Novel double contrast MRI technique for intramyocardial detection of percutaneously transplanted autologous cells

Magnetic resonance imaging tracking and assessing repair function of the bone marrow mesenchymal stem cells transplantation in a rat model of spinal cord injury

The transplantation of bone marrow mesenchymal stem cells (BMSCs) to repair spinal cord injury (SCI) has become a promising therapy. However, there is still a lack of visual evidence directly implicating the transplanted cells as the source of the improvement of spinal cord function. In this study, BMSCs were labeled with NF-200 promoter and lipase-activated gadolinium-containing nanoparticles (Gd-DTPA-FA). Double labeled BMSCs were implanted into spinal cord transaction injury in rat models in situ, the function recovery was evaluated on 1st, 7th, 14th, 28 th days by MRI, Diffusion Tensor Imaing, CT imaging and post-processing, and histological observations. BBB scores were used for assessing function recovery. After transplantation of BMSCs, the hypersignal emerged in spinal cord in T1WI starting at day 7 that was focused at the injection site, which then increased and extended until day 14. Subsequently, the increased signal intensity area rapidly spread from the injection site to entire injured segment lasting four weeks. The diffusion tensor tractography and histological analysis both showed the nerve fibre from dividing to connecting partly. Immunofluorescence showed higher expression of NF-200 in Repaired group than Injury group. Electron microscopy showed detachment and loose of myelin lamellar getting better in Repaired group compared with the Injury group. BBB scores in Repaired group were significantly higher than those of injury animals. Our study suggests that the migration and distribution of Gd-DTPA-FA labeled BMSCs can be tracked using MRI. Transplantation of BMSCs represents a promising potential strategy for the repair of SCI.

Effects of Iron Oxide Nanoparticle Labeling on Human Endothelial Cells

Iron oxide nanoparticles (INOPS) are a potential contrast agent for magnetic resonance (MR) tracking of transplanted endothelial cells. The objective of this study was to examine the effect of INOPS labeling on endothelial cells. The mixture of INOPS and poly-l-lysine (PLL) was used to label human endothelial cells. Labeling efficiency was examined by Prussian blue staining, transmission electron microscopy, and atomic absorption spectrometry. The effect of iron oxide concentration on cell viability and proliferation were determined. The correlation of reactive oxygen species (ROS) and apoptosis was also examined. In vitro MRI scanning was carried out using a 1.5T MR system. INOPS-PLL could be readily taken up by endothelial cells and subsequently induce MRI signal intensity changes. However, higher labeling concentration (>50 μg/ml) and longer incubation (48 h) can affect cell viability and proliferation. Mitochondrial damage, apoptosis, and autolysosmes were observed under high INOPS-PLL concentrations, which were correlated to ROS production. INOPS-PLL nanoparticles can be used to label transplanted endothelial cells. However, high concentration of INOPS can impair cell viability, possibly through ROS-mediated apoptosis and autophagy.

Value of MR contrast media in image-guided body interventions

In the past few years, there have been multiple advances in magnetic resonance (MR) instrumentation, in vivo devices, real-time imaging sequences and interventional procedures with new therapies. More recently, interventionists have started to use minimally invasive image-guided procedures and local therapies, which reduce the pain from conventional surgery and increase drug effectiveness, respectively. Local therapy also reduces the systemic dose and eliminates the toxic side effects of some drugs to other organs. The success of MR-guided procedures depends on visualization of the targets in 3D and precise deployment of ablation catheters, local therapies and devices. MR contrast media provide a wealth of tissue contrast and allows 3D and 4D image acquisitions. After the development of fast imaging sequences, the clinical applications of MR contrast media have been substantially expanded to include pre- during- and post-interventions. Prior to intervention, MR contrast media have the potential to localize and delineate pathologic tissues of vital organs, such as the brain, heart, breast, kidney, prostate, liver and uterus. They also offer other options such as labeling therapeutic agents or cells. During intervention, these agents have the capability to map blood vessels and enhance the contrast between the endovascular guidewire/catheters/devices, blood and tissues as well as direct therapies to the target. Furthermore, labeling therapeutic agents or cells aids in visualizing their delivery sites and tracking their tissue distribution. After intervention, MR contrast media have been used for assessing the efficacy of ablation and therapies. It should be noted that most image-guided procedures are under preclinical research and development. It can be concluded that MR contrast media have great value in preclinical and some clinical interventional procedures. Future applications of MR contrast media in image-guided procedures depend on their safety, tolerability, tissue specificity and effectiveness in demonstrating success of the interventions and therapies.

lacZ as a genetic reporter for real-time MRI

Molecular imaging based on MRI is currently hampered by the lack of genetic reporters for in vivo imaging. We determined that the commercially available substrate S-Gal can be used to detect genetically engineered beta-galactosidase expressing cells by MRI. The effect and specificity of the reaction between beta-galactosidase and S-Gal on MRI contrast were determined both in vitro and in vivo. beta-galactosidase activity in the presence of S-Gal resulted in enhanced T(2) and T*(2) MR-contrast, which was amplified with increasing magnetic field strengths (4.7-17.6 T) in phantom studies. Using both lacZ(+) transgenic animals and lacZ(+) tissue transplants, we were able to detect labeled cells in live animals in real time. Similar to phantom studies, detection of the labeled cells/tissues in vivo was enhanced at high magnetic fields. These results demonstrate that the genetic reporter, lacZ, can be used as an in vivo marker gene using high-field-strength MRI.

Long-term tracking of bone marrow progenitor cells following intracoronary injection post-myocardial infarction in swine using MRI

Magnetic resonance imaging (MRI) can track progenitor cells following direct intramyocardial injection. However, in the vast majority of post-myocardial infarction (MI) clinical trials, cells are delivered by the intracoronary (IC) route, which results in far greater dispersion within the myocardium. Therefore, we assessed whether the more diffuse distribution of cells following IC delivery could be imaged longitudinally with MRI. In 11 pigs (7 active, 4 controls), MI was induced by 90-min balloon occlusion of the left anterior descending coronary artery. Seven (0) days [median (interquartile range)] following MI, bone marrow progenitor cells (BMCs) were colabeled with an iron-fluorophore and a cell viability marker and delivered to the left anterior descending coronary artery distal to an inflated over-the-wire percutaneous transluminal coronary angioplasty balloon. T2*-weighted images were used to assess the location of the magnetically labeled cells over a 6-wk period post-MI. Immediately following cell delivery, hypointensity characteristic of the magnetic label was observed in the infarct border rather than within the infarct itself. At 6 wk, the cell signal hypointensity persisted, albeit with significantly decreased intensity. BMC delivery resulted in significant improvement in infarct volume and ejection fraction (EF): infarct volume in cell-treated animals decreased from 7.1 +/- 1.5 to 4.9 +/- 1.0 ml (P < 0.01); infarct volume in controls was virtually unchanged at 4.64 +/- 2.1 to 4.39 +/- 2.1 ml (P = 0.7). EF in cell-treated animals went from 30.4 +/- 5.2% preinjection to 34.5 +/- 2.5% 6 wk postinjection (P = 0.013); EF in control animals went from 34.3 +/- 4.7 to 31.9 +/- 6.8% (P = 0.5). Immunohistochemical analysis revealed intracellular colocalization of the iron fluorophore and cell viability dye with the labeled cells continuing to express the same surface markers as at baseline. MRI can track the persistence and distribution of magnetically labeled BMCs over a 6-wk period following IC delivery. Signal hypointensity declines with time, particularly in the first week following delivery. These cells maintain their original phenotype during this time course. Delivery of these cells appears safe and results in improvement in infarct size and left ventricular ejection fraction.

Imaging approaches for the study of cell-based cardiac therapies

Despite promising preclinical data, the treatment of cardiovascular diseases using embryonic, bone-marrow-derived, and skeletal myoblast stem cells has not yet come to fruition within mainstream clinical practice. Major obstacles in cardiac stem cell investigations include the ability to monitor cell engraftment and survival following implantation within the myocardium. Several cellular imaging modalities, including reporter gene and MRI-based tracking approaches, have emerged that provide the means to identify, localize, and monitor stem cells longitudinally in vivo following implantation. This Review will examine the various cardiac cellular tracking modalities, including the combinatorial use of several probes in multimodality imaging, with a focus on data from the past 5 years.

Gene and cell therapy for heart failure

Cardiac gene and cell therapy have both entered clinical trials aimed at ameliorating ventricular dysfunction in patients with chronic congestive heart failure. The transduction of myocardial cells with viral constructs encoding a specific cardiomyocyte Ca(2+) pump in the sarcoplasmic reticulum (SR), SRCa(2+)-ATPase has been shown to correct deficient Ca(2+) handling in cardiomyocytes and improvements in contractility in preclinical studies, thus leading to the first clinical trial of gene therapy for heart failure. In cell therapy, it is not clear whether beneficial effects are cell-type specific and how improvements in contractility are brought about. Despite these uncertainties, a number of clinical trials are under way, supported by safety and efficacy data from trials of cell therapy in the setting of myocardial infarction. Safety concerns for gene therapy center on inflammatory and immune responses triggered by viral constructs, and for cell therapy with myoblast cells, the major concern is increased incidence of ventricular arrhythmia after cell transplantation. Principles and mechanisms of action of gene and cell therapy for heart failure are discussed, together with the potential influence of reactive oxygen species on the efficacy of these treatments and the status of myocardial-delivery techniques for viral constructs and cells.

Cellular magnetic resonance imaging: potential for use in assessing aspects of cardiovascular disease

There is rapidly increasing interest in the use of magnetic resonance imaging (MRI) to track cell migration in vivo. Iron oxide MR contrast agents can be detected at micromolar concentrations of iron, and offer sufficient sensitivity for T2*-weighted imaging. Cellular MRI shows potential for assessing aspects of cardiovascular disease. Labeling in vivo and tracking macrophages using iron oxide nanoparticles has been a goal for cellular MRI because macrophages play a pivotal role in the pathophysiology of many human diseases, including atherosclerosis. Cellular MRI has also been using to track transplanted therapeutic cells in myocardial regeneration. This review looked at iron oxide nanoparticles, methods of cell labeling, image acquisition techniques and limitations encountered for visualization. Particular attention was paid to stem cells and macrophages for the cardiovascular system.

Composite MR Contrast Agents for Conditional Cell-Labeling

Gadolinium-chelates (Gd-DTPA) and superparamagnetic particles of iron oxide (SPIO) are two commonly used MR contrast agents that exhibit inherently different relaxation properties. These two agents have been used to label cells ex-vivo to generate signal contrast with respect to background tissue when introduced to a tissue-of-interest. Assuming minimal mutual interaction between these two agents, we were motivated to investigate the creation of composite relaxation properties by mixing the two in aqueous solutions for conditioning cell labeling. Concentration-dependent relaxivity coefficients were first obtained from each contrast agent, independently, in saline solution at 3 Tesla. These coefficients were then used to predict both the R(1) and R(2) relaxation rates of a composite contrast agent using a linear model combining the effects of both contrast media. The predicted relaxation rates were experimentally confirmed from 25 composite solutions (combinations of SPIO-concentration ranging from 0 to 1 mug/mL and Gd-DTPA-concentration ranging from 0 to 0.20 mM). We show that the combination of SPIO and Gd-DTPA in an aqueous solution exhibits unique and predictable relaxivity properties that are unattainable via the individual use of either agent. The method may be applied to create 'user-tunable' contrast conditions for the visualization of magnetically labeled cells in the context of cell replacement therapy.

Stem cell therapy: MRI guidance and monitoring

With the recent advances in magnetic resonance (MR) labeling of cellular therapeutics, it is natural that interventional MRI techniques for targeting would be developed. This review provides an overview of the current methods of stem cell labeling and the challenges that are created with respect to interventional MRI administration. In particular, stem cell therapies will require specialized, MR-compatible devices as well as integration of graphical user interfaces with pulse sequences designed for interactive, real-time delivery in many organs. Specific applications that are being developed will be reviewed as well as strategies for future translation to the clinical realm.

Local frequency density of states around field inhomogeneities in magnetic resonance imaging: effects of diffusion

A method describing NMR-signal formation in inhomogeneous tissue is presented which covers all diffusion regimes. For this purpose, the frequency distribution inside the voxel is described. Generalizing the results of the well-known static dephasing regime, we derive a formalism to describe the frequency distribution that is valid over the whole dynamic range. The expressions obtained are in agreement with the results obtained from Kubos line-shape theory. To examine the diffusion effects, we utilize a strong collision approximation, which replaces the original diffusion process by a simpler stochastic dynamics. We provide a generally valid relation between the frequency distribution and the local Larmor frequency inside the voxel. To demonstrate the formalism we give analytical expressions for the frequency distribution and the free induction decay in the case of cylindrical and spherical magnetic inhomogeneities. For experimental verification, we performed measurements using a single-voxel spectroscopy method. The data obtained for the frequency distribution, as well as the magnetization decay, are in good agreement with the analytic results, although experiments were limited by magnetic field gradients caused by an imperfect shim and low signal-to-noise ratio.

Magnetic resonance imaging of haemorrhage within reperfused myocardial infarcts: possible interference with iron oxide-labelled cell tracking?

AIMS

Magnetic resonance imaging (MRI) has been proposed as a tool to track iron oxide-labelled cells within myocardial infarction (MI). However, infarct reperfusion aggravates microvascular obstruction (MO) and causes haemorrhage. We hypothesized that haemorrhagic MI causes magnetic susceptibility-induced signal voids that may interfere with iron oxide-labelled cell detection.

METHODS AND RESULTS

Pigs (n = 23) underwent 2 h occlusion of the left circumflex artery. Cine, T2*-weighted, perfusion, and delayed enhancement MRI scans were performed at 1 and 5 weeks, followed by ex vivo high-resolution scanning. At 1 week, MO was observed in 17 out of 21 animals. Signal voids were observed on T2*-weighted scans in five out of eight animals, comprising 24 +/- 22% of the infarct area. A linear correlation was found between area of MO and signal voids (R2 = 0.87; P = 0.002). At 5 weeks, MO was observed in two out of 13 animals. Signal voids were identified in three out of seven animals. Ex vivo scanning showed signal voids on T2*-weighted scanning in all animals because of the presence of haemorrhage, as confirmed by histology. Signal voids interfered with the detection of iron oxide-labelled cells ex vivo (n = 21 injections).

CONCLUSION

Haemorrhage in reperfused MI produces MRI signal voids, which may hamper tracking of iron oxide-labelled cells.