (MRI) can provide high-resolution 3D maps of structural and functional information, yet its use of mapping in vivo gene expression has only recently been explored. A potential application for this technology is to noninvasively image transgene expression. The current study explores the latter using a nonregulatable internalizing engineered transferrin receptor (ETR) whose expression can be probed for with a superparamagnetic Tf-CLIO probe. Using an HSV-based amplicon vector system for transgene delivery, we demonstrate that: 1) ETR is a sensitive MR marker gene; 2) several transgenes can be efficiently expressed from a single amplicon; 3) expression of each transgene results in functional gene product; and 4) ETR gene expression correlates with expression of therapeutic genes when the latter are contained within the same amplicon. These data, taken together, suggest that MRI of ETR expression can serve as a surrogate for measuring therapeutic transgene expression.

Magnetite tattoos

BACKGROUND AND OBJECTIVES

Tattoo removal is a significant problem. The extraction of magnetite (Fe(3)O(4)) ink tattoos by a magnetic field was investigated, with and without Q-switched laser treatment.

STUDY DESIGN/MATERIALS AND METHODS

Magnetite particles (1.4 microm) were used to make mature, black skin tattoos on hairless albino rats. A Q-switched ruby laser (QSRL) 3.5 J/cm(2), 6.5-mm spot size, 40-nanosecond pulse width was used for treatment. Permanent magnets (1.4 T, 6-mm diameter) were tested to extract the magnetite particles, alone and after QSRL. Lightening of treated tattoos was measured from digital photographs, and the amount and distribution of magnetite in skin biopsies was scored blindly.

RESULTS

External application of magnets on mature magnetite tattoos without prior QSRL treatment, did not significantly extract, lighten, darken, or change their histologic appearance. A magnetic field applied immediately after QSRL treatment extracted some ink when epidermal injury was present, and caused significant redistribution of magnetite into the upper dermis with vertical banding along magnetic field lines. When applied for 3 weeks following QSRL, magnets caused darkening of tattoos.

CONCLUSIONS

Magnetite skin tattoos can be manipulated by external magnets, especially after Q-switched laser treatment. Magnetically-extractable tattoos may be feasible.

Preoperative detection of hepatocellular carcinoma: ferumoxides-enhanced versus mangafodipir trisodium-enhanced MR imaging

OBJECTIVE

The purpose of our study was to compare the diagnostic accuracy and lesion conspicuity of ferumoxides-enhanced MR imaging with those of mangafodipir trisodium-enhanced MR imaging for the preoperative detection of hepatocellular carcinoma.

SUBJECTS AND METHODS

Twenty-one patients with 39 hepatocellular carcinomas underwent ferumoxides-enhanced and mangafodipir trisodium-enhanced MR imaging. The diagnosis was established by pathologic examination after surgical resection in all patients. Five MR sequences were obtained 30 min after ferumoxides administration, and two MR sequences were obtained before and 15 min after mangafodipir trisodium administration. Three observers independently interpreted both MR images of all sequences on a segment-by-segment basis. The diagnostic accuracy of MR imaging was assessed using receiver operating characterizing analysis. Lesion (hepatocellular carcinoma > 10 mm in diameter)-to-liver contrast-to-noise ratio was calculated on MR images.

RESULTS

Ferumoxides-enhanced MR imaging (A(z) = 0.971) was significantly more accurate (p < 0.05) than mangafodipir trisodium-enhanced MR imaging (A(z) = 0.950). The mean sensitivity of ferumoxides-enhanced MR imaging (86%) was significantly greater (p < 0.05) than that of mangafodipir trisodium-enhanced MR imaging (44%) in lesions smaller than 10 mm. The mean lesion-to-liver contrast-to-noise ratio of hepatocellular carcinoma on ferumoxides-enhanced MR imaging (13.7 +/- 8.8) was significantly greater than on mangafodipir trisodium-enhanced MR imaging (5.4 +/- 5.1) (p < 0.01).

CONCLUSION

Ferumoxides-enhanced MR imaging has superior diagnostic accuracy in lesions smaller than 10 mm and superior lesion conspicuity compared with mangafodipir trisodium-enhanced MR imaging for the preoperative detection of hepatocellular carcinoma.

In vitro characterization of two different ultrasmall iron oxide particles for magnetic resonance cell tracking

RATIONALE AND OBJECTIVES

Comparison of two different ultrasmall superparamagnetic iron oxide (USPIO) particles in terms of their intracellular cell-labeling properties of macrophages and subsequent visualization by MR imaging.

MATERIALS AND METHODS

Cultures containing the macrophage cell line P-388D1 were incubated with a neutral carboxydextran-coated USPIO preparation (DDM 43/34/103) or an acidic citrate-coated USPIO (VSOP-C125). Experiments were performed in which incubation concentration and duration were varied and phagocytosis and pinocytosis suppressed by specific inhibitors. In cell culture specimens iron content was measured quantitatively and signal intensities determined by in vitro MR imaging.

RESULTS

VSOP-C125 is incorporated by cells much faster than DDM 43/34/103 and produces significantly higher final intracellular iron concentrations per cell (3420 vs. 727 ng/million cells). Both preparations show similar signal-reducing effects at MR imaging relative to the Fe content per cell. Intracellular USPIO has a much lower detection threshold at MR imaging (50/80 micromol/L) than extracellular USPIO in free solution (300 micromol/L).

CONCLUSIONS

Citrate-coated USPIO particles VSOP-C125 appear to have more favorable properties for magnetic labeling of macrophages than the carboxydextran-coated USPIO preparation DDM 43/34/103.

In vivo magnetic resonance tracking of magnetically labeled cells after transplantation

During the last few years, the therapeutic use of stem and progenitor cells as a substitute for malfunctioning endogenous cell populations has received considerable attention. Unlike their current use in animal models, the introduction of therapeutic cells in patients will require techniques that can monitor their tissue biodistribution noninvasively. Among the different imaging modalities, magnetic resonance (MR) imaging offers both near-cellular (i.e., 25- to 50-mu) resolution and whole-body imaging capability. In order to be visualized, cells must be labeled with an intracellular tracer molecule that can be detected by MR imaging. Methods have now been developed that make it possible to incorporate sufficient amounts of superparamagnetic iron oxide into cells, enabling their detection in vivo using MR imaging. This is illustrated for (neural stem cell-derived) magnetically labeled oligodendroglial progenitors, transplanted in the central nervous system of dysmyelinated rats. Cells can be followed in vivo for at least 6 weeks after transplantation, with a good histopathologic correlation including the formation of myelin. Now that MR tracking of magnetically labeled cells appears feasible, it is anticipated that this technique may ultimately become an important tool for monitoring the efficacy of clinical (stem) cell transplantation protocols.

Theory of nonexponential NMR signal decay in liver with iron overload or superparamagnetic iron oxide particles

A quantitative theory is proposed for the nonexponential NMR proton signal decay observed in liver with iron overload or superparamagnetic iron oxide particles. This effect occurs for Carr-Purcell-Meiboom-Gill (CPMG) sequences and is argued to be a direct consequence of the strong magnetic field inhomogeneities generated by the iron, rather than being due to tissue compartments. An approximate mathematical form is given for the signal decay, which is fit to experimental data for samples of rat liver with iron oxide particles, for samples of marmoset liver with hemosiderosis, and for in vivo human liver with hereditary hemochromatosis. The fitting parameters obtained are consistent with the pattern of iron deposition determined from histology. For the case of hereditary hemochromatosis, a good correlation is found between a parameter characterizing the nonexponential decay and the iron concentration. Implications for practical MR quantification of hepatic iron are discussed.

Contrast agents for hepatic magnetic resonance imaging

The current availability of liver-specific contrast media (LSCM) allows the possibility to obtain an accurate diagnosis when studying focal liver lesions (FLL). It is necessary to have an in-depth knowledge of the biologic and histologic characteristics of FLL and the enhancement mechanism of LSCM to gain significant accuracy in the differential diagnosis of FLL. It is possible to subdivide FLL into three main groups according to the kinetics of contrast enhancement: hypervascular FLL, hypovascular FLL, and FLL with delayed enhancement. Dynamic contrast-enhanced magnetic resonance imaging is an important tool in the identification and characterization of FLL. LSCM with a first phase of extracellular distribution give both dynamic (morphologic) and late phase (functional) information useful for lesion characterization. With LSCM it is possible to differentiate with high accuracy benign from malignant lesions and hepatocellular from nonhepatocellular lesions. To understand contrast behavior after injection of LSCM, it is necessary to correlate contrast enhancement with the biologic and histologic findings of FLL.

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.

Effect of resovist on rats with different severities of liver cirrhosis

RATIONALE AND OBJECTIVES

Whether the degree of diffuse hepatic damage is correlated with the signal change on MR images after injection of superparamagnetic iron oxide (SPIO) particles was investigated. In addition, we investigated whether hepatic function deteriorates after injection of SPIO.

METHODS

Seventy-six female Sprague-Dawley rats aged 3 to 4 weeks were given drinking water containing 0.03% thioacetamide (TAA) for 4 or 12 weeks to induce two grades of liver injury. Seventeen normal rats were served as a control. Normal and model rats were administered Resovist (10 micromol Fe/kg), and signal intensities in the liver on MR images obtained at 4.7 T were measured up to 60 minutes after injection (n = 5). The model rats were injected with 10 times the envisaged dose of Resovist (100 micromol Fe/kg) or saline as a control substance, and blood parameters were measured at 4, 24, and 48 hours after injection (n = 5 or 6). At 4 hours after injection, iron and Kupffer cells in the liver were stained (n = 3).

RESULTS

Maximal signal reduction in the liver occurred 15 minutes after injection in all groups. The reduced signal persisted for 60 minutes after injection. However, the degree of maximal signal reduction in the model rats was significantly less than that in the normal rats (P < 0.05, 0.01). Signal reduction in the 12-week group was less than that in the 4-week group. In control rats, the number of iron-positive cells increased by 22 cells per area (0.065 mm(2)) following treatment with Resovist. In the 4-week and 12-week groups, numbers of iron-positive cells increased by 13 and 11 cells, respectively. There was no statistically significant difference in the number of Kupffer cells between control and model rats. No significant change was observed in blood parameters with Resovist.

CONCLUSION

The MR signal induced by Resovist depended on the degree of phagocytic activity in the liver. The safety profiles of Resovist remained unchanged even at 10 times the imaging dose.

Annexin V-CLIO: a nanoparticle for detecting apoptosis by MRI

Annexin V, which recognizes the phosphatidylserine of apoptotic cells, was conjugated to crosslinked iron oxide (CLIO) nanoparticles, a functionalized superparamagnetic preparation developed for target-specific magnetic resonance imaging (MRI). The resulting nanoparticle had an average of 2.7 annexin V proteins linked per CLIO nanoparticle through disulfide bonds. Using camptothecin to induce apoptosis, a mixture of Jurkat T cells (69% healthy and 31% apoptotic) was incubated with annexin V-CLIO and was applied to magnetic columns. The result was an almost complete removal of the apoptotic cells (> 99%). In a phantom MRI experiment, untreated control cells (12% apoptotic cells, 88% healthy cells) and camptothecin-treated cells (65% apoptotic cells, 35% healthy cells) were incubated with either annexin V-CLIO (1.0, 0.5, and 0.1 microgram Fe/mL) or with unlabeled CLIO. A significant signal decrease of camptothecin-treated cells relative to untreated cells was observed even at the lowest concentration tested. Unmodified CLIO failed to cause a significant signal change of apoptotic cells. Hence, annexin V-CLIO allowed the identification of cell suspensions containing apoptotic cells by MRI even at very low concentrations of magnetic substrate. Conjugation of annexin V to CLIO affords a strategy for the development of a MRI imaging probe for detecting apoptosis.

Monomer-coated very small superparamagnetic iron oxide particles as contrast medium for magnetic resonance imaging: preclinical in vivo characterization

RATIONALE AND OBJECTIVES

Preclinical in-vivo characterization of a newly developed MR contrast medium consisting of very small superparamagnetic iron oxide particles (VSOP) coated with citrate (VSOP-C184).

METHODS

VSOP-C184 (core diameter: 4 nm; total diameter: 8.6 nm; relaxivities in water at 0.94 T (T1) 20.1 and (T2) 37.1 l/[mmol*sec]) was investigated to determine its pharmacokinetics, efficacy, acute single dose toxicity, repeated dose toxicity, and genotoxicity.

RESULTS

The plasma elimination half-life at 0.045 mmol Fe/kg was 21.3 +/- 5.5 minutes in rats and 36.1 +/- 4.2 minutes in pigs, resulting in a T1-relaxation time of plasma of < 100 milliseconds for 30 minutes in pigs. The particles are mainly cleared via the phagocytosing system of the liver. MR angiography at a dose of 0.045 mmol Fe/kg shows an excellent depiction of the thoracic and abdominal vasculature in rats and of the coronary arteries in pigs. The LD50 in mice is > 17.9 mmol Fe/kg. A good tolerance and safety profile was found.

CONCLUSIONS

The experiments indicate, that VSOP-C184 may be a well tolerated and safe contrast medium for MR imaging that can be effectively used for MR angiography including visualization of the coronary arteries.

Coronary MR angiography: experimental results with a monomer-stabilized blood pool contrast medium

PURPOSE

To evaluate the signal-enhancing characteristics of monomer-coated very small superparamagnetic iron oxide (SPIO) particles used as a blood pool contrast medium for magnetic resonance (MR) angiography in the coronary arteries.

MATERIALS AND METHODS

The particles used in this study were coated with citrate as the monomer (VSOP-C91). The particles have a total diameter of 7 nm and show the following relaxivities at 0.47 T: T1, 19 L/mmol. sec(-1); T2, 29 L/mmol. sec(-1). Fifteen cardiac MR examinations were performed at 1.5 T in five pigs. Images were acquired from immediately to 35 minutes (equilibrium phase) after intravenous injection of gadopentetate dimeglumine, gadobenate dimeglumine, and the very small SPIO particles (n = 5 for each substance).

RESULTS

Immediately after administration of gadopentetate dimeglumine, gadobenate dimeglumine, and the very small SPIO particles, respectively, increases in the signal-to-noise ratio in blood were 94%, 103%, and 102% and in myocardium were 83%, 83%, and 29% (P <.05, very small SPIO particles versus the low-molecular-weight gadolinium-based compounds). Differences in the blood-to-myocardium contrast-to-noise ratio and visualization of the coronary arteries and their branches were also significant.

CONCLUSION

VSOP-C91 significantly improves visualization of the coronary arteries at MR angiography from immediately to 35 minutes after injection.

Superparamagnetic iron oxide-enhanced magnetic resonance imaging of the liver in beagle dogs

The role of superparamagnetic iron oxide as a tissue-specific contrast medium has been established in humans, especially for hepatic imaging. Superparamagnetic iron oxide particles exhibit a tissue-specific biodistribution to the reticuloendothelial system, where they predominantly shorten transverse T2 relaxation time. Most hepatic tumors lack Kupffer cells; therefore, the T2 of tumors remains virtually unchanged after administration of superparamagnetic iron oxide. The resulting loss of signal intensity from the liver, with unchanged tumor signal intensity, increases lesion-to-liver contrast. In this study, MR images were acquired with fast gradient echo recalled at steady state (FGRE) in five Beagle dogs before and after injection of superparamagnetic iron oxide. The effect of superparamagnetic iron oxide on signal intensity of the liver with time was assessed. A signal intensity decrease of 65.7+/-10.0% was detected at 20 minutes, and it continued to decrease until the last time point of MR scanning (200 minutes). The liver intensity of all dogs dropped to half its value after 20 minutes. The effect of motion was minimized by breath holding. Superparamagnetic iron oxide did not have any adverse effects on the dogs.

USPIO-enhanced dynamic MRI: evaluation of normal and transplanted rat kidneys

To evaluate first-pass renal perfusion with ultrasmall superparamagnetic iron oxide (USPIO) particles by MRI, 40 normal rats (20 Dark Agouti (DA) rats and 20 Brown Norway (BN) rats) and 16 transplanted rats (12 allografts and four isografts) were studied on day 4 post-transplantation with different USPIO doses (3.0-18.1 mg Fe/kg/body weight). All animals underwent 128 consecutive snapshot fast low-angle shot (FLASH) coronal dynamic studies in 43 s. In the normal rats, a larger maximum signal decrease (MSD) in the cortex and the outer medulla is observed with an increasing dose of USPIO particles (P < 0.01). No significant differences were observed between the right and left kidneys at all doses studied. Higher MSD, time of occurrence of MSD (tMSD), and wash-in slope appear with higher doses of USPIO particles. The dynamic curves for DA rats show similar shapes when compared to those for BN rats. In the transplanted rats, allograft kidneys show lower MSD, longer tMSD, and lower wash-in slope compared to those in the normal kidneys. Isograft kidneys show perfusion patterns similar to those of normal kidneys in the cortex and the outer medulla. Histopathology indicates acute vascular rejection in all allografts and normal kidney architecture in all isografts. The results clearly show good agreement between the renal graft perfusion measurements and histopathological changes associated with rejection. This work also introduces a new signal analysis methodology for the automatic detection of transplanted organ rejection. This method compares the dynamics of the intrarenal signal intensities for native and transplanted kidneys. A quantitative measurement to detect significant differences between these signals was developed, and showed that this technique exhibits good performance in identifying renal rejection.

Magnetic labeling of activated microglia in experimental gliomas

Microglia, as intrinsic immunoeffector cells of the central nervous system (CNS), play a very sensitive, crucial role in the response to almost any brain pathology where they are activated to a phagocytic state. Based on the characteristic features of activated microglia, we investigated whether these cells can be visualized with magnetic resonance imaging (MRI) using ultrasmall superparamagnetic iron oxides (USPIOs). The hypothesis of this study was that MR microglia visualization could not only reveal the extent of the tumor, but also allow for assessing the status of immunologic defense. Using USPIOs in cell culture experiments and in a rat glioma model, we showed that microglia can be labeled magnetically. Labeled microglia are detected by confocal microscopy within and around tumors in a typical border-like pattern. Quantitative in vitro studies revealed that microglia internalize amounts of USPIOs that are significantly higher than those incorporated by tumor cells and astrocytes. Labeled microglia can be detected and quantified with MRI in cell phantoms, and the extent of the tumor can be seen in glioma-bearing rats in vivo. We conclude that magnetic labeling of microglia provides a potential tool for MRI of gliomas, which reflects tumor morphology precisely. Furthermore, the results suggest that MRI may yield functional data on the immunologic reaction of the CNS.

Time-to-echo optimization for spin echo magnetic resonance imaging of liver metastasis using superparamagnetic iron oxide particles

Superparamagnetic iron oxide (SPIO) particles are used as a contrast agent in liver magnetic resonance imaging (MRI). SPIO particles exert their greatest influence on T2-weighted MR signal intensity. The time-to-echo (TE) value that provides optimal contrast has not been systematically studied over the range of clinically relevant field strengths. The purpose of this study was to quantitatively evaluate the TE dependence of the post-SPIO tumor to liver contrast-to-noise ratio (CNR). The hypothesis was that there is a TE that provides an optimal CNR. Subjects having probable metastatic hepatic lesions secondary to colorectal carcinoma were studied. Pre- and post-SPIO images were acquired at TE-effective (TE(eff)) equal to 46, 76, and 106 msec by using a turbo spin echo pulse sequence at 0.2 T and 1.5 T. The CNR for all lesions greater than 1 cm in diameter was determined in pre- and post-SPIO images. A paired statistical design was used to identify TE-related CNR dependencies. The primary findings were as follows. (1) CNR differences attributable to TE(eff) variation over the range of 46-106 msec were less than 34%. For 0.2 T, TE(eff) = 46 msec yielded a statistically significantly greater CNR than did TE(eff) = 76 or 106 msec. The same was true at the higher field strength, but differences were not significant. (2) Signal-to-noise measures suggested that SPIO reduced the lesion signal. (3) Post-SPIO CNR was significantly greater at 1.5 T than at 0.2 T. The observations indicate that over the field strength range of 0.2-1.5 T, CNR differences attributable to the TE(eff) variation, while being statistically significant in some cases, are small relative to those resulting from the SPIO administration.

Hepatocyte-mediated transport to the bile of AMI-HS, a particulate contrast agent

RATIONALE AND OBJECTIVES

The elimination of hepatocyte-directed particulate contrast agents has not been studied in the same detail as particles eliminated mainly by the mononuclear phagocyte system. The aim of the present study was to elucidate the fate of these particles by a multidisciplinary approach.

METHODS

After intravenous injection of AMI-HS particles directed to the hepatocytes, rats were killed and cytological studies, by both electron microscopy and histochemistry, and spectroscopic studies of the bile were performed. The data were compared with a dynamic magnetic resonance study of the heart and liver.

RESULTS

The particles were rapidly cleared from the blood by Kupffer cells and hepatocytes and then found first in the vascular and later in the biliary pole of the hepatocytes. After 24 hours, a relaxometric characterization of the bile showed the presence of unchanged particles in the bile.

CONCLUSIONS

These results show the capacity of the liver to excrete unchanged AMI-HS particles directly into the bile.

A targeted contrast agent for magnetic resonance imaging of thrombus: implications of spatial resolution

A preparation of ultra-small superparamagnetic iron oxide (USPIO) particles coupled to an RGD peptide (RGD-USPIO) was investigated as an MR contrast agent, targeted to activated platelets, in both ex vivo and in vivo thrombus models. Thrombus visualization ex vivo was compared using RGD-USPIO and a non-targeted UPSIO. The influence of thrombus visualization on thrombus exposure time to RGD-USPIO (ex vivo) and on the spatial resolution of the MR image (ex vivo and in vivo) was assessed. RGD-USPIO resulted in better thrombus visualization than non-targeted USPIO ex vivo, and maximum enhancement was achieved after approximately one hour exposure time of the thrombus to RGD-USPIO. The ability to visualize the clots was highly dependent on the spatial resolution of the image. In vivo, an in-plane resolution of less than 0.2 x 0.2 mm(2) was required for good clot visualization after contrast enhancement. It is concluded that the achievable resolution and sensitivity is a potential limitation to the usefulness of active vascular targeting in MRI.

Magnetic resonance imaging of atherosclerotic plaque with ultrasmall superparamagnetic particles of iron oxide in hyperlipidemic rabbits

BACKGROUND

Based on the observation that ultrasmall superparamagnetic particles of iron oxides (USPIOs) are phagocytosed by cells of the mononuclear phagocytic system, the purpose of this study was to evaluate their use as a marker of atherosclerosis-associated inflammatory changes in the vessel wall before luminal narrowing is present.

METHODS AND RESULTS

Experiments were conducted on 6 heritable hyperlipidemic and 3 New Zealand White rabbits. 3D MR angiography (MRA) of the thoracic aorta was performed on all rabbits by use of a conventional paramagnetic contrast agent that failed to reveal any abnormalities. One week later, all rabbits except 1 of the hyperlipidemic animals were injected with a USPIO contrast agent (Sinerem, Guerbet) at a dose of 1 mmol Fe/kg. 3D MRA data sets collected over the subsequent 5 days showed increasing signal in the aortic lumen. Whereas the aortic wall of the control rabbits remained smooth and bright, marked susceptibility effects became evident on day 4 within the aortic walls of hyperlipidemic rabbits. Ex vivo imaging of aortic specimens confirmed the in vivo results. Histopathology documented marked Fe uptake in macrophages embedded in atherosclerotic plaque of the hyperlipidemic rabbits. Electron microscopy showed multiple cytoplasmic Fe particles in macrophages. No such changes were seen in control rabbits or in the hyperlipidemic rabbit that had not received Sinerem.

CONCLUSIONS

USPIOs are phagocytosed by macrophages in atherosclerotic plaques of the aortic wall of hyperlipidemic rabbits in a quantity sufficient to cause susceptibility effects detectable by MRI.

Nephrotoxic nephritis and obstructive nephropathy: evaluation with MR imaging enhanced with ultrasmall superparamagnetic iron oxide-preliminary findings in a rat model

PURPOSE

To evaluate the role of magnetic resonance (MR) imaging enhanced with ultrasmall superparamagnetic iron oxide (USPIO) in the evaluation and differentiation of different types of nephropathies.

MATERIALS AND METHODS

Two experimental rat models of nephropathies were studied: a model of nephrotoxic nephritis induced by means of intravenous injection of sheep anti-rat glomerular basement membrane serum (n = 43) and a model of obstructive nephropathy (n = 6). Imaging sessions were performed with a spectrometer operating at 4.7 T with fast low-angle shot, or FLASH, sequences. Signal intensity was measured in each kidney compartment before and 24 hours after intravenous injection of USPIO (90 micromol of iron per kilogram of body weight). MR findings were compared with histologic data and urine protein levels.

RESULTS

In the nephrotoxic nephritis model 24 hours after injection of USPIO, a significant signal intensity decrease (P: <.05) was present only in the cortex where the glomerular lesions were located. In the obstructive nephropathy model, the signal intensity decrease (P: <.05) was located in all kidney compartments in response to diffuse interstitial lesions. The decrease in signal intensity was due to iron uptake by either macrophages or mesangial cells gaining endocytic activity and was correlated, in the nephrotoxic nephritis model, to the degree of proteinuria.

CONCLUSION

Twenty-four-hour delayed USPIO-enhanced MR imaging may help identify and differentiate various types of nephropathies.

Characterization of focal hepatic lesions with ferumoxides-enhanced T2-weighted MR imaging

OBJECTIVE

The purpose of this study was to evaluate whether ferumoxides-enhanced MR imaging of focal hepatic lesions provides distinctive signal intensity and lesion-to-liver contrast changes for benign and malignant lesions, helping to further characterize and differentiate these lesions.

MATERIALS AND METHODS

Data analysis was performed on 70 patients, with previously identified focal hepatic lesions, who underwent MR imaging of the liver before and after IV administration of ferumoxides (10 micromol Fe/kg). Lesions analyzed with pathologically proven diagnoses included metastases (n = 40), hepatocellular carcinoma (n = 11), cholangiocarcinoma (n = 6), hemangioma (n = 4), focal nodular hyperplasia (n = 6), and hepatocellular adenoma (n = 3). Response variables measured and statistically compared included the percentage of signal-intensity change and lesion-to-liver contrast.

RESULTS

Focal nodular hyperplasia showed significant signal intensity loss on ferumoxides-enhanced T2-weighted images (mean, -43%+/-6.7%, p < 0.01). All other lesion groups showed no statistically significant change in signal intensity on ferumoxides-enhanced T2-weighted images, although signal intensity loss was seen in some individual hepatocellular adenomas (mean, -6.6%+/-24.0%) and hepatocellular carcinomas (mean, -3.3%+/-10.3%). All lesions, with the exception of hepatocellular carcinoma, had a marked increase in lesion-to-liver contrast on ferumoxides-enhanced T2-weighted images, which was statistically significant for metastases and hemangioma (p < 0.02).

CONCLUSION

Focal nodular hyperplasia shows significant decrease in signal intensity on ferumoxides-enhanced T2-weighted images, which may aid in the differentiation of focal nodular hyperplasia from other focal hepatic lesions. Other lesions, namely, hepatocellular adenoma and carcinoma, can have reticuloendothelial uptake, but usually to a lesser degree than that of focal nodular hyperplasia.

MR contrast agents in acute experimental cerebral ischemia: potential adverse impacts on neurologic outcome and infarction size

The purpose of the study was to investigate the effects of magnetic resonance (MR) contrast agents on neurologic outcome and infarction size in a rat stroke model. Focal cerebral ischemia was induced in 80 rats using an endovascular occlusion technique of the middle cerebral artery. Four hours after occlusion, 64 animals (4 groups of 16 each) received gadodiamide (Gd-DTPA-BMA) in a single (0.1 mmol/kg) or triple (0.3 mmol/kg) clinical dose or the ultrasmall superparamagnetic iron oxide particles contrast agent NC 100150 in a single (0.03 mmol/kg, 1.5 mg Fe(2+)/kg) or double (0.06 mmol/kg, 3.0 mg Fe(2+)/kg) clinical dose, respectively. Sixteen animals received equivolumetric saline (control group). Neurologic score and body weight were recorded every 8 hours. Twenty-four hours after vessel occlusion, infarction size was measured by 2,3, 5-triphenyl-tetrazolium-chloride (TTC) staining. Neither the normal nor the triple clinical dose of gadodiamide or NC 100150 in the single or double dose had any statistically significant effects on infarction volume, mortality, body weight, or neurologic outcome (P > 0.05). Our results suggest that bolus injection of gadodiamide and the ultrasmall superparamagnetic iron oxide particles NC 100150 in clinically relevant doses does not significantly affect infarction volume and clinical outcome of acute cerebral ischemia.

Tat peptide-derivatized magnetic nanoparticles allow in vivo tracking and recovery of progenitor cells

The ability to track the distribution and differentiation of progenitor and stem cells by high-resolution in vivo imaging techniques would have significant clinical and research implications. We have developed a cell labeling approach using short HIV-Tat peptides to derivatize superparamagnetic nanoparticles. The particles are efficiently internalized into hematopoietic and neural progenitor cells in quantities up to 10-30 pg of superparamagnetic iron per cell. Iron incorporation did not affect cell viability, differentiation, or proliferation of CD34+ cells. Following intravenous injection into immunodeficient mice, 4% of magnetically CD34+ cells homed to bone marrow per gram of tissue, and single cells could be detected by magnetic resonance (MR) imaging in tissue samples. In addition, magnetically labeled cells that had homed to bone marrow could be recovered by magnetic separation columns. Localization and retrieval of cell populations in vivo enable detailed analysis of specific stem cell and organ interactions critical for advancing the therapeutic use of stem cells.

Accuracy of pathologic techniques for the diagnosis of metastatic melanoma in sentinel lymph nodes

BACKGROUND

Sentinel lymph node (SLN) biopsy can accurately predict the presence of metastatic melanoma (MM) and has been used to identify patients with occult metastases. We present an analysis of the sensitivity and specificity of standard pathological techniques including intraoperative frozen section, permanent section, and immunohistochemistry in diagnosing MM within the SLN.

METHODS

Sixty-nine consecutive patients with primary malignant melanoma thickness of >1.0 mm or thinner lesions invading the reticular dermis (Clark level IV) who underwent SLN biopsy were reviewed. Lymph nodes were examined intraoperatively by frozen section (FS), permanent section (H&E), and by immunohistochemistry (IH) for S-100 protein and HMB45.

RESULTS

MM was found in 14 of 69 cases (20%). Permanent section H&E was performed in all cases, FS in 64 cases, and IH in 65 cases. FS analysis diagnosed MM in 4 of 14 cases (29%), was suspicious in 2 of 14 (14%), and falsely negative (FN) in 8 of 14 (57%) ultimately found to be positive with further workup. Within the FN group, MM was identified on review of the original FS slides in 3 of 8 cases (38%). Furthermore, within the FN group, the remaining 5 cases were identified as positive for MM by either permanent and/or deeper H&E sections and IH. IH alone with permanent H&E sections would have diagnosed MM in only 8 of 10 cases (80%) that were FS negative or suspicious. In no cases was MM identified by IH alone with the permanent and deeper H&E sections being negative. It is noteworthy that no false-positive cases were identified.

CONCLUSIONS

Intraoperative FS has low sensitivity in identifying MM within the SLN. IH alone does not increase the diagnostic yield. A combination of permanent H&E sections with deeper levels and S-100 and HMB45 IH dramatically increases the overall diagnostic sensitivity of SLN biopsy. Definitive diagnosis should await permanent H&E sections and IH staining.

Hepatic tumor imaging using iron oxide MRI: comparison with computed tomography, clinical impact, and cost analysis

BACKGROUND

The surgical management of hepatic tumors has traditionally relied on preoperative contrast-enhanced computed tomography (CECT) in combination with intraoperative ultrasonography (IOUS). Unfortunately, the ability to detect and characterize hepatic tumors by using CECT is limited, and IOUS frequently reveals additional disease that alters the operative approach. Recent advances in hepatic magnetic resonance imaging (MRI) may improve preoperative tumor detection and characterization; however, little is known about how MRI compares with CECT or about the clinical impact and cost considerations of liver MRI.

METHODS

A retrospective chart review was performed to compare iron oxide (Feridex [Fe])-MRI with CECT in the preoperative imaging of hepatic neoplasms, as well as to determine the clinical impact and overall healthcare costs associated with Fe-MRI.

RESULTS

Of approximately 1000 patients who underwent abdominal MRI at a single institution during a 20-month period, 57 were identified who underwent Fe-MRI evaluation of the liver. Indications for imaging included suspected metastases (n = 43), an indeterminate hepatic mass (n = 9), or primary hepatic cancer (n = 5). Overall, Fe-MRI identified a total of 157 lesions (mean, 2.75 per patient; range, 0-14). CECT was performed in 50 patients, of whom 35 had primary or metastatic cancer. Fe-MRI identified more lesions than CT (n = 136 vs. 77; P = .016), and the average size of lesion detected by Fe-MRI was significantly smaller than that by CECT (2.5 vs. 3.4 cm; P = .018). Comparison of CECT and Fe-MRI findings with IOUS and pathological specimens showed a significant difference in sensitivity (MRI, 86%; CECT, 58%; P<.001), and IOUS changed the operative approach in only 5% of those imaged with Fe-MRI. Overall, Fe-MRI altered the clinical management in 67% of patients imaged (n = 38 of 57), which corresponded to an overall net cost savings of $108,368 ($1,901 per patient).

CONCLUSIONS

Fe-MRI is a powerful imaging technique, with greater hepatic tumor detection sensitivity than CECT. Moreover, it is an economically feasible imaging method that will alter the clinical management in most patients imaged.

Dose-dependency and rate of decay of efficacy of Resovist on MR images in a rat cirrhotic liver model

RATIONALE AND OBJECTIVES

To investigate the dose-dependency and half-life of decay of the signal reduction by super-paramagnetic iron oxides on three different spin-echo images in a rat model of cirrhosis.

METHODS

For normal and cirrhotic liver, signal intensities were measured 15 minutes after injection of Resovist (range, 1-40 mumol Fe/kg) on three different spin-echo images. Subsequently, recovery of signal intensity was monitored up to 2 weeks after injection.

RESULTS

The dose-dependency of efficacy was somewhat less at all doses and imaging parameters in cirrhotic liver. However, the submaximal effect was obtained at a dose of 20 mumol Fe/kg, the same as in normal liver. The rate of decay of the efficacy in cirrhotic liver was similar to or faster than that in normal liver.

CONCLUSIONS

The dose-dependency and rate of decay of efficacy of Resovist in cirrhosis were similar to those in normal liver, although the efficacy was less in cirrhotic liver and both the dose-dependency and rate of decay of efficacy were dependent on imaging parameters.

Contrast-enhanced MR imaging of two superparamagnetic RES-contrast agents: functional assessment of experimental radiation-induced liver injury

The purpose of this study was to compare liver contrast-enhancing characteristics of two superparamagnetic reticuloendothelial system (RES)-directed agents with different particle sizes, polycrystalline iron oxide nanocompounds (PION) and carboxydextran-coated maghemite (DDM128N/389, later referred to as DDM128), in an experimental model of focal radiation-induced hepatitis. PION, for the small particle size (31 nm), and DDM128, for the large particle size (59 nm), RES-directed agents were compared for liver enhancement after radiation-induced liver injury. A single x-irradiation exposure varying from 10 to 60 Gy was delivered to one side of the liver. T2-weighted spinecho magnetic resonance imaging was performed 3 days after x-irradiation at 30 minutes post-contrast. Using the RES-directed PION, the normal, non-irradiated portion of the liver decreased in signal intensity with a maximum negative enhancement of -66%, while the irradiated portion of the liver decreased in signal intensity by -24% (60 Gy). The signal intensity decline of irradiated liver tissue using PION was dose dependent, but was found at all radiation dose levels (10-60 Gy). The difference in signal intensity between irradiated (-63%) and non-irradiated (-82%) portions was also statistically different using DDM128 at 60 Gy. However, lower irradiation doses (10 and 30 Gy) failed to produce a statistically significantly different enhancement in the irradiated and non-irradiated portion of the liver. Sensitivity of liver enhancement with RES-directed agents is size dependent. The smaller particle (PION) is more sensitive for detection of radiation-induced hepatitis than the larger particle (DDM128). The relative insensitivity of DDM128 enhancement for diffuse liver injury will be clinically advantageous for detecting focal lesions in the presence of diffuse hepatic injury.

Delivery of diagnostic agents for magnetic resonance imaging

A review of contrast agents used for magnetic resonance imaging was made with regard to methods of drug delivery using published literature. Since the clinical approval of Gd-DTPA in 1988, there has been extensive research towards developing organ- and tissue-specific contrast agents. Targeting strategies have consistently improved along with improvements in nuclear medicine imaging, and a broad spectrum of potential agents has accumulated. Liver, blood-pool targeted, and, due to their inherent convenience of delivery, intraorally administered gastrointestinal agents have been developed or are being developed. For intravenous contrast agents, collective magnetic labels with modifications for some specificities results in the larger-sized agents which can be an obstacle for the agent in accessing the targeted cells. In conclusion, the next step in the development of specific contrast agents for clinical use is to improve non-specific delivery to the extra-capillary space adjacent to targeted cells.

Early detection of radiation-induced liver injury in rat by superparamagnetic iron oxide-enhanced MR imaging

The detectability of early liver injury induced by irradiation was studied using magnetic resonance (MR) imaging enhanced with superparamagnetic iron oxide (SPIO), a tissue-specific contrast agent against the reticuloendothelial system (RES). In rat, 3 days after focal irradiation (0-10 Gy), MR imaging was performed and specimens were obtained to observe the phagocytic function of RES. The irradiated portion of the liver was visualized with a clear demarcation from the nonirradiated part by SPIO-enhanced MR images as a decrease in negative enhancement reflecting the function of RES (P < 0.05), whereas this was impossible with nonenhanced MR images. Significant regression was observed as a dose-related change of the signal intensity in the irradiated portion on SPIO-enhanced MR images (R = 0.867, P < 0.0001). SPIO-enhanced MR imaging was reliable for detecting the range and extent of liver injury a few days after low-dose irradiation, and it may be a useful procedure for verifying the target area in clinical cases of radiation therapy.

MRI with superparamagnetic iron oxide: efficacy in the detection and characterization of focal hepatic lesions

The purpose of this study was to evaluate the potential of superparamagnetic iron oxide particles (SPIO) as tissue specific contrast agent in magnetic resonance (MR) imaging in detection and characterization of focal hepatic lesions. We investigated 45 patients with focal hepatic lesions. T1-weighted SE (TR 650/TE 15 ms) and T2-weighted SE (TR 2015-2030/TE 45 and 90 ms) unenhanced images were obtained. After SPIO application we performed T1-weighted images with and T2-weighted images with and without fat suppression using the same image parameters. Liver signal intensity decreased by 74% (min 47%, max 83%) on T2-weighted images after application of the contrast agent. Benign lesions (FNH, adenoma) showed an average signal drop of 40% (min 20%, max 47%) whereas malignant lesions showed no significant change of signal intensity on post-contrast images. The mean tumor-to-liver contrast-to-noise ratio (C/N) was improved in all post-contrast sequences irrespective of the lesion type. An additional increase of tumor-to-liver contrast by use of fat suppression technique could be established in the slightly T2-weighted sequence (TE 45 ms). In metastases, divided in different size groups, we could determine a significant size relation of tumor-to-liver C/N. After SPIO application the number of detected lesions increased distinctly, especially small foci are more easily demonstrated. SPIO particles are a efficacious contrast agent for MR examinations of the liver. For tumor characterization T1- and T2-weighted pre- and post-contrast images are necessary. The T1-weighted sequences are helpful to differentiate benign lesions such as cysts and hemangiomas from malignant lesions. Detection and differential diagnoses of hepatic lesions are improved by use of the SPIO-particles.

The rete mirabile of the eel: a useful model for the study of transcapillary passage of MR contrast agents

Our purpose was to study the capillary leakage of MR contrast media using a pure capillary model, the rete mirabile of the eel. The rete is a countercurrent-exchange organ composed of an arterial and a venous capillary system that can be catheterized and perfused. Substances are introduced at the arterial input by a constant infusion, and their steady-state concentrations are measured at the arterial and venous outputs. The capillary leakage of four MR contrast agents--Gd-DOTA(MW = 561 D), carboxymethyldextran-Gd-DTPA (MW = 38,900 D), albumin-Gd-DTPA (MW = 92,000 D), AMI-227 (400,000 D<MW<900,000 D)--was characterized by reference to radioactive tracers (3HHO, 22Na, 14C-sucrose, 125I-albumin) by two parameters. These parameters were the concentration ratio of the venous output over the arterial input [C(VOUT)(%)] and the permeability coefficient (P). The transcapillary pathway mechanisms for carboxymethyldextran-Gd-DTPA and albumin-Gd-DTPA were studied by electron microscopy. P values for Gd-DOTA (9.4+/-3.6 x 10(-7) cm/s) and albumin-Gd-DTPA (11.8+/-5.5 x 10(-7) cm/s) were close to P values for 14C-sucrose, while P values for carboxymethyldextran-Gd-DTPA (6.4+/-4.9 x 10(-7) cm/sec) were similar to P values for 125I-albumin. The lowest permeability was observed with AMI-227 (2.7+/-2 x 10(-7) cm/sec). Vesicular transport was demonstrated for carboxymethyldextran-Gd-DTPA and albumin-Gd-DTPA. The transcapillary passage of several MR contrast agents can be characterized with the rete mirabile model. Molecular weight is the major factor influencing transport.

Ferumoxides-enhanced quantitative magnetic resonance imaging of the normal and abnormal bone marrow: preliminary assessment

The purpose of our study was to assess the effects of intravenous administration of ferumoxides on normal and abnormal vertebral bone marrow T1 and T2 relaxation times. Changes in bulk T1 and T2 relaxation times induced by intravenous administration of ferumoxides were determined in the normal vertebral marrow of two healthy subjects and four patients. In the four patients, changes in bulk T1 and T2 values induced by furomoxides injection were also determined in 12 vertebral metastases. Relative to precontrast relaxation time values, bulk T1 and T2 values of normal bone marrow had declined by a mean of 24% and 19%, respectively, in the two subjects and the four patients 45 minutes after ferumoxides administration. Relative to precontrast values, bulk T1 and T2 values of abnormal bone marrow had decreased by a mean of 16% and 2%, respectively. Decreases in bulk T1 and T2 values in normal bone marrow and in bulk T1 values in metastases were statistically significant (P<0.001). Changes in bulk T2 values observed in metastases were not statistically significant. Quantitative MRI demonstrates that ferumoxides infusion induces a decrease in bulk T1 and T2 relaxation times of normal bone marrow. It also suggests a lack of T2 shortening in bone metastases.

Safety and preliminary findings with the intravascular contrast agent NC100150 injection for MR coronary angiography

In this Phase I clinical study, a novel ultrasmall superparamagnetic iron oxide contrast agent, NC100150 Injection (Nycomed Imaging, Oslo, Norway, a part of Nycomed Amersham), was used in two-dimensional magnetic resonance coronary angiography (MRCA). Safety and imaging data were acquired from 18 healthy male volunteers at both 0.5 and 1.5 T, before and after the administration of NC100150 Injection. Through-plane and in-plane images of the right coronary artery were analyzed. The postcontrast imaging sequences used prepulses and a high flip angle, to introduce T1 weighting. At 1.5 T (TE 2.6 msec), the through-plane coronary artery signal-to-noise ratio (SNR) (P = 0.04), coronary artery-to-fat signal difference-to-noise ratio (SDNR) (P = 0.001), coronary artery-to-myocardium SDNR (P<0.001), and coronary artery delineation (P<0.001) were improved by the administration of NC100150 Injection. For in-plane imaging, coronary artery delineation improved, but there were no significant changes in the SNR and SDNR. At 0.5 T, with the longer TE (6.7 msec) imaging sequence used, there was a reduction in the SNR (P = 0.01), the fat SDNR (through-plane P = 0.02; in-plane P = 0.25), and the coronary artery diameter (P<0.01 in both imaging planes). There was a trend toward improvement in the myocardial SDNR and coronary artery delineation. In conclusion, NC 100150 Injection was given safely to 18 healthy subjects, with no major adverse reactions. Coronary artery delineation was improved in both imaging planes at 1.5 T, with a trend toward improvement at 0.5 T. At 1.5 T, with a short TE imaging sequence, the marked T1 shortening effects of NC100150 Injection were dominant, leading to an improvement in the quantitative parameters for the through-plane images. At 0.5 T, with a longer TE imaging sequence, the T2* effects of the contrast agent played a role in reducing the quantitative image parameters. With further optimization of imaging sequences, to take advantage of the long-lived intravascular T1 shortening effect of NC100150 Injection, further improvements in MRCA will be possible.

Hepatic lesion detection: comparison of MR imaging after the administration of superparamagnetic iron oxide with dual-phase CT by using alternative-free response receiver operating characteristic analysis

PURPOSE

To compare the performance of magnetic resonance (MR) imaging after the administration of superparamagnetic iron oxide (SPIO) and dual-phase computed tomography (CT) in the depiction of liver metastases.

MATERIALS AND METHODS

Fifty-one hepatic resection candidates with known colorectal metastases were examined. MR imaging comprised fast spin-echo (SE) T2-weighted imaging, T1-weighted gradient-echo (GRE) fast low-angle shot imaging before SPIO enhancement, dual-echo SE imaging, T2-weighted fast low-angle shot imaging, and T1-weighted GRE imaging after SPIO enhancement. CT was performed with 8-mm collimation and 1:1 pitch; imaging commenced 20 seconds and 65-70 seconds after injection of 150 mL of contrast medium. All images were reviewed independently by four blinded observers. The alternative-free response receiver operating characteristic (ROC) method was used to analyze the results, which were correlated with findings from surgery, intraoperative ultrasonography, and histopathologic studies in 31 patients and with consensus review together with all other imaging and clinical follow-up in 20 patients. Sensitivities were also calculated.

RESULTS

The mean sensitivity of MR was significantly higher than that of CT (p < .02): 79.8% for MR and 75.3% for CT for all lesions, and 80.6% for MR and 73.5% for CT for malignant lesions. The mean areas under the alternative-free response ROC curves were 0.83 for MR and 0.78 for CT (difference not significant).

CONCLUSION

SPIO-enhanced MR imaging was more sensitive than dual-phase CT in the depiction of colorectal metastases.