Use of rebinned spiral CT data to better approximate the 511 keV attenuation correction factors for PET/CT imaging

In reconstructing PET data in PET/CT imaging the determination of attenuation correction factors (ACFs) involves the scaling and reprojecting of the CT images. However in the case of truncation it is possible to obtain a better estimate of the ACFs passing through the CT field-of-view, since the ACFs at the CT energy (-70 keV) are known directly from the CT raw data. A proposed method for estimating the correct ACFs at 511 keV is to calculate the ACFs from the reconstructed CT images in the usual way at 511 keV, but also calculate them at 70 keV (i.e. without scaling). The correction to ACF at 511 keV is taken as the difference between the measured and reprojected ACF at 70 keV, scaled to 511 keV assuming it scales like water. It is expected that this procedure may be useful in obtaining better quantification of lesions within the CT field-of-view in the case of very large patients where the adipose tissue outside the CT field-of-view accounts for much of the attenuation.