Influence of residual oxygen-15-labeled carbon monoxide radioactivity on cerebral blood flow and oxygen extraction fraction in a dual-tracer autoradiographic method

Objective Cerebral blood flow (CBF), cerebral metabolic rate of oxygen (CMRO 2 ), oxygen extraction fraction (OEF), and cerebral blood volume (CBV) are quantitatively measured with PET with 15 O gases. Kudomi et al. developed a dual tracer autoradiographic (DARG) protocol that enables the duration of a PET study to be shortened by sequentially administrating 15 O 2 and C 15 O 2 gases. In this protocol, before the sequential PET scan with 15 O 2 and C 15 O 2 gases ( 15 O 2 –C 15 O 2 PET scan), a PET scan with C 15 O should be preceded to obtain CBV image. C 15 O has a high affinity for red blood cells and a very slow washout rate, and residual radioactivity from C 15 O might exist during a 15 O 2 –C 15 O 2 PET scan. As the current DARG method assumes no residual C 15 O radioactivity before scanning, we performed computer simulations to evaluate the influence of the residual C 15 O radioactivity on the accuracy of measured CBF and OEF values with DARG method and also proposed a subtraction technique to minimize the error due to the residual C 15 O radioactivity. Methods In the simulation, normal and ischemic conditions were considered. The 15 O 2 and C 15 O 2 PET count curves with the residual C 15 O PET counts were generated by the arterial input function with the residual C 15 O radioactivity. The amounts of residual C 15 O radioactivity were varied by changing the interval between the C 15 O PET scan and 15 O 2 –C 15 O 2 PET scan, and the absolute inhaled radioactivity of the C 15 O gas. Using the simulated input functions and the PET counts, the CBF and OEF were computed by the DARG method. Furthermore, we evaluated a subtraction method that subtracts the influence of the C 15 O gas in the input function and PET counts. Results Our simulations revealed that the CBF and OEF values were underestimated by the residual C 15 O radioactivity. The magnitude of this underestimation depended on the amount of C 15 O radioactivity and the physiological conditions. This underestimation was corrected by the subtraction method. Conclusions This study showed the influence of C 15 O radioactivity in DARG protocol, and the magnitude of the influence was affected by several factors, such as the radioactivity of C 15 O, and the physiological condition.