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Fluorine MRI ((19) F MRI) is receiving an increasing attention as a viable alternative to proton-based MRI ((1) H MRI) for dedicated application in molecular imaging. The (19) F nucleus has a high gyromagnetic ratio, a 100% natural abundance and is furthermore hardly present in human tissues allowing for hot spot MR imaging. The applicability of (19) F MRI as a molecular and cellular imaging technique has been exploited, ranging from cell tracking to detection and imaging of tumors in preclinical studies. In addition to applications, developing new contrast materials with improved relaxation properties has also been a core research topic in the field, since the inherently low longitudinal relaxation rates of perfluorocarbon compounds result in relatively low imaging efficiency. Borrowed from (1) H MRI, the incorporation of lanthanides, specifically Gd(III) complexes, as signal modulating ingredients in the nanoparticle formulation has emerged as a promising approach to improvement of the fluorine signal. Three different perfluorocarbon emulsions were investigated at five different magnetic field strengths. Perfluoro-15-crown-5-ether was used as the core material and Gd(III)DOTA-DSPE, Gd(III)DOTA-C6-DSPE and Gd(III)DTPA-BSA as the relaxation altering components. While Gd(III)DOTA-DSPE and Gd(III)DOTA-C6-DSPE were favorable constructs for (1) H NMR, Gd(III)DTPA-BSA showed the strongest increase in (19F) R(1). These results show the potential of the use of paramagnetic lipids to increase (19F) R(1) at clinical field strengths (1.5-3 T). At higher field strengths (6.3-14 T), gadolinium does not lead to an increase in (19F) R(1) compared with emulsions without gadolinium, but leads to an significant increase in (19F) R(2). Our data therefore suggest that the most favorable situation for fluorine measurements is at high magnetic fields without the inclusion of gadolinium constructs.

The virtual elastic sphere tool is applicable to CT, dual-energy CT, and MR angiography, and it improves reproducibility and efficiency over that achieved with manual stenosis measurements.

In vivo molecular imaging with targeted MRI contrast agents will require sensitive methods to quantify local concentrations of contrast agent, enabling not only imaging-based recognition of pathological biomarkers but also detection of changes in expression levels as a consequence of disease development, therapeutic interventions or recurrence of disease. In recent years, targeted paramagnetic perfluorocarbon emulsions have been frequently applied in this context, permitting high-resolution (1)H MRI combined with quantitative (19)F MR imaging or spectroscopy, under the assumption that the fluorine signal is not altered by the local tissue and cellular environment. In this in vitro study we have investigated the (19)F MR-based quantification potential of a paramagnetic perfluorocarbon emulsion conjugated with RGD-peptide to target the cell-internalizing α(ν)β(3)-integrin expressed on endothelial cells, using a combination of (1)H MRI, (19)F MRI and (19)F MRS. The cells took up the targeted emulsion to a greater extent than nontargeted emulsion. The targeted emulsion was internalized into large 1-7 µm diameter vesicles in the perinuclear region, whereas nontargeted emulsion ended up in 1-4 µm diameter vesicles, which were more evenly distributed in the cytoplasm. Association of the targeted emulsion with the cells resulted in different proton longitudinal relaxivity values, r(1), for targeted and control nanoparticles, prohibiting unambiguous quantification of local contrast agent concentration. Upon cellular association, the fluorine R(1) was constant with concentration, while the fluorine R(2) increased nonlinearly with concentration. Even though the fluorine relaxation rate was not constant, the (19)F MRI and (19)F MRS signals for both targeted nanoparticles and controls were linear and quantifiable as function of nanoparticle concentration.

This preclinical proof-of-concept study shows the in vivo quantification of iodine concentrations in tissues using spectral CT. Our multimodal imaging approach with spectral CT and SPECT using radiolabeled iodinated emulsions together with ICP-based quantification allows a direct comparison of all methods. Benchmarked against ICP-MS data, spectral CT in the present implementation shows a slight underestimation of organ iodine concentrations compared with SPECT but with a more narrow distribution. This slight deviation is most likely caused by experimental rather than technical issues.

The synthesis, design and subsequent pre-clinical testing of new molecular imaging tracers are topic of extensive research in healthcare. Quantitative dual-isotope SPECT imaging is proposed here as a tool in the design and validation of such tracers, as it can be used to quantify and compare the biodistribution of a specific ligand and its nonspecific control ligand, labeled with two different radionuclides, in the same animal. Since the biodistribution results are not blurred by experimental or physiological inter-animal variations, this approach allows determination of the ligand's net targeting effect. However, dual-isotope quantification is complicated by crosstalk between the two radionuclides used and the radionuclides should not influence the biodistribution of the tracer. Here, we developed a quantitative dual-isotope SPECT protocol using combined (111)Indium and (177)Lutetium and tested this tool for a well-known angiogenesis-specific ligand (cRGD peptide) in comparison to a potential nonspecific control (cRAD peptide). Dual-isotope SPECT imaging of the peptides showed a similar organ and tumor uptake to single-isotope studies (cRGDfK-DOTA, 1.5 ± 0.8%ID cm(-3); cRADfK-DOTA, 0.2 ± 0.1%ID cm(-3)), but with higher statistical relevance (p-value 0.007, n = 8). This demonstrated that, for the same relevance, seven animals were required in case of a single-isotope test design as compared with only three animals when a dual-isotope test was used. Interchanging radionuclides did not influence the biodistribution of the peptides. Dual-isotope SPECT after simultaneous injection of (111)In and (177)Lu-labeled cRGD and cRAD was shown to be a valuable method for paired testing of the in vivo target specificity of ligands in molecular imaging tracer design.

One of the major challenges of MR imaging is the quantification of local concentrations of contrast agents. Cellular uptake strongly influences different parameters such as the water exchange rate and the pool of water protons, and results in alteration of the contrast agent's relaxivity, therefore making it difficult to determine contrast agent concentrations based on the MR signal only. Here, we propose a multimodal radiolabeled paramagnetic liposomal contrast agent that allows simultaneous imaging with SPECT and MRI. As SPECT-based quantification allows determination of the gadolinium concentration, the MRI signal can be deconvoluted to get an understanding of the cellular location of the contrast agent. The cell experiments indicated a reduction of the relaxivity from 2.7 ± 0.1 m m(-1)  s(-1) to a net relaxivity of 1.7 ± 0.3 m m(-1)  s(-1) upon cellular uptake for RGD targeted liposomes by means of the contrast agent concentration as determined by SPECT. This is not observed for nontargeted liposomes that serve as controls. We show that receptor targeted liposomes in comparison to nontargeted liposomes are taken up into cells faster and into subcellular structures of different sizes. We suggest that the presented multimodal contrast agent provides a functional readout of its response to the biological environment and is furthermore applicable in in vivo measurements. As this approach can be extended to several MRI-based contrast mechanisms, we foresee a broader use of multimodal SPECT/MRI nanoparticles to serve as in vivo sensors in biological or medical research.

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