PET quantification: strategies for partial volume correction

Despite the continuous evolution of positron emission tomography (PET) technology, the spatial resolution (SR) of PET images remains poor (4–6 mm) compared with that of computed tomography (CT) and magnetic resonance imaging (<1 mm). The reasons for this lie intrinsically in the physics behind PET and in the scanner design. This poor SR and the consequent partial volume effect (PVE), affecting image quality and the estimation of radioactivity concentration in small structures, complicate the anatomical localisation of focal radiotracer uptake signals and the application of a quantitative approach in the evaluation of PET studies. Methods for PVE compensation are therefore needed to improve the quality and quantitative accuracy of PET images. The simplest of the PVE compensation methods is the one based on recovery coefficients. Other techniques exploit high-resolution structural information extracted from co-registered CT or MR images to correct for the PVE. Others still are aimed instead at recovering the degradation effect by exploiting the point spread function of the PET system within the reconstruction process or using post-processing deconvolution algorithms. Regardless of the method employed, PVE compensation techniques generally assume motion-free PET data. In neurological studies, this assumption may be verified; in cardiological and oncological applications, on the other hand, organ/lesion motion and the PVE should necessarily be taken into account and corrected for sequentially or simultaneously. This paper deals with the PVE, its impact on qualitative and quantitative PET imaging, and possible approaches for PVE compensation.