The strength of this is that the approach is feasible even if the background is heterogeneous or is found in high concentrations in organs of filtration. As long as the two tracers have similar delivery characteristics (i.e., their vascular permeability), nonspecific uptake, and pharmacokinetics 19-and the nonspecific fluorescence uptake is much greater than the level of autofluorescence 3-the untargeted tracer fluorescence data can be used as a scaled surrogate of the unbound component of the targeted tracer uptake, allowing a means of subtracting off the unbound (background) signal. 18 In the present study, an approach is proposed wherein the uptake of a second, untargeted fluorescent tracer is used to subtract off the nonspecific uptake of a simultaneously injected targeted fluorescent tracer.
6, 16, 17 However, while many of these approaches work well for autofluorescence removal or removal of homogeneously distributed fluorescent backgrounds, they are not ideal if the background is heterogeneous and predominantly comprised of nonspecific fluorescent tracer uptake, which may be expected in molecularly targeted FT studies at earlier time-points after targeted tracer injection. 6 In response, a number of approaches have been designed to mitigate the effects of background: such as constraint-based reconstruction algorithms, 7 – 11 spectral and lifetime removal of autofluorescence, 12 – 14 preinjection image subtraction, 15 and analytical modeling of background fluorescence.
The nonlinearity of image reconstruction algorithms in FT make background fluorescence severely detrimental to resolving bound tracer populations of interest. 5 This study develops one new approach toward advancing this area. 4 Furthermore, at these extended time points there may still be considerable nonspecific tracer concentrations in organs of filtration like the liver, kidneys, spleen, and bladder, confounding the ability to localize bound fractions of the tracer, at least in the abdomen. The problem with the approach is that at early timepoints there can be significant levels of background tracer uptake, 3 and by waiting the hours or even days necessary for the background to dissipate, it is more likely for effects, such as cellular internalization or metabolism of the tracer, to affect the relationship between fluorescence and receptor density. It is retained in areas where it has bound to the targeted biomolecule, thus providing fluorescent contrast only in tissues where the receptor is ‘over-expressed.’ The strength of this approach is that the targeted molecule can be synthesized to bind most biomolecules of interest, and as long as the chosen biomolecule is in a region that can be accessed by the tracer, the approach should be practicable. Immediately after injection, the tracer is dispersed throughout the body and over time it is cleared from the plasma and tissues that are devoid of the biomolecule of interest. 2 This study introduces an approach to doing diffuse FT that has some inherent benefits in image reconstruction accuracy, as well as peripheral benefits by allowing imaging of molecular binding at very short times after contrast injection in vivo.Ĭontrast in FT is typically based on the injection of a fluorescently labeled molecular drug that will preferentially bind to a specific biomolecule of interest (e.g., cellular receptors that are over-expressed in pathological processes). 1 This depth limitation has restricted applications of whole-body FT to small animal imaging to date, where it has shown great promise as a preclinical tool for in vivo investigations of biomolecular pathways in disease states. However, while FT is very sensitive, the propagation of light through tissue is hindered by a significantly greater amount of scattering and absorption than ionizing radiation (e.g., x-rays), limiting the depth of tissue that can be imaged through to several centimeters. Fluorescence tomography (FT) is a promising molecular imaging modality that is capable of mapping biomolecule distributions in tissue without requiring the use of ionizing radiation, such as is required in competing nuclear medicine modalities.
0 kommentar(er)
