LAUSR

Dear Sir, We have received the letter to the editor written by Dr. Renaud et al. [1], addressing our recent publication regarding system saturation with standard Rb doses for PET myocardial perfusion imaging [2]. Herein, we address their comments in a point-by-point fashion below. In their letter, the authors note that the injection activities of Rb employed in our manuscript exceed the proposed injected activities as published in the most recent (2018) joint-position statement between the Society of Nuclear Medicine and Molecular Imaging (SNMMI) Cardiovascular Council and American Society of Nuclear Cardiology (ASNC) [3]. This latest statement proposes injections of 1100 MBq (30 mCi) of Rb for the assessments of myocardial blood flow (MBF), which are slightly below the injected activities used in our study (1247 ± 196 MBq (33.7 ± 5.3 mCi))—considered as full-dose studies. The scans acquired in our study were performed (February–September 2016) [2] and were obtained before this position statement, using the most recent published guidelines from ASNC/SNMMI and the European Association of Nuclear Medicine/European Society of Cardiology [4, 5] which recommend 1100–1480 MBq (30–40 mCi) for 3D LYSO PET scanners—the scanner type evaluated in our report. Of note, the PET system saturation was observed in 20% (n = 4) of the cases with full-dose scans. In 3 of these 4 cases, the saturation occurred when using injected activities within 11% of 1100 MBq (30 mCi) of injection dose recommended in the latest joint ASNC/SNMMI position statement. Of these 3 patients, one had an injected activity even lower than the newly suggested dose (injected activity of 1006MBq (27.2 mCi)). On the other hand, we did not observe any PET system saturation effects in studies employing the half-dose protocol (662 ± 115 MBq (17.9 ± 3.1 mCi), maximum injected activity of 895 MBq (24.2 mCi)). Regarding the method for identification of PET system saturation, our study relied mainly on visual assessments. Although visual assessments are more subjective than objective quantitative criteria, this identification protocol provides an easily implementable assessment. The findings in our study are in alignment with those of Tout et al. [6] (15% saturation rate) and, thus, suggest that even visual identification of PET system saturation is feasible. Nonetheless, we agree that in future studies more automated methods should be explored. In a paper published by some authors of the letter [7], they have shown that non-conformities in the injected dose of up to 68% could be calibrated by phantom scans to estimate the maximum patient weight/injected dose ratio. They also suggest that weight-based dosing is possible even without recalibration of the cart when residual dose activities are taken into account.While this is feasible, in the paper byMoody et al. [8] cited in support of this approach, the authors showed nonconformities in the injected activities that might account for more than 10% when injecting doses ≤ 962 MBq (26 mCi), thus advocating for recalibration between each study when having patients with different weights. By using a half-dose protocol, only one calibration is needed per day which permits for the correction of the residual activity in the vials at different generator ages and various generators. Furthermore, the authors suggest that the commercial product RUBY-FILL generator permits the use of constant-activity infusion (slowinfusion) methods, which may avoid PET system saturation. This is however beyond the scope of our study, since we use a different type of the generator, which is widely used in many clinical centers. This article is part of the Topical Collection on Letter to the Editor