Impact of injected activity and scanning time to the image quality of PET/CT
FENG Yan lin *, HE Xiao hong, HUANG Ke min, YU Feng wen, LIU De jun, YUAN Jian wei, YUAN Bai hong, SU Shao di(PET/CT Center, Department of Nuclear Medicine, the First People's Hospital of Foshan, Foshan 528000, China)
Objective To probe the image quality of PET/CT effected by injected activity and scanning time from phantom and patients' studies. Methods For phantom studies, a Jaszczak phantom with smaller diameter of cold and hot inserts were imaged in 3 different scanning conditions. The first was higher activity level and shorter scanning time (81.918 MBq、2 min per bed), the second was recommended activity level and scanning time (42.654 MBq, 3 min per bed)by the manufacturer, and the last one was lower activity level and longer scanning time (15.947 MBq,4 min per bed). For patients' studies,the same patient position and scanning parameters were used after 50 min and 90 min with 18 F FDG injected, respectively. For both phantom and patients' studies,the corresponding X ray CT data were used for the attenuation correction,and the clinical whole body reconstruction modes were used for the relevant scanning data. Then the image quality, especially the spatial resolution and contrast of the images, were analyzed and compared visually and quantitatively. A volunteer with 74 kg was imaged with the total injected activity of 152.026 MBq and the scanning time of 3 min 30 s per bed. Results Diameter with 6.4 mm of cold and hot inserts were resolved in the 3 imaging methods with phantom studies, respectively.There were also no artifacts found in the uniformity regions. For patient studies,better image quality was contributed by the images acquired at 90 min after 18 F FDG injected,comparing with that of 50 min. Outline of part of the lesions in the 50 min's images was difficult to distinguish from the normal tissue, while the number and shape of lesions were increased and improved in the same area as the 90 min's images. The volunteer's images were also good enough for clinical diagnosis. Conclusion It was important to find out the tradeoffs between the total injected activity and scanning time to decrease the impact to the image quality of PET/CT. It was also a good idea to increase the interval more than 90 min between the 18 F FDG injected and the beginning acquisition time, combining prolonging the duration appropriately for better image quality of PET/CT.