Determining canine myocardial area at risk with manganese-enhanced MR imaging

Crosslinked albumin-manganese nanoaggregates with sensitized T1 relaxivity and indocyanine green loading for multimodal imaging and cancer phototherapy

Albumin-manganese-based nanocomposites (AMNs) characterized by simple preparation and good biocompatibility have been widely used for in vivo T₁-weighted magnetic resonance imaging (MRI) and cancer theranostics. Herein, an aggregation and crosslinking assembly strategy was proposed to achieve the sensitization to T₁ relaxivity of the albumin-manganese nanocomposite. At a relatively low Mn content (0.35%), the aggregation and crosslinking of bovine serum albumin-MnO₂ (BM) resulted in a dramatic increase of T₁ relaxivity from 5.49 to 67.2 mM^-1 s^-1. Upon the loading of indocyanine green (ICG) into the crosslinked BM nanoaggregates (C-BM), the T₁ relaxivity of the C-BM/ICG nanocomposite (C-BM/I) was further increased to 97.3 mM^-1 s^-1, which was much higher than those reported previously even at high Mn contents. Moreover, the presence of C-BM greatly enhanced the photoacoustic (PA) and photothermal effects of ICG at 830 and 808 nm, respectively, and the second near infrared fluorescence (NIR-II FL) of ICG also showed better stability. Therefore, the synthesized C-BM/ICG nanocomposite exhibited remarkable performance in in vivo multimodal imaging of tumors, such as T₁-weighted MRI, NIR-II FL imaging and PA imaging, and cancer phototherapy with little side effects. This work provided a highly efficient and promising multifunctional nanoprobe for breaking through the limits of cancer theranostics, and opened a new avenue for the development of high-relaxivity AMNs and multimodal imaging methodology.

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.

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.

Magnetic resonance imaging for area at risk, myocardial infarction, and myocardial salvage

Almost all published preclinical studies of cardioprotective agents include a measurement of area at risk, infarct size, and thus allow determination of myocardial salvage as an indicator of therapeutic benefit. Until recently, single-photon emission tomography (SPECT) imaging with injection of sestamibi prior to intervention was the only clinical method suitable for making similar assessments in patients. Over the past 5 years, a large number of articles have documented that magnetic resonance imaging (MRI) can noninvasively determine area at risk, infarct size, and myocardial salvage. While T2-weighted imaging has been the method used most commonly, precontrast T1-weighted images and early gadolinium enhancement (EGE) images can also determine the size of the area at risk. All 3 of these MRI methods detect the area at risk based on myocardial edema resulting from ischemia. Late gadolinium enhancement (LGE) images provide a well-accepted reference standard for infarct size in all of those methods. Finally, LGE images can also provide a single modality measure of myocardial salvage based on the "wave front" of myocardial injury associated with the progressively more severe damage associated with acute myocardial infarction (MI). Essentially, the LGE images can provide an endocardial-based snap shot of infarct size and salvaged myocardium is estimated as the viable myocardium within the circumferential extent of the infarct. Thus, the purpose of this review is to provide an overview of how MRI can determine the area at risk, infarct size, and thus measure myocardial salvage.

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.

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.

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.

Myocardial infarction quantification with Manganese-Enhanced MRI (MEMRI) in mice using a 3T clinical scanner

Manganese (Mn(2+)) was recognized early as an efficient intracellular MR contrast agent to assess cardiomyocyte viability. It had previously been used for the assessment of myocardial infarction in various animal models from pig to mouse. However, whether Manganese-Enhanced MRI (MEMRI) is also able to assess infarction in the acute phase of a coronary occlusion reperfusion model in mice has not yet been demonstrated. This model is of particular interest as it is closer to the situation encountered in the clinical setting. This study aimed to measure infarction volume taking TTC staining as a gold standard, as well as global and regional function before and after Mn(2+) injection using a clinical 3T scanner. The first step of this study was to perform a dose-response curve in order to optimize the injection protocol. Infarction volume measured with MEMRI was strongly correlated to TTC staining. Ejection fraction (EF) and percent wall thickening measurements allowed evaluation of global and regional function. While EF must be measured before Mn(2+) injection to avoid bias introduced by the reduction of contrast in cine images, percent wall thickening can be measured either before or after Mn(2+) injection and depicts accurately infarct related contraction deficit. This study is the first step for further longitudinal studies of cardiac disease in mice on a clinical 3T scanner, a widely available platform.

Manganese-enhanced MRI of acute cardiac ischemia and chronic infarction in pig hearts: kinetic analysis of enhancement development

To assess infarction development in pig hearts, Mn-enhanced and Gd-enhanced MRI were used. In domestic pigs (25-35 kg, n = 37), the first and second diagonal branches of the left anterior descending coronary artery were ligated to induce acute ischemia and infarction (ischemia+reperfusion) or chronic infarction of increasing duration (3- 28 days). Ex vivo experiments were performed on hearts perfused in the Langendorff mode with a 50:50 mixture of blood and Krebs-Henseleit buffer using a spin-echo sequence on a 7 T Bruker imaging system. Signal acquisition from the heart and two reference test tubes (H(2)O and H(2)O + 10 mM CuSO(4)) was gated by the left ventricular pressure wave. T(1)-weighted images of six 8 mm short-axis slices (2 mm interslice gaps) were obtained before and after the addition of 0.2 mM MnCl(2) every 5 min over a 30-45 min period. Signal intensities were normalized to those of the H(2)O reference and fitted by a monoexponential function. The rates of intensity increase and maximal increases were significantly lower in the ischemic/infarcted areas and showed a trend to rise on infarction progression. In vivo Gd-enhanced MRI (3 T Siemens scanner) and in vivo/ex vivo near-infrared imaging confirmed major Mn-enhanced MRI findings. Triphenyltetrazolium chloride staining revealed necrotic areas in all chronic infarctions and no necrosis after acute ischemia. We conclude that MnCl(2) highlights ischemic areas because of the low collateral flow characteristic of pig hearts, whereas in the infarcted areas with substantial perfusion, scar tissue components are important for contrast distribution.

In vivo myocardial infarct area at risk assessment in the rat using manganese enhanced magnetic resonance imaging (MEMRI) at 1.5T

The aim of this study was to measure the myocardial area at risk in rat, using MRI and manganese injection during a coronary occlusion/reperfusion model at 1.5T. A sequential protocol with occlusion and MnCl2 injection immediately followed by MRI was used with the assumption that MnCl2-induced contrast persistence is enough to accurately image the area at risk 90 min after occlusion. A total of 15 adult rats underwent a single 30-min episode of coronary occlusion followed by reperfusion. MnCl2 was injected (25 micromol/kg) at the beginning of the occlusion for 11 rats (group 1) and 6 h after reperfusion for four animals (group 2). A deficit of signal enhancement was observed in all rats. Hypoenhancement area in group 1 was correlated to the area at risk delineated by methylene blue (r=0.96, P<0.0001) whereas in group 2 it was correlated to the infarct area given by triphenyltetrazolium chloride (TTC) solution (r=0.98, P=0.003). The area at risk size was significantly correlated with left ventricle ejection fraction (LVEF), end-systolic volume and anterolateral wall thickening. This work demonstrates that hypoenhanced zone obtained after manganese injection during occlusion represents the area at risk and not only the infarct zone.

Feasibility of complementary spatial modulation of magnetization tagging in the rat heart after manganese injection

It has been shown that manganese-enhanced MRI (MEMRI) can safely depict the myocardial area at risk in models of coronary occlusion-reperfusion for at least 2 h after reperfusion. To achieve this, a solution of MnCl(2) is injected during coronary occlusion. In this model, the regional function quantification deficit of the stunning phase cannot be assessed before contrast injection using MR tagging. The relaxation effects of manganese (which remains in normal cardiac myocytes for several hours) may alter the tags by increasing tag fading and hence the quality of strain measurement. Therefore, we evaluated the feasibility of cardiac MR tagging after manganese injection in normal rats. Six normal Sprague-Dawley rats were imaged in vivo using complementary spatial modulation of magnetization (C-SPAMM) at 1.5 T, before and 15 min after intraperitoneal injection of MnCl(2) solution (~17.5 micromol kg(-1)). The contrast-to-noise ratio of the tag pattern increased significantly (P < 0.001) after injection and remained comparable to the control scan in spite of the higher myocardial relaxation rate caused by the presence of manganese. The measurements of circumferential strain obtained from harmonic phase imaging analysis of the tagged images after MnCl(2) injection did not differ significantly from the measurements before injection in the endocardial, mid-wall, and epicardial regions. In particular, the transmural strain gradient was preserved. Thus, our study suggests that MR tagging could be used in combination with MEMRI to study the acute phase of coronary artery disease.

Nanoshell Magnetic Resonance Imaging Contrast Agents

Nanocontrast agents have great potential in magnetic resonance (MR) molecular imaging applications for clinical diagnosis. We synthesized Au(3)Cu(1) (gold and copper) nanoshells that showed a promising MR contrast effect. For in vitro MR images, the large proton r1 relaxivities brightened T(1)-weighted images. As for the proton-dephasing effect in T(2), Au(3)Cu(1) lightened MR images at the low concentration of 0.125 mg mL(-1) (3.84 x 10(-7) mM), and then the signal continuously decreased as the concentration increased. For in vivo MR imaging, Au(3)Cu(1) nanocontrast agents enhanced the contrast of blood vessels and suggested their potential use in MR angiography as blood-pool agents. We propose that (1) the cooperativity originating from the form of the nanoparticles and (2) the large surface area coordinated to water from their porous hollow morphology are important for efficient relaxivity. In a cytotoxicity and animal survival assay, Au(3)Cu(1) nanocontrast agents showed a dose-dependent toxic effect: the viability rate of experimental mice reached 83% at a dose of 20 mg kg(-1) and as much as 100% at 2 mg kg(-1).