Evaluation of noise power spectra of CT images

Using a commercial clinical CT scanner (GE lightspeed), nine CT scans were performed on a 20 cm diameter plastic pipe filled with water. The mAs was varied from 10 to 400mAs and the beam energy was varied from 80 to 140kVp. For each scan three volume datasets were reconstructed using different filters. Noise power spectrum (NPS) curves were measured to examine the effect of varying kVp, mAs and reconstruction filter on the noise content. Sixteen slices from each of the reconstructed volumes were used to compute the NPS; the central 192 x 192 pixels of each slice were split into four overlapping regions of interest (ROI) of 128 x 128 pixels. A total of 64 ROI were used per scan. The magnitude squared of the 2D Fourier transform of each ROI was computed. The mean of the 64 2D results was averaged over radial frequency, yielding a 1D NPS. The overall shape of the NPS was dependent on the reconstruction filter used. The magnitude of the curves decreased with the increase of mAs or kVp. kVp, mAs, and the reconstruction filter can be adjusted to modulate- the amount of noise present in resulting CT volumes, but the effect these values have on the patient must be considered. The relationship between NPS and the Noise Equivalent Quanta (NEQ) makes trends in NPS important and is the motivation for this evaluation and future research.