Investigation of factors affecting the recovery coefficients in FDG PET/CT: Effects of target size, background activity and respiratory motion

Objectives Signal recovery can be hindered by Partial volume effect (PVE) due to the limited spatial resolution of PET scanners. PVE can be corrected using recovery coefficients (RCs). However, signal recovery is further degraded due to respiratory motion under clinical conditions. The aim of this study was to investigate number of various factors affecting the RCs under simulated clinical conditions.

Methods The RCs were determined using NEMA body phantom with six spheres (diameters 10-37mm). The influence of the following factors on the RC was assessed: object size, target-to-background activity ratio (TBR) and respiratory motion. All spheres and the background in the phantom were filled with F-18 and we obtained 3 different TBRs. To simulate respiratory motion, the phantom was sinusoidally driven in the superior-inferior direction with amplitudes of 0, 10 and 15mm and a breathing period of 3.5 seconds. Signal loss on PET images due to PVE and respiratory motion respectively measured. Furthermore, we evaluated the validity of these data in phantom study using clinical data.

Results For a stationary mode, signal loss was significant for small sphere. The RCs were only weakly influenced, particularity for small spheres measuring 22mm or less, of the activity ratio between the sphere and the background. The RC of 10mm decreased from 83% in a stationary mode with a TBR of 4 to 75 and 56% with 10 and 15mm motion amplitude, respectively. The RCs of the large sphere were relatively less affected by respiratory motion amplitude.

Conclusions A phantom study was performed to estimate the effects of object size, TBR and respiratory motion affecting the RCs under simulated clinical conditions. Our results demonstrate that the RCs are dominant for small lesions. Respiratory motion significantly deteriorates signal recovery and this loss depends on the amplitude of respiratory motion