Accuracy of CT-Based attenuation correction in bone imaging with PET/CT

PET/CT has become the most comprehensive diagnostic tool in oncology imaging providing improved lesion identification and localization. Bone is a common site of metastasis and the quantitative accuracy of PET images in bone tissue is important for assessing response to therapy. The use of CT images for attenuation correction is becoming a standard procedure in these scanners. However the impact of CT-based attenuation correction (CTAC) on the accuracy of PET tracer uptake values measured in bone has not been carefully evaluated, having only been carefully studied in soft tissue. We investigated the accuracy of CTAC on PET bone images by comparing the attenuation coefficients with PET transmission scans. For this, we imaged frozen bovine femur segments in a 20 x 20 cm cylindrical phantom. Different regions of the bones in both images were segmented by using thresholding and erosion methods to get equivalent volume masks. Differences in linear attenuation coefficients between the two images were then calculated. We repeated this analysis using patient images from the same patient imaged on the GE Advance PET scanner and the GE Discovery STE PET/CT scanner. The impact of the errors in the linear attenuation coefficients on PET SUV measurements was evaluated by simulations using the patent images with known bone disease and elevated levels of FDG uptake in bone (e.g. SUV = 5) at disease sites. The impact of the errors in the linear attenuation coefficients was then estimated by forward projection and reconstruction, after including the effects of attenuation and attenuation correction.