Reported Differences Between Digital and Analog PET/CT Studies

TO THE EDITOR: We read with interest the recent article by Koopman et al. entitled “Performance of Digital PET Compared with High-Resolution Conventional PET in Patients with Cancer” (1) published in the October issue of The Journal of Nuclear Medicine, describing the improved performance of the digital PET compared with the high-resolution analog PET in the detection of small lesions and image quality, which allows disease upstaging. These results are in line with our previous published data, showing superior performance of the digital PET/CT over the analog PET/CT in the detection rate and image quality in a group of oncologic (2) and nononcologic patients. In the discussion section, Koopman et al. indicated that our study showed 22 additional small lesions (<10 mm) on digital PET images in 100 oncologic patients. Indeed, the detection performance was even superior since in 17 out of the 100 oncologic patients, neither the digital nor the analog PET/CT revealed radiotracer uptake suggestive of malignancy (PET-negative). Therefore, the detection performance of the digital PET/CT was superior to the analog PET/CT in detecting subcentimeter lesions (<10 mm) in 22 out of the 83 PET-positive patients (26.5%) (P = 0.05, 95% confidence interval, 17.9–36.7). In addition, the authors considered invalid another previous study coming from our laboratory comparing the SUV_max between the digital PET/CT and the analog PET/CT (3) because of the differences in reconstruction parameters between both systems. It is well known that various technical and physics issues, such as detector performance, voxel size, and reconstruction parameters, influence SUV measurements. However, the aim of our study was not to evaluate the influence of such factors on SUV measurements, but to compare under standard clinical conditions the values rendered by both systems. The results of Koopman et al. (1) are again in line with our previous results showing increase of the SUV_max in the digital PET/CT as compared with the analog PET/CT. Koopman et al. compared digital and analog PET/CT using high-resolution reconstructions for both systems, whereas our study compared digital and analog PET/CT under standard clinical conditions as provided by the vendor (digital PET using high-resolution reconstructions and analog PET using standard-resolution reconstructions).

We believe that in the next years, digital PET/CT will coexist with analog PET/CT, and differences in performance and SUV measurements must be considered in follow-up studies. Differences in clinical and research performance will guide the selection of the appropriate system for future given indications, with impact on the diagnosis and therapy assessment of oncologic and nononcologic diseases.

• © 2020 by the Society of Nuclear Medicine and Molecular Imaging.