Intracellular manganese ions provide strongT1relaxation in rat myocardium

Manganese-Enhanced Magnetic Resonance Imaging of the Heart

Manganese-based contrast media were the first in vivo paramagnetic agents to be used in magnetic resonance imaging (MRI). The uniqueness of manganese lies in its biological function as a calcium channel analog, thus behaving as an intracellular contrast agent. Manganese ions are taken up by voltage-gated calcium channels in viable tissues, such as the liver, pancreas, kidneys, and heart, in response to active calcium-dependent cellular processes. Manganese-enhanced magnetic resonance imaging (MEMRI) has therefore been used as a surrogate marker for cellular calcium handling and interest in its potential clinical applications has recently re-emerged, especially in relation to assessing cellular viability and myocardial function. Calcium homeostasis is central to myocardial contraction and dysfunction of myocardial calcium handling is present in various cardiac pathologies. Recent studies have demonstrated that MEMRI can detect the presence of abnormal myocardial calcium handling in patients with myocardial infarction, providing clear demarcation between the infarcted and viable myocardium. Furthermore, it can provide more subtle assessments of abnormal myocardial calcium handling in patients with cardiomyopathies and being excluded from areas of nonviable cardiomyocytes and severe fibrosis. As such, MEMRI offers exciting potential to improve cardiac diagnoses and provide a noninvasive measure of myocardial function and contractility. This could be an invaluable tool for the assessment of both ischemic and nonischemic cardiomyopathies as well as providing a measure of functional myocardial recovery, an accurate prediction of disease progression and a method of monitoring treatment response. EVIDENCE LEVEL: 5: TECHNICAL EFFICACY: STAGE 5.

Effects of repeated manganese treatment on proton magnetic resonance spectra of the globus pallidus in rat brain

Excessive manganese is neurotoxic, which means that it can affect the concentrations of metabolite in ¹ H MRS. In addition, manganese is paramagnetic and it may influence the relaxation times of the metabolite. The aim of this study is to assess the sensitivity of the metabolite relaxation properties and concentrations to exogenous manganese deposition in the globus pallidus (GP) of rat brain after repeated manganese injection. Proton magnetic resonance spectroscopy (¹ H MRS) experiments in vivo and ex vivo were carried out to evaluate the changes in the metabolite concentration and the major metabolite relaxation times, and histological experiments were also performed after repeated manganese administration. Only the T₁ value for N-acetylaspartate (NAA) of the GP was significantly reduced after 1 day of manganese injection compared with that of the control group (p < 0.025). The T₁ and T₂ values for NAA and total creatine (tCr) (p < 0.025), along with the amounts of NAA, tCr, myo-inositol, choline, and glutamate (p < 0.0086) in the GP, were all significantly decreased after 5 days of manganese administration compared with that of the control group. The changes in the concentration and relaxation properties of NAA and tCr in the GP of rat brain indicated that manganese represented paramagnetism and neurotoxicity after repeated administration. Accurate knowledge of relaxation properties and concentrations of NAA and tCr in this study could help appropriate selection of sequence parameters to improve the ability to distinguish the brain regions affected in cases of manganese poisoning.

Design and Synthesis of Luminescent Lanthanide-Based Bimodal Nanoprobes for Dual Magnetic Resonance (MR) and Optical Imaging

Current biomedical imaging techniques are crucial for the diagnosis of various diseases. Each imaging technique uses specific probes that, although each one has its own merits, do not encompass all the functionalities required for comprehensive imaging (sensitivity, non-invasiveness, etc.). Bimodal imaging methods are therefore rapidly becoming an important topic in advanced healthcare. This bimodality can be achieved by successive image acquisitions involving different and independent probes, one for each mode, with the risk of artifacts. It can be also achieved simultaneously by using a single probe combining a complete set of physical and chemical characteristics, in order to record complementary views of the same biological object at the same time. In this scenario, and focusing on bimodal magnetic resonance imaging (MRI) and optical imaging (OI), probes can be engineered by the attachment, more or less covalently, of a contrast agent (CA) to an organic or inorganic dye, or by designing single objects containing both the optical emitter and MRI-active dipole. If in the first type of system, there is frequent concern that at some point the dye may dissociate from the magnetic dipole, it may not in the second type. This review aims to present a summary of current activity relating to this kind of dual probes, with a special emphasis on lanthanide-based luminescent nano-objects.

A multi-parametric PET/MRI study of breast cancer: Evaluation of DCE-MRI pharmacokinetic models and correlation with diffusion and functional parameters

46 patients with histologically confirmed breast cancer were enrolled and imaged with a 3T hybrid PET/MRI system, at staging. Diffusion, functional and perfusion parameters (measured by Tofts and shutter speed models) were compared. Results showed a good correlation between pharmacokinetic parameters and the SUV.

In Vivo Visualization of Active Polysynaptic Circuits With Longitudinal Manganese-Enhanced MRI (MEMRI)

Manganese-enhanced magnetic resonance imaging (MEMRI) is a powerful tool for in vivo non-invasive whole-brain mapping of neuronal activity. Mn²⁺ enters active neurons via voltage-gated calcium channels and increases local contrast in T₁-weighted images. Given the property of Mn²⁺ of axonal transport, this technique can also be used for tract tracing after local administration of the contrast agent. However, MEMRI is still not widely employed in basic research due to the lack of a complete description of the Mn²⁺ dynamics in the brain. Here, we sought to investigate how the activity state of neurons modulates interneuronal Mn²⁺ transport. To this end, we injected mice with low dose MnCl₂ 2. (i.p., 20 mg/kg; repeatedly for 8 days) followed by two MEMRI scans at an interval of 1 week without further MnCl₂ injections. We assessed changes in T₁ contrast intensity before (scan 1) and after (scan 2) partial sensory deprivation (unilateral whisker trimming), while keeping the animals in a sensory enriched environment. After correcting for the general decay in Mn²⁺ content, whole brain analysis revealed a single cluster with higher signal in scan 1 compared to scan 2: the left barrel cortex corresponding to the right untrimmed whiskers. In the inverse contrast (scan 2 > scan 1), a number of brain structures, including many efferents of the left barrel cortex were observed. These results suggest that continuous neuronal activity elicited by ongoing sensory stimulation accelerates Mn²⁺ transport from the uptake site to its projection terminals, while the blockage of sensory-input and the resulting decrease in neuronal activity attenuates Mn²⁺ transport. The description of this critical property of Mn²⁺ dynamics in the brain allows a better understanding of MEMRI functional mechanisms, which will lead to more carefully designed experiments and clearer interpretation of the results.

Surface impact on nanoparticle-based magnetic resonance imaging contrast agents

Magnetic resonance imaging (MRI) is one of the most widely used diagnostic tools in the clinic. To improve imaging quality, MRI contrast agents, which can modulate local T1 and T2 relaxation times, are often injected prior to or during MRI scans. However, clinically used contrast agents, including Gd3+-based chelates and iron oxide nanoparticles (IONPs), afford mediocre contrast abilities. To address this issue, there has been extensive research on developing alternative MRI contrast agents with superior r1 and r2 relaxivities. These efforts are facilitated by the fast progress in nanotechnology, which allows for preparation of magnetic nanoparticles (NPs) with varied size, shape, crystallinity, and composition. Studies suggest that surface coatings can also largely affect T1 and T2 relaxations and can be tailored in favor of a high r1 or r2. However, the surface impact of NPs has been less emphasized. Herein, we review recent progress on developing NP-based T1 and T2 contrast agents, with a focus on the surface impact.

Magnetic susceptibility anisotropy outside the central nervous system

Magnetic-susceptibility-based MRI has made important contributions to the characterization of tissue microstructure, chemical composition, and organ function. This has motivated a number of studies to explore the link between microstructure and susceptibility in organs and tissues throughout the body, including the kidney, heart, and connective tissue. These organs and tissues have anisotropic magnetic susceptibility properties and cellular organizations that are distinct from the lipid organization of myelin in the brain. For instance, anisotropy is traced to the epithelial lipid orientation in the kidney, the myofilament proteins in the heart, and the collagen fibrils in the knee cartilage. The magnetic susceptibility properties of these and other tissues are quantified using specific MRI tools: susceptibility tensor imaging (STI), quantitative susceptibility mapping (QSM), and individual QSM measurements with respect to tubular and filament directions determined from diffusion tensor imaging. These techniques provide complementary and supplementary information to that produced by traditional MRI methods. In the kidney, STI can track tubules in all layers including the cortex, outer medulla, and inner medulla. In the heart, STI detected myofibers throughout the myocardium. QSM in the knee revealed three unique layers in articular cartilage by exploiting the anisotropic susceptibility features of collagen. While QSM and STI are promising tools to study tissue susceptibility, certain technical challenges must be overcome in order to realize routine clinical use. This paper reviews essential experimental findings of susceptibility anisotropy in the body, the underlying mechanisms, and the associated MRI methodologies. Copyright © 2016 John Wiley & Sons, Ltd.

Magnetic susceptibility anisotropy of myocardium imaged by cardiovascular magnetic resonance reflects the anisotropy of myocardial filament α-helix polypeptide bonds

BACKGROUND

A key component of evaluating myocardial tissue function is the assessment of myofiber organization and structure. Studies suggest that striated muscle fibers are magnetically anisotropic, which, if measurable in the heart, may provide a tool to assess myocardial microstructure and function.

METHODS

To determine whether this weak anisotropy is observable and spatially quantifiable with cardiovascular magnetic resonance (CMR), both gradient-echo and diffusion-weighted data were collected from intact mouse heart specimens at 9.4 Tesla. Susceptibility anisotropy was experimentally calculated using a voxelwise analysis of myocardial tissue susceptibility as a function of myofiber angle. A myocardial tissue simulation was developed to evaluate the role of the known diamagnetic anisotropy of the peptide bond in the observed susceptibility contrast.

RESULTS

The CMR data revealed that myocardial tissue fibers that were parallel and perpendicular to the magnetic field direction appeared relatively paramagnetic and diamagnetic, respectively. A linear relationship was found between the magnetic susceptibility of the myocardial tissue and the squared sine of the myofiber angle with respect to the field direction. The multi-filament model simulation yielded susceptibility anisotropy values that reflected those found in the experimental data, and were consistent that this anisotropy decreased as the echo time increased.

CONCLUSIONS

Though other sources of susceptibility anisotropy in myocardium may exist, the arrangement of peptide bonds in the myofilaments is a significant, and likely the most dominant source of susceptibility anisotropy. This anisotropy can be further exploited to probe the integrity and organization of myofibers in both healthy and diseased heart tissue.

Rapid multislice T1 mapping of mouse myocardium: Application to quantification of manganese uptake in α-Dystrobrevin knockout mice

PURPOSE

The aim of this study was to develop a rapid, multislice cardiac T1 mapping method in mice and to apply the method to quantify manganese (Mn(2+)) uptake in a mouse model with altered Ca(2+) channel activity.

METHODS

An electrocardiography-triggered multislice saturation-recovery Look-Locker method was developed and validated both in vitro and in vivo. A two-dose study was performed to investigate the kinetics of T1 shortening, Mn(2+) relaxivity in myocardium, and the impact of Mn(2+) on cardiac function. The sensitivity of Mn(2+)-enhanced MRI in detecting subtle changes in altered Ca(2+) channel activity was evaluated in a mouse model with α-dystrobrevin knockout.

RESULTS

Validation studies showed strong agreement between the current method and an established method. High Mn(2+) dose led to significantly accelerated T1 shortening. Heart rate decreased during Mn(2+) infusion, while ejection ratio increased slightly at the end of imaging protocol. No statistical difference in cardiac function was detected between the two dose groups. Mice with α-dystrobrevin knockout showed enhanced Mn(2+) uptake in vivo. In vitro patch-clamp study showed increased Ca(2+) channel activity.

CONCLUSION

The saturation recovery method provides rapid T1 mapping in mouse hearts, which allowed sensitive detection of subtle changes in Mn(2+) uptake in α-dystrobrevin knockout mice.

Manganese G8 dendrimers targeted to oxidation-specific epitopes: in vivo MR imaging of atherosclerosis

PURPOSE

To determine if manganese (Mn) G8 dendrimers targeted to oxidation-specific epitopes (OSE) allow for in vivo detection of atherosclerotic lesions.

MATERIALS AND METHODS

OSE have been identified as key factors in atherosclerotic plaque progression and destabilization. Mn offers a potentially clinically translatable alternative to gadolinium-based agents when bioretention and potential toxicity of gadolinium is anticipated. However, to be effective, high payloads of Mn must accumulate intracellularly in macrophages. It was hypothesized that G8 dendrimers targeted to OSE may allow delivery of high Mn payloads, thereby enabling in vivo detection of macrophage-rich plaques. G8 dendrimers were modified to allow conjugation with MnDTPA (758 Mn ion) and the antibody MDA2 that is targeted to malondialdehyde (MDA)-lysine epitopes. Both the untargeted and targeted G8 dendrimers were characterized and their in vivo efficacy evaluated in apoE(-/-) mice over a 96-hour time period after bolus administration of a 0.05 mmol Mn/kg dose using a clinical MR system (3T).

RESULTS

Significant enhancement (normalized enhancement >60%, P = 0.0013) of atherosclerotic lesions was observed within a 72-hour time period following administration of the targeted dendrimers. The presence of Mn within atherosclerotic lesions was confirmed using spectroscopic methods (>8 μg Mn/g). Limited signal attenuation (<18%) and Mn deposition (<1 μg Mn/g) was observed in the arterial wall following injection of the untargeted material.

CONCLUSION

This study demonstrates that manganese-labeled dendrimers, allowing a high Mn payload, targeted to OSE may allow in vivo image of atherosclerotic lesions.

Understanding the metabolic fate and assessing the biosafety of MnO nanoparticles by metabonomic analysis

Recently, some types of MnO nanoparticle (Mn-NP) with favorable imaging capacity have been developed to improve the biocompatible profile of the existing Mn-based MRI contrast agent Mn-DPDP; however, the overall bio-effects and potential toxicity remain largely unknown. In this study, (1)H NMR-based metabolic profiling, integrated with traditional biochemical analysis and histopathological examinations, was used to investigate the absorption, distribution, metabolism, excretion and toxicity of Mn-NPs as candidates for MRI contrast agent. The metabolic responses in biofluids (plasma and urine) and tissues (liver, spleen, kidney, lung and brain) from rats could be divided into four classes following Mn-NP administration: Mn biodistribution-dependent, time-dependent, dose-dependent and complicated metabolic variations. The variations of these metabolites involved in lipid, energy, amino acid and other nutrient metabolism, which disclosed the metabolic fate and biological effects of Mn-NPs in rats. The changes of metabolic profile implied that the disturbance and impairment of biological functions induced by Mn-NP exposure were correlated with the particle size and the surface chemistry of nanoparticles. Integration of metabonomic technology with traditional methods provides a promising tool to understand the toxicological behavior of biomedical nanomaterials and will result in informed decision-making during drug development.

Brain processing of biologically relevant odors in the awake rat, as revealed by manganese-enhanced MRI

Background

So far, an overall view of olfactory structures activated by natural biologically relevant odors in the awake rat is not available. Manganese-enhanced MRI (MEMRI) is appropriate for this purpose. While MEMRI has been used for anatomical labeling of olfactory pathways, functional imaging analyses have not yet been performed beyond the olfactory bulb. Here, we have used MEMRI for functional imaging of rat central olfactory structures and for comparing activation maps obtained with odors conveying different biological messages.

Methodology/Principal Findings

Odors of male fox feces and of chocolate flavored cereals were used to stimulate conscious rats previously treated by intranasal instillation of manganese (Mn). MEMRI activation maps showed Mn enhancement all along the primary olfactory cortex. Mn enhancement elicited by male fox feces odor and to a lesser extent that elicited by chocolate odor, differed from that elicited by deodorized air. This result was partly confirmed by c-Fos immunohistochemistry in the piriform cortex.

Conclusion/Significance

By providing an overall image of brain structures activated in awake rats by odorous stimulation, and by showing that Mn enhancement is differently sensitive to different stimulating odors, the present results demonstrate the interest of MEMRI for functional studies of olfaction in the primary olfactory cortex of laboratory small animals, under conditions close to natural perception. Finally, the factors that may cause the variability of the MEMRI signal in response to different odor are discussed.

In vivo detection of oxidation-specific epitopes in atherosclerotic lesions using biocompatible manganese molecular magnetic imaging probes

OBJECTIVES

This study sought to evaluate the in vivo magnetic resonance imaging (MRI) efficacy of manganese [Mn(II)] molecular imaging probes targeted to oxidation-specific epitopes (OSE).

BACKGROUND

OSE are critical in the initiation, progression, and destabilization of atherosclerotic plaques. Gadolinium [Gd(III)]-based MRI agents can be associated with systemic toxicity. Mn is an endogenous, biocompatible, paramagnetic metal ion that has poor MR efficacy when chelated, but strong efficacy when released within cells.

METHODS

Multimodal Mn micelles were generated to contain rhodamine for confocal microscopy and conjugated with either the murine monoclonal IgG antibody MDA2 targeted to malondialdehyde (MDA)-lysine epitopes or the human single-chain Fv antibody fragment IK17 targeted to MDA-like epitopes ("targeted micelles"). Micelle formulations were characterized in vitro and in vivo, and their MR efficacy (9.4-T) evaluated in apolipoprotein-deficient (apoE(-/-)) and low-density lipoprotein receptor negative (LDLR(-/-)) mice (0.05 mmol Mn/kg dose) (total of 120 mice for all experiments). In vivo competitive inhibition studies were performed to evaluate target specificity. Untargeted, MDA2-Gd, and IK17-Gd micelles (0.075 mmol Gd/kg) were included as controls.

RESULTS

In vitro studies demonstrated that targeted Mn micelles accumulate in macrophages when pre-exposed to MDA-LDL with ∼10× increase in longitudinal relativity. Following intravenous injection, strong MR signal enhancement was observed 48 to 72 h after administration of targeted Mn micelles, with colocalization within intraplaque macrophages. Co-injection of free MDA2 with the MDA2-Mn micelles resulted in full suppression of MR signal in the arterial wall, confirming target specificity. Similar MR efficacy was noted in apoE(-/-) and LDLR(-/-) mice with aortic atherosclerosis. No significant differences in MR efficacy were noted between targeted Mn and Gd micelles.

CONCLUSIONS

This study demonstrates that biocompatible multimodal Mn-based molecular imaging probes detect OSE within atherosclerotic plaques and may facilitate clinical translation of noninvasive imaging of human atherosclerosis.

Quantitative measurement of changes in calcium channel activity in vivo utilizing dynamic manganese-enhanced MRI (dMEMRI)

The ability of manganese ions (Mn(2+)) to enter cells through calcium ion (Ca(2+)) channels has been used for depolarization dependent brain functional imaging with manganese-enhanced MRI (MEMRI). The purpose of this study was to quantify changes to Mn(2+) uptake in rat brain using a dynamic manganese-enhanced MRI (dMEMRI) scanning protocol with the Patlak and Logan graphical analysis methods. The graphical analysis was based on a three-compartment model describing the tissue and plasma concentration of Mn. Mn(2+) uptake was characterized by the total distribution volume of manganese (Mn) inside tissue (V(T)) and the unidirectional influx constant of Mn(2+) from plasma to tissue (K(i)). The measurements were performed on the anterior (APit) and posterior (PPit) parts of the pituitary gland, a region with an incomplete blood brain barrier. Modulation of Ca(2+) channel activity was performed by administration of the stimulant glutamate and the inhibitor verapamil. It was found that the APit and PPit showed different Mn(2+) uptake characteristics. While the influx of Mn(2+) into the PPit was reversible, Mn(2+) was found to be irreversibly trapped in the APit during the course of the experiment. In the PPit, an increase of Mn(2+) uptake led to an increase in V(T) (from 2.8±0.3 ml/cm(3) to 4.6±1.2 ml/cm(3)) while a decrease of Mn(2+) uptake corresponded to a decrease in V(T) (from 2.8±0.3 ml/cm(3) to 1.4±0.3 ml/cm(3)). In the APit, an increase of Mn(2+) uptake led to an increase in K(i) (from 0.034±0.009 min(-1) to 0.049±0.012 min(-1)) while a decrease of Mn(2+) uptake corresponded to a decrease in K(i) (from 0.034±0.009 min(-1) to 0.019±0.003 min(-1)). This work demonstrates that graphical analysis applied to dMEMRI data can quantitatively measure changes to Mn(2+) uptake following modulation of neural activity.

Discrepancy in T1 and T2 shortening of the globus pallidus in hepatic insufficiency: evaluation by susceptibility-weighted imaging

PURPOSE

We assessed the signal of the globus pallidus (GP) in cases of hepatic insufficiency, especially to evaluate the degree of discrepancy in paramagnetic effects on shortening of T(1) and T(2)* using susceptibility-weighted images (SWI).

MATERIALS AND METHODS

Seven patients with hepatic insufficiency underwent magnetic resonance (MR) examinations that included T(1)-weighted images (T(1)WI), T(2)-weighted images (T(2)WI), and SWI on a 1.5-tesla MR imager, and we compared their results to those of controls. On T(1)WI and T(2)WI, we measured signal intensity in the GP and posterior segment of the putamen (Put) to obtain a signal ratio (GP/Put ratio), and on SWI, we classified signal intensity into 4 grades: A, higher than the cortex; B, lower than the cortex and higher than the cerebrospinal fluid (CSF); C, lower than the CSF and higher than the red nucleus; and D, lower than the red nucleus.

RESULTS

In the 7 patients with hepatic insufficiency, the mean GP/Put ratio was significantly higher on T(1)WI and T(2)WI than those values in controls. On SWI, we classified 2 cases each as Grade A, Grade B, and Grade C, and one as Grade D. Although the signal of the GP was elevated on T(1)WI, there was no decrease in signal on T(2)WI. On SWI, we obtained no low signal intensity.

CONCLUSION

In patients with hepatic insufficiency, the globus pallidus did not show low signal intensity on either T(2)WI or SWI. Hyperintensity of the GP on T(1)WI without hypointensity on T(2)WI, or even SWI, suggests a discrepancy between paramagnetic effect on T(1) and T(2) shortening that reflects the accumulation of manganese and the presence of hepatic insufficiency.

Quantitative pancreatic β cell MRI using manganese-enhanced Look-Locker imaging and two-site water exchange analysis

Pancreatic β-cell imaging would be useful in monitoring the progression of and therapies for diabetes. The purpose of this study was to develop and evaluate quantitative β-cell MRI using manganese (Mn(2+)) labeling of β cells, T1 mapping, and a two-site water exchange model. Normal, pharmacologically-treated, and severely diabetic mice underwent injection of MnCl(2). Pancreatic water proton T1 relaxation was measured using Look-Locker MRI, and two-site water exchange analysis was used to estimate model parameters including the intracellular water proton relaxation rate constant (R1(ic)) and the intracellular fraction as indicators of β-cell function and mass, respectively. Logarithmic plots of T1 relaxation revealed two distinct proton pools relaxing with different T1s, and the two-site water exchange model fit the measured T1 relaxation data better than a monoexponential model. The intracellular R1(ic) time course revealed the kinetics of β-cell Mn(2+) labeling. Pharmacological treatments with nifedipine, tolbutamide, and diazoxide altered R1(ic), indicating that beta cell function was a determinant of Mn(2+) uptake. Intracellular fraction was significantly higher in mice with normal β cell mass than in diabetic mice (14.9% vs. 14.4%, P < 0.05). Two-site water exchange analysis of T1 relaxation of the Mn(2+)-enhanced pancreas is a promising method for quantifying β cell volume fraction and function.

Relationship between blood and myocardium manganese levels during manganese-enhanced MRI (MEMRI) with T1 mapping in rats

Manganese ions (Mn(2+) ) enter viable myocardial cells via voltage-gated calcium channels. Because of its shortening of T(1) and its relatively long half-life in cells, Mn(2+) can serve as an intracellular molecular contrast agent to study indirect calcium influx into the myocardium. One major concern in using Mn(2+) is its sensitivity over a limited range of concentrations employing T(1)-weighted images for visualization, which limits its potential in quantitative techniques. Therefore, this study assessed the implementation of a T(1) mapping method for cardiac manganese-enhanced MRI to enable a quantitative estimate of the influx of Mn(2+) over a wide range of concentrations in male Sprague-Dawley rats. This MRI method was used to compare the relationship between T(1) changes in the heart as a function of myocardium and blood Mn(2+) levels. Results showed a biphasic relationship between ΔR(1) and the total Mn(2+) infusion dose. Nonlinear relationships were observed between the total Mn(2+) infusion dose versus blood levels and left ventricular free wall ΔR(1) . At low blood levels of Mn(2+) , there was proportionally less cardiac enhancement seen than at higher levels of blood Mn(2+) . We hypothesize that Mn(2+) blood levels increase as a result of rate-limiting excretion by the liver and kidneys at these higher Mn(2+) doses.

Manganese-enhanced magnetic resonance imaging

Manganese-enhanced magnetic resonance imaging (MEMRI) relies on contrasts that are due to the shortening of the T (1) relaxation time of tissue water protons that become exposed to paramagnetic manganese ions. In experimental animals, the technique combines the high spatial resolution achievable by MRI with the biological information gathered by tissue-specific or functionally induced accumulations of manganese. After in vivo administration, manganese ions may enter cells via voltage-gated calcium channels. In the nervous system, manganese ions are actively transported along the axon. Based on these properties, MEMRI is increasingly used to delineate neuroanatomical structures, assess differences in functional brain activity, and unravel neuronal connectivities in both healthy animals and models of neurological disorders. Because of the cellular toxicity of manganese, a major challenge for a successful MEMRI study is to achieve the lowest possible dose for a particular biological question. Moreover, the interpretation of MEMRI findings requires a profound knowledge of the behavior of manganese in complex organ systems under physiological and pathological conditions. Starting with an overview of manganese pharmacokinetics and mechanisms of toxicity, this chapter covers experimental methods and protocols for applications in neuroscience.

Experimental protocol for activation-induced manganese-enhanced MRI (AIM-MRI) based on quantitative determination of Mn content in rat brain by fast T1 mapping

In activation-induced manganese-enhanced MRI (AIM-MRI) experiments, differential accumulation of Mn in activated and silent brain areas is generally assessed using T(1)-weighted images and quantified by the enhancement of signal intensity (SI), calculated with reference to SI before Mn administration or to SI of brain regions unaffected by the specific stimulus. However, SI enhancement can be unreliable when animals are removed from and reinserted into the magnet. We have developed an experimental protocol based on repeated intraperitoneal (i.p.) injections of Mn, quantitative determination of T(1), and coregistration of images to a rat brain atlas that allows absolute quantification of Mn concentration in selected brain areas. Results showed that interanimal variability of postcontrast T(1) values was very low (compared to the experimental error in T(1) determinations) allowing detection of differential regional Mn uptake in stimulated and unstimulated animals. In addition we have determined in vivo relaxivity of Mn in brain tissue and its frequency dependence.

Assessing manganese efflux using SEA0400 and cardiac T1-mapping manganese-enhanced MRI in a murine model

The sodium-calcium exchanger (NCX) is one of the transporters contributing to the control of intracellular calcium (Ca(2+)) concentration by normally mediating net Ca(2+) efflux. However, the reverse mode of the NCX can cause intracellular Ca(2+) concentration overload, which exacerbates the myocardial tissue injury resulting from ischemia. Although the NCX inhibitor SEA0400 has been shown to therapeutically reduce myocardial injury, no in vivo technique exists to monitor intracellular Ca(2+) fluctuations produced by this drug. Cardiac manganese-enhanced MRI (MEMRI) may indirectly assess Ca(2+) efflux by estimating changes in manganese (Mn(2+)) content in vivo, since Mn(2+) has been suggested as a surrogate marker for Ca(2+). This study used the MEMRI technique to examine the temporal features of cardiac Mn(2+) efflux by implementing a T(1)-mapping method and inhibiting the NCX with SEA0400. The change in (1)H(2)O longitudinal relaxation rate, Delta R(1), in the left ventricular free wall, was calculated at different time points following infusion of 190 nmol/g manganese chloride (MnCl(2)) in healthy adult male mice. The results showed 50% MEMRI signal attenuation at 3.4 +/- 0.6 h post-MnCl(2) infusion without drug intervention. Furthermore, treatment with 50 +/- 0.2 mg/kg of SEA0400 significantly reduced the rate of decrease in Delta R(1). At 4.9-5.9 h post-MnCl(2) infusion, the average Delta R(1) values for the two groups treated with SEA0400 were 2.46 +/- 0.29 and 1.72 +/- 0.24 s(-1) for 50 and 20 mg/kg doses, respectively, as compared to the value of 1.27 +/- 0.28 s(-1) for the control group. When this in vivo data were compared to ex vivo absolute manganese content data, the MEMRI T(1)-mapping technique was shown to effectively quantify Mn(2+) efflux rates in the myocardium. Therefore, combining an NCX inhibitor with MEMRI may be a useful technique for assessing Mn(2+) transport mechanisms and rates in vivo, which may reflect changes in Ca(2+) transport.

Temporal changes in the T1 and T2 relaxation rates (DeltaR1 and DeltaR2) in the rat brain are consistent with the tissue-clearance rates of elemental manganese

Temporal changes in the T(1) and T(2) relaxation rates (DeltaR(1) and DeltaR(2)) in rat olfactory bulb (OB) and cortex were compared with the absolute manganese (Mn) concentrations from the corresponding excised tissue samples. In vivo T(1) and T(2) relaxation times were measured before, and at 1, 7, 28, and 35 d after intravenous infusion of 176 mg/kg MnCl(2). The values of DeltaR(1), DeltaR(2), and absolute Mn concentration peaked at day 1 and then declined to near control levels after 28 to 35 d. The Mn bioelimination rate from the rat brain was significantly faster than that reported using radioisotope techniques. The R(1) and R(2) relaxation rates were linearly proportional to the underlying tissue Mn concentration and reflect the total absolute amount of Mn present in the tissue. The in vivo Mn r(1) and r(2) tissue relaxivities were comparable to the in vitro values for aqueous Mn(2+). These results demonstrate that loss of manganese-enhanced MRI (MEMRI) contrast after systemic Mn(2+) administration is due to elimination of Mn(2+) from the brain.

A model for the analysis of competitive relaxation effects of manganese and iron in vivo

Manganese (Mn) and iron (Fe) are both paramagnetic species that can affect magnetic resonance relaxation rates. They also share common transport systems in vivo and thus in experimental models of metal exposure their effects on relaxation rates may interact in a complex fashion. Here we present a novel model to interpret the combined effects of Mn and Fe on MRI relaxation rates. To achieve varying levels of both metals, adult rats were separated into four groups; a control group and three groups treated with weekly intravenous injections of 3 mg Mn/kg body for 14 weeks. The three treated groups were fed either a normal diet, Fe deficient or Fe enriched diet. All rats were scanned using MRI at the 14th week to measure regional water relaxation rates. Rat brains were removed at the end of the study (14th week) and dissected into regions for measurement of Mn and Fe by atomic absorption spectroscopy. For the normal diet groups, R(1) was strongly correlated with tissue Mn concentrations. However, the slopes of the linear regression fits varied significantly among different brain regions, and a simple linear model failed to explain the changes in relaxation rate when both Mn and Fe contents changed. We propose a competition model, which is based on the ability of Mn and Fe to compete in vivo for common binding sites. The combined effect of Mn and Fe on the relaxation rates is complicated and additional studies will be necessary to explain how MRI signals are affected when the levels of both metals are varied.

Manipulation of tissue contrast using contrast agents for enhanced MR microscopy in ex vivo mouse brain

Detailed 3D mouse brain images may promote better understanding of phenotypical differences between normal and transgenic/mutant mouse models. Previously, a number of magnetic resonance microscopy (MRM) studies have successfully established brain atlases, revealing genotypic traits of several commonly used mouse strains. In such studies, MR contrast agents, mainly gadolinium (Gd) based, were often used to reduce acquisition time and improve signal-to-noise ratio (SNR). In this paper, we intended to extend the utility of contrast agents for MRM applications. Using Gd-DTPA and MnCl(2), we exploited the potential use of MR contrast agents to manipulate image contrast by drawing upon the multiple relaxation mechanisms and tissue-dependent staining properties characteristic of each contrast agent. We quantified r(1) and r(2) of Gd-DTPA and MnCl(2) in both aqueous solution and brain tissue and demonstrated the presence of divergent relaxation mechanisms between solution and tissue for each contrast agent. Further analyses using nuclear magnetic resonance dispersion (NMRD) of Mn(2+) in ex vivo tissue strongly suggested macromolecule binding of Mn(2+), leading to increased T(1) relaxation. Moreover, inductively coupled plasma (ICP) mass spectroscopy revealed that MnCl(2) had higher tissue affinity than Gd-DTPA. As a result, multiple regions of the brain stained by the two agents exhibited different image contrasts. Our results show that differential MRM staining can be achieved using multiple MR contrast agents, revealing detailed cytoarchitecture, and may ultimately offer a window for investigating new techniques by which to understand biophysical MR relaxation mechanisms and perhaps to visualize tissue anomalies even at the molecular level.

Effect of paramagnetic manganese cations on (1)H MRS of the brain

Manganese cations (Mn(2+)) can be used as an intracellular contrast agent for structural, functional and neural pathway imaging applications. However, at high concentrations, Mn(2+) is neurotoxic and may influence the concentration of (1)H MR-detectable metabolites. Furthermore, the paramagnetic Mn(2+) cations may also influence the relaxation of the metabolites under investigation. Consequently, the purpose of this study was to investigate the effect of paramagnetic Mn(2+) cations on (1)H-MR spectra of the brain using in vivo and phantom models at 4.7 T. To investigate the direct paramagnetic effects of Mn(2+) cations on the relaxation of N-acetylaspartate (NAA), creatine and choline, T(1) relaxation times of metabolite solutions, with and without 5% albumin, and containing different Mn(2+) concentrations were determined. Relaxivity values with/without 5% albumin for NAA (4.8/28.1 s(-1) mM(-1)), creatine (2.8/2.8 s(-1) mM(-1)) and choline (1.8/1.1 s(-1) mM(-1)) showed NAA to be the most sensitive metabolite to the relaxation effects of the cations. Using an in vivo optic tract tracing imaging model, we obtained two adjacent regions of interest in the superior colliculi with different water T(1) values (Mn(2+)-enhanced = 1.01 s; unenhanced = 1.14 s) 24 h after intravitreal injection of 3 microL 50 mM MnCl(2). Using phantom and in vivo water relaxation time data, we estimated the in vivo Mn(2+) concentration to be 2-8 microM. The phantom data suggest that limited metabolite relaxation effects would be expected at this concentration. Consequently, this study indicates that, in this model, the presence of Mn(2+) cations does not significantly affect (1)H-MR spectra despite possible toxic and paramagnetic effects.

Manganese cell labeling of murine hepatocytes using manganese(III)-transferrin

Manganese(III)-transferrin [Mn(III)-Tf] was investigated as a way to accomplish manganese-labeling of murine hepatocytes for MRI contrast. It is postulated that Mn(III)-Tf can exploit the same transferrin-receptor-dependent and -independent metabolic pathways used by hepatocytes to transport the iron analog Fe(III)-Tf. More specifically, it was investigated whether manganese delivered by transferrin could give MRI contrast in hepatocytes. Comparison of the T1 and T2 relaxation times of Mn(III)-Tf and Fe(III)-Tf over the same concentration range showed that the r1 relaxivities of the two metalloproteins are the same in vitro, with little contribution from paramagnetic enhancement. The degree of manganese cell labeling following incubation for 2-7 h in 31.5 microm Mn(III)-Tf was comparable to that of hepatocytes incubated in 500 microm Mn2+ for 1 h. The intrinsic manganese tissue relaxivity between Mn(III)-Tf-labeled and Mn2+-labeled cells was found to be the same, consistent with Mn(III) being released from transferrin and reduced to Mn2+. For both treatment regimens, manganese uptake by hepatocytes appeared to saturate in the first 1-2 h of the incubation period and may explain why the efficiency of hepatocyte cell labeling by the two methods appeared to be comparable in spite of the approximately 16-fold difference in effective manganese concentration. Hepatocytes continuously released manganese, as detected by MRI, and this was the same for both Mn2+- and Mn(III)-Tf-labeled cells. Manganese release may be the result of normal hepatocyte function, much in the same way that hepatocytes excrete manganese into the bile in vivo. This approach exploits a biological process-namely receptor binding, endocytosis and endosomal acidification-to initiate the release of an MRI contrast agent, potentially conferring more specificity to the labeling process. The ubiquitous expression of transferrin receptors by eukaryotic cells should make Mn(III)-Tf particularly useful for manganese labeling of a wide variety of cells both in culture and in vivo.

Magnetic resonance imaging of cortical connectivity in vivo

Magnetic resonance imaging of neuronal connectivity in vivo opens up the possibility of performing longitudinal investigations on neuronal networks. This is one main reason for the attention that paramagnetic ion manganese (Mn2+) has attracted as a potential anterograde neuronal tracer for MRI experiments. However, the correct and possibly repeated use of this tracer--or of any tracer for that matter, including heavy metals--requires the development of an administration strategy that minimizes its toxic effects. Here we first investigated the conditions that maximize the tracing efficiency of Mn2+ and preserve viability and tissue architectonics in combined MRI and histology experiments in rats. We demonstrate that most common protocols for neuronal tract tracing using Mn2+ result in large neuronal and glial lesions. The toxicity of manganese is distinct during intracortical injections and blocks the transfer of the tracer. After optimizing the technique, we could show that extensive cortical connectivity maps can be generated, with no sign of neuronal damage. Importantly, preservation of tissue viability improves the efficiency of Mn2+ in tracing neuronal connections. We have successfully used this technique to trace corticofugal somatosensory and motor pathways in individual animals and describe a connectivity index (CnI) based on Mn2+ transport that quantitatively reveals cortical heterogeneities in interhemispheric communication. Finally, we have significantly improved the resolution of the technique by continuously infusing very low concentrations of Mn2+ into the target area using osmotic pumps coupled to chronically implanted brain cannulae. The specific, nontoxic and quantitative nature of the neuronal tracings described here indicates the value of this tracer for chronic studies of development and plasticity as well as for studies of brain pathology.

Dynamic water changes in excised rat myocardium assessed by continuous distribution of T1 and T2

Ischemic changes in excised rat myocardium were followed by series of T1 or T2 measurements from 1 to 60 min after isolated perfusion cessation, and the influence of manganese enhancement was investigated. An inverse Laplace transformation (ILT) of T1 or T2 data was used to resolve the number, time constants, and fractions of tissue water components in a continuous distribution. For T1 distributions, one single tissue component approximately 900 ms was significantly shortened and dispersed by manganese enhancement (25 and 200 microM MnCl2). For T2 distributions, three tissue components (approximately 30, approximately 100, and approximately 350 ms) were obtained initially. The two shortest components merged after approximately 10 min to one component (approximately 40 ms). Both T1 and T2 tissue components became shorter with time. In particular, the T2 distribution dynamics might be compatible with complex sequential changes in tissue water fractions during ischemia.

Core/shell quantum dots with high relaxivity and photoluminescence for multimodality imaging

A series of core/shell CdSe/Zn1-xMnxS nanoparticles were synthesized for use in dual-mode optical and magnetic resonance (MR) imaging techniques. Mn2+ content was in the range of 0.6-6.2% and varies with the thickness of the shell or amount of Mn2+ introduced to the reaction. These materials showed high quantum yield (QY), reaching 60% in organic solvent. Water-soluble nanoparticles were obtained by capping the core/shell particles with amphiphilic polymer, and the QY values in water reached 21%. These materials also demonstrated high relaxivity with r1 values in the range of 11-18 mM-1 s-1 (at room temperature, 7 T). Both optical and MR imaging were performed on nanoparticles in aqueous solution and applied to cells in culture. The results showed that the QY and manganese concentration in the particles was sufficient to produce contrast for both modalities at relatively low concentrations of nanoparticles.

An apparent unidirectional influx constant for manganese as a measure of myocardial calcium channel activity

PURPOSE

To develop an in vivo MR method for evaluation of myocardial calcium channel activity through quantification of apparent unidirectional manganese influx constants following manganese dipyridoxyl-diphosphate (MnDPDP) infusions.

MATERIALS AND METHODS

A total of 10 healthy volunteers were divided in two groups, and received 5 micromol of MnDPDP per kg of body weight intravenously in a 1.5 Tesla scanner over five or 30 minutes, respectively. A fast inversion recovery gradient echo sequence was used to estimate pre- and postcontrast R1 values and to measure signal changes following infusions. By assuming equal longitudinal relaxivity (r1) of the contrast in all tissue compartments, signal changes in blood and myocardial tissue yielded temporal input and tissue contrast concentrations respectively. Through a two-tissue compartment model, apparent unidirectional influx constants (Ki) for myocardial manganese accumulation were estimated.

RESULTS

Consistent values for Ki in left ventricular wall were found, with a mean value of 5.96 mL/100 g/minute (SD=0.49; N=10). No statistical significant differences in Ki were found between the two infusion groups.

CONCLUSION

Since unidirectional manganese accumulation depends upon intact myocyte membranes with functioning calcium channels, the use of unidirectional manganese influx rates may be a valuable research tool for in vivo studies of myocyte functioning in myocardial disease.

Manganese dipyridoxyl-diphosphate (MnDPDP) as a viability marker in patients with myocardial infarction

PURPOSE

To evaluate contrast accumulation in left ventricular (LV) myocardium after manganese dipyridoxyl-diphosphate (MnDPDP) administration in patients with recent first time myocardial infarction.

MATERIALS AND METHODS

MnDPDP (5 micromol/kg) was administered to 10 patients with recent myocardial infarction (three to 12 weeks). One slice of interest (SOI) likely to traverse the infarction was chosen, and sectorial pre- and postcontrast longitudinal relaxivity rates (R(1)) and signal changes during infusion were estimated with a fast gradient echo sequence. LV volume and wall thickening were measured in short-axis cine recordings. Infarct localization from R(1) and wall thickening data were compared by vector analyses.

RESULTS

Reduced wall thickening was associated with reduced precontrast R(1) and reduced contrast enhancement. Both remote and infarcted regions showed rapid initial contrast accumulation. In remote regions, this was followed by a continuing slow increase. Mean precontrast R(1) was 0.87 +/- 0.06 second(-1) in infarcted regions and 0.96 +/- 0.03 second(-1) in remote regions (P < 0.001). Mean R(1) change over one hour was 0.24 +/- 0.07 second(-1) in infarcted regions and 0.38 +/- 0.03 second(-1) in remote regions (P < 0.0001).

CONCLUSION

Remote regions showed larger increases in R(1) than infarcted regions. This is most likely due to selective and slow Mn accumulation in viable myocytes.

Manganese-enhanced magnetic resonance imaging (MEMRI) of rat brain after systemic administration of MnCl2: Changes in T1 relaxation times during postnatal development

PURPOSE

To measure regional T(1) changes in the postnatal rat brain following systemic administration of the contrast agent manganese chloride (MnCl(2)).

MATERIALS AND METHODS

MnCl(2) (120 mM) was administered intravenously (i.v.) at 1.25 mL/hour to a dose of 175 mg/kg body weight. MRI experiments were performed on anaesthetized animals (32 male Wistar rats, postnatal days (PDs) 11, 16, 21, and 31) at 2.0 T. Regions of interest (ROIs) were drawn in sagittal slices and placed over five brain regions: olfactory bulb, cerebellum, cortex, thalamus, and hypothalamus. The signal intensities of each ROI were measured and fitted to a three-parameter function to estimate T(1) values.

RESULTS

In the brains of animals who did not receive the contrast agent (control group), we observed a consistent age-dependent decrease in T(1) values. In the brains of manganese-infused animals (manganese group), however, T(1) values were significantly lower than in the control group, indicating the uptake of manganese, but no dependence of T(1) on age was found.

CONCLUSION

Our T(1) measurements indicate that the relative Mn(2+) concentrations are higher in neonates and decrease with brain development. An estimate of the relative cortical concentration of manganese shows a two-fold drop from PD 11 to PD 31.

Analyzing equilibrium water exchange between myocardial tissue compartments using dynamical two-dimensional correlation experiments combined with manganese-enhanced relaxography

Water compartments were identified and equilibrium water exchange was studied in excised rat myocardium enriched with intracellular manganese (Mn(2+)). Standard relaxographic measurements were supplemented with diffusion-T(2) and T(1)-T(2) correlation measurements. In nonenriched myocardium, one T(1) component (800 ms) and three T(2) components (32, 120, and 350 ms) were identified. The correlation measurements revealed fast- and slow-diffusing water fractions with mean diffusion coefficients of 1.2 x 10(-5) and 3.0 x 10(-5) cm(2) s(-1). The two shortest T(2) components, which had different diffusivities, both originated from water in intracellular compartments. A component with longer relaxation time (T(1) approximately equal 2200 ms; T(2) approximately equal 1200 ms), originating from extra-tissue water, was also observed. The presence of this component may lead to erroneous estimations of water exchange rates from multiexponential relaxographic analyses of excised tissues. The tissue T(1) value is strongly reduced with increasing enrichment of Mn(2+), and eventually a second tissue T(1) component emerges, indicating a shift in the equilibrium water exchange between intra- and extracellular compartments from the fast-exchange limit to the slow-exchange regime. Using a two-site water exchange analysis, the lifetime of intracellular water, T(ic), was found to be 475 ms, with a fraction, p(ic), of 0.71.

Contrast enhancement of manganese-hydroxypropyl-tetraacetic acid, an MR contrast agent with potential for detecting differences in myocardial blood flow

PURPOSE

To determine whether the contrast agent MnHPTA has potential for detecting differences in myocardial blood flow.

MATERIALS AND METHODS

R1 in the myocardium was calculated from MR signal intensity measurements in 18 pigs after intravenous injection of 5, 15, or 25 micromol MnHPTA/kg body weight. Measurements were made in each animal after administration at rest and during dobutamine-induced stress.

RESULTS

A difference of approximately 0.1 sec-1 in the R1 increase between rest and stress still remained 31 minutes after administration of 25 micromol MnHPTA/kg body weight. When two consecutive MnHPTA injections were performed, the second injection induced a lower R1 increase than the corresponding first injection.

CONCLUSION

MnHPTA at a dose of 25 micromol/kg body weight (b.w.) has the potential to detect perfusion differences in myocardium. When two consecutive injections of MnHPTA were administered, the R1 change after the second injection was affected by the earlier administration. Therefore, a protocol including more than one administration is not ideal for this contrast agent.

The use of magnetic resonance imaging (MRI) in the study of manganese neurotoxicity

Manganese (Mn), an element found in many foods, is an important and essential nutrient for proper health and maintenance. It is toxic in high doses, however, and exposure to excessive levels can result in the onset of a neurological disorder similar to, but distinct from, Parkinson's disease. Historically, Mn neurotoxicity was most commonly associated with various occupations, such as Mn mining, welding and steel production. More recently, increases in both blood and brain Mn levels have been observed in persons with liver disease or those receiving prolonged parenteral nutrition. Additionally, rodent data suggest that iron deficiency and anemia may be risk factors for Mn neurotoxicity. Clinically, brain Mn accumulation can be monitored in vivo using non-invasive magnetic resonance imaging (MRI) due to the paramagnetic nature of this element. Indeed, MRI has been used in a variety of settings to evaluate the brain Mn deposition in various populations. This review focuses on the use of MRI technology in studies related specifically to Mn neurotoxicity. Thus, we will examine reports using MRI to confirm brain Mn accumulation in human populations, and conclude with data from non-human primate and rodent models of Mn neurotoxicity.

Manganese-calcium interactions with contrast media for cardiac magnetic resonance imaging: a study of manganese chloride supplemented with calcium gluconate in isolated Guinea pig hearts

OBJECTIVES

Manganese ions (Mn) enter cardiomyocytes via calcium (Ca) channels and enhance relaxation intracellularly. To prevent negative inotropy, new Mn-releasing contrast agents have been supplemented with high Ca. The study aim was to investigate how this affects cardiac function and magnetic resonance efficacy.

MATERIALS AND METHODS

MnCl2 based contrast agents, manganese and manganese-calcium (Ca:Mn 10:1), were infused during 4 repeated washin-washout sequences in perfused guinea pig hearts. [Mn] were 10, 50, 100 and 500 microM.

RESULTS

During washin, manganese depressed left ventricular developed pressure (LVDP) by 4, 9, 17, and 53% whereas manganese-calcium increased LVDP by 13, 18, 25, and 56%. After experiments, tissue Mn contents (nmol/g dry wt) were control <40, manganese 3720, and manganese-calcium 1620. T1 was reduced by 85-92% in Mn-enriched hearts.

CONCLUSIONS

High Ca supplements to Mn-releasing contrast agents may be counterproductive by inducing a strong positive inotropic response and by reducing the magnetic resonance efficacy.

Manganese-enhanced MRI of the optic visual pathway and optic nerve injury in adult rats

PURPOSE

To evaluate manganese (Mn2+)-enhanced MRI in a longitudinal study of normal and injured rat visual projections.

MATERIALS AND METHODS

MRI was performed 24 hours after unilateral intravitreal injection of MnCl2 (150 nmol) into adult Fischer rats that were divided into four groups: 1) controls (N = 5), 2) dose-response (N = 10, 0.2-200 nmol), 3) time-response with repeated MRI during 24-168 hours post injection (N = 4), and 4) optic nerve crush (ONC) immediately preceding the MnCl2 injection (N = 7). Control and ONC animals were reinjected with MnCl2 20 days after the first injection, and MRI was performed 24 hours later.

RESULTS

In the control group, the optic projection was visualized from the retina to the superior colliculus, with indications of transsynaptic transport to the cortex. There was a semilogarithmic relationship between the Mn2+ dose and Mn2+ enhancement from 4 to 200 nmol, and the enhancement decayed gradually to 0 by 168 hours. No Mn2+-enhanced signal was detected distal to the ON crush site. In the control group, similar enhancement was obtained after the first and second MnCl2 injections, while in the ONC group the enhancement proximal to the crush site was reduced 20 days post lesion (20 dpl).

CONCLUSION

Mn2+-enhanced MRI is a viable method for temporospatial visualization of normal and injured ON in the adult rat. The observed reduction in the Mn2+ signal proximal to the ONC is probably a result of retrograde damage to the retinal ganglion cells, and not of Mn2+ toxicity.

Practical Route to Relative Diffusion Coefficients and Electronic Relaxation Rates of Paramagnetic Metal Complexes in Solution by Model-Independent Outer-Sphere NMRD. Potentiality for MRI Contrast Agents

The relaxation of electronic spins S of paramagnetic species is studied by the field-dependence of the longitudinal, transverse, and longitudinal in the rotating frame relaxation rates R1, R2, and R1rho of nuclear spins I carried by dissolved probe solutes. The method rests on the model-independent low-frequency dispersions of the outer-sphere (OS) paramagnetic relaxation enhancement (PRE) of these rates due to the three-dimensional relative diffusion of the complex with respect to the probe solute. We propose simple analytical formulas to calculate these enhancements in terms of the relative diffusion coefficient D, the longitudinal electronic relaxation time T1e, and the time integral of the time correlation function of the I-S dipolar magnetic interaction. In the domain of vanishing magnetic field, these parameters can be derived from the low-frequency dispersion of R1 thanks to sensitivity improvements of fast field-cycling nuclear relaxometers. At medium field, we present various approaches to obtain these parameters by combining the rates R1, R2, and R1rho. The method is illustrated by a careful study of the proton PREs of deuterated water HOD, methanol CH3OD, and tert-butyl alcohol (CH3)3COD in heavy water in the presence of a recently reported nonacoordinate Gd(III) complex. The exceptionally slow electronic relaxation of the Gd(III) spin in this complex is confirmed and used to test the accuracy of the method through the self-consistency of the low- and medium-field results. The study of molecular diffusion at a few nanometer scale and of the electronic spin relaxation of other complexed metal ions is discussed.

Cellular MR Imaging

Cellular MR imaging is a young field that aims to visualize targeted cells in living organisms. In order to provide a different signal intensity of the targeted cell, they are either labeled with MR contrast agents in vivo or prelabeled in vitro. Either (ultrasmall) superparamagnetic iron oxide [(U)SPIO] particles or (polymeric) paramagnetic chelates can be used for this purpose. For in vivo cellular labeling, Gd3+- and Mn2+- chelates have mainly been used for targeted hepatobiliary imaging, and (U)SPIO-based cellular imaging has been focused on imaging of macrophage activity. Several of these magneto-pharmaceuticals have been FDA-approved or are in late-phase clinical trials. As for prelabeling of cells in vitro, a challenge has been to induce a sufficient uptake of contrast agents into nonphagocytic cells, without affecting normal cellular function. It appears that this issue has now largely been resolved, leading to an active research on monitoring the cellular biodistribution in vivo following transplantation or transfusion of these cells, including cell migration and trafficking. New applications of cellular MR imaging will be directed, for instance, towards our understanding of hematopoietic (immune) cell trafficking and of novel guided (stem) cell-based therapies aimed to be translated to the clinic in the future.

Shutter-speed analysis of contrast reagent bolus-tracking data: Preliminary observations in benign and malignant breast disease

The standard pharmacokinetic model applied to contrast reagent (CR) bolus-tracking (B-T) MRI (dynamic-contrast-enhanced) data makes the intrinsic assumption that equilibrium transcytolemmal water molecule exchange is effectively infinitely fast. Theory and simulation have suggested that this assumption can lead to significant errors. Recent analyses of animal model experimental data have confirmed two predicted signature inadequacies: a specific temporal mismatch with the B-T time-course and a CR dose-dependent underestimation of model parameters. The most parsimonious adjustment to account for this aspect leads to the "shutter-speed" pharmacokinetic model. Application of the latter to the animal model data mostly eliminates the two signature inadequacies. Here, the standard and shutter-speed models are applied to B-T data obtained from routine human breast examinations. The signature standard model temporal mismatch is found for each of the three invasive ductal carcinoma (IDC) cases and for each of the three fibroadenoma (FA) cases studied. It is effectively eliminated by use of the shutter-speed model. The size of the mismatch is considerably greater for the IDC lesions than for the FA lesions, causing the shutter-speed model to exhibit improved discrimination of malignant IDC tumors from the benign FA lesions compared with the standard model. Furthermore, the shutter-speed model clearly reveals focal "hot spots" of elevated CR perfusion/permeation present in only the malignant tumors.

Manganese-enhanced magnetic resonance imaging (MEMRI): methodological and practical considerations

Manganese-enhanced MRI (MEMRI) is being increasingly used for MRI in animals due to the unique T1 contrast that is sensitive to a number of biological processes. Three specific uses of MEMRI have been demonstrated: to visualize activity in the brain and the heart; to trace neuronal specific connections in the brain; and to enhance the brain cytoarchitecture after a systemic dose. Based on an ever-growing number of applications, MEMRI is proving useful as a new molecular imaging method to visualize functional neural circuits and anatomy as well as function in the brain in vivo. Paramount to the successful application of MEMRI is the ability to deliver Mn2+ to the site of interest at an appropriate dose and in a time-efficient manner. A major drawback to the use of Mn2+ as a contrast agent is its cellular toxicity. Therefore, it is critical to use as low a dose as possible. In the present work the different approaches to MEMRI are reviewed from a practical standpoint. Emphasis is given to the experimental methodology of how to achieve significant, yet safe, amounts of Mn2+ to the target areas of interest.