Molecular imaging is a rapidly emerging research tool and clinical discipline aimed at noninvasive, quantitative visualization of in vivo molecular processes occurring at cellular and subcellular levels. At present, advancement of the molecular imaging field is driven by the development of improved imaging hardware for use in preclinical and clinical settings, the identification and validation of new, biologically relevant imaging targets, and the development of improved imaging probes derived from novel chemistries. Of these 3 essential facets, which comprise a majority of current molecular imaging research, hardware development and novel target discovery significantly outpace the development and clinical advancement of new molecular imaging probes, particularly with respect to cancer imaging.
Abstract. Simple, quantitative assays to measure pH in tissue could improve the study of complicated biological processes and diseases such as cancer. We evaluated multispectral fluorescence imaging (MSFI) to quantify extracellular pH (pH e ) in dye-perfused, surgically-resected tumor specimens with commercially available instrumentation. Utilizing a water-soluble organic dye with pH-dependent fluorescence emission (SNARF-4F), we used standard fluorimetry to quantitatively assess the emission properties of the dye as a function of pH. By conducting these studies within the spectroscopic constraints imposed by the appropriate imaging filter set supplied with the imaging system, we determined that correction of the fluorescence emission of deprotonated dye was necessary for accurate determination of pH due to suboptimal excitation. Subsequently, employing a fluorimetry-derived correction factor (C F ), MSFI data sets of aqueous dye solutions and tissuelike phantoms could be spectrally unmixed to accurately quantify equilibrium concentrations of protonated (HA) and deprotonated (A − ) dye and thus determine solution pH. Finally, we explored the feasibility of MSFI for high-resolution pH e mapping of human colorectal cancer cell-line xenografts. Data presented suggest that MSFI is suitable for quantitative determination of pH e in ex vivo dye-perfused tissue, potentially enabling measurement of pH across a variety of preclinical models of disease. C 2011 Society of Photo-Optical Instrumentation Engineers (SPIE).
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