Assessment of [18F]FAZA tumor-to-muscle ratios in CT26 colon carcinomas under different breathing protocols. A, we used three different breathing protocols: (P0) air; (P1) 100% oxygen 1 h prior injection until 3 h p.i.; (P2) 100% oxygen breathing starting 2 min prior tracer injection until 1 h p.i. and during the PET scans; mice were breathing air between the 2 h and 3 h 10 min static scans. B, an example of microPET images of CT26 colon carcinoma-bearing mice (arrowheads indicate tumors). Images were taken 1 h, 2 h and 3 h after injection of [18F]FAZA following the P0, P1 and P2 protocols. C, [18F]FAZA clearance (relative to 1 h after injection) from tumor and muscle tissue with a clear oxygen dependency (* p < 0.05; ** p < 0.01; P0: n = 10, P1: n = 7, P2: n = 7).