Improved time of flight timing resolution provides high quality PET images

Purpose: ¹⁸F FDG-PET/CT is effective in detecting various malignant tumors, staging, and prognostic stratification. Adequate image quality and quantitative accuracy are important for accurate diagnosis using PET examination. Time-of-flight (TOF) is the difference in the arrival time of the detectors between two annihilation photons and reduces the probability distribution that annihilation events occur along the line of response (LOR). An improved TOF is expected to reduce noise and to improve the lesion detectability. This study evaluated the influence of different TOF timing resolutions on PET images. Materials and Method: The TOF timing resolutions on Biograph Vision and Biograph mCT (Siemens Healthineers) were 214 psec and 540 psec, respectively. The NEMA IEC Body Phantom with two spherical shell phantoms (outer diameter, 60/37 mm; inner diameter, 37/22 mm) and a 37-mm-diameter sphere was filled with an ¹⁸F-FDG solution (outer layer, 14.58 kBq/mL; inner layer, 0 kBq/mL; background, 3.58 kBq/mL). The 3D list mode data was obtained in 30 min. The PET data were reconstructed with the filter back projection (FBP), FBP + TOF, ordered-subsets expectation maximization (OSEM), and OSEM + TOF. The PET images of mCT were reconstructed using iteration 3 and subset 24 in OSEM, and iteration 2 and subset 21 in OSEM + TOF. Those of Vision were reconstructed using iteration 3 and subset 5 in both OSEM and OSEM + TOF. The image quality was assessed using the coefficient of variation of background (CV_background), the contrast of the cold part of a 37-mm-diameter and a 22-mm-diameter (Q_C,37mm and Q_C,22mm, respectively), and the contrast of the hot part of a 37-mm-diameter (Q_H,37mm). Result: The CV_background of mCT was 5.66% for FBP + TOF, and 3.32% for OSEM + TOF, whereas that of Vision was 4.54% for FBP + TOF and 2.37% for OSEM + TOF. Those of Vision were superior over those of mCT. The improvement rate by TOF in Q_C was 31.9% in the Vision image, whereas it was 4.1% in the image of mCT. Q_H,37mm of mCT was 66.7% for FBP + TOF, and 69.9% for OSEM + TOF; that of Vision was 64.3% for FBP + TOF, and 69.3% for OSEM + TOF. However, Q_H,37mm was not different between mCT and Vision. Conclusion: A more superior image quality is obtained with a higher TOF timing resolution, especially in the low activity area.