Determining the Minimal Required Ultra-Low-Dose CT Dose Level for Reliable Attenuation Correction of ¹⁸F-FDG PET/CT: A Phantom Study

Abstract

Our purpose was to investigate the minimal required submillisievert ultra-low-dose CT dose level and the corresponding tube current and voltage for reliable attenuation correction and semiquantitation in ¹⁸F-FDG PET/CT in an effort toward radiation dose reduction. Methods: We performed a PET/CT investigational study using a National Electrical Manufacturers Association torso phantom containing 6 spheres (10, 13, 17, 22, 28, and 37 mm in diameter) filled with a fixed concentration of 60 kBq/mL and a background of 15 kBq/mL of ¹⁸F-FDG. Two sets of PET images, separated by 2 h, were acquired for 3 min at a single bed position using 3-dimensional mode in a Discovery 690 scanner. Several sets of CT images were acquired for attenuation correction with different combinations of tube voltage (80, 100, and 120 kVp) and effective mAs (tube current–time product divided by pitch), using the maximum beam collimation (64 × 0.625 mm). The lowest CT dose acquisition technique available on this scanner is 10 mA, 0.4 s, and 1.375 for the tube current, tube rotation time, and pitch, respectively. The CT radiation dose was estimated on the basis of CT dose index volume measurements performed following the standard methodology and the Imaging Performance Assessment of CT Scanners (ImPACT) calculator. Each of the CT techniques was applied for attenuation correction to the same PET acquisition, using ordered-subset expectation maximization with 24 subsets and 2 iterations. The maximal and average radioactivity (kBq/mL) and SUVs of the spheres were measured. The minimal ultra-low-dose CT dose level for attenuation correction was determined by reproducible SUV measurements (±10%) compared with our reference CT protocol of 100 kVp and 80 mA for a 0.5-s rotation. Results: The minimal ultra-low-dose CT dose level for reproducible quantification in all spheres (<10% relative difference) was determined to be 0.3 mSv for a combination of 100 kVp and 10 mA at a 0.5-s rotation and a helical pitch of 0.984 (0.26 mGy measured CT dose index volume). From these results. we could confidently determine the CT parameters for reliable attenuation correction of PET images while significantly reducing the associated radiation dose. Conclusion: Our phantom study provided guidance in using ultra-low-dose CT for precise attenuation correction and semiquantification of ¹⁸F-FDG PET imaging, which can further reduce the CT dose and radiation exposure to patients in clinical PET/CT studies. On the basis of the data, we can further reduce the radiation dose to the submillisievert level using an ultra-low-dose CT protocol for reliable attenuation correction in clinical ¹⁸F-FDG PET/CT studies.