Delay time of arterial input function measured by an external radiation detector must be estimated correctly in order to accurately measure rCBF or CMRO2 with O-15 tracer and PET by autoradiographic method. Conventionally, in order to estimate the delay time, the measured arterial input curve is compared with tissue time activity curve (TTAC) from reconstructed image and the delay time is determined by the fitting procedure. Instead of getting TTAC from the reconstructed image (image based method), Shidahara et al1 proposed a method to get TTAC from the sinogram data (sinogram based method), which enables more rapid estimation of the delay time. CO2 or O2 gas is usually supplied by a mask and a subject inhales the radioactive gas through the mask. During PET acquisition, radioactive gas in mask and nasal cavity contributes large artifact on the sinogram data and it is difficult to estimate delay time from the sinogram data. We proposed a new method to estimate the delay time using the sinogram data and the attenuation map (attenuation weighted sinogram method). In the present method, the attenuation map was used to eliminate the effect of the gas outside the brain region from the sinogram data. For the validation of the present method, PET data with CO2 (n=10) were analyzed. Three methods, namely the image base method, the sinogram base method and the attenuation weighted sinogram method were used to estimate the delay time. The estimated delay times and calculated rCBF images by three methods were compared. Due to the radioactivity outside of the brain, the sinogram method significantly overestimated the delay time and thus underestimated the rCBF value compared with the image base method. On the other hand, there were good agreements between the delay times estimated by the attenuation weighted sinogram method and the image base method. The present method can eliminate the effect of the radioactivity outside of the brain on the sinogram data and estimate the delay time accurately and fast enough for clinical use.
