title: Absorption reconstruction improves biodistribution assessment of fluorescent nanoprobes using hybrid fluorescence-mediated tomography.
abstract: OBJECTIVE
  Fluorescence-mediated tomography (FMT) holds potential for accelerating diagnostic and theranostic drug development. However, for proper quantitative fluorescence reconstruction, knowledge on optical scattering and absorption, which are highly heterogeneous in different (mouse) tissues, is required. We here describe methods to assess these parameters using co-registered micro Computed Tomography (µCT) data and nonlinear whole-animal absorption reconstruction, and evaluate their importance for assessment of the biodistribution and target site accumulation of fluorophore-labeled drug delivery systems.
  
  
  METHODS
  Besides phantoms with varying degrees of absorption, mice bearing A431 tumors were imaged 15 min and 48 h after i.v. injection of a fluorophore-labeled polymeric drug carrier (pHPMA-Dy750) using µCT-FMT. The outer shape of mice and a scattering map were derived using automated segmentation of the µCT data. Furthermore, a 3D absorption map was reconstructed from the trans-illumination data. We determined the absorption of five interactively segmented regions (heart, liver, kidney, muscle, tumor). Since blood is the main near-infrared absorber in vivo, the absorption was also estimated from the relative blood volume (rBV), determined by contrast-enhanced µCT. We compared the reconstructed absorption with the rBV-based values and analyzed the effect of using the absorption map on the fluorescence reconstruction.
  
  
  RESULTS
  Phantom experiments demonstrated that absorption reconstruction is possible and necessary for quantitative fluorescence reconstruction. In vivo, the reconstructed absorption showed high values in strongly blood-perfused organs such as the heart, liver and kidney. The absorption values correlated strongly with the rBV-based absorption values, confirming the accuracy of the absorption reconstruction. Usage of homogenous absorption instead of the reconstructed absorption map resulted in reduced values in the heart, liver and kidney, by factors of 3.5, 2.1 and 1.4, respectively. For muscle and subcutaneous tumors, which have a much lower rBV and absorption, absorption reconstruction was less important.
  
  
  CONCLUSIONS
  Quantitative whole-animal absorption reconstruction is possible and can be validated in vivo using the rBV. Usage of an absorption map is important when quantitatively assessing the biodistribution of fluorescently labeled drugs and drug delivery systems, to avoid a systematic underestimation of fluorescence in strongly absorbing organs, such as the heart, liver and kidney.
authors.affiliation: 1. Department of Experimental Molecular Imaging, University Clinic and Helmholtz Institute for Biomedical Engineering, RWTH Aachen University, Aachen, Germany;
authors.forename: Felix
authors.initials: F
authors.lastname: Gremse
authors.affiliation: 1. Department of Experimental Molecular Imaging, University Clinic and Helmholtz Institute for Biomedical Engineering, RWTH Aachen University, Aachen, Germany;
authors.forename: Benjamin
authors.initials: B
authors.lastname: Theek
authors.affiliation: 1. Department of Experimental Molecular Imaging, University Clinic and Helmholtz Institute for Biomedical Engineering, RWTH Aachen University, Aachen, Germany;
authors.forename: Sijumon
authors.initials: S
authors.lastname: Kunjachan
authors.affiliation: 1. Department of Experimental Molecular Imaging, University Clinic and Helmholtz Institute for Biomedical Engineering, RWTH Aachen University, Aachen, Germany;
authors.forename: Wiltrud
authors.initials: W
authors.lastname: Lederle
authors.affiliation: 2. Department of Experimental Medicine and Immunotherapy, Institute of Applied Medical Engineering, University Hospital RWTH Aachen, Aachen, Germany;
authors.forename: Alessa
authors.initials: A
authors.lastname: Pardo
authors.affiliation: 2. Department of Experimental Medicine and Immunotherapy, Institute of Applied Medical Engineering, University Hospital RWTH Aachen, Aachen, Germany; ; 3. Department of Pharmaceutical Product Development, Fraunhofer Institute for Molecular Biology and Applied Ecology, Aachen, Germany.
authors.forename: Stefan
authors.initials: S
authors.lastname: Barth
authors.affiliation: 1. Department of Experimental Molecular Imaging, University Clinic and Helmholtz Institute for Biomedical Engineering, RWTH Aachen University, Aachen, Germany; ; 4. Department of Pharmaceutics, Utrecht Institute for Pharmaceutical Sciences, Utrecht University, Utrecht, The Netherlands ; 5. Department of Controlled Drug Delivery, MIRA Institute for Biomedical Technology and Technical Medicine, University of Twente, Enschede, The Netherlands.
authors.forename: Twan
authors.initials: T
authors.lastname: Lammers
authors.affiliation: 6. LuFG Informatik 12, RWTH Aachen University, Aachen, Germany.
authors.forename: Uwe
authors.initials: U
authors.lastname: Naumann
authors.affiliation: 1. Department of Experimental Molecular Imaging, University Clinic and Helmholtz Institute for Biomedical Engineering, RWTH Aachen University, Aachen, Germany;
authors.forename: Fabian
authors.initials: F
authors.lastname: Kiessling
mesh_terms: D000818:Animals; D045744:Cell Line, Tumor; D005456:Fluorescent Dyes / Q000737:chemistry / Q000493:pharmacokinetics*; D006801:Humans; D008807:Mice, Inbred BALB C; D008819:Mice, Nude; D053758:Nanoparticles / Q000737:chemistry; D009368:Neoplasm Transplantation; D019047:Phantoms, Imaging; D013050:Spectrometry, Fluorescence; D014018:Tissue Distribution; D055114:X-Ray Microtomography / Q000379:methods*
chemical_list: D005456:Fluorescent Dyes
keywords: Diffuse Optical Tomography; Drug delivery; Fluorescence-mediated Tomography; Micro-Computed Tomography; Multimodal Imaging; Nanomedicine.