Applications of manganese-enhanced magnetic resonance imaging (MEMRI) to imaging of the heart

A Bis(Aquated) Mn(II)-Based MRI Contrast Agent with a Rigid Hydroquinazoline Unit: Synthesis, Characterization, and in Vivo MR Imaging Study

Since the finding of nephrogenic systemic fibrosis (NFS) in patients with renal impairment and the long-term accumulation of Gd(III) ions in the central nervous system, the search for nongadolinium ion-based MRI contrast agents made of nutrient metal ions has drawn paramount attention. In this context, the development of Mn(II)-based MRI contrast agents has been a subject of interest for the last few decades. Herein, we report a pentadentate ligand (Li2[BenzPic2]) composed of two picolinate moieties and a rigid 1,2,3,4-tetrahydroquinazoline unit and the corresponding bis(aquated) Mn(II) complex (Complex 1). The complex exhibited high thermodynamic stability (log Kcond = 11.62) and kinetic inertness similar to that of the clinically approved Gd(III)-based contrast agent Magnevist. Complex 1 exerted longitudinal relaxivity (r1) of 5.32 mM-1 s-1 at 1.41 T, 37 °C, pH 7.4, and it increased by 3.6-fold in the presence of serum albumin protein, confirming a substantial rigidifying interaction (albumin association constant KA = 1.66 × 103 M-1) between the protein and the amphiphilic (log P = -0.45) contrast agent. An intravenous dose of 0.08 mmol/kg in a healthy mouse, excellent MRI signal intensity enhancement in the vasculature of the mouse liver, and brightened images of the gallbladder, kidney, and liver were realized.

Myocardial Calcium Handling in Type 2 Diabetes: A Novel Therapeutic Target

Type 2 diabetes (T2D) is a multisystem disease with rapidly increasing global prevalence. Heart failure has emerged as a major complication of T2D. Dysregulated myocardial calcium handling is evident in the failing heart and this may be a key driver of cardiomyopathy in T2D, but until recently this has only been demonstrated in animal models. In this review, we describe the physiological concepts behind calcium handling within the cardiomyocyte and the application of novel imaging techniques for the quantification of myocardial calcium uptake. We take an in-depth look at the evidence for the impairment of calcium handling in T2D using pre-clinical models as well as in vivo studies, following which we discuss potential novel therapeutic approaches targeting dysregulated myocardial calcium handling in T2D.

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.

Repeatability and reproducibility of cardiac manganese-enhanced magnetic resonance imaging

Manganese-enhanced magnetic resonance imaging can provide a surrogate measure of myocardial calcium handling. Its repeatability and reproducibility are currently unknown. Sixty-eight participants: 20 healthy volunteers, 20 with acute myocardial infarction, 18 with hypertrophic and 10 with non-ischemic dilated cardiomyopathy underwent manganese-enhanced magnetic resonance imaging. Ten healthy volunteers were re-scanned at 3 months. Native T1 values and myocardial manganese uptake were assessed for intra and inter-observer repeatability. Scan-rescan reproducibility was assessed in ten healthy volunteers. Intra-observer and inter-observer correlation was excellent in healthy volunteers for mean native T1 mapping [Lin's correlation coefficient (LCC) 0.97 and 0.97 respectively] and myocardial manganese uptake (LCC: 0.99 and 0.96 respectively). Scan-rescan correlation for native T1 and myocardial manganese uptake was also excellent. Similarly, intra-observer correlations for native T1 and myocardial manganese uptake in patients with acute myocardial infarction (LCC: 0.97 and 0.97 respectively), hypertrophic (LCC: 0.98 and 0.97 respectively) and dilated cardiomyopathy (LCC: 0.99 and 0.95 respectively) were excellent. Limits of agreement were broader in patients with dilated cardiomyopathy. Manganese-enhanced magnetic resonance imaging has high repeatability and reproducibility in healthy myocardium and high repeatability in diseased myocardium. However, further study is needed to establish robustness in pathologies with diffuse myocardial fibrosis.

Nanodiamonds Doped with Manganese for Applications in Magnetic Resonance Imaging

Nanodiamonds (NDs) are emerging with great potential in biomedical applications like biomarking through fluorescence and magnetic resonance imaging (MRI), targeted drug delivery, and cancer therapy. The magnetic and optical properties of NDs could be tuned by selective doping. Therefore, we report multifunctional manganese-incorporated NDs (Mn-NDs) fabricated by Mn ion implantation. The fluorescent properties of Mn-NDs were tuned by inducing the defects by ion implantation and enhancing the residual nitrogen vacancy density achieved by a two-step annealing process. The cytotoxicity of Mn-NDs was investigated using NCTC clone 929 cells, and the results revealed no cytotoxicity effect. Mn-NDs have demonstrated dual mode contrast enhancement for both T ₁- and T ₂-weighted in vitro MR imaging. Furthermore, Mn-NDs have illustrated a significant increase in longitudinal relaxivity (fivefold) and transversal relaxivity (17-fold) compared to the as-received NDs. Mn-NDs are employed to investigate their ability for in vivo MR imaging by intraperitoneal (ip) injection of Mn-NDs into mice with liver tumors. After 2.5 h of ip injection, the enhancement of contrast in T ₁- and T ₂-weighted images has been observed via the accumulation of Mn-NDs in liver tumors of mice. Therefore, Mn-NDs have great potential for in vivo imaging by MR imaging in cancer therapy.

Albumin-templated manganese carbonate nanoparticles for precise magnetic resonance imaging of acute myocardial infarction

Myocardial infarction (MI) is a major cause of death worldwide. Early and precise diagnosis of myocardial viability after MI is extremely important for effective treatment and prognosis evaluation. Herein, we developed the BSA-templated manganese carbonate (MnCO₃@BSA) nanoparticles as an MR imaging contrast agent for accurate detection of the infarcted regions. The chemophysical features, targeting capability toward the infarct, and biocompatibility were evaluated. The nanoparticles showed superior chemical stability. In vitro study suggested that the MnCO₃@BSA nanoparticles do not enter normal cardiomyocytes. MR imaging indicated that the MnCO₃@BSA with a high longitudinal (r₁) relaxivity of 5.84 mM^-1s^-1 at physiological condition specifically accumulated into the infarcted regions of myocardial ischemia/reperfusion (I/R) mice. In addition, the MnCO₃@BSA nanoparticles exhibited low cytotoxicity to cardiomyocytes, no damage to organs and good hemocompatibility. Thereby, the MnCO₃@BSA nanoparticles manifested great potential as an extracellular contrast agent of MR imaging for sensitive and specific detection of the infarcted regions during acute myocardial I/R injury.

Myocardial Viability Imaging using Manganese-Enhanced MRI in the First Hours after Myocardial Infarction

Early measurements of tissue viability after myocardial infarction (MI) are essential for accurate diagnosis and treatment planning but are challenging to obtain. Here, manganese, a calcium analogue and clinically approved magnetic resonance imaging (MRI) contrast agent, is used as an imaging biomarker of myocardial viability in the first hours after experimental MI. Safe Mn2+ dosing is confirmed by measuring in vitro beating rates, calcium transients, and action potentials in cardiomyocytes, and in vivo heart rates and cardiac contractility in mice. Quantitative T1 mapping-manganese-enhanced MRI (MEMRI) reveals elevated and increasing Mn2+ uptake in viable myocardium remote from the infarct, suggesting MEMRI offers a quantitative biomarker of cardiac inotropy. MEMRI evaluation of infarct size at 1 h, 1 and 14 days after MI quantifies myocardial viability earlier than the current gold-standard technique, late-gadolinium-enhanced MRI. These data, coupled with the re-emergence of clinical Mn2+ -based contrast agents open the possibility of using MEMRI for direct evaluation of myocardial viability early after ischemic onset in patients.

MnDPDP: Contrast Agent for Imaging and Protection of Viable Tissue

The semistable chelate manganese (Mn) dipyridoxyl diphosphate (MnDPDP, mangafodipir), previously used as an intravenous (i.v.) contrast agent (Teslascan™, GE Healthcare) for Mn-ion-enhanced MRI (MEMRI), should be reappraised for clinical use but now as a diagnostic drug with cytoprotective properties. Approved for imaging of the liver and pancreas, MnDPDP enhances contrast also in other targets such as the heart, kidney, glandular tissue, and potentially retina and brain. Transmetallation releases paramagnetic Mn²⁺ for cellular uptake in competition with calcium (Ca²⁺), and intracellular (IC) macromolecular Mn²⁺ adducts lower myocardial T₁ to midway between native values and values obtained with gadolinium (Gd³⁺). What is essential is that T₁ mapping and, to a lesser degree, T₁ weighted imaging enable quantification of viability at a cellular or even molecular level. IC Mn²⁺ retention for hours provides delayed imaging as another advantage. Examples in humans include quantitative imaging of cardiomyocyte remodeling and of Ca²⁺ channel activity, capabilities beyond the scope of Gd³⁺ based or native MRI. In addition, MnDPDP and the metabolite Mn dipyridoxyl diethyl-diamine (MnPLED) act as catalytic antioxidants enabling prevention and treatment of oxidative stress caused by tissue injury and inflammation. Tested applications in humans include protection of normal cells during chemotherapy of cancer and, potentially, of ischemic tissues during reperfusion. Theragnostic use combining therapy with delayed imaging remains to be explored. This review updates MnDPDP and its clinical potential with emphasis on the working mode of an exquisite chelate in the diagnosis of heart disease and in the treatment of oxidative stress.

Manganese-enhanced T1 mapping to quantify myocardial viability: validation with 18F-fluorodeoxyglucose positron emission tomography

Gadolinium chelates are widely used in cardiovascular magnetic resonance imaging (MRI) as passive intravascular and extracellular space markers. Manganese, a biologically active paramagnetic calcium analogue, provides novel intracellular myocardial tissue characterisation. We previously showed manganese-enhanced MRI (MEMRI) more accurately quantifies myocardial infarction than gadolinium delayed-enhancement MRI (DEMRI). Here, we evaluated the potential of MEMRI to assess myocardial viability compared to gold-standard 18F-fluorodeoxyglucose (18F-FDG) positron emission tomography (PET) viability. Coronary artery ligation surgery was performed in male Sprague-Dawley rats (n = 13) followed by dual MEMRI and 18F-FDG PET imaging at 10-12 weeks. MEMRI was achieved with unchelated (EVP1001-1) or chelated (mangafodipir) manganese. T1 mapping MRI was followed by 18F-FDG micro-PET, with tissue taken for histological correlation. MEMRI and PET demonstrated good agreement with histology but native T1 underestimated infarct size. Quantification of viability by MEMRI, PET and MTC were similar, irrespective of manganese agent. MEMRI showed superior agreement with PET than native T1. MEMRI showed excellent agreement with PET and MTC viability. Myocardial MEMRI T1 correlated with 18F-FDG standard uptake values and influx constant but not native T1. Our findings indicate that MEMRI identifies and quantifies myocardial viability and has major potential for clinical application in myocardial disease and regenerative therapies.

Effective MR Molecular Imaging of Triple Negative Breast Cancer With an EDB-Fibronectin-Specific Contrast Agent at Reduced Doses

MR molecular imaging (MRMI) of abundant oncogenic biomarkers in tumor microenvironment has the potential to provide precision cancer imaging in high resolution. Extradomain-B fibronectin (EDB-FN) is an oncogenic extracellular matrix protein, highly expressed in aggressive triple negative breast cancer. A targeted macrocyclic gadolinium-based contrast agent (GBCA) ZD2-N3-Gd(HP-DO3A) (MT218), specific to EDB-FN, was developed for MRMI of aggressive breast cancer. The effectiveness of different doses of MT218 for MRMI was tested in MDA-MB-231 and Hs578T human triple negative breast cancer models. At clinical dose of 0.1 and subclinical dose of 0.04 mmol Gd/kg, MT218 rapidly bound to the extracellular matrix EDB-FN and produced robust tumor contrast enhancement in both the tumor models, as early as 1-30 min post-injection. Substantial tumor enhancement was also observed in both the models with MT218 at doses as low as 0.02 mmol Gd/kg, which was significantly better than the clinical agent Gd(HP-DO3A) at 0.1 mmol Gd/kg. Little non-specific enhancement was observed in the normal tissues including liver, spleen, and brain for MT218 at all the tested doses, with renal clearance at 30 min. These results demonstrate that MRMI with reduced doses of MT218 is safe and effective for sensitive and specific imaging of aggressive breast cancers.

Manganese-enhanced MRI of the myocardium

Gadolinium-based contrast media are widely used in cardiovascular MRI to identify and to highlight the intravascular and extracellular space. After gadolinium, manganese has the second highest paramagnetic moment and was one of the first MRI contrast agents assessed in humans. Over the last 50 years, manganese-enhanced MRI (MEMRI) has emerged as a complementary approach enabling intracellular myocardial contrast imaging that can identify functional myocardium through its ability to act as a calcium analogue. Early progress was limited by its potential to cause myocardial depression. To overcome this problem, two clinical formulations of manganese were developed using either chelation (manganese dipyridoxyl diphosphate) or coadministration with a calcium compound (EVP1001-1, Eagle Vision Pharmaceuticals). Preclinical studies have demonstrated the efficacy of MEMRI in quantifying myocardial infarction and detecting myocardial viability as well as tracking altered contractility and calcium handling in cardiomyopathy. Recent clinical data suggest that MEMRI has exciting potential in the quantification of myocardial viability in ischaemic cardiomyopathy, the early detection of abnormalities in myocardial calcium handling, and ultimately, in the development of novel therapies for myocardial infarction or heart failure by actively quantifying viable myocardium. The stage is now set for wider clinical translational study of this novel and promising non-invasive imaging modality.

A novel manganese chelated macromolecular MRI contrast agent based on O-carboxymethyl chitosan derivatives

Currently used Gd-based and Mn-based small molecular MRI contrast agents fail to meet the requirements for the long-term monitoring, and the potential safety risk under high administration dose or repeat dosing needs to be considered. In the present study, a biocompatible macromolecular magnetic resonance imaging (MRI) contrast agents based on O-carboxymethyl chitosan (CMCS), CMCS-(Mn-DTPA)_n was designed and synthesized. The relaxivity of CMCS-(Mn-DTPA)_n is approximately 3.5 and 5.5 times higher than that of Gd-DTPA and Mn-DPDP in aqueous solution, respectively. The MRI signal intensity in the kidney and liver of Sprague Dawley (SD) rats is significantly increased at a dose of 0.03 mM Mn/kg b.w. CMCS-(Mn-DTPA)_n accompanied by a long effective imaging window. According to in vitro studies, CMCS-(Mn-DTPA)_n exhibits good cellular and blood biocompatibility at the dose necessary for MRI imaging. Based on the results from in vivo studies, manganese (Mn) is completely excreted from SD rats within ten days after administration and does not exert a pathological effect on the liver. CMCS-(Mn-DTPA)_n represents a potentially novel MRI contrast agent due to its excellent relaxivity, long effective imaging window and good biocompatibility.

Myocardial viability of the peri-infarct region measured by T1 mapping post manganese-enhanced MRI correlates with LV dysfunction

BACKGROUND

Manganese-enhanced MRI (MEMRI) detects viable cardiomyocytes based on the intracellular manganese uptake via L-type calcium-channels. This study aimed to quantify myocardial viability based on manganese uptake by viable myocardium in the infarct core (IC), peri-infarct region (PIR) and remote myocardium (RM) using T1 mapping before and after MEMRI and assess their association with cardiac function and arrhythmogenesis.

METHODS

Fifteen female swine had a 60-minute balloon ischemia-reperfusion injury in the LAD. MRI (Signa 3T, GE Healthcare) and electrophysiological study (EPS) were performed 4 weeks later. MEMRI and delayed gadolinium-enhanced MRI (DEMRI) were acquired on LV short axis. The DEMRI positive total infarct area was subdivided into the regions of MEMRI-negative non-viable IC and MEMRI-positive viable PIR. T1 mapping was performed to evaluate native T1, post-MEMRI T1, and delta R1 (R1_post-R1_pre, where R1 equals 1/T1) of each territory. Their correlation with LV function and EPS data was assessed.

RESULTS

PIR was characterized by intermediate native T1 (1530.5 ± 75.2 ms) compared to IC (1634.7 ± 88.4 ms, p = 0.001) and RM (1406.4 ± 37.9 ms, p < 0.0001). Lower post-MEMRI T1 of PIR (1136.3 ± 99.6 ms) than IC (1262.6 ± 126.8 ms, p = 0.005) and higher delta R1 (0.23 ± 0.08 s^-1) of PIR than IC (0.18 ± 0.09 s^-1, p = 0.04) indicated higher myocardial manganese uptake of PIR compared to IC. Post-MEMRI T1 (r = -0.57, p = 0.02) and delta R1 (r = 0.51, p = 0.04) of PIR correlated significantly with LVEF.

CONCLUSIONS

PIR is characterized by higher manganese uptake compared to the infarct core. In the subacute phase post-IR, PIR viability measured by post-MEMRI T1 correlates with cardiac function.

Manganese-Enhanced T1 Mapping in the Myocardium of Normal and Infarcted Hearts

Background

Manganese-enhanced MRI (MEMRI) has the potential to identify viable myocardium and quantify calcium influx and handling. Two distinct manganese contrast media have been developed for clinical application, mangafodipir and EVP1001-1, employing different strategies to mitigate against adverse effects resulting from calcium-channel agonism. Mangafodipir delivers manganese ions as a chelate, and EVP1001-1 coadministers calcium gluconate. Using myocardial T1 mapping, we aimed to explore chelated and nonchelated manganese contrast agents, their mechanism of myocardial uptake, and their application to infarcted hearts.

Methods

T1 mapping was performed in healthy adult male Sprague-Dawley rats using a 7T MRI scanner before and after nonchelated (EVP1001-1 or MnCl2 (22 μmol/kg)) or chelated (mangafodipir (22-44 μmol/kg)) manganese-based contrast media in the presence of calcium channel blockade (diltiazem (100-200 μmol/kg/min)) or sodium chloride (0.9%). A second cohort of rats underwent surgery to induce anterior myocardial infarction by permanent coronary artery ligation or sham surgery. Infarcted rats were imaged with standard gadolinium delayed enhancement MRI (DEMRI) with inversion recovery techniques (DEMRI inversion recovery) as well as DEMRI T1 mapping. A subsequent MEMRI scan was performed 48 h later using either nonchelated or chelated manganese and T1 mapping. Finally, animals were culled at 12 weeks, and infarct size was quantified histologically with Masson's trichrome (MTC).

Results

Both manganese agents induced concentration-dependent shortening of myocardial T1 values. This was greatest with nonchelated manganese, and could be inhibited by 30-43% with calcium-channel blockade. Manganese imaging successfully delineated the area of myocardial infarction. Indeed, irrespective of the manganese agent, there was good agreement between infarct size on MEMRI T1 mapping and histology (bias 1.4%, 95% CI -14.8 to 17.1 P>0.05). In contrast, DEMRI inversion recovery overestimated infarct size (bias 11.4%, 95% CI -9.1 to 31.8 P=0.002), as did DEMRI T1 mapping (bias 8.2%, 95% CI -10.7 to 27.2 P=0.008). Increased manganese uptake was also observed in the remote myocardium, with remote myocardial ∆T1 inversely correlating with left ventricular ejection fraction after myocardial infarction (r=-0.61, P=0.022).

Conclusions

MEMRI causes concentration and calcium channel-dependent myocardial T1 shortening. MEMRI with T1 mapping provides an accurate assessment of infarct size and can also identify changes in calcium handling in the remote myocardium. This technique has potential applications for the assessment of myocardial viability, remodelling, and regeneration.

Bimodal Phosphorescence-Magnetic Resonance Imaging Nanoprobes for Glutathione Based on MnO2 Nanosheet-Ru(II) Complex Nanoarchitecture

Bimodal fluorescence-magnetic resonance imaging (MRI) technique has shown great utilities in bioassays because it combines the advantages of both optical imaging and MRI to provide more sufficient information over any modality alone. In this work, on the basis of a MnO₂ nanosheet-Ru(II) complex nanoarchitecture, a bimodal phosphorescence-MRI nanoprobe for glutathione (GSH) has been constructed. The nanoprobe, Ru(BPY)₃@MnO₂, was constructed by integrating MnO₂ nanosheets with a phosphorescent Ru(II) complex [Ru(BPY)₃](PF₆)₂ (BPY = 2,2'-bipyridine), which resulted in complete phosphorescence quenching of the Ru(II) complex, accompanied by very low longitudinal and transverse relaxivity. Upon exposure to GSH, the reduction of MnO₂ nanosheets by GSH triggers a recovery of phosphorescence and simultaneously produces a number of Mn²⁺ ions, a perfect MRI contrast agent. The as-prepared nanoprobe showed good water dispersion and biocompatibility and a rapid, selective, and sensitive response toward GSH in the phosphorescence and MR detection modes. The practicability of the nanoprobe was proved by time-gated luminescence assay of GSH in human serum, phosphorescent imaging of endogenous GSH in living cells, zebrafish, and tumor-bearing mice, as well as the MRI of GSH in tumor-bearing mice. The research outcomes suggested the potential of Ru(BPY)₃@MnO₂ for the bimodal phosphorescence-MRI sensing of GSH in vitro and in vivo.

A rapid T1 mapping method for assessment of murine kidney viability using dynamic manganese-enhanced magnetic resonance imaging

PURPOSE

Dynamic manganese-enhanced MRI (MEMRI) allows assessment of tissue viability by tracing manganese uptake. We aimed to develop a rapid T1 mapping method for dynamic MEMRI to facilitate assessments of murine kidney viability.

METHODS

A multi-slice saturation recovery fast spin echo (MSRFSE) was developed, validated, and subsequently applied in dynamic MEMRI at 16.4T on ischemic mouse kidneys after 4 weeks of unilateral renal artery stenosis (RAS). Baseline T1 values and post-contrast R1 (1/T1 ) changes were measured in cortex (CO), outer (OSOM), inner (ISOM) strips of outer medulla, and inner medulla (IM).

RESULTS

Validation studies showed strong agreement between MSRFSE and an established saturation recovery Look-Locker method. Baseline T1 (s) increased in the stenotic kidney CO (2.10 [1.95-2.56] vs. 1.88 [1.81-2.00], P = 0.0317) and OSOM (2.17 [2.05-2.33] vs. 1.96 [1.87-2.00], P = 0.0075) but remained unchanged in ISOM and IM. This method allowed a temporal resolution of 1.43 min in dynamic MEMRI. Mn2+ uptake and retention decreased in stenotic kidneys, particularly in the OSOM (ΔR1 : 0.48 [0.38-0.56] vs. 0.64 [0.61-0.69] s-1 , P < 0.0001).

CONCLUSION

Dynamic MEMRI by MSRFSE detected decreased cellular viability and discerned the regional responses to RAS. This technique may provide a valuable tool for noninvasive evaluation of renal viability. Magn Reson Med 80:190-199, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Manganese Complex of Ethylenediaminetetraacetic Acid (EDTA)-Benzothiazole Aniline (BTA) Conjugate as a Potential Liver-Targeting MRI Contrast Agent

A novel manganese(II) complex based on an ethylenediaminetetraacetic acid (EDTA) coordination cage bearing a benzothiazole aniline (BTA) moiety (Mn-EDTA-BTA) was designed and synthesized for use as a liver-specific MRI contrast agent with high chelation stability. In addition to forming a hydrophilic, stable complex with Mn²⁺, this new Mn chelate was rapidly taken up by liver hepatocytes and excreted by the kidneys and biliary system. The kinetic inertness and R₁ relaxivity of the complex were much higher than those of mangafodipir trisodium (MnDPDP), a clinically approved liver-specific MRI contrast agent. The diagnostic utility of this new Mn complex in MRI was demonstrated by high-sensitivity tumor detection in an animal model of liver cancer.

Ability of Mn(2+) to Permeate the Eye and Availability of Manganese-enhanced Magnetic Resonance Imaging for Visual Pathway Imaging via Topical Administration

Background:

Manganese-enhanced magnetic resonance imaging (MEMRI) for visual pathway imaging via topical administration requires further research. This study investigated the permeability of the corneal epithelium and corneal toxicity after topical administration of Mn²⁺ to understand the applicability of MEMRI.

Methods:

Forty New Zealand rabbits were divided into 0.05 mol/L, 0.10 mol/L, and 0.20 mol/L groups as well as a control group (n = 10 in each group). Each group was further subdivided into epithelium-removed and epithelium-intact subgroups (n = 5 in each subgroup). Rabbits were given 8 drops of MnCl₂ in 5 min intervals. The Mn²⁺ concentrations in the aqueous and vitreous humors were analyzed using inductively coupled plasma-mass spectrometry at different time points. MEMRI scanning was carried out to image the visual pathway after 24 h. The corneal toxicity of Mn²⁺ was evaluated with corneal imaging and pathology slices.

Results:

Between the aqueous and vitreous humors, there was a 10 h lag for the peak Mn²⁺ concentration times. The intraocular Mn²⁺ concentration increased with the concentration gradients of Mn²⁺ and was higher in the epithelium-removed subgroup than that in the epithelium-intact subgroup. The enhancement of the visual pathway was achieved in the 0.10 mol/L and 0.20 mol/L epithelium-removed subgroups. The corresponding peak concentrations of Mn²⁺ were 5087 ± 666 ng/ml, 22920 ± 1188 ng/ml in the aqueous humor and 884 ± 78 ng/ml, 2556 ± 492 ng/ml in the vitreous body, respectively. Corneal injury was evident in the epithelium-removed and 0.20 mol/L epithelium-intact subgroups.

Conclusions:

The corneal epithelium is a barrier to Mn²⁺, and the iris and lens septum might be another intraocular barrier to the permeation of Mn²⁺. An elevated Mn²⁺ concentration contributes to the increased permeation of Mn²⁺, higher MEMRI signal, and corneal toxicity. The enhancement of the visual pathway requires an effective Mn²⁺ concentration in the vitreous body.

Rapid T2 mapping of mouse heart using the carr-purcell-meiboom-gill sequence and compressed sensing reconstruction

PURPOSE

To develop and prove preliminary validation of a fast in vivo T2 mapping technique for mouse heart.

MATERIALS AND METHODS

Magnetic resonance imaging (MRI) experiments were performed on a 7T animal scanner. The standard Carr-Purcell-Meiboom-Gill (CPMG) sequence was modified to minimize the effect of stimulated echoes for accurate T2 quantification. The acquisition was further accelerated with the compressed sensing approach. The accuracy of the proposed method was first validated with both phantom experiments and numerical simulations. In vivo T2 measurement was performed on seven mice in a manganese-enhanced MRI study.

RESULTS

In phantom studies, T2 values obtained with the modified CPMG sequence are in good agreement with the standard spin-echo method (P > 0.05). Numerical simulations further demonstrated that with the application of the compressed sensing approach, fast T2 quantification with a spatial resolution of 2.3 mm can be achieved at a high temporal resolution of 1 minute per slice. With the proposed technique, an average T2 value of 25 msec was observed for mouse heart at 7T and this number decreased significantly after manganese infusion (P < 0.001).

CONCLUSION

A rapid T2 mapping technique was developed and assessed, which allows accurate T2 quantification of mouse heart at a temporal resolution of 1 minute per slice. J. Magn. Reson. Imaging 2016;44:375-382.

The positive influence of fullerene derivatives bonded to manganese(III) porphyrins on water proton relaxation

Manganese-porphyrin compounds as MRI contrast agents have drawn particular attention due to high relaxivities and unique biodistribution. It has been reported that the charge density of the metal center and steric decompression of the substituents, rather than rotational correlation time, were the key factors to determine the relaxivities of manganese(III) porphyrins. In this study, [6,6]-phenyl-C61-butyric acid (PC61BA) was introduced into 5-(4-aminophenyl)-10,15,20-tris (4-sulfonatophenyl) porphyrin (APTSPP) to investigate the influence on water proton relaxation. The obtained PC61BA-APTSPP-Mn possesses a relaxivity of 19.2 mM(-1) s(-1), which is greater than that of Mn-APTSPP (11.2 mM(-1) s(-1)) and clinically used Gd-DTPA (4.1 mM(-1) s(-1)) at 0.5 T, and even more effective compared with those binding manganese(III) porphyrins to certain macromolecules. It was reasonably speculated that the high relaxivity of PC61BA-APTSPP-Mn should ascribe to the charge density variation of Mn(III) and steric decompression induced by PC61BA. Both fluorescence emission spectra and cyclic voltammetry results verified the presence of electronic communication between PC61BA and APTSPP-Mn. In addition, the hydrodynamic diameter of PC61BA-APTSPP-Mn aggregates was much smaller than that of APTSPP-Mn aggregates, which may contribute to the higher relaxivity by inhibiting the formation of dimers of APTSPP-Mn. Therefore, the introduction of fullerene derivatives is suggested to be a good strategy for the improvement of the relaxivities of manganese(III) porphyrins.

Small animal cardiovascular MR imaging and spectroscopy

The use of MR imaging and spectroscopy for studying cardiovascular disease processes in small animals has increased tremendously over the past decade. This is the result of the remarkable advances in MR technologies and the increased availability of genetically modified mice. MR techniques provide a window on the entire timeline of cardiovascular disease development, ranging from subtle early changes in myocardial metabolism that often mark disease onset to severe myocardial dysfunction associated with end-stage heart failure. MR imaging and spectroscopy techniques play an important role in basic cardiovascular research and in cardiovascular disease diagnosis and therapy follow-up. This is due to the broad range of functional, structural and metabolic parameters that can be quantified by MR under in vivo conditions non-invasively. This review describes the spectrum of MR techniques that are employed in small animal cardiovascular disease research and how the technological challenges resulting from the small dimensions of heart and blood vessels as well as high heart and respiratory rates, particularly in mice, are tackled.

Manganese-enhanced MRI detection of impaired calcium regulation in a mouse model of cardiac hypertrophy

The aim of this study was to use manganese (Mn)-enhanced MRI (MEMRI) to detect changes in calcium handling associated with cardiac hypertrophy in a mouse model, and to determine whether the impact of creatine kinase ablation is detectable using this method. Male C57BL/6 (C57, n = 11) and male creatine kinase double-knockout (CK-M/Mito(-/-) , DBKO, n = 12) mice were imaged using the saturation recovery Look-Locker T1 mapping sequence before and after the development of cardiac hypertrophy. Hypertrophy was induced via subcutaneous continuous 3-day infusion of isoproterenol, and sham mice not subjected to cardiac hypertrophy were also imaged. During each scan, the contrast agent Mn was administered and the resulting change in R1 (=1/T1) was calculated. Two anatomical regions of interest (ROIs) were considered, the left-ventricular free wall (LVFW) and the septum, and one ROI in an Mn-containing standard placed next to the mouse. We found statistically significant (p < 0.05) decreases in the uptake of Mn in both the LVFW and septum following the induction of cardiac hypertrophy. No statistically significant decreases were detected in the standard, and no statistically significant differences were found among the sham mice. Using a murine model, we successfully demonstrated that changes in Mn uptake as a result of cardiac hypertrophy are detectable using the functional contrast agent and calcium mimetic Mn. Our measurements showed a decrease in the relaxivity (R1) of the myocardium following cardiac hypertrophy compared with normal control mice.

Calmangafodipir [Ca4Mn(DPDP)5], mangafodipir (MnDPDP) and MnPLED with special reference to their SOD mimetic and therapeutic properties

Reactive oxygen species (ROS) and reactive nitrogen species (RNS) participate in pathological tissue damage. Mitochondrial manganese superoxide dismutase (MnSOD) normally keeps ROS and RNS in check. During development of mangafodipir (MnDPDP) as a magnetic resonance imaging (MRI) contrast agent, it was discovered that MnDPDP and its metabolite manganese pyridoxyl ethyldiamine (MnPLED) possessed SOD mimetic activity. MnDPDP has been tested as a chemotherapy adjunct in cancer patients and as an adjunct to percutaneous coronary intervention in patients with myocardial infarctions, with promising results. Whereas MRI contrast depends on release of Mn(2+), the SOD mimetic activity depends on Mn(2+) that remains bound to DPDP or PLED. Calmangafodipir [Ca4Mn(DPDP)5] is stabilized with respect to Mn(2+) and has superior therapeutic activity. Ca4Mn(DPDP)5 is presently being explored as a chemotherapy adjunct in a clinical multicenter Phase II study in patients with metastatic colorectal cancer.

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.

Self-organized Mn2+-Block Copolymer Complexes and Their Use for In Vivo MR Imaging of Biological Processes

Manganese-block copolymer complexes (MnBCs) that contain paramagnetic Mn ions complexed with ionic-nonionic poly(ethylene oxide-b-poly(methacrylate) have been developed for use as a T1-weighted MRI contrast agent. By encasing Mn ion within ionized polymer matrices, r1 values could be increased by 250-350 % in comparison with free Mn ion at relative high fields of 4.7 to 11.7 T. MnBCs were further manipulated by treatment with NaOH to achieve more stable complexes (iMnBCs). iMnBCs delayed release of Mn2+ which could be accelerated by low pH, indeed by cellular uptake via endocytosis into acidic compartments. Both complexes exhibited good T1 contrast signal enhancement in liver following intravenous infusion. The contrast was observed in gallbladder due to the clearance of Mn ion from liver to biliary process. iMnBCs, notably, showed a delayed contrast enhancement profile in gallbladder, which was interpreted to be due to degradation and excretion of Mn2+ ions into the gallbladder. Intracortical injection of iMnBCs into the rat brain also led to delayed neuronal transport to thalamus. The delayed enhancement feature may have benefits for targeting MRI contrast to specific cells and surface receptors that are known to be internalized by endocytosis.

Manganese: a new contrast agent for lung imaging?

Lung parenchyma remains one of the most difficult tissues to be imaged by means of magnetic resonance imaging (MRI). Several MRI techniques are routinely used for lung imaging. However, manganese-enhancement MRI (MEMRI) technique has not been associated with pulmonary MRI. Here, we evaluated T(1) -enhancement in the rat lung after a manganese instillation, using a 4.7 T magnet with a radial ultrashort echo time sequence. Our data showed that the signal intensity was increased in lungs receiving a manganese solution compared with a control solution to the lungs. MR signal enhancements above 30% were measured in lung parenchyma following 200 µl instillation of a 1 mm manganese chloride solution. MEMRI, therefore, may be a useful novel tool for enhancing signal intensity and image contrast in lung tissue.

Superior therapeutic index of calmangafodipir in comparison to mangafodipir as a chemotherapy adjunct

Mangafodipir is a magnetic resonance imaging contrast agent with manganese superoxide dismutase (MnSOD) mimetic activity. The MnSOD mimetic activity protects healthy cells against oxidative stress-induced detrimental effects, e.g., myelosuppressive effects of chemotherapy drugs. The contrast property depends on in vivo dissociation of Mn(2+) from mangafodipir-about 80% dissociates after injection. The SOD mimetic activity, however, depends on the intact Mn complex. Complexed Mn(2+) is readily excreted in the urine, whereas dissociated Mn(2+) is excreted slowly via the biliary route. Mn is an essential but also a potentially neurotoxic metal. For more frequent therapeutic use, neurotoxicity due to Mn accumulation in the brain may represent a serious problem. Replacement of 4/5 of Mn(2+) in mangafodipir with Ca(2+) (resulting in calmangafodipir) stabilizes it from releasing Mn(2+) after administration, which roughly doubles renal excretion of Mn. A considerable part of Mn(2+) release from mangafodipir is governed by the presence of a limited amount of plasma zinc (Zn(2+)). Zn(2+) has roughly 10(3) and 10(9) times higher affinity than Mn(2+) and Ca(2+), respectively, for fodipir. Replacement of 80% of Mn(2+) with Ca(2+) is enough for binding a considerable amount of the readily available plasma Zn(2+), resulting in considerably less Mn(2+) release and retention in the brain and other organs. At equivalent Mn(2+) doses, calmangafodipir was significantly more efficacious than mangafodipir to protect BALB/c mice against myelosuppressive effects of the chemotherapy drug oxaliplatin. Calmangafodipir did not interfere negatively with the antitumor activity of oxaliplatin in CT26 tumor-bearing syngenic BALB/c mice, contrary calmangafodipir increased the antitumor activity.

Inhibition of the sodium-calcium exchanger via SEA0400 altered manganese-induced T1 changes in isolated perfused rat hearts

Manganese (Mn(2+) )-enhanced MRI (MEMRI) provides the potential for the in vivo evaluation of calcium (Ca(2+) ) uptake in the heart. Recent studies have also suggested the role of the sodium-calcium (Na(+) -Ca(2+) ) exchanger (NCX) in Mn(2+) retention, which may have an impact on MEMRI signals. In this study, we investigated whether MEMRI with fast T(1) mapping allowed the sensitive detection of changes in NCX activity. We quantified the dynamics of the Mn(2+) -induced T(1) changes in isolated perfused rat hearts in response to SEA0400, an NCX inhibitor. The experimental protocol comprised 30 min of Mn(2+) perfusion (wash-in), followed by a 30-min wash-out period. There were three experimental groups: 1, NCX inhibition by 1 µ m SEA0400 during Mn(2+) wash-in only (SEAin, n=6); 2, NCX inhibition by 1 µ m SEA0400 during Mn(2+) wash-out only (SEAout, n=6); 3, no NCX inhibition during both wash-in and wash-out to serve as the control group (CNTL, n=5). Rapid T(1) mapping at a temporal resolution of 3 min was performed throughout the perfusion protocol using a triggered saturation-recovery Look-Locker sequence. Our results showed that NCX inhibition during Mn(2+) wash-in caused a significant increase in relaxation rate (R(1) ) at the end of Mn(2+) perfusion. During the wash-out period, NCX inhibition led to less reduction in R(1) . Further analysis of Mn(2+) content in myocardium with flame atomic absorption spectroscopy was consistent with the MRI findings. These results suggest that Mn(2+) accumulation and retention in rat hearts are, in part, dependent on NCX activity. Hence, MEMRI may provide an imaging method that is also sensitive to changes in NCX activity.

Longitudinal MRI contrast enhanced monitoring of early tumour development with manganese chloride (MnCl2) and superparamagnetic iron oxide nanoparticles (SPIOs) in a CT1258 based in vivo model of prostate cancer

Background

Cell lines represent a key tool in cancer research allowing the generation of neoplasias which resemble initial tumours in in-vivo animal models. The characterisation of early tumour development is of major interest in order to evaluate the efficacy of therapeutic agents. Magnetic resonance imaging (MRI) based in-vivo characterisation allows visualisation and characterisation of tumour development in early stages prior to manual palpation. Contrast agents for MRI such as superparamagnetic iron oxide nanoparticles (SPIOs) and manganese chloride (MnCl₂) represent powerful tools for the in-vivo characterisation of early stage tumours. In this experimental study, we labelled prostate cancer cells with MnCl₂ or SPIOs in vitro and used 1 T MRI for tracing labelled cells in-vitro and 7 T MRI for tracking in an in-vivo animal model.

Methods

Labelling of prostate cancer cells CT1258 was established in-vitro with MnCl₂ and SPIOs. In-vitro detection of labelled cells in an agar phantom was carried out through 1 T MRI while in-vivo detection was performed using 7 T MRI after subcutaneous (s.c.) injection of labelled cells into NOD-Scid mice (n = 20). The animals were scanned in regular intervals until euthanization. The respective tumour volumes were analysed and corresponding tumour masses were subjected to histologic examination.

Results

MnCl₂in-vitro labelling resulted in no significant metabolic effects on proliferation and cell vitality. In-vitro detection-limit accounted 10⁵ cells for MnCl₂ as well as for SPIOs labelling. In-vivo 7 T MRI scans allowed detection of 10³ and 10⁴ cells. In-vivo MnCl₂ labelled cells were detectable from days 4–16 while SPIO labelling allowed detection until 4 days after s.c. injection. MnCl₂ labelled cells were highly tumourigenic in NOD-Scid mice and the tumour volume development was characterised in a time dependent manner. The amount of injected cells correlated with tumour size development and disease progression. Histological analysis of the induced tumour masses demonstrated characteristic morphologies of prostate adenocarcinoma.

Conclusions

To the best of our knowledge, this is the first study reporting direct in-vitro MnCl₂ labelling and 7 T based in-vivo MRI tracing of cancer cells in a model of prostate cancer. MnCl₂ labelling was found to be suitable for in-vivo tracing allowing long detection periods. The labelled cells kept their highly tumourigenic potential in-vivo. Tumour volume development was visualised prior to manual palpation allowing tumour characterisation in early stages of the disease.

Oral manganese as an MRI contrast agent for the detection of nociceptive activity

The ability of divalent manganese to enter neurons via calcium channels makes manganese an excellent MRI contrast agent for the imaging of nociception, the afferent neuronal encoding of pain perception. There is growing evidence that nociceptive neurons possess increased expression and activity of calcium channels, which would allow for the selective accumulation of manganese at these sites. In this study, we show that oral manganese chloride leads to increased enhancement of peripheral nerves involved in nociception on T(1)-weighted MRI. Oral rather than intravenous administration was chosen for its potentially better safety profile, making it a better candidate for clinical translation with important applications, such as pain diagnosis, therapy and research. The spared nerve injury (SNI) model of neuropathic pain was used for the purposes of this study. SNI rats were given, sequentially, increasing amounts of manganese chloride (lowest, 2.29 mg/100 g weight; highest, 20.6 mg/100 g weight) with alanine and vitamin D(3) by oral gavage. Compared with controls, SNI rats demonstrated increased signal-to-background ratios on T(1)-weighted fast spin echo MRI, which was confirmed by and correlated strongly with spectrometry measurements of nerve manganese concentration. We also found the difference between SNI and control rats to be greater at 48 h than at 24 h after dosing, indicating increased manganese retention in addition to increased manganese uptake in nociceptive nerves. This study demonstrates that oral manganese is a viable method for the imaging of nerves associated with increased nociceptive activity.

Manganese kinetics demonstrated double contrast in acute but not in chronic infarction in a mouse model of myocardial occlusion reperfusion

Manganese (Mn(2+)) is considered as a specific MRI contrast agent that enters viable cardiomyocytes through calcium pathways. Compared to extracellular gadolinium based contrast agents, it has the potential to assess cell viability. To date, only information from the washout phase after recirculation has been used for the detection and characterization of myocardial infarct. This study showed for the first time that in a mouse model of coronary occlusion-reperfusion, Mn(2+) wash-in kinetics are different at 24 h after surgery (acute infarction) than at eight days after surgery (chronic infarction). A fast but transient entry of Mn(2+) into the acute infarct area led to a double contrast between infarct and remote areas, whereas entry of Mn(2+) into the chronic infarct area remained reduced compared to remote regions during both wash-in and washout phases. The main hypothesis is that extracellular space is largely enhanced in acute infarction due to cell membrane rupture and interstitial edema, whereas scar tissue is densely composed of collagen fibers that reduce the distribution volume of free Mn(2+) ions. In addition to its ability to accurately depict the infarct area during the redistribution phase, Mn(2+) is also able to discriminate acute versus chronic injury by the observation of double-contrast kinetics in a mouse model of ischemia reperfusion.

Manganese-based MRI contrast agents: past, present and future

Paramagnetic and superparamagnetic metals are used as contrast materials for magnetic resonance (MR) based techniques. Lanthanide metal gadolinium (Gd) has been the most widely explored, predominant paramagnetic contrast agent until the discovery and association of the metal with nephrogenic systemic fibrosis (NSF), a rare but serious side effects in patients with renal or kidney problems. Manganese was one of the earliest reported examples of paramagnetic contrast material for MRI because of its efficient positive contrast enhancement. In this review, manganese based contrast agent approaches are discussed with a particular emphasis on their synthetic approaches. Both small molecules based typical blood pool contrast agents and more recently developed novel nanometer sized materials are reviewed focusing on a number of successful molecular imaging examples.

Murine cardiac hemodynamics following manganese administration under isoflurane anesthesia

This study examines (a) the temporal stability of hemodynamic indices of systolic and diastolic function in C57BL/6 mice under 1.5% isoflurane (ISO) (v/v) anesthesia conditions in 50:50 O(2)/N(2)O (v/v) within 90 min post-induction, and (b) the effects of Mn(2+) on the mouse hemodynamic response in male C57BL/6 mice (n = 16). Left ventricular catheterizations allowed estimation of the hemodynamic indices. Hypertonic saline infusion (10%) allowed absolute volume quantification in conjunction with a separate series of aortic flow experiments (n = 3). In a separate cohort of mice (n = 6), MnCl(2) (190 nmoles/g/bw) was infused via the left jugular for 29-39 min, following 11 min of baseline recording, to assess temporal responses. Stable temporal hemodynamic responses were achieved in control mice under ISO anesthesia. Hemodynamic indices during control, time-matched-control, baseline-Mn, and Mn-infused periods, were within normal expected ranges. No chronotropic changes were observed. Significant differences in systolic and diastolic cardiac indices of function (HR, EF, ESP, dP/dt (max), dP/dt (min), PAMP, τ(glantz), and τ(weiss)) resulted between baseline-Mn and Mn-infused time periods in Mn-treated mice at the 1% significance (p < 0.001). Transient positive, or negative, or positive followed by negative evoked pressure-volume loop shifts were observed (exemplified through changes in the end-systolic pressure-volume relationship and dP/dt (max)) in Mn-infusion studies. It is concluded that Mn(2+) can be used safely for prolonged mouse imaging studies, however, the significant variations elicited in cardiovascular hemodynamics post-manganese infusion, necessitate further investigations for its suitability and appropriateness for quantification of global cardiac function in image-based phenotyping.

Dual manganese-enhanced and delayed gadolinium-enhanced MRI detects myocardial border zone injury in a pig ischemia-reperfusion model

BACKGROUND

Gadolinium (Gd)-based delayed-enhancement MRI (DEMRI) identifies nonviable myocardium but is nonspecific and may overestimate nonviable territory. Manganese (Mn(2+))-enhanced MRI (MEMRI) denotes specific Mn(2+) uptake into viable cardiomyocytes. We performed a dual-contrast myocardial assessment in a porcine ischemia-reperfusion (IR) model to test the hypothesis that combined DEMRI and MEMRI identifies viable infarct border zone (BZ) myocardium in vivo.

METHODS AND RESULTS

Sixty-minute left anterior descending coronary artery IR injury was induced in 13 adult swine. Twenty-one days post-IR, 3-T cardiac MRI was performed. MEMRI was obtained after injection of 0.7 mL/kg Mn(2+) contrast agent. DEMRI was then acquired after injection of 0.2 mmol/kg Gd. Left ventricular (LV) mass, infarct, and function were analyzed. Subtraction of MEMRI defect from DEMRI signal identified injured BZ myocardium. Explanted hearts were analyzed by 2,3,5-triphenyltetrazolium chloride stain and tissue electron microscopy to compare infarct, BZ, and remote myocardium. Average LV ejection fraction was reduced (30±7%). MEMRI and DEMRI infarct volumes correlated with 2,3,5-triphenyltetrazolium chloride stain analysis (MEMRI, r=0.78; DEMRI, r=0.75; P<0.004). MEMRI infarct volume percentage was significantly lower than that of DEMRI (14±4% versus 23±4%; P<0.05). BZ MEMRI signal-to-noise ratio (SNR) was intermediate to remote and core infarct SNR (7.5±2.8 versus 13.2±3.4 and 2.9±1.6; P<0.0001), and DEMRI BZ SNR tended to be intermediate to remote and core infarct SNR (8.4±5.4 versus 3.3±0.6 and 14.3±6.6; P>0.05). Tissue electron microscopy analysis exhibited preserved cell structure in BZ cardiomyocytes despite transmural DEMRI enhancement.

CONCLUSIONS

The dual-contrast MEMRI-DEMRI detects BZ viability within DEMRI infarct zones. This approach may identify injured, at-risk myocardium in ischemic cardiomyopathy.

Indirectly probing Ca(2+) handling alterations following myocardial infarction in a murine model using T(1)-mapping manganese-enhanced magnetic resonance imaging

Prolonged ischemia causes cellular necrosis and myocardial infarction (MI) via intracellular calcium (Ca(2+)) overload. Manganese-enhanced MRI indirectly assesses Ca(2+) influx movement in vivo as manganese (Mn(2+)) is a Ca(2+) analog. To characterize myocardial Mn(2+) efflux properties, T(1)-mapping manganese-enhanced MRI studies were performed on adult male C57Bl/6 mice in which Ca(2+) efflux was altered using pharmacological intervention agents or MI-inducing surgery. Results showed that (1) Mn(2+) efflux rate increased exponentially with increasing Mn(2+) doses; (2) SEA0400 (a sodium-calcium exchanger inhibitor) decreased the rate of Mn(2+) efflux; and (3) dobutamine (a positive inotropic agent) increased the Mn(2+) efflux rate. A novel analysis technique also delineated regional features in the MI mice, which showed an increased Mn(2+) efflux rate in the necrosed and peri-infarcted tissue zones. The T(1)-mapping manganese-enhanced MRI technique characterized alterations in myocardial Mn(2+) efflux rates following both pharmacologic intervention and an acute MI. The Mn(2+) efflux results were consistent with those in ex vivo studies showing an increased Ca(2+) concentration under similar conditions. Thus, T(1)-mapping manganese-enhanced MRI has the potential to indirectly identify and quantify intracellular Ca(2+) handling in the peri-infarcted tissue zones, which may reveal salvageable tissue in the post-MI myocardium.

The role of imaging and molecular imaging in the early detection of metabolic and cardiovascular dysfunctions

Despite intense effort, obesity is still rising throughout the world. Links between obesity and cardiovascular diseases are now well established. Most of the cardiovascular changes related to obesity can be followed by magnetic resonance imaging (MRI) or by magnetic resonance spectroscopy (MRS). In particular, we will see in this review that MRI/MRS is extremely well suited to depict (1) changes in cardiac mass and function, (2) changes in stroke volume, (3) accumulation of fat inside the mediastinum or even inside the cardiomyocytes, (4) cell viability and (5) molecular changes during early cardiovascular diseases.

A fast black-blood sequence for four-dimensional cardiac manganese-enhanced MRI in mouse

The increasing number of mouse models of cardiac diseases requires improvements in the current MRI tools. Anatomic and functional cardiac phenotyping by MRI calls for both time and space resolution in three dimensions. Black-blood contrast is often needed for the accurate delineation of myocardium and chambers, and is consistent with manganese contrast enhancement. In this article, we propose a fast, three-dimensional, time-resolved (four-dimensional), black-blood MRI sequence that allows mouse heart imaging at 10 periods of the cardiac cycle within 30  min at an isotropic resolution of 200  µm. Two-dimensional imaging was possible within 80  s. Blood cancellation was achieved by employing bipolar gradients without the use of a double inversion recovery preparation scheme. Saturation slices were added in two-dimensional experiments for better blood nulling. The rapidity of the two-dimensional acquisition protocol allowed the measurement of the time course of contrast enhancement on manganese infusion. Owing to the very high contrast-to-noise ratio, manganese-enhanced MRI in four dimensions made possible the accurate assessment of regional cardiac volumes in healthy animals. In experimentally infarcted mice, the size of the ischemic zone could be measured easily with this method. The technique might be valuable in evaluating mouse heart diseases and their follow-up in longitudinal studies.

Emerging MRI methods in translational cardiovascular research

Cardiac magnetic resonance imaging (CMR) has become a reference standard modality for imaging of left ventricular (LV) structure and function and, using late gadolinium enhancement, for imaging myocardial infarction. Emerging CMR techniques enable a more comprehensive examination of the heart, making CMR an excellent tool for use in translational cardiovascular research. Specifically, emerging CMR methods have been developed to measure the extent of myocardial edema, changes in ventricular mechanics, changes in tissue composition as a result of fibrosis, and changes in myocardial perfusion as a function of both disease and infarct healing. New CMR techniques also enable the tracking of labeled cells, molecular imaging of biomarkers of disease, and changes in calcium flux in cardiomyocytes. In addition, MRI can quantify blood flow velocity and wall shear stress in large blood vessels. Almost all of these techniques can be applied in both pre-clinical and clinical settings, enabling both the techniques themselves and the knowledge gained using such techniques in pre-clinical research to be translated from the lab bench to the patient bedside.

Interdependence of cardiac iron and calcium in a murine model of iron overload

Iron cardiomyopathy in ß-thalassemia major patients is associated with a vitamin D deficiency. Stores of 25-OH-D3 are markedly reduced, whereas the active metabolite, 1-25-(OH)-D3, is normal or increased. Interestingly, the ratio of 25-OH-D3 to 1-25-(OH)-D3 (a surrogate for parathyroid hormone [PTH]) is the strongest predictor of cardiac iron. Increased PTH and 1-25-OH-D3 levels have been shown to up-regulate L-type voltage-gated calcium channels (LVGCC), the putative channel for cardiac iron uptake. Therefore, we postulate that a vitamin D deficiency increases cardiac iron by altering LVGCC regulation. Hemojuvelin knockout mice were calcitriol treated, PTH treated, vitamin D-depleted, or untreated. Half of the animals in each group received the Ca(2+)-channel blocker verapamil. Mn(2+) was infused to determine LVGCC activity. Hearts and livers were harvested for iron, calcium, and manganese measurements as well as histology. Cardiac iron did not differ among the treatment groups; however, liver iron was increased in vitamin D-depleted animals (P < 0.0003). Cardiac iron levels did not correlate with manganese uptake but were proportional to cardiac calcium levels (r(2) = 0.6; P < 0.0001). Verapamil treatment reduced both cardiac (P < 0.02) and hepatic (P < 0.003) iron levels significantly by 34% and 28%, respectively. The association between cardiac iron and calcium levels was maintained after verapamil treatment (r(2) = 0.3; P < 0.008). Vitamin D depletion is associated with an increase in liver, but not cardiac, iron accumulation. Cardiac iron uptake was strongly correlated with cardiac calcium stores and was significantly attenuated by verapamil, suggesting that cardiac calcium and iron are related.

Manganese-enhanced MRI of salivary glands and head and neck tumors in living subjects

Manganese-enhanced MRI has previously been used for visualization of brain architecture and functional mapping of neural pathways. The present work investigated the potential of manganese-enhanced MRI for noninvasive imaging of salivary glands in living subjects. Marked shortening of T(1) was observed in salivary glands of naïve mice (n = 5) 24-48 h after systemic administration of MnCl(2) (0.4 mmol/kg, intraperitoneally). Three-dimensional MR microscopy confirmed selective contrast enhancement of salivary gland tissues post-MnCl(2) injection. Ectopic and orthotopic head and neck tumor xenografts also showed an increase in R(1) at 24 h following MnCl(2) injection (0.2 mmol/kg, intraperitoneally). However, tumor enhancement was minimal compared to salivary gland tissue. Salivary gland R(1) values were lower in mice bearing orthotopic head and neck tumors compared to naïve mice. These results demonstrate, for the first time, the usefulness of manganese-enhanced MRI in the visualization of salivary glands and head and neck tumors in vivo.

Preliminary assessment of cardiac short term safety and efficacy of manganese chloride for cardiovascular magnetic resonance in humans

BACKGROUND

Manganese based agents are intracellular and accumulate inside myocytes allowing for different imaging strategies compared to gadolinium contrasts. While previous agents release manganese very slowly in the circulation, MnCl2 allows for rapid Mn2+ uptake in myocytes, creating a memory effect that can be potentially explored. Data on animal models are very encouraging but the safety and efficacy of this approach in humans has not yet been investigated. Therefore, our objectives were to study the safety and efficacy of a rapid infusion of manganese chloride (MnCl2) for cardiovascular magnetic resonance (CMR) in humans.

METHODS

Fifteen healthy volunteers underwent a CMR scan on a 1.5 T scanner. Before the infusion, cardiac function was calculated and images of a short axis mid-ventricular slice were obtained using a 2D and 3D gradient-echo inversion recovery (GRE-IR) sequence, a phase-sensitive IR sequence and a single breath-hold segmented IR prepared steady-state precession acquisition for T1 calculations. MnCl2 was infused over three minutes at a total dose of 5 μMol/kg. Immediately after the infusion, and at 15 and 30 minutes later, new images were obtained and cardiac function re-evaluated.

RESULTS

There was a significant decrease in T1 values compared to baseline, sustained up to 30 minutes after the MnCl2 infusion (pre,839 ± 281 ms; 0 min, 684 ± 99; 15 min, 714 ± 168; 30 min, 706 ± 172, P = 0.003). The 2D and 3D GRE-IR sequence showed the greatest increase in signal-to-noise ratio compared to the other sequences (baseline 6.6 ± 4.2 and 9.7 ± 5.3; 0 min, 11.3 ± 4.1 and 15.0 ± 8.7; 15 min, 10.8 ± 4.0 and 16.9 ± 10.2; 30 min, 10.6 ± 5.2 and 16.5 ± 8.3, P < 0.001 for both). There was a slight increase in systolic pressure and heart rate after three and four minutes of the infusion with normalization of these parameters thereafter. Patients showed good tolerance to MnCl2 with no major adverse events, despite all reporting transient facial flush.

CONCLUSIONS

In the short term, MnCl2 appears safe for human use. It effectively decreases myocardium T1, maintaining this effect for a relatively long period of time and allowing for the development of new imaging strategies in CMR, especially in ischemia research.

Manganese-enhanced magnetic resonance imaging (MEMRI)

The use of manganese ions (Mn(2+)) as an MRI contrast agent was introduced over 20 years ago in studies of Mn(2+) toxicity in anesthetized rats (1). Manganese-enhanced MRI (MEMRI) evolved in the late nineties when Koretsky and associates pioneered the use of MEMRI for brain activity measurements (2) as well as neuronal tract tracing (3). Currently, MEMRI has three primary applications in biological systems: (1) contrast enhancement for anatomical detail, (2) activity-dependent assessment and (3) tracing of neuronal connections or tract tracing. MEMRI relies upon the following three main properties of Mn(2+): (1) it is a paramagnetic ion that shortens the spin lattice relaxation time constant (T(1)) of tissues, where it accumulates and hence functions as an excellent T(1) contrast agent; (2) it is a calcium (Ca(2+)) analog that can enter excitable cells, such as neurons and cardiac cells via voltage-gated Ca(2+) channels; and (3) once in the cells Mn(2+) can be transported along axons by microtubule-dependent axonal transport and can also cross synapses trans-synaptically to neighboring neurons. This chapter will emphasize the methodological approaches towards the use of MEMRI in biological systems.

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.