Objective: The purpose of this study was to investigate the effect of attenuation correction (AC) on lesion detection for a hybrid PET system.
Material and Method: Experimental list-mode data were acquired from hot spheres inside a uniform cylindrical
phantom with an elliptical cross-section using a Siemens E.CAM+ dual-camera hybrid PET system. Spheres with inner diameters of 0.8- and 1-cm and the cylindrical phantom were filled with F-18 to simulate lesions with lesion-to-background (L/B) ratios of 14:1 and 8:1, respectively, found in clinical PET studies. The list-mode data of each sphere size were regrouped into sinograms with peak-to-peak energy window settings at 30% and 20% for the 0.8- and 1-cm diameter lesion, respectively. They were then rebinned using the single slice rebinning method. Attenuation correction was applied assuming uniform attenuation. The
sinograms with and without AC were reconstructed using 5 iterations of OS-EM algorithm with 8 angles/subset and postfiltered with a Butterworth filter with n = 5 and fc = 0.52 cycles/cm. Human observer performance study and localization receiver operating characteristic (LROC) analysis were used to evaluate the reconstructed images for maximum lesion detection. Average areas under the LROC curves (ALROC) across 8 observers obtained with and without AC were determined. The null hypothesis that there was no difference between with AC and without AC was tested using a two-tailed t-test with 95% confidence interval.
Results: The results indicated that for the 0.8-cm lesion with 14:1 L/B ratio, the ALROC decreases from 0.66 to 0.62 when AC is applied as compared to without AC and from 0.69 to 0.63 for the 1.0-cm lesion with 8:1 L/B ratio, but no statistical significant difference (p > 0.05).
Conclusion: The authors conclude that for a phantom with hot lesions embedded in a uniform background, AC decreases lesion detectability compared to without AC using a hybrid PET system for small lesion sizes.

Keywords: Hybrid PET, Attenuation correction, LROC study