Compared to conventional PET/CT scanners, silicon-photomultiplier-based PET/CT scanners show higher arterial 18F-FDG uptake in whole-body 18F-FDG-PET/CT

Variants of physiological FDG vascular activity on digital PET

OBJECTIVE

Fluorodeoxyglucose PET/computed tomography (FDG PET/CT) is effective in detecting large vessel vasculitis. Digital PET cameras have improved spatial resolution compared with analog PET, resulting in more prominent physiological uptake in arterial walls. This study's goal was to define qualitative normal variants of arterial activity on digital PET/CT.

METHODS

We retrospectively reviewed 126 oncological PET/CT studies. Exclusion criteria included history of vasculitis, immunosuppressant therapy, hyperglycemia, or altered FDG biodistribution. Qualitative vessel wall activity (common carotid, brachiocephalic, subclavian, aorta, and femoral) was visually graded by two nuclear physicians with guideline-proposed criteria: 0: ≤mediastinum, 1: liver, where grade 3 is compatible, 2 is possible, and <2 is negative for vasculitis. Cranial artery uptake was visually graded as follows: grade 0: ≤surrounding tissues, grade 1: just above surrounding tissues, and grade 2: significantly above surrounding tissues, with grades 1 and 2 considered positive for cranial artery vasculitis.

RESULTS

Large vessel uptake was grade 3 in 0 subjects, grade 2 in four subjects (3%), grade 1 in 87 subjects (69%), and grade 0 in 35 subjects (28%). In studies acquired ≥75 min post-injection, 1/15 subjects had grade 2 uptake. Four subjects (3%) had grade 1 vertebral artery uptake. No subjects had temporal, maxillary, or occipital artery uptake.

CONCLUSION

A minority of our subjects presented with grade 2 large vessel uptake, which was associated with longer uptake times, or grade 1 cranial artery uptake, which was associated with higher age and glycemia. These findings should be interpreted with caution in patients referred for suspected vasculitis, as they may represent normal variants on digital PET.

A low-dose protocol in pediatric 18F-FDG scans using 30-cm axis field of view PET/CT

OBJECTIVE

To explore the feasibility of a low-dose 18F-FDG protocol for the 30-cm standard axial field of view (SAFOV) PET/CT imaging in pediatric patients.

METHODS

A retrospective analysis was conducted on 112 pediatric patients who underwent a full-dose (3.7 MBq/kg) 18F-FDG PET/CT imaging, and a prospective analysis was performed on 55 patients who received a low-dose (2.5 MBq/kg) imaging. PET images were reconstructed at 1.0-min/bed intervals, and labeled as G1.0, G2.0, G3.0 for the full-dose imaging and G1.0', G2.0', G3.0' for the low-dose imaging. Patients were categorized into three age groups, and the image quality was assessed using the Likert scale and signal-to-noise ratio (SNR); Lesion detectability was evaluated using lesion detection rates and the target-to-liver ratio (TLR).

RESULTS

In G2.0 and G3.0, all cases (112/112) achieved an image score of ≥ 3 and a lesion detection rate of 100% (98/98). There were no significant differences in SNR between G2.0 and G3.0 (11.09 ± 2.31 vs. 11.88 ± 2.58, p = 0.39), nor between age groups ≤ 5 years and 6-10 years groups (9.52 ± 3.16 vs. 9.53 ± 3.19, p = 0.99). In G3.0', 98.2% of cases (54/55) had an image score ≥ 3 and a lesion detection rate of 100% (43/43). The SNR of every age group for G3.0' was comparable to that of G2.0, and no significant differences between ≤ 5 years and 6-10 years groups (9.32 ± 1.94 vs. 9.99 ± 2.28, p = 0.82).

CONCLUSIONS

A 2.5 MBq/kg dose with a 3.0 min/bed acquisition protocol is feasible for 18F-FDG 30-cm SAFOV PET/CT imaging in pediatric patients, and SNR and TLR demonstrated age-dependent discrepancies.

Simultaneous evaluation of brain metastasis and thoracic cancer using semiconductor 11C-methionine PET/CT imaging

OBJECTIVE

To investigate the potential of whole-body digital 11C-methionine (MET) PET/CT imaging for simultaneous evaluation of thoracic cancer patients suspected of local recurrence (LR) after stereotactic radiosurgery (SRS) for brain metastasis.

METHODS

A total of 45 lung or breast cancer patients suspected of LR after SRS were investigated using brain and whole-body MET-PET/CT scans. We compared the tumor-to-normal ratio (TNR) and maximum standardized uptake values (SUVmax) between patients with LR and radiation necrosis (RN) and performed receiver operating characteristic (ROC) analyses. We also investigated associations among extracranial recurrence, intracranial recurrence, primary site, and initial treatment type.

RESULTS

A total of 44 LR and 14 RN lesions were analyzed. In the ROC analyses for differentiating LR from RN, TNR showed higher area under the curve (AUC) (0.82) than SUVmax (0.79), and the cutoff TNR value (2.12) was higher than current cutoff values of conventional PET systems. The whole-body scans detected extracranial recurrences in 31.1% of the patients. Recurrence rates were not significantly correlated with existence of intracranial recurrence or primary site, but patients who underwent non-surgical treatment (consisting of stage III/ IV patients according to the Union for International Cancer Control TNM classification or small-cell lung cancer patients) showed significantly higher recurrence than the surgically treated patients (68.8% vs. 10.3%, p = 0.0001).

CONCLUSION

In digital MET-PET/CT imaging, TNR was a more useful parameter to differentiate LR from RN than SUVmax, and the cutoff value was higher than those with conventional PET systems. Additional whole-body scans could detect extracranial recurrence and would be especially useful for advanced thoracic cancer patients who underwent non-surgical treatment.