In our study we aimed to determine the maximum 18F-FGD dose reduction on a BGO PET/CT camera that would not compromise the image quality. The minimum feasible dose was defined by proxy – we checked if reduction of acquisition time can be directly translated into dose reduction. We showed that there is a linear correlation between time per bed, administered activity and number of total prompts detected by the scanner. Hence, we demonstrated that time per bed reduction is directly proportional to dose reduction. In visual analysis we reported that time per bed can be safely reduced by 25% (from 2 min to 1.5 min) without significant compromise in image quality (only 2 min and 1.5 min scans received a mean grade ≥ 7). This in turn could be translated into a 25% tracer dose reduction – from 3.0 to 2.25 MBq/kg. We reported that administration of a reduced dose with 2 min time per bed acquisition is a feasible protocol that does not compromise the image quality.
The radiation dose reduction from PET/CT scans can be achieved by optimising either PET or CT scanning protocols. It has been robustly shown that various alterations in CT protocols may cut the used radiation by as much as 1/3 (from 8.1 mSv to 5.5 mSv) [11]. On the other hand, the literature on the radiotracer activity reduction is rather scant.
According to the European Association of Nuclear Medicine (EANM) recommendations the minimum administered activity for a gamma camera like we used in the study (with ≤ 30% bed overlap) and a 2 min time per bed should be 7 MBq/kg. The guidelines also state that the dose can be lowered for PET/CT systems with higher sensitivity or improved performance [3]. In our institution—accordingly to national regulations, instructions from the producer of our PET/CT system, available literature and our personal experience—we routinely administer activities of about 3.7 MBq/kg with acquisition time of 1.5 min [12, 13].
Experimentally, for the purpose of the study, we have reduced the administered dose to 3.0 MBq/kg. We show that further reduction to 2.25 MBq/kg with 2 min acquisition time might be feasible. It is in accordance with a study performed on the same type of scanner (GE Discovery IQ) that showed similarly high sensitivity and performance of the camera after the injection of 2.5 MBq/kg of 18F-FDG. High sensitivity of the scanner was achieved by adopting several technological solutions such as the 3-dimensional mode, extension of the axial field of view (FOV) and increasing number of detector rings from 2 to 5 along FOV [14, 15]. The system also uses a new reconstruction algorithm (Bayesian penalised likelihood algorithm named Q.Clear) that improves signal-to-noise ratio and standardised uptake value (SUV) quantification. In Q.Clear the noise suppression is controlled by a penalty term beta (the only one user-adjusted term in the algorithm). The algorithm also incorporates point-spread-function modelling [16–18].
Prieto et al. showed that average 18F-FDG dose reduction of 23.4% (down to 3.57 MBq/kg) is feasible without significant impairment of image quality. Yet, the study was performed on a 4-ring lutetium oxyorthosilicate PET/CT scanner [19]. On the other hand, Murray et al. reported that emission scans as short as 15 sec per bed position sufficiently identified tumour lesions for quantification. The scans were performed on a Gemini TF PET/CT system after injection of 269–411 MBq of 18F-FDG (3.8–5.9 MBq for a 70 kg patient) [20]. As shown, contemporary PET/CT technology allows for a notable reduction of radiotracer doses compared to the current guidelines.
The findings of this study have to be seen in light of some limitations. First, the research focused on a single clinical condition – Hodgkin lymphoma. It is highly probable that a 25% dose reduction would also be feasible in other aggressive, FDG-avid lymphomas and malignancies yet it may not be so in case of indolent tumours. Hence, further research on more heterogenous groups of patients is needed to explore this subject. Our research is a retrospective study and might show some of method’s inherent limitations such as selection bias or confounding.