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Purpose: 18F-fluoromisonidazole (18F-FMISO) is the most widely used positron emission tomography (PET) tracer for imaging tumor hypoxia. Previous reports suggested that the time from injection to the scan may affect the assessment of 18F-FMISO uptake. Herein, we directly compared the images at 2 h and 4 h after a single injection of 18F-FMISO. Methods: Twenty-three patients with or suspected of having a brain tumor were scanned twice at 2 and 4 h following an intravenous injection of 18F-FMISO. We estimated the mean standardized uptake value (SUV) of the gray matter and white matter and the gray-to-white matter ratio in the background brain tissue from the two scans. We also performed a semi-quantitative analysis using the SUVmax and maximum tumor-to-normal ratio (TNR) for the tumor. Results: At 2 h, the SUVmean of gray matter was significantly higher than that of white matter (median 1.23, interquartile range (IQR) 1.10–1.32 vs. 1.04, IQR 0.95–1.16, p < 0.0001), whereas at 4 h, it significantly decreased to approach that of the white matter (1.10, IQR 1.00–1.23 vs. 1.02, IQR 0.93–1.13, p = NS). The gray-to-white matter ratio thus significantly declined from 1.17 (IQR 1.14–1.19) to 1.09 (IQR 1.07–1.10) (p < 0.0001). All 7 patients with glioblastoma showed significant increases in the SUVmax (2.20, IQR 1.67–3.32 at 2 h vs. 2.65, IQR 1.74–4.41 at 4 h, p = 0.016) and the TNR (1.75, IQR 1.40–2.38 at 2 h vs. 2.34, IQR 1.67–3.60 at 4 h, p = 0.016). Conclusion: In the assessment of hypoxic tumors, 18F-FMISO PET for hypoxia imaging should be obtained at 4 h rather than 2 h after the injection. [ABSTRACT FROM AUTHOR]