Dynamic whole-body PET imaging: principles, potentials and applications

Objectives: To demonstrate that dynamic whole-body (DWB) PET is a clinical imaging tool with significant potential. DWB PET can be performed within reasonable clinical imaging times and enables generation of multiple types of PET images with complementary information in a single imaging session (<30min). Methods: Current clinical PET protocols mirror the pattern established for traditional radiology in that they are optimized for qualitative as opposed to quantitative assessment, with documented limitations [1]. Radiotracer distribution is a dynamic process that varies substantially between organs (particularly tumors) and between patients. The uptake periods used in clinical protocols are somewhat arbitrary and are not expected to be optimal for all clinical cases. We argue that a new paradigm of DWB-PET is both feasible and has significant potential. It is made possible by ongoing technical developments; significant advancements in PET hardware, combined with statistical image reconstruction have made it possible to acquire multi-pass eyes-to-thighs imaging in clinically feasible times, achieving adequate statistical quality in less than 5 min/pass. Patlak analysis [2] is particularly suited for generation of parametric images from DWB PET because it is applicable to FDG and it does not require PET scans to sample the early tracer kinetics. Thus, DWB imaging can be used to produce parametric images of (i) Patlak slope (influx rate Ki) and (ii) intercept (distribution volume V), while also providing (iii) conventional SUV images by summation of dynamic frames. There are also means to reduce noise in the parametric Patlak slope and intercept images, by use of constrained statistical regression [3, 4], and direct 4D reconstruction of parametric images from sinogram data [5]. Results: Parametric images of both Patlak slope Ki and intercept V can be generated, in addition to conventional SUV images by summation of the dynamic frames (Figure 1a). Thus, three distinct images can be obtained from the same imaging session. The Ki image generally shows reduced normal organ uptake, particularly in the liver. Furthermore, direct 4D parametric image reconstruction can significantly improve quality and quantitative accuracy of DWB images (Figure 1b). We also have results demonstrating application of DWB imaging to beyond FDG PET/CT, for instance to PET/MR imaging, as well as Ga-68 DOTATOC PET/CT. Conclusions: DWB-PET has a number of advantages. It can minimize time dependence of SUV activity: SUV uptake changes in time in direct proportion to changes in image uptake. Given variable scan times inherent in a busy clinical practice, this is an issue, and the proposed measures are expected to be less subject to such alterations. Furthermore, it can remove background uptake, allow small and less FDG avid tumors to be identified, and produce more quantitative estimates of tumor uptake. Overall, a new paradigm of DWB PET imaging (applicable to both PET/CT and PET/MRI) generates quantitative measures that may contribute significantly added clinical value to conventional SUV images.

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