Measurement of input functions in rodents: challenges and solutions.
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.
Laforest
Richard
R
Division of Radiological Sciences, Mallinckrodt Institute of Radiology, Washington University School of Medicine, St. Louis, MO 63110, USA. laforestr@mir.wustl.edu
Sharp
Terry L
TL
Engelbach
John A
JA
Fettig
Nicole M
NM
Herrero
Pilar
P
Kim
Joonyoung
J
Lewis
Jason S
JS
Rowland
Douglas J
DJ
Tai
Yuan-Chuan
YC
Welch
Michael J
MJ
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
D019275:Radiopharmaceuticals; D019788:Fluorodeoxyglucose F18