Neuroimaging: do we really need new contrast agents for MRI?

In vivo magnetic nanoparticle hyperthermia: a review on preclinical studies, low-field nano-heaters, noninvasive thermometry and computer simulations for treatment planning

Magnetic nanoparticle hyperthermia (MNH) is a promising nanotechnology-based cancer thermal therapy that has been approved for clinical use, together with radiation therapy, for treating brain tumors. Almost ten years after approval, few new clinical applications had appeared, perhaps because it cannot benefit from the gold standard noninvasive MRI thermometry technique, since static magnetic fields inhibit heat generation. This might limit its clinical use, in particular as a single therapeutic modality. In this article, we review the in vivo MNH preclinical studies, discussing results of the last two decades with emphasis on safety as a clinical criteria, the need for low-field nano-heaters and noninvasive thermal dosimetry, and the state of the art of computational modeling for treatment planning using MNH. Limitations to more effective clinical use are discussed, together with suggestions for future directions, such as the development of ultrasound-based, computed tomography-based or magnetic nanoparticle-based thermometry to achieve greater impact on clinical translation of MNH.

Influence of Oxidation Degree of Graphene Oxide on Its Nuclear Relaxivity and Contrast in MRI

Graphene oxide (GO) serves as a versatile platform for various applications, with the oxygen content of GO playing an important role in governing its properties. In the present study, different GO types covering a wide range of oxidation degree were prepared using our newly developed two-step method involving ball milling of graphite followed by its oxidation to GO. In addition to the variations in their physicochemical properties, the different GO types exhibited differences in proton relaxivity due to their paramagnetic nature. Nuclear magnetic resonance spectroscopy studies showed that the degree of oxidation of GO perturbs its nuclear relaxation properties and, together with intercalated Mn2+ ions, provides large contrast variation in magnetic resonance imaging (MRI). The study for the first time reveals that the surface chemistry of GO affects its relaxivity and opens up new avenues for developing tunable GO-based contrast agents in magnetic resonance imaging for diagnostics and therapies.

Contrast Extravasation versus Hemorrhage after Thrombectomy in Patients with Acute Stroke

BACKGROUND AND PURPOSE

Intra-arterial recanalization postprocedural imaging in stroke patients can result in diagnostic complications due to hyperdensities on noncontrast computed tomography (CT), which may represent either contrast extravasation or intracranial hemorrhage. If these lesions are hemorrhage, then they are risk factors becoming symptomatic, which, if not distinguished, can alter clinical management. We investigate the effects of iodinated contrast on postprocedural magnetic resonance imaging (MRI) and prevalence of equivocal imaging interpretations of postprocedural extravasated contrast versus hemorrhage while identifying protocol pitfalls.

METHODS

We identified 10 patients diagnosed with ischemic stroke who underwent intra-arterial recanalization in a 5-year period. These patients demonstrated a hyperdensity on a postprocedural CT within 24 hours, underwent an MRI within 48 hours, and an additional confirmatory noncontrast CT at least 72 hours postprocedure.

RESULTS

Postprocedural MRI in all 10 stroke patients demonstrated T1 - and T2 -relaxation time changes due to residual iodine contrast agents. This lead to false positive postprocedural hemorrhage MRI interpretations in 2/10 patients, 3/10 false negative interpretations of contrast extravasation, and 5/10 equivocal interpretations suggesting extravasation or hemorrhage. Of these five cases, two were performed with gadolinium.

CONCLUSION

MRI done within 48 hours postprocedure can lead to false positive hemorrhage or false negative contrast extravasation interpretations in stroke patients possibly due to effects from the administered angiographic contrast. Additionally, MRI should be done both after 72 hours for confirmation and without gadolinium contrast as the effects of the gadolinium contrast and residual angiographic contrast could lead to misdiagnosis.

Tuning of synthesis conditions by thermal decomposition towards gadolinium-doped manganese carbonate nanoparticles with uniform size and high relaxivity

A low temperature thermal decomposition method has been developed to synthesize uniform-sized Gd-doped MnCO₃ nanoparticles.

A mathematical model of tumor growth and its response to single irradiation

BACKGROUND

Mathematical modeling of biological processes is widely used to enhance quantitative understanding of bio-medical phenomena. This quantitative knowledge can be applied in both clinical and experimental settings. Recently, many investigators began studying mathematical models of tumor response to radiation therapy. We developed a simple mathematical model to simulate the growth of tumor volume and its response to a single fraction of high dose irradiation. The modelling study may provide clinicians important insights on radiation therapy strategies through identification of biological factors significantly influencing the treatment effectiveness.

METHODS

We made several key assumptions of the model. Tumor volume is composed of proliferating (or dividing) cancer cells and non-dividing (or dead) cells. Tumor growth rate (or tumor volume doubling time) is proportional to the ratio of the volumes of tumor vasculature and the tumor. The vascular volume grows slower than the tumor by introducing the vascular growth retardation factor, θ. Upon irradiation, the proliferating cells gradually die over a fixed time period after irradiation. Dead cells are cleared away with cell clearance time. The model was applied to simulate pre-treatment growth and post-treatment radiation response of rat rhabdomyosarcoma tumors and metastatic brain tumors of five patients who were treated with Gamma Knife stereotactic radiosurgery (GKSRS).

RESULTS

By selecting appropriate model parameters, we showed the temporal variation of the tumors for both the rat experiment and the clinical GKSRS cases could be easily replicated by the simple model. Additionally, the application of our model to the GKSRS cases showed that the α-value, which is an indicator of radiation sensitivity in the LQ model, and the value of θ could be predictors of the post-treatment volume change.

CONCLUSIONS

The proposed model was successful in representing both the animal experimental data and the clinically observed tumor volume changes. We showed that the model can be used to find the potential biological parameters, which may be able to predict the treatment outcome. However, there is a large statistical uncertainty of the result due to the small sample size. Therefore, a future clinical study with a larger number of patients is needed to confirm the finding.

Synthesis and characterization of a porphyrazine-Gd(III) MRI contrast agent and in vivo imaging of a breast cancer xenograft model

Porphyrazines (Pz), or tetraazaporphyrins, are being studied for their potential use in detection and treatment of cancer. Here, an amphiphilic Cu-Pz-Gd(III) conjugate has been prepared via azide-alkyne Huisgen cycloaddition or 'click' chemistry between an azide functionalized Pz and alkyne functionalized DOTA-Gd(III) analog for use as an MRI contrast agent. This agent, Cu-Pz-Gd(III), is synthesized in good yield and exhibits solution-phase ionic relaxivity (r1  = 11.5 mM(-1) s(-1)) that is approximately four times higher than that of a clinically used monomeric Gd(III) contrast agent, DOTA-Gd(III). Breast tumor cells (MDA-MB-231) associate with Cu-Pz-Gd(III) in vitro, where significant contrast enhancement (9.336 ± 0.335 contrast-to-noise ratio) is observed in phantom cell pellet MR images. This novel contrast agent was administered in vivo to an orthotopic breast tumor model in athymic nude mice and MR images were collected. The average T1 of tumor regions in mice treated with 50 mg kg(-1) Cu-Pz-Gd(III) decreased relative to saline-treated controls. Furthermore, the decrease in T1 was persistent relative to mice treated with the monomeric Gd(III) contrast agent. An ex vivo biodistribution study confirmed that Cu-Pz-Gd(III) accumulates in the tumors and is rapidly cleared, primarily through the kidneys. Differential accumulation and T1 enhancement by Cu-Pz-Gd(III) in the tumor's core relative to the periphery offer preliminary evidence that this agent would find application in the imaging of necrotic tissue.

Fluorescent magnetic nanoparticles with specific targeting functions for combinded targeting, optical imaging and magnetic resonance imaging

Superparamagnetic iron oxides nanoparticles possess specific magnetic properties to be an efficient contrast agent for magnetic resonance imaging (MRI) to enhance the detection and characterization of tissue lesions within the body. To endow specific properties to nanoparticles that can target cancer cells and prevent recognition by the reticuloendothelial system (RES), the surface of the nanoparticles was modified with folic-acid-conjugated poly(ethylene glycol) (FA-PEG). In this study, we investigated the multifunctional fluorescent magnetic nanoparticles (IOPFC) that can specifically target cancer cells and be monitored by both MRI and optical imaging. IOPFC consists of an iron oxide superparamagnetic nanoparticle conjugated with a layer of PEG, which was terminal modified with either Cypher5E or folic acid molecules. The core sizes of IOPFC nanoparticles are around 10 nm, which were visualized by transmission electron microscope (TEM). The hysteresis curves, generated with superconducting quantum interference device (SQUID) magnetometer analysis, demonstrated that IOPFC nanoparticles are superparamagnetic with insignificant hysteresis. IOPFC displays higher intracellular uptake into KB and MDA-MB-231 cells due to the over-expressed folate receptor. This result is confirmed by laser confocal scanning microscopy (LCSM) and atomic flow cytometry. Both in vitro and in vivo MRI studies show better IOPFC uptake by the KB cells (folate positive) than the HT1080 cells (folate negative) and, hence, stronger T 2-weighted signals enhancement. The in vivo fluorescent image recorded at 20 min post injection show strong fluorescence from IOPFC which can be observed around the tumor region. This multifunctional nanoparticle can assess the potential application of developing a magnetic nanoparticle system that combines tumor targeting, as well as MRI and optical imaging.

Surface Modification of Superparamagnetic Nanoparticles for in-vivo Bio-medical Applications

Chemical modifications of Superparamagnetic Iron Oxide Nanoparticles (SPION) surfaces by attachment of functional groups and further covalent coupling with biodegradable substances have been studied. Based on computer-assisted chemical equilibrium calculations, several optimum operation conditions for a coprecipitation process of magnetite nanoparticles were predicted. These particles were immobilized by ultra-thin films of PVA, Dextran, Dextrin, PEG and MPEG to obtain a biocompatible particle surface for further functionalization purposes. The effect of surface modification of the superparamagnetic nanoparticles in terms of chemical and physical properties of the samples was investigated with several techniques, including microelectrophoresis measurement. The feasibility of using SPION in biomedical applications was investigated by in-vivo treatment in rat brains.

Use of templates to fabricate nanoscale spherical structures for defined architectural control

Architectural design of biomaterial structures is essential to reach the full potential of the materials' chemical and biological properties. Clinically, these properties depend on the targeted applications of delivery, such as tissue regeneration, imaging, or cancer. To get an efficient material for biological applications, key properties are needed, such as degradability, low toxicity, cell specificity, relative efficiency, and capability of delivering multiple molecules. In recent years, significant progress has been made through either the design of the material itself (synthetic or natural polymers, dendrimers, crosslinking) or the fabrication technique (nozzle reactor, emulsion, and template). The combination of these materials and techniques results in a large variety of biomaterials that have varied shape and physico-chemical and biological properties. Nevertheless, these inherent properties are not sufficient and interest in discovering and developing new techniques that present these biomaterials in different light is now under focus. A useful strategy to prepare biomaterials with unique and novel architectures is through the use of templates that have defined geometrical features. This holds great promise, especially for the development of hollow structures, such as spheres. The nanoscale structural design of biomaterials via the use of templates and their potential clinical applications are discussed. In addition, the conceptual hurdles that must be overcome to produce applications that are clinically relevant are examined.

No aging phenomena in ferrofluids: the influence of coating on interparticle interactions of maghemite nanoparticles

The influence of coating on interparticle interactions in ferrofluids has been investigated using various techniques such as Mossbauer spectroscopy, magnetometry, transmission electron microscopy, photon correlation spectroscopy, X-ray diffraction, X-ray photoelectron, and resonance micro-Raman spectroscopy. Aging and spin-glass-like behavior was investigated in frozen ferrofluids of various concentrations from dense, initial value of 40 mg of coated nanoparticles per 1 mL of water, to dilute 1:10 (4 mg/mL). The as-prepared nanoparticles, core size 7-8 nm, were subsequently coated with a gummic acid corona of 20 nm thickness, which was observed to prevent agglomeration and to delay aggregation even in dense ferrofluids. The resulting separation of magnetic cores due to the coating eliminated all magnetic interparticle interaction mechanisms, such as exchange and dipoledipole, thus ensuring no aging effects of the magnetic particle system, as manifested in particle agglomeration and precipitation.

In vitro studies on ultrasmall superparamagnetic iron oxide nanoparticles coated with gummic acid for T2 MRI contrast agent

Ultrasmall superparamagnetic iron oxide nanoparticles coated with gummic acid have been investigated as possible constituents of aqueous ferrofluids for biomedical applications and especially for MRI contrast agent. The structural characteristics and the size of the nanoparticles have been analyzed as well as the magnetic properties. In order to evaluate any possible capabilities as a contrast agent, the relaxation time, T2, of hydrogen protons in the colloidal solutions of nanoparticles have been measured in order to gain information on the relaxation behavior compared to other MRI contrast agents. The in vitro cytotoxicity of the obtained magnetic nanoparticles of iron oxide coated with gummic acid was investigated by two separate methods (MTT and FACS analysis) and by using three different normal and transformed cell lines. Our results showed that the synthesized nanoparticles had no toxic effect on any of the cell lines used.

Imaging cerebral gene transcripts in live animals

To circumvent the limitations of using postmortem brain in molecular assays, we used avidin-biotin binding to couple superparamagnetic iron oxide nanoparticles (SPIONs) (15-20 nm) to phosphorothioate-modified oligodeoxynucleotides (sODNs) with sequence complementary to c-fos and beta-actin mRNA (SPION-cfos and SPION-beta-actin, respectively) (14-22 nm). The Stern-Volmer constant for the complex of SPION and fluorescein isothiocyanate (FITC)-sODN is 3.1 x 10(6)/m. We studied the feasibility of using the conjugates for in vivo magnetic resonance imaging (MRI) to monitor gene transcription, and demonstrated that these complexes at 40 mug of Fe per kilogram of body weight were retained at least 1 d after intracerebroventricular infusion into the left ventricle of C57Black6 mice. SPION retention measured by MRI as T(2)* or R(2)* maps (R(2)* = 1/T(2)*) was compared with histology of iron oxide (Prussian blue) and FITC-labeled sODN. We observed significant reduction in magnetic resonance (MR) T(2)* signal in the right cortex and striatum; retention of SPION-cfos and SPION-beta-actin positively correlated with c-fos and beta-actin mRNA maps obtained from in situ hybridization. Histological examination showed that intracellular iron oxide and FITC-sODN correlated positively with in vivo MR signal reduction. Furthermore, in animals that were administered SPION-cfos and amphetamine (4 mg/kg, i.p.), retention was significantly elevated in the nucleus accumbens, striatum, and medial prefrontal cortex of the forebrain. Control groups that received SPION-cfos and saline or that received a SPION conjugate with a random-sequence probe and amphetamine showed no retention. These results demonstrated that SPION-sODN conjugates can detect active transcriptions of specific mRNA species in living animals with MRI.

Drug delivery to brain tumours: challenges and progress

Nearly 12.5 million new cancer cases are diagnosed worldwide each year. Although new treatments have been developed, most new anticancer drugs that are effective outside the brain have failed in clinical trials against brain tumours, in part due to poor penetration across the blood-brain barrier and the blood-brain tumour barrier. This review will discuss the challenges of drug delivery across the blood-brain barrier/blood-brain tumour barrier to cancer cells, as well as progress made so far. This will include a biochemical modulation strategy that transiently opens the barrier to increase anticancer drug delivery selectively to brain tumours. It will also briefly discuss a quantitative non-invasive method to measure permeability changes and tumour response to treatment in the human brain.

Synthesis and characterization of HE-24.8: a polymeric contrast agent for magnetic resonance angiography

The physical and biological properties of a water-soluble polymeric contrast agent based on a complex of N-(2-hydroxypropyl)methacrylamide copolymer with gadolinium (HE-24.8) were investigated, and its potential for experimental magnetic resonance (MR) angiography was assessed. Relaxivities of Gd-DTPA-BMA, Gd-DTPA-HSA (human serum albumin), and HE-24.8 were determined at 1.5 T. Thermic stability and biocompatibility of HE-24.8 were assessed in vitro and by analyzing kinetics and organ distribution in rats for up to 2 weeks. For comparison, HE-24.8- and Gd-DTPA-HSA-enhanced micro-MR angiographies of brain, chest, and subcutaneous tumors in rats were performed. T1 relaxivity of HE-24.8 (21.3 +/- 1.1 mM(-1) s(-1)) was 5-fold higher than that of Gd-DTPA-BMA (4.1 +/- 0.1 mM(-1) s(-1)) and twice as high as that of Gd-DTPA-HSA (12.4 +/- 0.2 mM(-1) s(-1)). Varying the molecular weight of the polymer (15-46 kDa) did not significantly change the T1 relaxivity. In rats, 20 and 10% of the injected dose of HE-24.8 was detected at 24 and 168 h postinjection, respectively. Upon a relatively rapid initial renal clearance, no specific retention in any organ was noted, with some exception for the reticulo-endothelial system. No measurable release of gadolinium from the polymer-Gd complex or cell toxicity was observed during its incubation in aqueous environment. Excellent display of rat and tumor vascularization was achieved with Gd-DTPA-HSA and HE-24.8; however, contrast of vessels was higher in HE-24.8-enhanced scans. HE-24.8 is considered a macromolecular contrast agent highly suited for experimental MR studies.

Quantification of Gd-BOPTA uptake and biliary excretion from dynamic magnetic resonance imaging in rat livers: model validation with 153Gd-BOPTA

OBJECTIVES

We sought to develop and validate a pharmacokinetic model allowing description of the magnetic resonance (MR) signal intensity induced by the hepatobiliary contrast agent Gd-BOPTA and to quantify the overall Gd-BOPTA transport in rat liver.

MATERIALS AND METHODS

MR signal intensity was recorded during the perfusion of rat livers with Gd-DTPA, an extracellular contrast agent, and Gd-BOPTA, a hepatobiliary contrast agent. Similar experiments were conducted with Gd-labeled contrast agents for quantitative measurement in liver, bile and perfusate.

RESULTS

A complete 6-compartment, 8 parameter open model was first developed to describe the pharmacokinetics of the compound based on the radioactivity data analysis. Because perfusate and bile data were not available in MRI experiments, a reduced model (6-compartment, 5 parameters) was considered for the MRI data. The performance of the reduced model was tested using the radioactivity data. The reduced model successfully described the contrast agent amount in the liver and correctly predicted amounts in bile and perfusate.

CONCLUSIONS

Pharmacokinetic modeling of MR signal intensity induced by Gd-BOPTA permits quantification of Gd-BOPTA uptake and biliary excretion in rat livers.

Do highly concentrated gadolinium chelates improve MR brain perfusion imaging? Intraindividually controlled randomized crossover concentration comparison study of 0.5 versus 1.0 mol/L gadobutrol

PURPOSE

To assess the potential advantages of using a 1.0 mol/L versus 0.5 mol/L gadobutrol formulation for magnetic resonance (MR) brain perfusion imaging.

MATERIALS AND METHODS

Forty-three healthy volunteers were enrolled in an intraindividually controlled, randomized crossover comparison study. Two gadobutrol formulations-0.5 and 1.0 mol/L- were randomly injected during two separate treatment periods. For intraindividual comparison of effectiveness parameters, single-section gradient-echo brain perfusion MR imaging was performed under identical conditions for both investigations. Quantitative and qualitative evaluations were performed. Differences between the two gadobutrol formulations were evaluated at analysis of covariance and tested for statistical significance (P <.05) with a t test.

RESULTS

Use of 1.0 mol/L gadobutrol resulted in a significantly smaller bolus width at half maximum signal intensity decrease, a smaller mean peak time, a higher contrast and contrast-to-noise ratio between gray and white matter, and significant increases in both maximum change in transverse relaxation rate (DeltaR2max) and differences in peak enhancement in gray matter among all volunteers (P <.001). In white matter, increases in DeltaR2max (P =.262) and in differences in peak enhancement (P =.262) were smaller and not significant (P =.292). Parameter map analysis revealed improved quality and superior contrast in relative regional cerebral blood flow (P =.034) and mean transit time (P <.001). The lack of difference regarding relative regional cerebral blood volume maps was consistent with the use of the same dose of each gadobutrol formulation.

CONCLUSION

Brain perfusion images obtained with 1.0 mol/L gadobutrol were superior to those obtained with 0.5 mol/L gadobutrol in healthy volunteers examined with the described MR imaging protocol.

Albumin-binding MR blood pool agents as MRI contrast agents in an intracranial mouse glioma model

Intravenous MRI contrast agents are commonly used to improve the detection of intracranial tumors and other central nervous system (CNS) lesions for diagnosis and treatment planning. Two small-molecule, albumin-binding blood pool contrast agents (MP-2269 and MS-325) of potential clinical significance were evaluated at 1.5 Tesla in a mouse glioma model and compared with an extracellular contrast agent (OptiMARK). Tumor image contrast was significantly enhanced and long-lived following administration of 30 micromole/kg of the blood pool agents: specifically, contrast enhancement peaked slowly at 25-30 min following administration, remained constant for >3 hr, and returned to baseline within 20 hr. Comparable but "transient" enhancement was achieved using 100 micromole/kg OptiMARK: specifically, contrast enhancement peaked rapidly at 2-5 min following administration and then declined over 40 min. The blood pool contrast agents demonstrated an approximately threefold increased dose-effectiveness and a lengthened window of tumor contrast enhancement in comparison to commonly available extracellular contrast agents. This demonstrates the potential of alternative contrast-enhanced (CE) MRI examination protocols for tumor detection.

Magnetic resonance angiography and evaluation of cervical arteries

Multiple clinical trials have demonstrated the efficacy of endarterectomy in selected groups of patients based primarily on percent diameter stenosis. Although measurement of stenosis in the clinical trials was established by conventional angiography, there is considerable interest in noninvasive alternatives. Magnetic resonance angiography, performed using time-of-flight methods or with contrast enhancement, is one of several alternatives for noninvasive carotid evaluation. Screening examinations are routinely performed for carotid stenosis. Preoperative evaluations based on one or a combination of noninvasive tests have been proposed, although these proposals are the subject of ongoing controversy. Evaluation of the vertebral arteries is more difficult and less well studied: however, the increasing availability of therapies for posterior circulation atherosclerotic narrowing is resulting in increased interest in this problem.

New advances in brain tumor imaging

During the period of this review, there has been continued use and development of neuroimaging techniques in brain tumor diagnosis and management. Although no monumental developments or improvements in neuroimaging techniques or technology have occurred, important studies continue to be published showing the clinical impact and utility of various neuroimaging techniques to improve the care of patients with brain tumors. Several studies have shown the power of functional neuroimaging techniques with both magnetic resonance imaging (MRI) and positron-emission tomography (PET) to map eloquent cortex and assist in the planning of surgical and radiation therapy. New nuclear imaging radiopharmaceuticals, including various PET ligands and single photon emission computed tomography (SPECT) agents, have also been developed and show their potential power in the evaluation of brain tumor patients. New MRI pulse sequences to improve image quality and shorten imaging time have also been developed. Several excellent reviews of the use of fluorodeoxyglucose (FDG)-PET and MRI techniques were also published. This article reviews the relevant and important neuroimaging literature related to brain tumor that was published during the defined time period of November 1, 1999 to October 31, 2000. Discussion is organized by modality, including nuclear imaging techniques (SPECT and PET) and MRI (pulse sequence development, contrast agent development, functional MRI developments, and general MRI-related information).