CT tube current for attenuation map in a combined PET/CT system: obese patient simulated phantom study

The CT portion of PET/CT provides attenuation correction of the PET emission scan. This study was performed to evaluate how much the CT tube current can be lowered while still providing attenuation maps on PET images.

Two body phantoms (outside diameters of 300 and 500 mm) were used to investigate, and PET/CT acquisitions were performed with an Aquiduo PCA-7000B (Toshiba Medical Systems, Otawara, Japan). The CT scan was performed with the following parameters (120 kVp; 0.5-s rotation; 10, 20, 40, 80, 160, 200, 320, 460 mA). After the CT scan, PET images for (18)F-FDG (5.3 kBq/mL) were obtained for 4 min/bed position. The linear attenuation coefficients for (18)F-FDG in 300- and 500-mm phantoms, pixel values and SD of CT images, radioactivity concentration values and hot- and cold-sphere contrast on PET images in the 500-mm phantom were evaluated.

In the 300-mm phantom, all eight tube currents gave average linear attenuation coefficients of approximately 0.095 cm(-1). In contrast, the average linear attenuation coefficients of the 500-mm phantom at 10, 20, and 40 mA were significantly decreased (0.081, 0.087, and 0.092 cm(-1), respectively; p < 0.05) as compared to 0.096 cm(-1) of the other tube currents. Further, CT pixel values decreased 10 and 20 mA. Thus, the background radioactivity concentration values at 10 and 20 mA were substantially underestimated to be 57 and 80%, respectively (p < 0.05); the hot-sphere contrast values at 10 and 20 mA were 0.26 and 0.29; the cold-sphere contrast values at 10, 20, and 40 mA were -0.33, -0.16, and 0.08.

Although the linear attenuation coefficients in the 300-mm phantom remained the same with varying CT tube currents, the 500-mm phantom yielded significant differences in the range 10-40 mA. Therefore, the CT tube currents for attenuation correction should be adjusted over 40 mA in obese patients.