Digital vs. analog PET/CT: intra-subject comparison of the SUVmax in target lesions and reference regions

The purpose of this study was to assess whether digital photon counting technology in digital PET/CT influences the quantification of SUVmax in target lesions and regions of reference compared to analog PET/CT before an interchangeable use of either system in follow up studies.

From January to June of 2018, 100 oncological patients underwent successive PET/CT imaging with digital and analog systems in the same day. Fifty-eight patients underwent analog imaging first and digital imaging thereafter, and 42 patients the other way round. SUVmax was measured in reference regions (liver and mediastinal blood pool) and in the most metabolically active target lesion in each patient. According to the sequence order of PET/CT acquisition, two groups of SUVmax values were obtained, i.e. group 1: analog PET/CT performed first; group 2: digital PET/CT performed first.

Mean SUVmax in the total sample (regardless of the order of PET/CT acquisition) in the target lesions with the analog PET/CT was 8.14 ± 6.39 and the digital 9.97 ± 6.14 (P = 0.000). Total mean SUVmax in the liver with the analog was 4.39 ± 2.59 and the digital 4.46 ± 3.18 (P = 0.477). Total mean SUVmax in the mediastinal blood pool with the analog was 2.30 ± 0.67 and the digital 2.54 ± 0.74 (P = 0.000). Group 1: mean SUVmax in the target lesions with the analog system was 6.64 ± 4.71 and the digital 9.48 ± 5.60 (P = 0.000). Mean liver SUVmax with the analog was 4.70 ± 2.90 and the digital 4.80 ± 3.72 (P = 0.088). Mediastinal blood pool SUVmax with the analog was 2.33 ± 0.66 and the digital 2.45 ± 0.73 (P = 0.041). Group 2: mean SUVmax in target lesions with the digital system was 10.63 ± 6.88 and the analog 10.16 ± 7.76 (P = 0.046). Mean liver SUVmax with the digital was 3.99 ± 2.20 and the analog 3.96 ± 2.04 (P = 0.218). Mediastinal blood pool SUVmax with the digital was 2.66 ± 0.75 and the analog 2.27 ± 0.68 (P = 0.000). No significant differences between both time delays were found.

Improved photon counting technology in the digital PET/CT, and the effect of delayed increased uptake and retention significantly increases SUVmax values. This has to be taken into account before interchangeable use of either system in follow up studies.