Injection-to-scan delay correction improves scan-rescan reproducibility sodium fluoride (¹⁸F-NaF) coronary plaque imaging.

Objectives: High test-retest reproducibility is required for the translation of coronary ¹⁸F-NaF PET imaging into clinical practice. In this study we investigate the feasibility of correcting for variations in the injection-to-scan delays and its relative impact on the test-retest reproducibility.

Methods: Twenty patients with coronary artery disease who underwent repeated hybrid PET/CT angiography (CTA) imaging within 3 weeks were recruited. Both imaging sessions included a 30-min PET acquisition following injection of 248±9 MBq ¹⁸F-NaF (injection-to-scan delay = 60 min) and a CTA-scan. All PET datasets were reconstructed in end-diastolic phase (25% of the counts). Coronary lesions were identified on CTA images in arteries with diameter ≥2mm, with stenosis of >25% in the coronary segment and no prior stents. Lesion uptake was quantified from spherical Volume of Interests (VOIs) (radius=5 mm), while background activities were obtained in the right atrium using a cylindrical VOI (length=15 mm, radius=10 mm). Lesions with TBR>1.25 were considered ¹⁸F-NaF avid, while lesions with TBR <1.25 were considered ¹⁸F-NaF negative. We corrected injection to imaging intervals for all acquisitions by normalizing the injection-to-scan delays to 60 minutes post injection (equation 1): SUV_(Background Corrected) = SUV_(Background)[asterisk] ((1.5092[asterisk]exp(-0.004[asterisk]60)/(1.5092[asterisk]exp(-0.004[asterisk]t)), based on prior study (1). The correction was applied to the extracted blood pool activities for all the acquisitions. We report the injection-to-scan delays and the TBR before and after background blood pool correction (BC). The test-retest evaluations before and after BC were reported as Bland-Altman analyses and coefficient of reproducibility measures.

Results: A total of 47 unique coronary lesions (15 ¹⁸F-NaF-avid) were identified on CTA. Average injection-to-scan delays were found to be (66±9 min, range 59-101 min). The increased injection-to-scan delays (>60 min) introduced a BC correction factor >1 and, thus, a reduction in the TBR following BC (TBR: Non-corrected=1.18±0.48, BC=1.14±0.46, relative reduction: 2.5±3.8%, range: -0.4% to 17.8%, p=0.98). Correcting for BC increased test-retest reproducibility for all lesions by 19.7% (Coefficient of reproducibility: Non-corrected = 0.437, BC= 0.365, p<0.001). Importantly, in a sub-analysis of ¹⁸F-NaF-avid lesions only, the test-retest reproducibility was improved by 25.3% (Coefficient of reproducibility: Non-corrected = 0.628, BC=0.501, p<0.001) (Figure 1).

Conclusions: BC technique significantly improves reproducibility of the coronary plaque assessment utilizing only the PET-image data. (1) Kwiecinski J, Berman DS, Lee S-E, et al. Three-hour delayed imaging improves assessment of coronary ¹⁸ F-sodium fluoride PET. J Nucl Med. 2018:jnumed.118.217885.

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