Anatomy of SUV
Introduction
Standardized uptake value (SUV), under various names, is a popular index used in clinical [F-18]fluorodeoxyglucose (FDG) studies to differentiate malignant from benign tumors 1, 2, 10, 11, 15, 19, 21, 23, 25, 26, 31 and to assess the efficacy of therapy 5, 9, 28. Despite its popularity, the reliability of SUV is still somewhat controversial (17). In the following, I will compare the SUV method with modeling approach to examine its merits as well as its variability. Issues related to physical measurements, such as attenuation correction, object size, imaging resolution, and regions of interest (ROI), that have been addressed separately by others will not be discussed extensively here.
Common factors that can influence the amount of tracer uptake in tissue are listed in Table 1. In addition to the primary biochemical process, there are many other confounding factors that can influence the amount of tracer uptake. Although not every factor listed in Table 1 would play a role for every tracer, they all potentially could affect significantly the tissue uptake of a particular tracer. Tracer kinetic modeling that describes mathematically the mechanism of transport and biochemical reactions of the tracer in tissue is a formal way to remove the effects of the confounding factors (13). Modeling approach usually requires taking series of blood samples from the studied subject to give the time course of the tracer delivery, and requires measuring the dynamics of the radiolabel in local tissues. Frequently, model fitting or regression analysis is needed to give the desired biological information.
For some tracers and for some studies, there are simplified approaches that can achieve a similar goal of reducing the effect of confounding factors. The SUV for FDG can be viewed as one of these approaches. Although the SUV formula in its original form has been criticized (17), the simplicity of the approach makes it extremely attractive for routine clinical use. Changes/modifications of the formula have been made to overcome the original deficiencies 8, 17, 18, 36. The following is a review of the basis of the approach to show how it is related to the modeling approach. Based on such a foundation, the merits and limitations of the SUV approach (with its various modifications) can be assessed more easily.
Section snippets
Relationship between glucose utilization rate and SUV
Quantitation of glucose utilization rate with FDG 14, 27 followed the original work of Sokoloff et al. (30) who had laid out a solid foundation for the use of C-14-labeled deoxyglucose (DG) and autoradiograph for the quantitation of cerebral metabolic rate of glucose. In this method, the time course of tracer delivery to local tissue is provided by plasma concentration of FDG. The transport of FDG across the capillary/cellular membrane is accounted for by two rate constants K1 and k2 (for
Variability of SUV
As shown in the above derivation of the SUV formula, the validity of SUV depends on a few important assumptions and approximations. The most critical one is the validity of the approximation of the time integral of plasma FDG TAC with Eq. (4). It does not require the plasma FDG TAC to have a fixed shape, but the integral of the curve is assumed to be proportional to the injected dose and inversely proportional to body weight or body surface area. While these assumptions are quite reasonable,
Acknowledgements
This work was supported in part by Department of Energy Grant DE-FC0387-ER60615.
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