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.
Adaptive Statistical Iterative Reconstruction (ASIR) is a new imaging reconstruction technique recently introduced by
General Electric (GE). This technique, when combined with a conventional filtered back-projection (FBP) approach, is
able to improve the image noise reduction. To quantify the benefits provided on the image quality and the dose reduction
by the ASIR method with respect to the pure FBP one, the standard deviation (SD), the modulation transfer function
(MTF), the noise power spectrum (NPS), the image uniformity and the noise homogeneity were examined.
Measurements were performed on a control quality phantom when varying the CT dose index (CTDIvol) and the
reconstruction kernels. A 64-MDCT was employed and raw data were reconstructed with different percentages of ASIR
on a CT console dedicated for ASIR reconstruction. Three radiologists also assessed a cardiac pediatric exam
reconstructed with different ASIR percentages using the visual grading analysis (VGA) method. For the standard, soft
and bone reconstruction kernels, the SD is reduced when the ASIR percentage increases up to 100% with a higher
benefit for low CTDIvol. MTF medium frequencies were slightly enhanced and modifications of the NPS shape curve
were observed. However for the pediatric cardiac CT exam, VGA scores indicate an upper limit of the ASIR benefit.
40% of ASIR was observed as the best trade-off between noise reduction and clinical realism of organ images. Using
phantom results, 40% of ASIR corresponded to an estimated dose reduction of 30% under pediatric cardiac protocol
conditions. In spite of this discrepancy between phantom and clinical results, the ASIR method is as an important option
when considering the reduction of radiation dose, especially for pediatric patients.