Compensation for partial volume effect and tumor motion in PET for NSCLC without respiratory gating

Objectives In PET, partial volume effect (PVE) and tumor motion degrade the maximum activity concentration (MAC) of the lung lesion. PVE can be corrected by the recovery coefficient (RC) of the imaging system. Tumor motion can be reduced by respiratory gating or 4DPET, which requires long acquisition time, and needs 4DCT for attenuation correction. 4DPET/CT is impractical in time, radiation dose and complexity. Alternatively, we developed a practical technique to estimate the tumor motion for quantifying the MAC in PET/CT with a low-dose cine CT scan and without respiratory gating, and applied the method on patient data.

Methods We derived a RC as function of tumor size and tumor motion on the GE PET/CT scanner with a NEMA IEC thorax phantom. The 6 spheres (diameters of 10 to 37 mm) and the tank were filled to 6 signal-to-background ratios of 5, 10, 15, 20, 30 and 50. The phantom was scanned on a moving platform of 0 to 3 cm motion at an increment of 0.5 cm. We added a cine CT scan after the PET/CT scan to obtain the cine CT data to attenuation correct the PET data with average CT, and to estimate the magnitude of tumor motion. We applied this technique to quantify the 11 lesions of 10 patients whose tumor sizes were between 1 and 2 cm.

Results The average statistics of the 11 tumors were 1.4+/-0.2 cm in size, 1.2+/-0.9 cm in motion, 19+/-10% increase in MAC after compensation for only PVE, and 68+/-60% increase after compensation for both PVE and tumor motion. An increase of > 30% occurred on 6 of the 11 tumors compensated for the PVE and tumor motion, and on only one tumor compensated for the PVE. Two of the three tumors with motion > 2 cm changed from 2.4 to 5.2 and 4.7 to 14.1 in maximum SUV, suggesting a more definite FDG-avid uptake. There was a statistically significant difference with the inclusion of tumor motion in compensation (p<0.05).

Conclusions Compensation for the PVE and tumor motion can be practically achieved with a low-dose cine CT scan and without gating. Tumor motion can be a significant factor in the diagnosis of the small and mobile lesions