Improving the spatial alignment in PET/CT using amplitude-based respiration-gated PET and patient-specific breathing-instructed CT

Objectives Appropriate attenuation correction is important for accurate quantification of standardized uptake values (SUVs) in PET/CT. Patient respiratory motion can introduce a spatial mismatch between PET and CT images, which reduces this accuracy. In this still ongoing study, the effect of patient-specific breathing-instructed CT on the spatial alignment between CT and amplitude-based optimal respiration-gated (ORG) PET images is investigated.

Methods Whole body (WB) FDG PET/CT imaging will be performed in a total of 20 patients. At the time of writing, 8 patients with 14 lung lesions were included. Images were acquired using a Siemens Biograph 40 mCT PET/CT scanner using optimally respiratory gating (ORG) (HDChest). In addition to the standard WB low-dose (LD) CT (CT_std) acquired during free breathing, an additional LD CT was made with patient-specific breathing instructions (CT_bh). The amplitude limits of the ORG PET were used to instruct the patients to hold their breath at a similar amplitude-level. For both types of CT, the mismatch was quantified using the position differences between the lung-liver transition in PET and CT images (Δ_LL), the distance between PET and CT lesions’ centroids (Δ_C) and the amount of overlap indicated by the Jaccard similarity coefficient (JSC). Furthermore, the effect on attenuation correction was quantified by SUV_max for both types of CT scans. Statistical analysis was performed using the Wilcoxon Signed Ranks test, statistical significance was defined for p<0.05.

Results All patients found the breathing instructions feasible, however 1 patient did not follow the instructions and was excluded. Regarding quantification of spatial overlap, the following values were found when comparing CT_std and CT_bh, respectively: Δ_LL was 0.75±1.3 and 1.0±1.2 mm (p=0.564), Δ_C was 4.1±1.8 and 2.5±1.5 mm (p=0.039), JSC was 0.34±0.15 and 0.43±0.13 (p=0.055), and SUV_max was 14.6±8.1 and 15.1±8.3 g/cm³ (p=0.173).

Conclusions Even though there is not an improvement at the lung-liver transition (near the respiratory tracking sensor) when patient-specific breathing instructed CT was used (the alignment was already good), there is a better match for the lung lesions. Indicating that the breathing instructions improve the spatial alignment between ORG PET and CT.