PET-CT acquisition and reconstruction parameters

Three Discovery MI (GE Healthcare, Milwaukee, WI, USA) PET-CT systems were used for image acquisition. The systems were configured with four rings of detector blocks with lutetium yttrium oxyorthosilicate crystals coupled to an array of SiPM. The PET-detector has a transaxial field of view of 70 cm, an axial field of view of 20 cm and an overlap of 24% between bed positions. The sensitivity, according to NEMA standards, was 13 cps/kBq. The PET system was combined with a 128 slice CT.

All patients received an intravenous injection of 4 MBq/kg body weight of ¹⁸F-FDG with an accumulation time of 60 min before imaging and after at least 4 h of fasting and a glucose level ≤ 10 mM. If no contraindications existed, the patients were administered with beta-blockers before the examination. Patients were scanned from the inguinal region to the base of the skull. Acquisition time was 1.5 min per bed position. CT images were acquired for attenuation correction and anatomic correlation of the PET images. A diagnostic CT with intravenous and oral contrast or a low-dose CT without contrast was performed. In our clinical routine, a low-dose is performed if a previous diagnostic CT has been performed within 4 weeks. For diagnostic CTs, tube current modulation was applied by adjusting the tube current for each individual with a noise index of 42.25 and a tube voltage of 100 kV. For low-dose CT, the tube voltage was 120 kV with a noise index of 45. If a diagnostic CT was performed, it was used for attenuation correction (delayed venous phase of intravenous contrast). The same CT was used for attenuation correction for all PET reconstructions. The adaptive statistical iterative reconstruction technique (ASiR-V) was applied for all CT reconstructions.

In order to compare the reconstruction algorithms, we reconstructed different data series, where the selected reconstruction parameters were based on phantom measurements in accordance with the EARL standard [10]. The EARL standard defines lower and upper limits for the resolution recovery coefficient (RRC) for different sized spheres in the NEMA-phantom and limits of the noise level. Two reconstructions were made corresponding to the lower (EARL_lower) and upper level (EARL_upper) of the RRCs. The ordered subset expectation maximization (OSEM) algorithm was used without resolution recovery or time of flight. For the upper level, the images were reconstructed with 4 iterations, 16 subsets and a Gaussian post filter with a FWHM of 5 mm. For the lower-level images, the reconstructions were performed with 3 iterations, 8 subsets and a post-filter with 7 mm FWHM. A new EARL standard has been proposed where the RRC limits have been substantially increased to accommodate modern systems [9]. One reconstruction was made which yields EARL results that fall near the upper level of the new EARL standard. The QC reconstruction algorithm was used, with a beta value of 500 [11]. The slice thickness for all three reconstructions was 2.79 mm, the matrix and pixel size were 192 × 192 and 3.64 mm for the EARL reconstructions and 256 × 256 and 2.73 mm for the QC reconstruction.