The noise power spectrum (NPS) is the reference metric for understanding the noise content in computed tomography
(CT) images. To evaluate the noise properties of clinical multidetector (MDCT) scanners, local 2D and 3D NPSs were
computed for different acquisition reconstruction parameters.
A 64- and a 128-MDCT scanners were employed. Measurements were performed on a water phantom in axial and
helical acquisition modes. CT dose index was identical for both installations. Influence of parameters such as the pitch,
the reconstruction filter (soft, standard and bone) and the reconstruction algorithm (filtered-back projection (FBP),
adaptive statistical iterative reconstruction (ASIR)) were investigated. Images were also reconstructed in the coronal
plane using a reformat process. Then 2D and 3D NPS methods were computed.
In axial acquisition mode, the 2D axial NPS showed an important magnitude variation as a function of the z-direction
when measured at the phantom center. In helical mode, a directional dependency with lobular shape was observed while
the magnitude of the NPS was kept constant. Important effects of the reconstruction filter, pitch and reconstruction
algorithm were observed on 3D NPS results for both MDCTs. With ASIR, a reduction of the NPS magnitude and a shift
of the NPS peak to the low frequency range were visible. 2D coronal NPS obtained from the reformat images was
impacted by the interpolation when compared to 2D coronal NPS obtained from 3D measurements.
The noise properties of volume measured in last generation MDCTs was studied using local 3D NPS metric. However,
impact of the non-stationarity noise effect may need further investigations.