Direct 4D whole-body PET/CT parametric image reconstruction: Concept and comparison vs. indirect parametric imaging

Objectives Recently, indirect whole-body parametric PET/CT imaging, using Patlak method, has been introduced to enhance tumor quantification and detectability in regions with high background activity, where SUV performs relatively poorly. However, in tumor regions of low-uptake the noise can be significant, due to short dynamic frames employed, affecting contrast and quantitative accuracy. Our aim is to apply a direct 4D reconstruction algorithm to reduce the noise, especially in low-uptake regions.

Methods Indirect whole-body parametric imaging consists of two distinct phases: initially, independent 3D OSEM reconstructions are conducted at each frame, followed by linear regression, to estimate the slope and intercept parameters of Patlak. Here, we propose a novel direct 4D whole-body reconstruction algorithm that unifies the two steps into a closed-form OSEM scheme, by incorporating the kinetic modeling within the system matrix and, thus, allowing for estimation of the parametric images directly from the available dynamic sinograms. To compare the performance of the two methods, we conducted Monte Carlo simulations of a 4D PET acquisition consisting of a 6min dynamic scan of the heart, followed by 6 whole-body scans. Data from literature were used to model the dynamics of FDG tracer in a set of liver and lung spherical tumors and their background regions in the XCAT phantom.

Results By comparing the slope parametric images, we observe enhanced tumor to background contrast (average over 15 realizations) in direct 4D reconstruction, particularly for the low-uptake tumors (lung).

Conclusions Direct 4D parametric image reconstruction can enhance tumor detectability and quantification compared to indirect. In addition to image data size reduction and faster generation of parametric images, our proposed method is designed to address challenges particularly associated with whole-body PET parametric imaging, such as presence of high noise, thus contributing to its clinical adoption.