title: Measurement of input functions in rodents: challenges and solutions.
abstract: BACKGROUND
  Tracer kinetic modeling used in conjunction with positron emission tomography (PET) is an excellent tool for the noninvasive quantification of physiological, biological and molecular processes and their alterations due to disease. Currently, complex multi-compartment modeling approaches are being applied in a variety of clinical studies to determine myocardial perfusion, viability and glucose utilization as well as fatty acid metabolism and oxidation in the normal and diseased heart. These kinetic models require two key measurements of tracer activity over time, tracer activity in arterial blood (input function) and its corresponding activity in the organ of interest. The alteration in the time course of tracer activity as it travels from blood to the organ of interest describes the kinetics of the tracer. To be able to implement these approaches in rodent models of disease using small-animal PET (microPET), it is imperative that the input function is measured accurately.
  
  
  METHODS
  The blood input functions in rodent experiments were obtained by (1) direct blood sampling, (2) direct measurement of blood activity by a beta-detecting probe that counts the activity in the blood, (3) an arterial-venous bypass (A/V shunt), (4) factor analysis of dynamic structures from dynamic PET images and (5) measurement from region-of-interest (ROI) analysis of dynamic PET images. Direct blood sampling was used as the reference standard to which the results of the other techniques were compared.
  
  
  RESULTS
  Beta probes are difficult to operate and may not provide accurate blood input functions unless they are used intravenously, which requires complicated microsurgery. A similar limitation applies to the A/V shunt. Factor analysis successfully extracts the blood input function for mice and rats. The ROI-based method is less accurate due to limited image resolution of the PET system, which results in severe partial volume effect and spillover from myocardium.
  
  
  CONCLUSIONS
  The current reference standard, direct blood sampling, is more invasive and has limited temporal resolution. With current imaging technology, image-based extraction of blood input functions is possible by factor analysis, while forthcoming technological developments are likely to allow extraction of input function directly from the images. These techniques will reduce the level of complexity and invasiveness for animal experiments and are likely to be used more widely in the future.
authors.affiliation: Division of Radiological Sciences, Mallinckrodt Institute of Radiology, Washington University School of Medicine, St. Louis, MO 63110, USA. laforestr@mir.wustl.edu
authors.forename: Richard
authors.initials: R
authors.lastname: Laforest
authors.forename: Terry L
authors.initials: TL
authors.lastname: Sharp
authors.forename: John A
authors.initials: JA
authors.lastname: Engelbach
authors.forename: Nicole M
authors.initials: NM
authors.lastname: Fettig
authors.forename: Pilar
authors.initials: P
authors.lastname: Herrero
authors.forename: Joonyoung
authors.initials: J
authors.lastname: Kim
authors.forename: Jason S
authors.initials: JS
authors.lastname: Lewis
authors.forename: Douglas J
authors.initials: DJ
authors.lastname: Rowland
authors.forename: Yuan-Chuan
authors.initials: YC
authors.lastname: Tai
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Welch
mesh_terms: D000818:Animals; D003198:Computer Simulation; D019788:Fluorodeoxyglucose F18 / Q000097:blood* / Q000493:pharmacokinetics; D007090:Image Interpretation, Computer-Assisted / Q000379:methods*; D008657:Metabolic Clearance Rate; D051379:Mice; D008955:Models, Cardiovascular; D049268:Positron-Emission Tomography / Q000662:veterinary*; D011865:Radioisotope Dilution Technique; D019275:Radiopharmaceuticals / Q000097:blood / Q000493:pharmacokinetics; D051381:Rats
chemical_list: D019275:Radiopharmaceuticals; D019788:Fluorodeoxyglucose F18