Affiliations: Department of Imaging Sciences, University of
Rochester Medical Center, Rochester, NY 14642, USA | Department of Physics and Astronomy, University of
Rochester. Rochester, NY 14627, USA
Abstract: The Noise Power Spectrum (NPS) is a function that yields information
about the spatial frequency composition of noise in images obtained by a
system. It is evaluated by calculating the absolute value squared of the noise
image and normalizing it with respect to the voxel and matrix sizes.
Consequently, the NPS has been one of the physical characteristics that is
commonly used to quantitatively measure the physical performance of a system.
In this article, we evaluated the NPS of a Cone Beam CT Breast Imaging system
by considering the following factors. First, we evaluated its symmetry around
the x- and y-axis along with the influence of the cone angle and the matrix
size on the NPS. Then, an analytical curve was suggested to best represent the
NPS. Second, we analyzed the influence on the NPS of a set of seven parameters,
namely the pixel size, exposure level, kVp value, number of projections
acquired, voxel size, back projection filter, and the reconstruction algorithm
employed. In addition, since the breast induced scattering in the image, we
investigated the effect of the scattering-correction algorithm used in this
system. Finally, we evaluated the uniformity of the NPS as a function of
z with the matrix center located at {r =
0 mm}. The results demonstrate that the proposed curve is an ideal candidate
that best represents the NPS. Hence, two parameters, the amplitude (A) and the
width (σ), can be used to characterize the curve. The
results also demonstrate that the voxel size and the cone angle are the only
two parameters investigated in this study that do not affect the NPS. On the
other hand, the matrix and pixel sizes, the back-projection filter and the
reconstruction algorithm, the exposure level and the scattering correction, all
influence the NPS. Finally, the results of the last part of this investigation
suggest that this imaging system does not have a 3D isotropic noise
distribution along the z-axis; yielding less noisy images at around
z = 0.00 m and z = 80 mm.
