Detection of pancreatic islet allograft impairment in advance of functional failure using magnetic resonance imaging.

This study evaluated the ability of magnetic resonance imaging (MRI) to predict failure of pancreatic islets (PI) transplanted into the hepatic portal vein. Brown-Norway (n = 18) and Lewis (n = 6) rats received islets isolated from Lewis donors. The rejection process in Brown-Norway recipients was mitigated by two different immunosuppressive regimens [tacrolimus + hydrocortisone for 3 months (n = 6) or tacrolimus for 12 days (n = 12)]. Longitudinal MRI monitoring of recipients at post-transplantation weeks 1, 2, 3, 4, 6, 8, 10, and 12 confirmed the ability to detect SPIO labeled PI after transplantation into the liver. The relative number of MRI signals related to PI isografts remained stable up to study completion. Recipients of PI allografts were normoglycemic until the end of study; signals declined gradually to 44 ± 17% in these animals. In animals with islets failure during post-transplant week 12, the number of signals decreased to 25 ± 10% of initial values. The difference between groups (islet function/failed) became significant post-transplant week 3. Our data demonstrate that the MRI changes attributable to rejection become apparent within 3 weeks after transplantation, i.e. at least 8 weeks before functional allograft failure.

Positive contrast visualization of SPIO-labeled pancreatic islets using echo-dephased steady-state free precession.

OBJECTIVE: MRI has recently been introduced as a promising method of monitoring the transplanted pancreatic islets labelled with superparamagnetic iron oxide (SPIO). However, the traditional [Formula: see text]-weighted approach frequently yields ambiguous results because of the negative contrast of the SPIO particles on the background of other body components. This obstacle could be overcome with the use of a novel method known as echo-dephased steady state free precession (SSFP), generating positive contrast in the presence of paramagnetic material. METHODS: In phantoms, we achieved exact localisation and clear positive contrast visualisation of human SPIO labelled islets. Using the proposed method we demonstrated the ability to detect even a single pancreatic islet against a homogeneous background. RESULTS: In vivo experiments in rats confirmed reliable and accurate localisation of transplanted SPIO labelled islets. CONCLUSION: The echo-dephased SSFP technique could successfully visualise SPIO-labelled human and rat pancreatic islets yielding a positive contrast.

A broad tuneable birdcage coil for mouse 1H/19F MR applications.

In this paper, we present the design and implementation of a 1H/19F volume coil for mouse body magnetic resonance (MR) imaging and spectroscopy using a high magnetic field (4.7 T). By changing the geometry of the coil rungs to include both nuclei for MR experiments, this innovative coil can be tuned over an extremely wide range of frequency. The coil, 45 mm in diameter and 55 mm in length, consists of a 12-rung birdcage-like structure. Using two types of tuning, the coil can generate a sufficiently homogeneous B1+ electromagnetic field within a working volume optimized for laboratory mouse. The first tuning involves changing the resonance frequency over a large frequency range. The electrical capacitance between the wires can be adjusted to reflect changes in the length of the coil. The second tuning comprises a habitual tuning transformer for precise detection in a narrow band. In contrast to widely used multinuclear coils, the coil presented here features only one resonance peak and can be manipulated according to the Larmor frequencies given for 1H and 19F. The coil was successfully tested using full-wave simulations of magnetic and electric field distributions under in vivo MR conditions.

A Trimodal Imaging Platform for Tracking Viable Transplanted Pancreatic Islets In Vivo: F-19 MR, Fluorescence, and Bioluminescence Imaging.

PURPOSE: Combining specific and quantitative F-19 magnetic resonance imaging (MRI) with sensitive and convenient optical imaging provides complementary information about the distribution and viability of transplanted pancreatic islet grafts. In this study, pancreatic islets (PIs) were labeled with positively charged multimodal nanoparticles based on poly(lactic-co-glycolic acid) (PLGA-NPs) with encapsulated perfluoro-15-crown-5-ether and the near-infrared fluorescent dye indocyanine green. PROCEDURES: One thousand and three thousand bioluminescent PIs were transplanted into subcutaneous artificial scaffolds, which served as an alternative transplant site. The grafts were monitored using in vivo F-19 MR, fluorescence, and bioluminescence imaging in healthy rats for 2 weeks. RESULTS: Transplanted PIs were unambiguously localized in the scaffolds by F-19 MRI throughout the whole experiment. Fluorescence was detected in the first 4 days after transplantation only. Importantly, in vivo bioluminescence correlated with the F-19 MRI signal. CONCLUSIONS: We developed a trimodal imaging platform for in vivo examination of transplanted PIs. Fluorescence imaging revealed instability of the fluorescent dye and its limited applicability for longitudinal in vivo studies. A correlation between the bioluminescence signal and the F-19 MRI signal indicated the fast clearance of PLGA-NPs from the transplantation site after cell death, which addresses a major issue with intracellular imaging labels. Therefore, the proposed PLGA-NP platform is reliable for reflecting the status of transplanted PIs in vivo.

Magnetic resonance imaging of pancreatic islets transplanted into the right liver lobes of diabetic mice.

INTRODUCTION: Pancreatic islets (PI) labeled with Feridex can be visualized using magnetic resonance imaging (MRI) after transplantation into the liver. However, there is still no accurate method of quantifying the signal loss caused by the iron contrast agent within transplanted tissue. The aim of this study was to test a new method for quantifying signal loss during the early posttransplantation period. METHODS: Isolated mouse PI (C57BL/6 and BALB/c) were labeled in CMRL-1066 culture media supplemented with Feridex. Two hundred twenty PI were injected directly into the right liver lobes of BALB/c diabetic (streptozotocine 220 mg/kg) recipients (isografts, n = 3; allografts, n = 3). Animals were scanned at 3 T, on a whole body scanner equipped with a high-performance gradient insert and using the 3D FIESTA sequence, on days 1, 7, and 14. Signal loss was quantified by comparison of liver tissue with and without labeled PI. Signal loss detected on the first scan was rated as 100% and subsequent measurements were recalculated as relative numbers. RESULTS: While the function of the isografts remained stable throughout the study, the allografts failed on days 5 and 10. A decrease in the amount of signal loss was observed in all animals and was comparable after the first week in both groups. However, there was a difference between groups after the second week (mean +/- SD; isografts, 100% --> 61.8 +/- 6.74% --> 47.18 +/- 7.14%; allografts, 100% --> 59.39 +/- 8.54% --> 38.16 +/- 6.81%). CONCLUSION: Disappearance of signal loss is comparable in all animals during the first week and seems to be independent of acute rejection.

Mannan-based conjugates as a multimodal imaging platform for lymph nodes.

We show that mannan-based conjugates possess exceptional features for multimodal imaging because of their biocompatibility, biodegradability and self-targeting properties. Two new mannan conjugates, containing a gadolinium complex and a fluorescent probe, one based only on polysaccharide and the other one comprising polysaccharide with poly(2-methyl-2-oxazoline) grafts, were prepared and simultaneously visualized in vitro and in vivo by magnetic resonance and fluorescence imaging. The synthesis of these mannan-based complexes was based on alkylation with allyl bromide or grafting with poly(2-methyl-2-oxazoline) chains, followed by a thiol-ene click reaction with cysteamine to introduce primary amino groups into their structure. Finally, the obtained conjugates were functionalized with contrast labels using the corresponding N-hydroxysuccinimide esters. When used to detect lymph nodes, the polymers showed better imaging properties than a commercially available contrast agent.

Lesion evolution after gamma knife irradiation observed by magnetic resonance imaging.

PURPOSE: Our study is focused on the magnetic resonance imaging (MRI) observation of lesion development and hippocampus related functional impairments in rats after irradiation with a Leksell Gamma knife (LGK). MATERIALS AND METHODS: We exposed 32 three-month-old Long-Evans rats to various radiation doses (25 Gy, 50 Gy or 75 Gy). The rats were scanned by a 4.7 T magnetic resonance (MR) spectrometer at several timepoints (1 - 18 months) after irradiation. The lesion size was evaluated by manual segmentation; the animals were behaviorally tested in a Morris water maze and examined histologically. RESULTS: We found that a dose of 25 Gy induced no edema, necrosis or behavioral change. The response of the rats to higher doses was not uniform; the first occurrence of lesions in the rat brains irradiated with 50 and 75 Gy was detected six months post-irradiation. Functional impairment correlated well with the lesion size and histology. CONCLUSIONS: Rat brains showed the development of expanding delayed lesions after 50 or 75 Gy doses from the LGK during the first year after irradiation.

Ln(iii)-complexes of a DOTA analogue with an ethylenediamine pendant arm as pH-responsive PARACEST contrast agents.

A novel macrocyclic DO3A derivative containing a linear diamine pendant arm, H3do3aNN, was prepared and its protonation and complexation properties were studied by means of potentiometry. It determined ligand consecutive protonation constants log K(An) = 12.62, 10.28, 9.67, 8.30, 3.30 and 1.58 and stability constants of selected lanthanide (Eu(iii), Yb(iii)) complexes log K(EuL) = 23.16 and log KYbL = 22.76. The complexes could be protonated on the pendant amino group(s) with log K(HLM) ≈ 5.6 and log K(H2LM) ≈ 4.8. Solution structures of both complexes were studied by NMR spectroscopy. The study revealed that the complex species exist exclusively in the form of twisted-square-antiprismatic (TSA) isomers. The complexes show significant pH dependence of the Chemical Exchange Saturation Transfer (CEST) between their amino groups and the bulk water molecules in the pH range of 5-8. Thus, the pH dependence of the magnetization transfer ratio of CEST signals can be used for pH determination using magnetic resonance imaging techniques in a pH range relevant for in vivo conditions.

Magnetic resonance imaging of intrahepatically transplanted islets using paramagnetic beads.

Superparamagnetic agents can be reliably used for magnetic resonance imaging (MRI) of pancreatic islets located in the liver sinusoids. However, the main disadvantages seemed to be the rather long culture time necessary for islet labeling and the low specificity of these agents. In the present study we investigated a more specific approach with a shorter labeling time using immunomagnetic particles. Isolated rat islets were cultivated with immunomagnetic beads coated with antibody against rat MHC class I antigen. Labeled islets were transplanted into the livers of syngeneic rats. The animals were examined weekly by MRI or livers explanted 10 minutes after islet transplantation for in vitro experiments. In both in vitro and in vivo studies, labeled transplanted islets were imaged as hypointensive spots, diffusely distributed throughout the liver. This experiment represents an alternative way of islet imaging by magnetic resonance, which is as effective as the use of known superparamagnetic contrast agents and more specific owing to targeting to specific donor antigens.

Effect of mesenchymal stem cells on the vascularization of the artificial site for islet transplantation in rats.

An adequate vascularization of the artificially created cavity is crucial for subsequent transplantation of isolated pancreatic islets. In a reported study, dynamic contrast-enhanced magnetic resonance imaging was used to assess the effect of mesenchymal stem cells on neoangiogenesis within connective tissue surrounding an implantable biocompatible device. The signal increase detected after injection of magnetic resonance contrast agent in each target region was considered to be an effect of contrast agent, which was related to the blood supply. To minimize the influence of variability in contrast agent application, all outcomes measured in the implanted devices were normalized to the signal intensity of kidney tissue. When supported by mesenchymal stem cells, the mean signal increase intra-abdominally was 42%, 41%, and 64% and within subcutaneously implanted devices was 23%, 54%, and 52% of that measured in kidney.

Dynamic contrast-enhanced magnetic resonance imaging as a tool to monitor the blood supply to an artificial cavity used as a site for islet transplantation in rats.

BACKGROUND: The transplantation of islets of Langerhans isolated from one donor pancreas can rarely release a diabetic recipient from insulin injections. The major reason is the destruction of 50%-60% of the transplanted tissue, which proceeds typically within a few hours after the insertion of the islets into the portal vein. Therefore, several groups have focused on development of an artificial site for islet transplantation. The main aim of the present study was to test the efficacy of dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) to evaluate the blood supply feeding the artificially created cavities for islet transplantation. METHODS: Two rounded devices were implanted: one device subcutaneously and the second one into the greater omentum of each animal. On the day of implantation as well as 1, 3, and 4 weeks later, we quickly injected the vascular specific MR contrast agent Vasovist (0.05 mL/100 g) intravenously. Penetration of the contrast agent was monitored by DCE-MRI. The influence of the contrast agent on the signal intensity observed within selected target areas was calculated with the use of ImageJ software. RESULTS: The penetration of the contrast agent was detected by the increase in signal intensity within implanted devices. The signal increase caused by the contrast compound was normalized to kidney tissue. On day of implantation of the device, no signal due to the contrast agent was detected in all devices. However, over the following weeks, there was an increase in signal detection within the omental device to 34%, 21%, and 14% of that of the kidney. Within the subcutaneously implanted devices there was an increase in signal detection up to 11%, 10%, and 7% of that detected in the kidney. CONCLUSIONS: The optimal time for transplantation of pancreatic islets into our omental device was 1 week after implantation of the scaffold. Also, the blood supply feeding the subcutaneous devices was regarded to be inadequate.

Vascularization of artificial beds for pancreatic islet transplantation in a rat model.

An alternative prevascularized bed with a subcutaneously located entrance would substantially improve islet engraftment, requiring much less invasive surgery. Studies have described times necessary for the creation of an artificial pouch suitable for subsequent islet transplantation. Polymeric mesh shaped in rounded scaffolds were implanted both subcutaneously and into the major omentum of Brown Norway female rats (n = 7). The connective tissue together with vessels were embedded into scaffolds at 1 week without regard to site. In contrast to the major omentum, vessels within the subcutaneous connective tissue surrounding the devices started to decline in 2 weeks and almost disappeared 1 week later. Magnetic resonance imaging (MRI) detected changes in fibrous tissue surrounding the wall, but only large veins located beside the devices were visible using basic MRI. The blood supply to the internal surface of the created beds was important for islet engraftment, but information could be obtained only by using dynamic contrast-enhanced MRI.

Classification of calf muscle MR images by texture analysis.

AIM: To evaluate a method of texture analysis (TA) for the description of magnetic resonance (MR) images of healthy and diseased calf muscles and to compare this method with standard radiological evaluation. METHODS: A total of 93 subjects (20 controls, seven healthy children of hypertonic parents, five diabetic patients and 61 subjects with muscle malfunction of various origin) underwent MR imaging of the calf muscle and texture analysis of images was performed. The results of TA were analysed by t-statistics and principal component analysis. Images of subjects were divided into four groups according to the assessment of three radiologists and this categorization of subjects was compared with the results from TA. RESULTS: We extracted seven features (from a total number of 282) which were successfully used for the description of the texture of T1w MR images of calf muscles. The results of classification by TA are in 80% agreement with the categorization made by the radiologists. In some cases, TA is able to describe changes not apparent by visual inspection. CONCLUSION: The TA of MR images of calf muscles can be used for the objective description of changes in muscles and could help radiologists to distinguish between healthy and diseased tissue.