Accounting for nonspecific enhancement in neuronal tract tracing using manganese enhanced magnetic resonance imaging

Manganese enhanced MRI (MEMRI) is an emerging technique for tracing neuronal pathways in vivo. However, manganese may leak into blood vessels or cerebrospinal fluid (CSF) after local injection and can be circulated to and taken up by brain regions that may not have connections to the targeted pathways. Comparing enhancement time courses after intranasal injection with intravenous infusion of MnCl₂ in rats, the early enhancements in the pituitary gland (Pit) and hippocampus indicate the contrasts in those regions in the olfactory tract-tracing experiment were caused by such systemic effects. Since the Pit has easy access to manganese from the blood and its signal is proportional to other brain regions after intravenous infusion, it was used as an internal reference for the systemic effects. Applying intensity normalization by the Pit signal to tract-tracing data from the olfactory bulb led to reduced contrast in the hippocampus. These results demonstrate that nonspecific enhancements in MEMRI tract-tracing studies may have to be taken into account and that normalization by the Pit signal can compensate these effects.     

Regional distribution of manganese found in the brain after injection of a single dose of manganese-based contrast agents

Manganese (Mn) complexes are unstable and dissociate in vivo. Because of the release of this metal, there exists some concern about the potential long-term neurotoxicity associated with the use of Mn-based contrast agents. This latter problem arises because manganese is known to accumulate in specific regions of the brain of people intoxicated by this metal. It was previously demonstrated that Mn can accumulate in the mice brain after administration of 5 μmol/kg of MnCl₂, Mn-diethylenetriaminepentaacetate (Mn-DTPA), or Mn-dipyridoxal diphosphate (Mn-DPDP). In order to better characterize the behavior of Mn complexes after administration, this study assesses the regional distribution of Mn in the brain after i.v. injection of a single dose of MnCl₂ or Mn-DTPA. Male Wistar rats received an i.v. injection of 5 μmol/kg of ⁵⁴Mn as MnCl₂ or Mn-DTPA. The rats were killed at one and two weeks post exposure. The distribution of the radioactivity in the slices was monitored by autoradiography. For both MnCl₂ or Mn-DTPA, we observed that the radioactivity was dispersed in the entire brain, but the radioactivity was higher in several regions. No difference was observed between MnCl₂ or Mn-DTPA in the regional distribution of Mn, and no difference was observed between the two times of exposure (1 week or 2 weeks). The uptake of Mn was minimal in corpus callosum. Maximal Mn concentration was observed in the hippocampal region, thalamus, colliculi, amygdala, olfactory nuclei, and cerebellum.     

Neuronal projections from ventral tegmental area to forebrain structures in rat studied by manganese-enhanced magnetic resonance imaging

Manganese-enhanced magnetic resonance imaging (MEMRI) has been widely applied to trace neuronal tracts and to monitor morphological and functional responses of specific brain circuits to changes in physiological and/or environmental conditions. In this study, we traced the efferent axonal projections from ventral tegmental area (VTA) to forebrain structures, an integrating part of the reward circuit implicated in drug addiction, in rats using MEMRI. Urethane- and chloral hydrate-anesthetized rats received injection of 100 nl of 200 mM MnCl(2) solution into the right VTA. Mn(2+)-induced signal enhancements were monitored 24 h after injection. The dose of MnCl(2) injection was shown, by histological evaluation, to have minor toxic effects to the neurons in/near the injection site. Dynamic Mn(2+)-induced signal intensity changes in urethane-anesthetized rats during a 24-h period were fit to a sigmoidal function to obtain parameters slope and t(50), which describe the dynamics of apparent Mn(2+)accumulation. The results showed that most of the forebrain structures known to receive neuronal projections from the VTA, including prefrontal cortex, nucleus accumbens, globus pallidus and caudate putaman, were enhanced at 24 h after injection of MnCl(2) into the ipsilateral VTA, and anesthesia seemed have little effects on the amount of Mn(2+)being transported from the VTA to these structures.     

Empirical mathematical model for dynamic manganese-enhanced MRI of the murine pancreas for assessment of β-cell function

Autoimmune ablation of pancreatic β-cells and alteration of its microvasculature may be a predictor of Type I diabetes development. A dynamic manganese-enhanced MRI (MEMRI) approach and an empirical mathematical model were developed to monitor whole pancreatic β-cell function and vasculature modifications in mice. Normal and streptozotocin-induced diabetic FVB/N mice were imaged on a 9.4 T MRI system using a 3D magnetization prepared rapid acquisition gradient echo pulse sequence to characterize low dose manganese kinetics in the pancreas head, body and tail. Average signal enhancement in the pancreas (head, body, and tail) as a function of time was fit by a novel empirical mathematical model characterizing contrast uptake/washout rates and yielding parameters describing peak signal, initial slope, and initial area under the curve. Signal enhancement from glucose-induced manganese uptake was fit by a linear function. The results demonstrated that the diabetic pancreatic tail had a significantly lower contrast uptake rate, smaller initial slope/initial area under the curve, and a smaller rate of Mn uptake following glucose activation (p < 0.05) compared to the normal pancreatic tail. These observations parallel known patterns of β-cell loss and alteration in supportive vasculature associated with diabetes. Dynamic MEMRI is a promising technique for assessing β-cell functionality and vascular perfusion with potential applications for monitoring diabetes progression and/or therapy.     

Real-time animal functional magnetic resonance imaging and its application to neuropharmacological studies

In pharmacological magnetic resonance imaging (phMRI) with anesthetized animals, there is usually only a single time window to observe the dynamic signal change to an acute drug administration since subsequent drug injections are likely to result in altered response properties (e.g., tolerance). Unlike the block-design experiments in which fMRI signal can be elicited with multiple repetitions of a task, these single-event experiments require stable baseline in order to reliably identify drug-induced signal changes. Such factors as subject motion, scanner instability and/or alterations in physiological conditions of the anesthetized animal could confound the baseline signal. The unique feature of such functional MRI (fMRI) studies necessitates a technique that is able to monitor MRI signal in a real-time fashion and to interactively control certain experimental procedures. In the present study, an approach for real-time MRI on a Bruker scanner is presented. The custom software runs on the console computer in parallel with the scanner imaging software, and no additional hardware is required. The utility of this technique is demonstrated in manganese-enhanced MRI (MEMRI) with acute cocaine challenge, in which temporary disruption of the blood-brain barrier (BBB) is a critical step for MEMRI experiments. With the aid of real-time MRI, we were able to assess the outcome of BBB disruption following bolus injection of hyperosmolar mannitol in a near real-time fashion prior to drug administration, improving experimental success rate. It is also shown that this technique can be applied to monitor baseline physiological conditions in conventional fMRI experiments using blood oxygenation level-dependent (BOLD) contrast, further demonstrating the versatility of this technique.     

MRI contrast-dose relationship of manganese(III)tetra(4-sulfonatophenyl) porphyrin with human xenograft tumors in nude mice at 2.0 T

Previously we reported that Mn(III)tetra(4-sulfonatophenyl) porphyrin, MnTPPS₄, is a contrast agent which can effectively enhance tumor detection by MRI. By imaging 30 additional athymic nude mice bearing subcutaneous MCF-7 WT human breast carcinoma xenografts, we have extended dose-contrast relationships over a wide range of intraperitoneal (IP) doses ranging from 0.025 to 0.50 mmol/kg. The benefits of IP injection are higher possible doses on a volume basis and a reduction in toxicity versus IV administration. Full coronal cross-section images have been obtained on a 2-T spectrometer. Although individual tumor masses displayed different distribution patterns, reflective of their internal morphology, single doses of 0.10 mmol/kg or greater were necessary to produce a detectable effect. At a dose of 0.50 mmol/kg, marked enhancement was produced. Multiple small dosages administered over the course of several days before imaging did not produce increased enhancement. Preliminary results on the new porphyrin derivative, MnTPPS₃, indicate that the ratio of the toxic dose to the effective dose may be adjustable to render this class of agents clinically useful.     

Ocular integrity following manganese labeling of the visual system for MRI

Injection of manganese into the eye will enhance the contrast of visual system neuronal pathways imaged by MRI (MEMRI). The present study was undertaken to determine the effect of a range of MnCl₂ doses upon the integrity of various ocular structures. Anesthetized mice received ocular anterior chamber injections of 50–500 nmol of MnCl₂. One week later, the eyes were fixed, embedded in paraffin, sectioned, and stained with hematoxylin and eosin. Additional animals received 50 nmol of MnCl₂ injected into the anterior chamber and were later imaged using T1-weighted 7 T MRI. Following 500 and 300 nmol MnCl₂, the corneal stroma and endothelium were degenerated, the anterior chamber contained a dense fibrin matrix with extensive inflammatory cell infiltration, a plaque often formed on the anterior lens, and significant retinal degeneration was observed. Following 100 nmol MnCl₂, retinal preservation of ocular structures was significantly better than at higher doses. In addition, there was no difference from vehicle control retina in cell counts within the ganglion cell layer, or in the width of the inner nuclear layer or outer nuclear layer. Also, there was no difference in the thickness of the inner plexiform layer. However, there was thinning of the peripheral outer plexiform layer, as well as in the outer segment layer. Visual system elements labeled in MRI of mice that received 100 nmol MnCl₂ included the retina, optic nerve, lateral geniculate nucleus, and superior colliculus. The preservation of ganglion cell layer cell counts and inner plexiform layer thickness following 100 nmol MnCl₂ suggests there was negligible injury to RGCs following this dose. These results support using 100 nmol MnCl₂ in mouse eyes for in vivo assessment of the integrity of RGC projections to target neurons in the brain.     

Iterative algorithm for spatial and intensity normalization of MEMRI images. Application to tract-tracing of rat olfactory pathways

Manganese (Mn)-enhanced magnetic resonance imaging (MEMRI) is an emerging technique for visualizing neuronal pathways and mapping brain activity modulation in animal models. Spatial and intensity normalizations of MEMRI images acquired from different subjects are crucial steps as they can influence the results of groupwise analysis. However, no commonly accepted procedure has yet emerged. Here, a normalization method is proposed that performs both spatial and intensity normalizations in a single iterative process without the arbitrary choice of a reference image. Spatial and intensity normalizations benefit from this iterative process. On one hand, spatial normalization increases the accuracy of region of interest (ROI) positioning for intensity normalization. On the other hand, improving the intensity normalization of the different MEMRI images leads to a better-averaged target on which the images are spatially registered. After automatic fast brain segmentation and optimization of the normalization process, this algorithm revealed the presence of Mn up to the posterior entorhinal cortex in a tract-tracing experiment on rat olfactory pathways. Quantitative comparison of registration algorithms showed that a rigid model with anisotropic scaling is the best deformation model for intersubject registration of three-dimensional MEMRI images. Furthermore, intensity normalization errors may occur if the ROI chosen for intensity normalization intersects regions where Mn concentration differs between experimental groups. Our study suggests that cross-comparing Mn-injected animals against a Mn-free group may provide a control to avoid bias introduced by intensity normalization quality. It is essential to optimize spatial and intensity normalization as the detectability of local between-group variations in Mn concentration is directly tied to normalization quality.     

Gd-enhanced MR imaging of brain metastases: Contrast as a function of dose and lesion size

MR imaging contrast of brain metastases after cumulative doses of gadolinium chelate is quantitated and compared in order to assess the clinical utility of high dosage. T1-weighted spin-echo MR images of 39 patients with metastatic brain tumors were made before and after each of three equal doses cumulating to 0.1, 0.2 and 0.3 mmol Gd-complex per kg body weight. Quantitation of MRI contrast was limited to homogeneous brain metastases larger than 3 mm (n = 246). Post-Gd MRI contrast doubled with dose escalation from 0.1 to 0.3 mmol/kg and also increased with lesion size, by a factor of 2.5 between metastases of 3 and 16 mm diameter, that is after correcting for partial volume effect. At 0.2 and 0.3 mmol/kg the respective numbers of visible metastases increased by 15% and 43% compared with 0.1 mmol/kg (p < 0.0001, both). Image contrast figures differed significantly between doses (p = 0.018). Both the number of metastases and the image contrast is significantly higher when dose escalation is performed. It is indicated that the number of detected metastases will increase further at Gd doses beyond 0.3 mmol/kg. Post-Gd MRI contrast increases with lesion size, to an extent that can not be attributed to partial volume attenuation.     

Behavioral,electrophysiological and histopathological consequences of systemic manganese administration in MEMRI

Manganese (Mn²⁺)-enhanced magnetic resonance imaging (MEMRI) offers the possibility to generate longitudinal maps of brain activity in unrestrained and behaving animals. However, Mn²⁺ is a metabolic toxin and a competitive inhibitor for Ca²⁺, and therefore, a yet unsolved question in MEMRI studies is whether the concentrations of metal ion used may alter brain physiology. In the present work we have investigated the behavioral, electrophysiological and histopathological consequences of MnCl₂ administration at concentrations and dosage protocols regularly used in MEMRI. Three groups of animals were sc injected with saline, 0.1 and 0.5 mmol/kg MnCl₂, respectively. In vivo electrophysiological recordings in the hippocampal formation revealed a mild but detectable decrease in both excitatory postsynaptic potentials (EPSP) and population spike (PS) amplitude under the highest MnCl₂ dose. The EPSP to PS ratio was preserved at control levels, indicating that neuronal excitability was not affected. Experiments of pair pulse facilitation demonstrated a dose dependent increase in the potentiation of the second pulse, suggesting presynaptic Ca²⁺ competition as the mechanism for the decreased neuronal response. Tetanization of the perforant path induced a long-term potentiation of synaptic transmission that was comparable in all groups, regardless of treatment. Accordingly, the choice accuracy tested on a hippocampal-dependent learning task was not affected. However, the response latency in the same task was largely increased in the group receiving 0.5 mmol/kg of MnCl₂. Immunohistological examination of the hippocampus at the end of the experiments revealed no sign of neuronal toxicity or glial reaction. Although we show that MEMRI at 0.1 mmol/Kg MnCl₂ may be safely applied to the study of cognitive networks, a detailed assessment of toxicity is strongly recommended for each particular study and Mn²⁺ administration protocol.     

A comparison of the sensitivity of MRI after double- and triple-dose Gd-DTPA for detecting enhancing lesions in multiple sclerosis

We compared the number and volume of enhancing lesions detected in patients with multiple sclerosis (MS) seen on post-contrast T(1)-weighted scans obtained after the injection of different gadolinium-DTPA (Gd) doses. Enhanced magnetic resonance imaging (MRI) scans were obtained from 16 patients with relapsing remitting or secondary progressive MS on two different occasions separated by an interval of approximately 24 h. On the first occasion, enhanced scans were obtained 15 min after the injection of a double dose of Gd (0.2 mmol/Kg), on the second 15 min after the injection of a triple dose (0.3 mmol/Kg) of Gd. Scans were assessed by consensus in a random order by two observers unaware of the dose of Gd used. We counted the same 30 enhancing lesions on both double dose and triple dose scans from 9 patients. The mean (SD) volumes of enhancing lesions were 1.7 (2.7) mL on double dose and 1.9 (3.4) mL on triple-dose scans. This difference was not statistically significant. This study demonstrated that double dose of Gd has a sensitivity for detecting MS activity similar to that of a triple dose, with the advantage of a significant cost saving.     

Influence of the hepatobiliary contrast agent mangafodipir trisodium (Mn-DPDP) on the imaging properties of abdominal organs

To assess the influence of Mangafodipir Trisodium on the imaging properties of abdominal organs when using T₁-weighted gradient-echo (GE) and T₂-weighted turbo spin-echo (TSE) sequences, thirty patients with focal lesions in the liver were examined at a field strength of 1.5 T before and after intravenous administration of Mangafodipir Trisodium (dose: 5 μmol/kg of body weight).Administration of Mangafodipir Trisodium led to a significant increase in the signal intensity of the liver tissue (p < 0.001), the spleen (p < 0.01), the pancreas (p < 0.001), and the kidneys (p < 0.001) in the T₁-weighted GE sequence, while there was no relevant enhancement in fatty tissue and the musculature. In the T₂-weighted turbo spin-echo sequence, there was no relevant change in the signal following administration of a contrast agent. The contrast-to-noise ratio (C/N) between the lesions and the liver tissue increased significantly in the post-contrast T₁-weighted GE sequence (p < 0.001), while there was no change in the contrast-to-noise ratio in the post-contrast T₂-weighted turbo spin-echo sequence. The contrast-to-noise ratio of the plain T₂-weighted TSE sequence was significantly higher than that in the post-contrast T₁-weighted GE sequence (p < 0.001). Although Mangafodipir Trisodium was primarily developed as a hepatobiliary contrast agent for demonstration and differentiation of liver lesions, it also affects the signal levels in the pancreas, spleen, and kidneys in the T₁-weighted image. Awareness of this effect on the extrahepatic tissue makes it easier to interpret pathological findings in magnetic resonance imaging (MRI) of the abdomen.     

柠檬醛长时程嗅觉吸入的脑功能活动MEMRI研究

本文使用设计的嗅觉吸入装置使大鼠吸入挥发性柠檬醛或无气味空气，之后使用锰增强磁共振成像（MEMRI）的方法，观察长时程（24 h）吸入柠檬醛之后，大鼠脑功能活动变化情况．基于体素分析和感兴趣区（ROI）分析结果显示，与对照组相比，长时程吸入柠檬醛组大鼠伏隔核中心部（AcbC）、嗅小球层（GL）等脑区功能活动增强；而该组大鼠视皮层（VC）、听觉皮层（AC）、压后皮层（RSC）等脑区锰累积显著减少．且长时程吸入柠檬醛之后，大鼠脑区GL与关联的脑区功能相关性显著增强．从而表明MEMRI可用于长时程嗅觉吸入刺激的脑功能研究，并极具应用前景．     

Intratympanic manganese administration revealed sound intensity and frequency dependent functional activity in rat auditory pathway

The cochlear plays a vital role in the sense and sensitivity of hearing; however, there is currently a lack of knowledge regarding the relationships between mechanical transduction of sound at different intensities and frequencies in the cochlear and the neurochemical processes that lead to neuronal responses in the central auditory system. In the current study, we introduced manganese-enhanced MRI (MEMRI), a convenient in vivo imaging method, for investigation of how sound, at different intensities and frequencies, is propagated from the cochlear to the central auditory system. Using MEMRI with intratympanic administration, we demonstrated differential manganese signal enhancements according to sound intensity and frequencies in the ascending auditory pathway of the rat after administration ofintratympanicMnCl₂.Compared to signal enhancement without explicit sound stimuli, auditory structures in the ascending auditory pathway showed stronger signal enhancement in rats who received sound stimuli of 10 and 40 kHz. In addition, signal enhancement with a stimulation frequency of 40 kHz was stronger than that with 10 kHz. Therefore, the results of this study seem to suggest that, in order to achieve an effective response to high sound intensity or frequency, more firing of auditory neurons, or firing of many auditory neurons together for the pooled neural activity is needed.     

Identifying non-toxic doses of manganese for manganese-enhanced magnetic resonance imaging to map brain areas activated by operant behavior in trained rats

Manganese-enhanced magnetic resonance imaging (MEMRI) offers unique advantages such as studying brain activation in freely moving rats, but its usefulness has not been previously evaluated during operant behavior training. Manganese in a form of MnCl₂, at a dose of 20 mg/kg, was intraperitoneally infused. The administration was repeated and separated by 24 h to reach the dose of 40 mg/kg or 60 mg/kg, respectively. Hepatotoxicity of the MnCl₂ was evaluated by determining serum aspartate aminotransferase, alanine aminotransferase, total bilirubin, albumin and protein levels. Neurological examination was also carried out. The animals were tested in visual cue discriminated operant task. Imaging was performed using a 3T clinical MR scanner. T1 values were determined before and after MnCl₂ administrations. Manganese-enhanced images of each animal were subtracted from their baseline images to calculate decrease in the T1 value (ΔT1) voxel by voxel. The subtracted T1 maps of trained animals performing visual cue discriminated operant task, and those of naive rats were compared. The dose of 60 mg/kg MnCl₂ showed hepatotoxic effect, but even these animals did not exhibit neurological symptoms. The dose of 20 and 40 mg/kg MnCl₂ increased the number of omissions and did not affect the accuracy of performing the visual cue discriminated operant task. Using the accumulated dose of 40 mg/kg, voxels with a significant enhanced ΔT1 value were detected in the following brain areas of the visual cue discriminated operant behavior performed animals compared to those in the controls: the visual, somatosensory, motor and premotor cortices, the insula, cingulate, ectorhinal, entorhinal, perirhinal and piriform cortices, hippocampus, amygdala with amygdalohippocampal areas, dorsal striatum, nucleus accumbens core, substantia nigra, and retrorubral field. In conclusion, the MEMRI proved to be a reliable method to accomplish brain activity mapping in correlation with the operant behavior of freely moving rodents.     

Enhanced MRI of tumors utilizing a new nitroxyl spin label contrast agent

Nitroxyl spin labels have been shown to be effective in vivo contrast agents for magnetic resonance imaging (MRI) of the central nervous system, myocardium, and urinary tract. A new pyrrolidine nitroxyl contrast agent (PCA) with better resistance to in vivo metabolic inactivation than previously tested agents was studied for its potential to enhance subcutaneous neoplasms in an animal model. Twenty-two contrast enhancement trials were performed on a total of 15 animals 4-6 weeks after implantation with human renal adenocarcinoma. Spin echo imaging was performed using a .35 T animal imager before and after intravenous administration of PCA in doses ranging from 0.5 to 3mM/kg. The intensity of tumor tissue in the images increased an average of 35% in animals receiving a dose of 3 mM/kg. The average enhancement with smaller doses was proportionately less. Tumor intensity reached a maximum within 15 min of injection. The average intensity difference between tumor and adjacent skeletal muscle more than doubled following administration of 3 mM/kg of PCA. Well-perfused tumor tissue was more intensely enhanced than adjacent poorly perfused and necrotic tissue.     

Manganese-enhanced magnetic resonance imaging investigation of the interferon-α model of depression in rats

Therapeutic effects of interferon-α (IFN-α) are known to be associated with CNS toxicity in humans, and in particular with depression symptoms. Animal models of IFN-α-induced depression (sickness behaviour) have been developed in rodents using various preparations, dosing schedules or routes of administrations. In this work, Manganese Enhanced MRI (MEMRI) has been applied to investigate an experimental model of sickness behaviour induced by administration of IFN-α in rats. IFN-α (3.10⁵ U/kg), or vehicle, was daily administered i.p., for 7 days in rats (n = 20 IFN-α treated and n = 20 controls). After treatment, animals were assigned to behavioural (n = 10 treated, n = 10 control) or MRI (n = 10 treated and n = 10 control) studies. Animals assigned to the MRI study received two repeated i.p. injections of MnCl₂, before image acquisition. Images were acquired at 4.7 T using T₁ mapping for determination of Mn concentration in brain. After co-registration of T₁ maps to a digital brain atlas, differences between brains of treated and untreated animals were assessed pixel-to-pixel by statistical analysis.     

Characterization of cryoinjury-induced infarction with manganese-and gadolinium-enhanced MRI and optical spectroscopy in pig hearts

Purpose

To investigate progression of cryoinjury in pigs using contrast-enhanced magnetic resonance imaging (MRI) as well as optical spectroscopy and imaging.

Methods

Cryoinjury was produced in 16 pigs in vivo and investigated using Gd-and Mn-enhanced MRI, optical imaging/spectroscopy and histology in acute and chronic setting up to 4 weeks after the injury.

Results

(1) Acute cryoinjury resulted in formation of a lesion with a severely reduced rate of sub-epicardial indocyanine green (intravascular optical flow tracer) passage. In vivo late Gd-enhanced MRI showed a ∼10 mm deep hypointense area that was surrounded by a hyperintense rim while ex vivo Mn-enhanced MRI (MEMRI) detected a homogenous hypointense zone. Histological and spectroscopic examination revealed embolic erythrocytes blockages within the cryolesion with a thin necrotic rim neighboring the normal myocardium. (2) Chronic 4-week cryoinjury was characterized by uniform Gd-enhancement, whereas MEMRI revealed reduced Mn²⁺enhancement. Histological examination showed replacement of the cryoinjured myocardium by scar tissue.

Conclusions

Acute cryoinjury resulted in formation of a no-reflow core embolized by erythrocytes and surrounded by a rim of necrotic tissue. Upon injury progression, the no-reflow zone shrunk and was completely replaced with scar tissue by 4 weeks after injury.     

Proton relaxation enhancement by manganese(III)TPPS4 in a model tumor system

Managanese(III)tetraphenylporphine sulfonate [Mn(III)TPPS4] has been investigated as a tumor specific paramagnetic contrast agent for magnetic resonance imaging (MRI) of L1210 solid tumors in mice. Mn(III)TPPS4 was found to clear rapidly from the blood and concentrate in the kidneys, tumor and liver. Although relatively high ratios of tumor to normal tissues could be obtained (e.g., greater than 90 for tumor/muscle), the kidneys were found to have the highest concentration of the metalloporphyrin at all doses and time periods tested. A significant decrease in the longitudinal relaxation time was measured for excised tissues (kidney, tumor, liver, muscle) from mice that were treated with Mn(III)TPPS4. A linear correlation was observed between the longitudinal relaxation rate determined for L1210 tumor and the corresponding concentration of Mn(III)TPPS4 found at various injected doses and time intervals between the injection and analysis. A small animal radiofrequency receiver coil designed for use with a 0.15-T clinical imager was employed to evaluate the ability of Mn(III)TPPS4 to selectively increase the signal intensity of the implanted L1210 tumor. The images show a conspicuous enhancement in the contrast between the tumor and adjacent tissue upon treatment with this agent. The results indicate that Mn(III)TPPS4 is a useful prototype paramagnetic metalloporphyrin MRI contrast agent with a significant affinity for the L1210 tumor.     

Manganese-enhanced MRI of rat spinal cord injury

The potential of the manganese-enhanced MRI (MEI) technique in labeling the intact neuronal circuitry of rat spinal cord was examined. Experiments were conducted on normal and injured cords at 9.4-T magnetic field strength using an implantable rf coil. The contrast agent manganese (Mn) was locally delivered within the parenchyma at a dose of 25 mmol/L in 10 nL. The transport, uptake and accumulation of Mn in tissue were then followed remotely on T1-weighted images that were acquired serially from the cord. In MEIs of normal cord, Mn was observed to be transported in directions both rostral and caudal to the site of injection. In the cord that was subjected to hemisection, signal enhancement was on the contralesional side of the cord, but not at the ipsilesional side. The sensitivity and specificity of the MEI technique in labeling the neurons that are functional were also validated with a traditional track-tracing method using biotinylated dextran amine.