Dose reduction and image quality in CT examinations using an iterative reconstruction algorithm: a phantom study

The purpose of this work is to evaluate the potential dose reduction in computed tomography (CT) examinations using an iterative algorithm compared to a filtered backprojection (FBP) reconstruction, analyzing image quality parameters. A 256 slice CT scanner, equipped with software for both FBP and iterative reconstruction, and a CT dedicated phantom were used. The image quality was evaluated by analyzing five different parameters: CT number constancy, signal to noise ratio (SNR), contrast to noise ratio (CNR), non-uniformity and axial spatial resolution (modulation transfer function, MTF). The CNR was evaluated on a low contrast insert and on a high contrast teflon insert, while the MTF was evaluated on a high density bead. The analysis was performed both on images obtained with FBP reconstruction and with the iterative algorithm (level 1 to 6). CT numbers were consistent between standard FBP and the iterative reconstruction. Non-uniformity and spatial resolution were not affected by the use of the iterative algorithm, and the SNR and the CNR were improved with the iterative algorithm, as a function of the level of iteration used. Comparing FBP to the sixth level of iteration, in a 120 kV scan performed at 200 mAs, a mean 43.6% noise reduction was observed and for the SNR and the CNR, a mean improvement of 70.9% and 70.6%, respectively, was obtained. A mean dose reduction of 36.5% was estimated using iterative reconstruction at level 3 without loss of image quality. The use of the iterative reconstruction method offers the possibility to improve image quality or to reduce patient dose in CT examinations, or a combination of these aspects. In our experience, reducing the tube current by 30% compared to the FBP scan and reconstructing the images by means of the iterative algorithm (level 3) resulted in equivalent image quality for both methods.