Computed Tomography-guided Time-domain Diffuse Fluorescence Tomography in Small Animals for Localization of Cancer Biomarkers

Abstract

Small animal fluorescence molecular imaging (FMI) can be a powerful tool for preclinical drug discovery and development studies 1 . However, light absorption by tissue chromophores (e.g., hemoglobin, water, lipids, melanin) typically limits optical signal propagation through thicknesses larger than a few millimeters 2 . Compared to other visible wavelengths, tissue absorption for red and near-infrared (near-IR) light absorption dramatically decreases and non-elastic scattering becomes the dominant light-tissue interaction mechanism. The relatively recent development of fluorescent agents that absorb and emit light in the near-IR range (600-1000 nm), has driven the development of imaging systems and light propagation models that can achieve whole body three-dimensional imaging in small animals 3 .Despite great strides in this area, the ill-posed nature of diffuse fluorescence tomography remains a significant problem for the stability, contrast recovery and spatial resolution of image reconstruction techniques and the optimal approach to FMI in small animals has yet to be agreed on. The majority of research groups have invested in charge-coupled device (CCD)-based systems that provide abundant tissue-sampling but suboptimal sensitivity [4][5][6][7][8][9] , while our group and a few others 10-13 have pursued systems based on very high sensitivity detectors, that at this time allow dense tissue sampling to be achieved only at the cost of low imaging throughput. Here we demonstrate the methodology for applying single-photon detection technology in a fluorescence tomography system to localize a cancerous brain lesion in a mouse model.The fluorescence tomography (FT) system employed single photon counting using photomultiplier tubes (PMT) and information-rich time-domain light detection in a non-contact conformation 11. This provides a simultaneous collection of transmitted excitation and emission light, and includes automatic fluorescence excitation exposure control 14 , laser referencing, and co-registration with a small animal computed tomography (microCT) system 15 . A nude mouse model was used for imaging. The animal was inoculated orthotopically with a human glioma cell line (U251) in the left cerebral hemisphere and imaged 2 weeks later. The tumor was made to fluoresce by injecting a fluorescent tracer, IRDye 800CW-EGF (LI-COR Biosciences, Lincoln, NE) targeted to epidermal growth factor receptor, a cell membrane protein known to be overexpressed in the U251 tumor line and many other cancers 18 . A second, untargeted fluorescent tracer, Alexa Fluor 647 (Life Technologies, Grand Island, NY) was also injected to account for non-receptor mediated effects on the uptake of the targeted tracers to provide a means of quantifying tracer binding and receptor availability/density 27 . A CT-guided, time-domain algorithm was used to reconstruct the location of both fluorescent tracers (i.e., the location of the tumor) in the mouse brain and their ability to localize the tumor was verified by contrast-enhanced magnet...

Tomography of epidermal growth factor receptor binding to fluorescent Affibodyin vivostudied with magnetic resonance guided fluorescence recovery in varying orthotopic glioma sizes

“…This tumor line was chosen because it is known to overexpress EGFR, enabling specific targeting using an engineered peptide with high affinity for EGFR, the anti-EGFR targeted Affibody (Affibody, Sweden). 3,[13][14][15] Transfection of the cell line with GFP allows the GFP signal to be measured ex vivo and provides definitive tumor localization. 16 The growth of the tumors was monitored using gadoliniumenhanced T1-weighted magnetic resonance imaging (MRI), which was also the method used to gather anatomical prior information for fluorescence tomography.…”

Fast single photon avalanche photodiode-based time-resolved diffuse optical tomography scanner

“…Diffuse optical tomography (DOT) and fluorescence mediated tomography (FMT) are emerging as important tools in biomedical research [1][2][3][4][5][6][7][8][9][10]. DOT utilizes multiple measurements between source and detector pairs through bulk biological tissue, and by solving the resulting inverse problem retrieves the internal distribution of intrinsic optical properties.…”

Targeting tumor hypoxia with 2-nitroimidazole-indocyanine green dye conjugates

“…[7][8][9][10] Cancer NIR molecular imaging relies greatly on the development of stable, highly specific and sensitive molecular probes. [11][12][13][14][15][16] Organic dyes such as indocyanine green have been used as nontargeted agents for optical imaging of cancer and have been used clinically for many years.…”