We conducted a phantom study to optimize the computed tomography (CT) acquisition protocol, to reduce patient dose without compromising the diagnostic quality of PET-CT study.
We conducted 42 PET-CT phantom studies. The jaszczak phantom was filled with 18F-fluorodeoxyglucose activity (1.4, 4, 6, 8, 10, and 12 mCi, on different settings) and PET-CT was acquired at different tube currents of 16, 25, 30, 35, 40, 45, and 50 mAs, keeping all the other parameters constant. Two nuclear medicine physicians qualitatively evaluated 252 images for their diagnostic quality (contrast, resolution, and number of lesion detected). The images were also evaluated qualitatively for noise on CT (the physical measure of image quality) and standardized uptake value (mean) on PET images. The weighted CT dose index (CTDIw) was measured for calculating the changes in the radiation dose with the change in tube current.
The 42 images provided same diagnostic information in CT images at different tube currents. However, there was a gradual increase in noise in the image acquired at lower values of the tube current, in mAs. The modulation of CT tube current had no effect on the PET images as well as the PET-CT images. Standardized uptake value mean values were also not affected with lowering of tube current. On reducing the tube current from 50 to 16 mAs, a reduction in the radiation dose (CTDIw) to almost 68% was achieved.
It is possible to reduce the tube current of the CT tube of PET-CT to a much lower level, without any loss of diagnostic information of a PET-CT image. Moreover, this can significantly reduce the radiation dose.
aDepartment of Nuclear Medicine, All India Institute of Medical Sciences
bDepartment of Health Physics, Institute Rotary Cancer Hospital, All India Institute of Medical Sciences, New Delhi, India
Correspondence to Dr Rakesh Kumar, MD, PhD, E-81, Ansari Nagar (East), AIIMS Campus, New Delhi 110029, India Tel: +91 112 658 8017; fax: +91 112 658 8663; e-mail: email@example.com
Received August 23, 2011
Accepted October 11, 2011