Purpose: Iterative reconstruction (IR) is a statistical reconstruction method that may be influenced by high background activity such as in the liver. Recently developed combined positron emission tomography (PET) and computerized tomography (CT) instrumentation utilizes CT attenuation correction that may also differ compared to 68Germanium (68Ge) segmented attenuation correction (IR SAC). Quantitative differences could affect the clinical interpretation of metastatic foci and subsequent response to therapy. The purpose of this study is to characterize potential quantitative differences specifically in hepatic metastases.
Procedures: Thirty-three metastatic liver lesions in 23 patients were evaluated. Whole-body 2-deoxy-2[18F]fluoro-D-glucose(FDG) PET images were obtained on a dedicated PET-CT device and reconstructed with IR and CT measured attenuation correction (IR CT MAC), IR and 68Ge segmented attenuation correction, and also with filtered back-projection and segmented attenuation correction (FBP SAC). Regions of interest over the liver lesions (L) and liver background (B) were drawn on FBP SAC images and superimposed on the co-registered IR CT MAC and IR SAC images for comparison. To identify the individual effects of IR and CT attenuation correction, IR SAC was first compared to FBP SAC, followed by a comparison of IR CT MAC and IR SAC. Differences were expressed as bias (%) and compared to FBP SAC.
Results: Compared to FBP SAC, IR SAC showed significantly lower liver lesion (mean bias -7.1%; P < 0.00001), significantly higher liver background (mean bias -3.5%; P = 0.005), and significantly lower L/B ratio (mean bias -10.1%; P < 0.00001). Compared to IR SAC, the IR CT MAC showed small but not statistically significant increases in liver lesion (mean bias 3.0%; P = 0.1) and liver background (mean bias 4.3%; P = 0.09), and no significant difference in L/B ratio (mean bias -1.3%; P = 0.5). The overall effect of IR CT MAC compared to FBP SAC was a significant decrease in liver lesion (mean bias -4.2%; P = 0.002), significant increase in liver background (mean bias 8.7%; P = 0.002), and significantly lower L/B (mean bias -11.1%; P < 0.00001) compared to FBP SAC.
Conclusion: Compared to FBP, IR resulted in significantly lower mean hepatic tumor activity, higher mean liver background activity, and lower L/B activity, but these differences were modest. These differences were similarly seen in the CT corrected IR CT MAC when compared to FBP SAC. CT attenuation correction did not result in significant differences when compared to 68Ge correction. Although the differences in IR CT MAC were modest, consistent reconstruction methods are important to decrease the measurement variability and improve reproducibility.