Detection of acute radiation damage to the spleen in mice by using fluorine-19 MR imaging

Systems Engineering Approach Towards Sensitive Cellular Fluorine-19 MRI

In vivo fluorine-19 MRI using F-based tracer media has shown utility and versatility for a wide range of biomedical uses, particularly immune and stem cell detection, as well as biosensing. As with many advanced MRI acquisition techniques, the sensitivity and limit of detection (LOD) in vivo is a key consideration for a successful study outcome. In this review, we analyze the primary factors that limit cell LOD. The achievable sensitivity is strongly dependent on the specific composition of tracer, cell type of interest, cell activity, data acquisition and reconstruction methods, and MRI hardware design. Recent innovations in molecular 19F tracer design and image acquisition-reconstruction methods have achieved significant leaps in 19F MRI sensitivity, and integration of these new materials and methods into studies can result in > 10-fold improvement in LOD. These developments will help unlock the full potential of clinical 19F MRI for biomedical applications.

In Vivo Fluorine Imaging Using 1.5 Tesla MRI for Depiction of Experimental Myocarditis in a Rodent Animal Model

The usefulness of perfluorocarbon nanoemulsions for the imaging of experimental myocarditis has been demonstrated in a high-field 9.4 Tesla MRI scanner. Our proof-of-concept study investigated the imaging capacity of PFC-based ¹⁹F/¹H MRI in an animal myocarditis model using a clinical field strength of 1.5 Tesla. To induce experimental myocarditis, five male rats (weight ~300 g, age ~50 days) were treated with one application per week of doxorubicin (2 mg/kg BW) over a period of six weeks. Three control animals received the identical volume of sodium chloride 0.9% instead. Following week six, all animals received a single 4 ml injection of an 20% oil-in-water perfluorooctylbromide nanoemulsion 24 hours prior to in vivo¹H/¹⁹F imaging on a 1.5 Tesla MRI. After euthanasia, cardiac histology and immunohistochemistry using CD68/ED1 macrophage antibodies were performed, measuring the inflamed myocardium in μm² for further statistical analysis to compare the extent of the inflammation with the ¹⁹F-MRI signal intensity. All animals treated with doxorubicin showed a specific signal in the myocardium, while no myocardial signal could be detected in the control group. Additionally, the doxorubicin group showed a significantly higher SNR for ¹⁹F and a stronger CD68/ED1 immunhistoreactivity compared to the control group. This proof-of-concept study demonstrates that perfluorocarbon nanoemulsions could be detected in an in vivo experimental myocarditis model at a currently clinically relevant field strength.

Recent Advances in 19Fluorine Magnetic Resonance Imaging with Perfluorocarbon Emulsions

The research roots of ¹⁹fluorine (¹⁹F) magnetic resonance imaging (MRI) date back over 35 years. Over that time span, ¹H imaging flourished and was adopted worldwide with an endless array of applications and imaging approaches, making magnetic resonance an indispensable pillar of biomedical diagnostic imaging. For many years during this timeframe, ¹⁹F imaging research continued at a slow pace as the various attributes of the technique were explored. However, over the last decade and particularly the last several years, the pace and clinical relevance of ¹⁹F imaging has exploded. In part, this is due to advances in MRI instrumentation, ¹⁹F/¹H coil designs, and ultrafast pulse sequence development for both preclinical and clinical scanners. These achievements, coupled with interest in the molecular imaging of anatomy and physiology, and combined with a cadre of innovative agents, have brought the concept of ¹⁹F into early clinical evaluation. In this review, we attempt to provide a slice of this rich history of research and development, with a particular focus on liquid perfluorocarbon compound-based agents.

In Vivo Quantification of Inflammation in Experimental Autoimmune Encephalomyelitis Rats Using Fluorine-19 Magnetic Resonance Imaging Reveals Immune Cell Recruitment outside the Nervous System

Progress in identifying new therapies for multiple sclerosis (MS) can be accelerated by using imaging biomarkers of disease progression or abatement in model systems. In this study, we evaluate the ability to noninvasively image and quantitate disease pathology using emerging “hot-spot” ¹⁹F MRI methods in an experimental autoimmune encephalomyelitis (EAE) rat, a model of MS. Rats with clinical symptoms of EAE were compared to control rats without EAE, as well as to EAE rats that received daily prophylactic treatments with cyclophosphamide. Perfluorocarbon (PFC) nanoemulsion was injected intravenously, which labels predominately monocytes and macrophages in situ. Analysis of the spin-density weighted ¹⁹F MRI data enabled quantification of the apparent macrophage burden in the central nervous system and other tissues. The in vivo MRI results were confirmed by extremely high-resolution ¹⁹F/¹H magnetic resonance microscopy in excised tissue samples and histopathologic analyses. Additionally, ¹⁹F nuclear magnetic resonance spectroscopy of intact tissue samples was used to assay the PFC biodistribution in EAE and control rats. In vivo hot-spot ¹⁹F signals were detected predominantly in the EAE spinal cord, consistent with the presence of inflammatory infiltrates. Surprising, prominent ¹⁹F hot-spots were observed in bone-marrow cavities adjacent to spinal cord lesions; these were not observed in control animals. Quantitative evaluation of cohorts receiving cyclophosphamide treatment displayed significant reduction in ¹⁹F signal within the spinal cord and bone marrow of EAE rats. Overall, ¹⁹F MRI can be used to quantitatively monitored EAE disease burden, discover unexpected sites of inflammatory activity, and may serve as a sensitive biomarker for the discovery and preclinical assessment of novel MS therapeutic interventions.

In vivo MRI cell tracking using perfluorocarbon probes and fluorine-19 detection

This article presents a brief review of preclinical in vivo cell-tracking methods and applications using perfluorocarbon (PFC) probes and fluorine-19 ((19) F) MRI detection. Detection of the (19) F signal offers high cell specificity and quantification ability in spin density-weighted MR images. We discuss the compositions of matter, methods and applications of PFC-based cell tracking using ex vivo and in situ PFC labeling in preclinical studies of inflammation and cellular therapeutics. We also address the potential applicability of (19) F cell tracking to clinical trials.

Study of radiation induced changes of phosphorus metabolism in mice by (31)P NMR spectroscopy

Background

The aim of this study was to examine whether ³¹P NMR can efficiently detect X-ray radiation induced changes of energy metabolism in mice. Exposure to ionizing radiation causes changes in energy supply that are associated with the tissue damage because of oxidative stress and uncoupled oxidative phosphorylation. This has as a consequence decreased phosphocreatine to adenosine triphosphate ratio (Pcr/ATP) as well as increased creatine kinase (CK) and liver enzymes (transaminases AST and ALT) levels in serum.

Materials and methods

In this study, experimental mice that received 7 Gy of X-ray radiation and a control group were studied by ³¹P NMR spectroscopy and biochemically by measuring CK and liver enzyme levels in plasma. Mice (irradiated and control) were measured at regular time intervals for the next three weeks after the exposure to radiation.

Results

A significant change in the Pcr/ATP ratio, determined from corresponding peaks of ³¹P NMR spectra, was observed in the 7 Gy group 2 days or more after the irradiation, while no significant change in the Pcr/ATP ratio, was observed in the control group. This result was supported by parallel measurements of CK levels that were highly increased immediately after the irradiation which correlates with the observed decrease of the Pcr/ATP ratio and with it associated drop of muscle energy supply.

Conclusions

The ³¹P NMR measurements of the Pcr/ATP ratio can in principle serve as an instantaneous and noninvasive index for assessment of the received dose of irradiation.

Fluorine-containing nanoemulsions for MRI cell tracking

In this article we review the chemistry and nanoemulsion formulation of perfluorocarbons used for in vivo(19)F MRI cell tracking. In this application, cells of interest are labeled in culture using a perfluorocarbon nanoemulsion. Labeled cells are introduced into a subject and tracked using (19)F MRI or NMR spectroscopy. In the same imaging session, a high-resolution, conventional ((1)H) image can be used to place the (19)F-labeled cells into anatomical context. Perfluorocarbon-based (19)F cell tracking is a useful technology because of the high specificity for labeled cells, ability to quantify cell accumulations, and biocompatibility. This technology can be widely applied to studies of inflammation, cellular regenerative medicine, and immunotherapy.

Effect of carbogen on tumor oxygenation: combined fluorine-19 and proton MRI measurements

PURPOSE

Blood oxygen level dependent (BOLD) contrast in magnetic resonance imaging (MRI) has been widely used for noninvasive evaluation of the effects of tumor-oxygenating agents. However, there have been few tests of the validity of this method. The goal of the present work was to use the T(1) of fluorine-19 in perfluorocarbon (PFC) emulsions as a "gold standard" for comparison with BOLD MRI. MATHODS AND MATERIALS: Rats bearing R3230AC tumors implanted in the hind limb were injected with an emulsion of perfluoro-15-crown-5-ether for 2-3 days before experiments, which ensured that the PFC emulsion concentrated in the tumors. We correlated changes in tumor oxygenation caused by carbogen inhalation measured by (1)H BOLD MRI with quantitative (19)F measurements. The (19)F spin-lattice relaxation rate R(1) (= 1/T(1)) was measured to determine initial oxygen tension (pO(2)) in each image pixel containing the PFC, and changes in pO(2) during carbogen (95% O(2), 5% CO(2)) breathing. In a second carbogen breathing period, changes in water signal linewidth were measured using high spectral and spatial resolution imaging. (19)F and (1)H measurements were used to classify pixels as responders to carbogen (pixels where oxygen increased significantly) or nonresponders (no significant change in tumor oxygenation).

RESULTS

The (19)F and (1)H measurements agreed in 65% +/- 11% of pixels (n = 14). Agreement was even stronger among pixels where (1)H showed increased oxygenation; (19)F measurements agreed with (1)H measurements in over 79% +/- 11% of these pixels. Similarly, there was strong agreement between the two modalities in pixels where (19)F reported no change in pO(2); (1)H also showed no changes in 76% +/- 18% of these pixels. Quantitative correlation of changes T(2)* (DeltaT(2)*) in (1)H and changes R(1) (DeltaR(1)) in (19)F was weak during carbogen breathing, and averaged over the whole tumor was approximately 0.40 for 14 experiments. However, the spatial patterns of (1)H and (19)F changes were qualitatively very similar. In hypoxic regions that were identified based on long (19)F T(1) (>2.53 s), (19)F and (1)H MRI agreed that carbogen had relatively weak effects.

CONCLUSIONS

These results suggest that (1)H BOLD MRI reliably identifies increases in tumor pO(2). In hypoxic regions where increases in pO(2) are most desirable, carbogen was ineffective. The data suggest that (19)F and (1)H MRI can be used individually or in combination to guide the design of improved tumor-oxygenating agents.